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How is the effective dose calculated when only part of the body is irradiated?

How is the effective dose calculated when only part of the body is irradiated?


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For calculating the effective radiation dose in Sv, the equivalent dose absorbed by each body part is averaged according to tissue-specific weighting factors, which sum up to 1.

If not the whole body is irradiated, how are these factors used?


Not sure that I understood your question correctly.

The concept of "effective dose" was specially introduced to provide a mechanism for assessing the radiation detriment from partial body irradiations in terms of data derived from whole body irradiations. The effective dose is the mean absorbed dose from a uniform whole-body irradiation that results in the same total radiation detriment as from the nonuniform, partial-body irradiation in question.

What is probably misleading you is the sum of the weighting factors to be 1. This is because we try to liken the partial body irradiation and the whole-body irradiation and the weighting factors represent the relative tissue sensitivity and susceptibility to the radiation. Your radiation should not go over all possible tissues and organs, you just assume the irradiation to be zero at them that cancels their summands so that they can be ignored in the final calculation. In other words, it is absolutely fine (and it is even so "by design") to have only some of the WT*HT factors in your formula.


Purpose: Late complications related to total body irradiation (TBI) as part of the conditioning regimen for hematopoietic stem cell transplantation have been increasingly noted. We reviewed and compared the results of treatments with various TBI regimens and tried to derive a dose–effect relationship for the endpoint of late renal dysfunction. The aim was to find the tolerance dose for the kidney when TBI is performed.

Methods and Materials: A literature search was performed using PubMed for articles reporting late renal dysfunction. For intercomparison, the various TBI regimens were normalized using the linear-quadratic model, and biologically effective doses (BEDs) were calculated.

Results: Eleven reports were found describing the frequency of renal dysfunction after TBI. The frequency of renal dysfunction as a function of the BED was obtained. For BED >16 Gy an increase in the frequency of dysfunction was observed.


Contents

In general, ionizing radiation is harmful and potentially lethal to living beings but can have health benefits in radiation therapy for the treatment of cancer and thyrotoxicosis.

Most adverse health effects of radiation exposure may be grouped in two general categories:

  • deterministic effects (harmful tissue reactions) due in large part to the killing/ malfunction of cells following high doses and
  • stochastic effects, i.e., cancer and heritable effects involving either cancer development in exposed individuals owing to mutation of somatic cells or heritable disease in their offspring owing to mutation of reproductive (germ) cells. [1]

Stochastic Edit

Some effects of ionizing radiation on human health are stochastic, meaning that their probability of occurrence increases with dose, while the severity is independent of dose. [2] Radiation-induced cancer, teratogenesis, cognitive decline, and heart disease are all examples of stochastic effects.

Its most common impact is the stochastic induction of cancer with a latent period of years or decades after exposure. The mechanism by which this occurs is well understood, but quantitative models predicting the level of risk remain controversial. The most widely accepted model posits that the incidence of cancers due to ionizing radiation increases linearly with effective radiation dose at a rate of 5.5% per sievert. [3] If this linear model is correct, then natural background radiation is the most hazardous source of radiation to general public health, followed by medical imaging as a close second. Other stochastic effects of ionizing radiation are teratogenesis, cognitive decline, and heart disease.

Quantitative data on the effects of ionizing radiation on human health is relatively limited compared to other medical conditions because of the low number of cases to date, and because of the stochastic nature of some of the effects. Stochastic effects can only be measured through large epidemiological studies where enough data has been collected to remove confounding factors such as smoking habits and other lifestyle factors. The richest source of high-quality data comes from the study of Japanese atomic bomb survivors. In vitro and animal experiments are informative, but radioresistance varies greatly across species.

The added lifetime risk of developing cancer by a single abdominal CT of 8 mSv is estimated to be 0.05%, or 1 one in 2,000. [4]

Deterministic Edit

Deterministic effects are those that reliably occur above a threshold dose, and their severity increases with dose. [2]

High radiation dose gives rise to deterministic effects which reliably occur above a threshold, and their severity increases with dose. Deterministic effects are not necessarily more or less serious than stochastic effects either can ultimately lead to a temporary nuisance or a fatality. Examples of deterministic effects are:

    , by acute whole-body radiation , from radiation to a particular body surface , a potential side effect from radiation treatment against hyperthyroidism , from long-term radiation. , from for example radiation therapy to the lungs , and infertility. [2]

The US National Academy of Sciences Biological Effects of Ionizing Radiation Committee "has concluded that there is no compelling evidence to indicate a dose threshold below which the risk of tumor induction is zero". [5]

Phase Symptom Whole-body absorbed dose (Gy)
1–2 Gy 2–6 Gy 6–8 Gy 8–30 Gy > 30 Gy
Immediate Nausea and vomiting 5–50% 50–100% 75–100% 90–100% 100%
Time of onset 2–6 h 1–2 h 10–60 min < 10 min Minutes
Duration < 24 h 24–48 h < 48 h < 48 h N/A (patients die in < 48 h)
Diarrhea None None to mild (< 10%) Heavy (> 10%) Heavy (> 95%) Heavy (100%)
Time of onset 3–8 h 1–3 h < 1 h < 1 h
Headache Slight Mild to moderate (50%) Moderate (80%) Severe (80–90%) Severe (100%)
Time of onset 4–24 h 3–4 h 1–2 h < 1 h
Fever None Moderate increase (10–100%) Moderate to severe (100%) Severe (100%) Severe (100%)
Time of onset 1–3 h < 1 h < 1 h < 1 h
CNS function No impairment Cognitive impairment 6–20 h Cognitive impairment > 24 h Rapid incapacitation Seizures, tremor, ataxia, lethargy
Latent period 28–31 days 7–28 days < 7 days None None
Illness Mild to moderate Leukopenia
Fatigue
Weakness
Moderate to severe Leukopenia
Purpura
Hemorrhage
Infections
Alopecia after 3 Gy
Severe leukopenia
High fever
Diarrhea
Vomiting
Dizziness and disorientation
Hypotension
Electrolyte disturbance
Nausea
Vomiting
Severe diarrhea
High fever
Electrolyte disturbance
Shock
N/A (patients die in < 48h)
Mortality Without care 0–5% 5–95% 95–100% 100% 100%
With care 0–5% 5–50% 50–100% 99–100% 100%
Death 6–8 weeks 4–6 weeks 2–4 weeks 2 days – 2 weeks 1–2 days
Table source [6]

By type of radiation Edit

When alpha particle emitting isotopes are ingested, they are far more dangerous than their half-life or decay rate would suggest. This is due to the high relative biological effectiveness of alpha radiation to cause biological damage after alpha-emitting radioisotopes enter living cells. Ingested alpha emitter radioisotopes such as transuranics or actinides are an average of about 20 times more dangerous, and in some experiments up to 1000 times more dangerous than an equivalent activity of beta emitting or gamma emitting radioisotopes. If the radiation type is not known then it can be determined by differential measurements in the presence of electrical fields, magnetic fields, or varying amounts of shielding.

In pregnancy Edit

The risk for developing radiation-induced cancer at some point in life is greater when exposing a fetus than an adult, both because the cells are more vulnerable when they are growing, and because there is much longer lifespan after the dose to develop cancer.

Possible deterministic effects include of radiation exposure in pregnancy include miscarriage, structural birth defects, Growth restriction and intellectual disability. [7] The determinstistic effects have been studied at for example survivors of the atomic bombings of Hiroshima and Nagasaki and cases where radiation therapy has been necessary during pregnancy:

Gestational age Embryonic age Effects Estimated threshold dose (mGy)
2 to 4 weeks 0 to 2 weeks Miscarriage or none (all or nothing) 50 - 100 [7]
4 to 10 weeks 2 to 8 weeks Structural birth defects 200 [7]
Growth restriction 200 - 250 [7]
10 to 17 weeks 8 to 15 weeks Severe intellectual disability 60 - 310 [7]
18 to 27 weeks 16 to 25 weeks Severe intellectual disability (lower risk) 250 - 280 [7]

The intellectual deficit has been estimated to be about 25 IQ-points per 1,000 mGy at 10 to 17 weeks of gestational age. [7]

These effects are sometimes relevant when deciding about medical imaging in pregnancy, since projectional radiography and CT scanning exposes the fetus to radiation.

Also, the risk for the mother of later acquiring radiation-induced breast cancer seems to be particularly high for radiation doses during pregnancy. [8]

The human body cannot sense ionizing radiation except in very high doses, but the effects of ionization can be used to characterize the radiation. Parameters of interest include disintegration rate, particle flux, particle type, beam energy, kerma, dose rate, and radiation dose.

The monitoring and calculation of doses to safeguard human health is called dosimetry and is undertaken within the science of health physics. Key measurement tools are the use of dosimeters to give the external effective dose uptake and the use of bio-assay for ingested dose. The article on the sievert summarises the recommendations of the ICRU and ICRP on the use of dose quantities and includes a guide to the effects of ionizing radiation as measured in sieverts, and gives examples of approximate figures of dose uptake in certain situations.

The committed dose is a measure of the stochastic health risk due to an intake of radioactive material into the human body. The ICRP states "For internal exposure, committed effective doses are generally determined from an assessment of the intakes of radionuclides from bioassay measurements or other quantities. The radiation dose is determined from the intake using recommended dose coefficients". [9]

Absorbed, equivalent and effective dose Edit

The Absorbed dose is a physical dose quantity D representing the mean energy imparted to matter per unit mass by ionizing radiation. In the SI system of units, the unit of measure is joules per kilogram, and its special name is gray (Gy). [10] The non-SI CGS unit rad is sometimes also used, predominantly in the USA.

To represent stochastic risk the equivalent dose H T and effective dose E are used, and appropriate dose factors and coefficients are used to calculate these from the absorbed dose. [11] Equivalent and effective dose quantities are expressed in units of the sievert or rem which implies that biological effects have been taken into account. These are usually in accordance with the recommendations of the International Committee on Radiation Protection (ICRP) and International Commission on Radiation Units and Measurements (ICRU). The coherent system of radiological protection quantities developed by them is shown in the accompanying diagram.

The International Commission on Radiological Protection (ICRP) manages the International System of Radiological Protection, which sets recommended limits for dose uptake. Dose values may represent absorbed, equivalent, effective, or committed dose.

Other important organizations studying the topic include

    (ICRU) (UNSCEAR)
  • US National Council on Radiation Protection and Measurements (NCRP)
  • UK Public Health England
  • US National Academy of Sciences (NAS through the BEIR studies)
  • French Institut de radioprotection et de sûreté nucléaire (IRSN) (ECRR) the stage of radiation depends on the stage the body parts are affected

External Edit

External exposure is exposure which occurs when the radioactive source (or other radiation source) is outside (and remains outside) the organism which is exposed. Examples of external exposure include:

  • A person who places a sealed radioactive source in his pocket
  • A space traveller who is irradiated by cosmic rays
  • A person who is treated for cancer by either teletherapy or brachytherapy. While in brachytherapy the source is inside the person it is still considered external exposure because it does not result in a committed dose.
  • A nuclear worker whose hands have been dirtied with radioactive dust. Assuming that his hands are cleaned before any radioactive material can be absorbed, inhaled or ingested, skin contamination is considered external exposure.

External exposure is relatively easy to estimate, and the irradiated organism does not become radioactive, except for a case where the radiation is an intense neutron beam which causes activation.

By type of medical imaging Edit

Internal Edit

Internal exposure occurs when the radioactive material enters the organism, and the radioactive atoms become incorporated into the organism. This can occur through inhalation, ingestion, or injection. Below are a series of examples of internal exposure.

  • The exposure caused by potassium-40 present within a normal person.
  • The exposure to the ingestion of a soluble radioactive substance, such as 89 Sr in cows' milk.
  • A person who is being treated for cancer by means of a radiopharmaceutical where a radioisotope is used as a drug (usually a liquid or pill). A review of this topic was published in 1999. [15] Because the radioactive material becomes intimately mixed with the affected object it is often difficult to decontaminate the object or person in a case where internal exposure is occurring. While some very insoluble materials such as fission products within a uranium dioxide matrix might never be able to truly become part of an organism, it is normal to consider such particles in the lungs and digestive tract as a form of internal contamination which results in internal exposure. (BNCT) involves injecting a boron-10 tagged chemical that preferentially binds to tumor cells. Neutrons from a nuclear reactor are shaped by a neutron moderator to the neutron energy spectrum suitable for BNCT treatment. The tumor is selectively bombarded with these neutrons. The neutrons quickly slow down in the body to become low energy thermal neutrons. These thermal neutrons are captured by the injected boron-10, forming excited (boron-11) which breaks down into lithium-7 and a helium-4alpha particle both of these produce closely spaced ionizing radiation.This concept is described as a binary system using two separate components for the therapy of cancer. Each component in itself is relatively harmless to the cells, but when combined together for treatment they produce a highly cytocidal (cytotoxic) effect which is lethal (within a limited range of 5-9 micrometers or approximately one cell diameter). Clinical trials, with promising results, are currently carried out in Finland and Japan.

When radioactive compounds enter the human body, the effects are different from those resulting from exposure to an external radiation source. Especially in the case of alpha radiation, which normally does not penetrate the skin, the exposure can be much more damaging after ingestion or inhalation. The radiation exposure is normally expressed as a committed dose.

Although radiation was discovered in late 19th century, the dangers of radioactivity and of radiation were not immediately recognized. Acute effects of radiation were first observed in the use of X-rays when Wilhelm Röntgen intentionally subjected his fingers to X-rays in 1895. He published his observations concerning the burns that developed, though he misattributed them to ozone, a free radical produced in air by X-rays. Other free radicals produced within the body are now understood to be more important. His injuries healed later.

As a field of medical sciences, radiobiology originated from Leopold Freund's 1896 demonstration of the therapeutic treatment of a hairy mole using a new type of electromagnetic radiation called x-rays, which was discovered 1 year previously by the German physicist, Wilhelm Röntgen. After irradiating frogs and insects with X-rays in early 1896, Ivan Romanovich Tarkhanov concluded that these newly discovered rays not only photograph, but also "affect the living function". [16] At the same time, Pierre and Marie Curie discovered the radioactive polonium and radium later used to treat cancer.

The genetic effects of radiation, including the effects on cancer risk, were recognized much later. In 1927 Hermann Joseph Muller published research showing genetic effects, and in 1946 was awarded the Nobel prize for his findings.

More generally, the 1930s saw attempts to develop a general model for radiobiology. Notable here was Douglas Lea, [17] [18] whose presentation also included an exhaustive review of some 400 supporting publications. [19] [ page needed ] [20]

Before the biological effects of radiation were known, many physicians and corporations had begun marketing radioactive substances as patent medicine and radioactive quackery. Examples were radium enema treatments, and radium-containing waters to be drunk as tonics. Marie Curie spoke out against this sort of treatment, warning that the effects of radiation on the human body were not well understood. Curie later died of aplastic anemia caused by radiation poisoning. Eben Byers, a famous American socialite, died of multiple cancers (but not acute radiation syndrome) in 1932 after consuming large quantities of radium over several years his death drew public attention to dangers of radiation. By the 1930s, after a number of cases of bone necrosis and death in enthusiasts, radium-containing medical products had nearly vanished from the market.

In the United States, the experience of the so-called Radium Girls, where thousands of radium-dial painters contracted oral cancers— [21] but no cases of acute radiation syndrome— [22] popularized the warnings of occupational health associated with radiation hazards. Robley D. Evans, at MIT, developed the first standard for permissible body burden of radium, a key step in the establishment of nuclear medicine as a field of study. With the development of nuclear reactors and nuclear weapons in the 1940s, heightened scientific attention was given to the study of all manner of radiation effects.

The atomic bombings of Hiroshima and Nagasaki resulted in a large number of incidents of radiation poisoning, allowing for greater insight into its symptoms and dangers. Red Cross Hospital surgeon Dr. Terufumi Sasaki led intensive research into the Syndrome in the weeks and months following the Hiroshima bombings. Dr Sasaki and his team were able to monitor the effects of radiation in patients of varying proximities to the blast itself, leading to the establishment of three recorded stages of the syndrome. Within 25–30 days of the explosion, the Red Cross surgeon noticed a sharp drop in white blood cell count and established this drop, along with symptoms of fever, as prognostic standards for Acute Radiation Syndrome. [23] Actress Midori Naka, who was present during the atomic bombing of Hiroshima, was the first incident of radiation poisoning to be extensively studied. Her death on August 24, 1945 was the first death ever to be officially certified as a result of radiation poisoning (or "Atomic bomb disease").

The interactions between organisms and electromagnetic fields (EMF) and ionizing radiation can be studied in a number of ways:

The activity of biological and astronomical systems inevitably generates magnetic and electrical fields, which can be measured with sensitive instruments and which have at times been suggested as a basis for "esoteric" ideas of energy.

Radiobiology experiments typically make use of a radiation source which could be:


How to Analyse DICOM Dose Reports in FGI

About 20 years ago, exposure parameters from FGI were usually recorded manually in a Radiology or Hospital Information System (RIS/HIS) or paper based. Later, storage was provided together with the angiographic images in a picture archiving and communication system (PACS) as bitmap report. All of these recording methods allow only difficult analysis of the patient exposure. Today DICOM RDSR is available in most new angiography systems and provides an easy solution to collect dose parameters. This includes all exposure parameters for each fluoroscopic scene, all radiographic images or cine series with kV, mAs, geometrical parameters of C-arm, detector and more. Table 1 shows an excerpt of an angiography RDSR. In radiography and CT, the exposure data can be extracted relatively reliable from the DICOM image data even without RDSR. This is not the case with fluoroscopic procedures, as fluoroscopy scenes are usually not stored in the PACS. The DICOM image data therefore lack the dose contribution from fluoroscopy, which can easily exceed 50% of the total dose depending on the type of intervention. Recording and processing of patient exposure was driven by the EU-BSS which requires member states of the European Union to ensure justification and optimisation of radiological procedures and store information on patient exposure for analysis and quality assurance [4, 5]. Various commercial dose management systems (DMSs) with varying characteristics are available today [6]. In contrast to radiography and CT, complex RDSR reports are not always correctly and completely saved as DICOM objects in FGI and are not always correctly and completely evaluated by DMS providers. This is particularly important because all contributions from fluoroscopy and radiography / cine series are required to determine the total exposure of a patient. Furthermore, a complete recording of all individual radiation events is required to calculate the dose distribution on the patient's surface and to identify locations where overlapping radiation fields can lead to a high peak skin dose (PSD) and thus to potential deterministic skin injuries.

The most commonly used exposure parameters are Kerma-area product (KAP) and Air kerma at the patient entrance reference point (Ka,r). KAP is used for diagnostic reference levels (DRLs) in most countries and is also displayed or transmitted by the manufacturers of all angiography systems. KAP is a public tag in both the Radio Fluoroscopy (RF) and X-ray angiography (XA) DICOM Service Class objects. The second most important parameter is Ka,r, which correlates more than KAP with the skin dose, followed by the total fluoroscopy time in the number of cine series or images. KAP and Ka,r are usually transmitted cumulatively in RDSR for an entire examination, while the dose distribution on the body surface with PSD has to be calculated from all individual exposure events. Since dosimetric data are usually transferred to a PACS after an examination has been completed, the dose distribution is only available in a DMS after the procedure. An online display of the skin dose distribution on the modality screen during the intervention would be desirable in order to avoid high PSD by changing the projection direction and hence the skin entrance field from time to time.


RESULTS

TLD Calibration

The TLD calibration result is given with the formula E = ƒ · C, where E is the radiation exposure in air (in milliroentgens) measured in the ion chamber, C is the reading of the TLD chips (in nanocoulombs), and ƒ is the calibration factor. For tube potentials of 120 and 140 kV, the calibration factors were computed to be 13.3 and 13.0 mR/nC, respectively, with an error of 1%.

Radiation Doses

The CT effective doses for protocol B calculated with ImPACT were 16.10 and 16.40 mSv for female and male patients, respectively. For protocol A with 100 and 300 mA, respectively, the effective doses to female patients were calculated to be 6.40 and 19.10 mSv, and the effective doses to male patients were calculated to be 6.60 and 19.70 mSv.

The measured organ doses and effective doses from CT scanning are summarized in , Table 3. The effective doses of the three CT protocols A, B, and C, respectively, were 7.22, 18.56, and 25.68 mSv for female patients and 7.42, 18.57, and 25.95 mSv for male patients. The radiation doses to the lens of the eye from CT scanning with the three protocols A, B, and C, respectively, were measured to be 8.1, 18.4, and 27.2 mSv for female patients and 8.3, 18.6, and 27.3 mSv for male patients.

The effective dose from PET scanning was 6.23 mSv ( , Table 4). Doses from PET scanning to the gonads, uterus, and bladder were higher than to the other organs and were 5.0, 7.8, and 59.2 mSv, respectively. This is because of the final accumulation of 18 F in the bladder. Other organ doses ranged from 2.5 to 4.8 mSv.

The total effective doses of the combined PET/CT studies, calculated by summing the effective doses of CT and PET scanning, were 13.45, 24.79, and 31.91 mSv for female patients and 13.65, 24.80, and 32.18 mSv for male patients for protocols A, B, and C, respectively. The CT component contributed 54%–81% of the total combined dose.

LAR of Cancer Incidence Estimated for U.S. and Hong Kong Populations

The LAR table of cancer incidence for the U.S. and Hong Kong populations demonstrated that excess risks for female patients were higher than those for male patients, except for the colon and bladder ( , Table 5). The estimated LARs of cancer incidence were particularly high in younger ages and decreased with increasing age ( , Figure , , ). For example, LARs were up to 0.514% and 0.323% for 20-year-old U.S. women and men, respectively. These risks for the Hong Kong population were higher than for the U.S. population for both sexes and all ages ( , Figure). For example, at age 20 years, the LARs of cancer incidence in the Hong Kong population were 5.5%–20.9% higher than those for the U.S. population, and at age 80 years, the LARs of cancer incidence were 6.5%–47.9% higher. The difference in risks between these two populations was larger for female patients than for male patients, at older ages, and with higher-dose CT protocols. To explain the cause of the higher LAR in the Hong Kong population compared with the U.S. population, we referred to the life expectancy and cancer statistics data of the most frequent five cancers among the Hong Kong and U.S. populations ( , Table 6). We found the explanation to be related to the differences in life table and cancer statistics data between the two populations. First, Hong Kong residents have a longer life expectancy than Americans in which to develop cancer after radiation exposure. Second, the baseline cancer incidences of the organs more sensitive to radiation are higher in Hong Kong than in the United States. For example, the most prevalent cancer in Hong Kong is in the lung, which is given a high tissue weighting factor of 0.12 ( , 10), while in the United States, it is in the prostate, with a tissue weighting factor of 0.013.


What level of patient dose should be allowed?

Radiation dose delivered to a patient is a necessary consequence of acquiring the X-ray images used to define the anatomical and patho-physiological processes and make a diagnosis. Because X-rays are carcinogenic and have an associated risk, it is important to ensure that benefits of making an accurate diagnosis outweigh the risks of being exposed to ionizing radiation. Fortunately, the risks of being exposed to ionizing radiation in quantities typically used for medically indicated imaging procedures are quite low and similar to other risks that are deemed acceptable for everyday life. Thus, the radiographic study should be optimized in terms of achieving necessary image quality at the lowest possible radiation dose, in order to maximize the benefit-to-risk ratio. Note that this is not necessarily the lowest dose possible, but the minimal radiation dose that results in image quality sufficient to allow a competent radiologist to make a confident diagnosis. As long as the examination is appropriate, the benefit to an individual patient (to confirm or eliminate disease or trauma) will far outweigh the associated risk.


Principles of Radiation Safety

    (Illustration) (HHS/CDC) (YouTube - 1:44 min) (DOE/TEPP/MERRTT) (YouTube - 2:15 min)
  • Factors that Decrease Radiation from Exposure - Time, Distance, Shielding (Illustration) (EPA) ("As Low As Reasonably Achievable"): making every reasonable effort to maintain exposures to ionizing radiation as far below the dose limits as practical.
  • Assessment of Occupational Exposure Due to Intakes of Radionuclides Safety Guide (PDF - 316 KB) (IAEA Safety Standards Series No. RS-G-1.2, 1999)
  • Assessment of Occupational Exposure Due to External Sources of Radiation Safety Guide (PDF - 307 KB) (IAEA Safety Standards Series No. RS-G-1.3, 1999)

Method

Equipment

The CBCT acquisition studied was the 8 s DynaCT digital radiography acquisition (8sDR) on a Siemens Artis zee interventional fluoroscopy unit (Siemens Medical Solutions, Erlangen, Germany).

The acquisition consists of 396 fluoroscopy images taken over a 200° rotation around the patient moving from a left anterior oblique (LAO) projection posteriorly to a right anterior oblique (RAO) projection. The acquisition was set up to deliver 0.36 μGy per frame to the imaging plate and no extra copper filtration was selected. The target kV was set to be 70 kV, although pre-acquisition screening determined the actual exposure parameters (kV and mA) required. The uncollimated X-ray beam was measured to be 27×36 cm at the imaging plate (craniocaudal×lateral directions, respectively). The field could be collimated in the craniocaudal direction only and the focus–image distance was 120 cm.

Doses were measured with a 100-mm-long 3-cm 3 2025-series Radcal pencil ionisation chamber and electrometer with a calibration traceable to primary standards. The chamber was used with a PMMA CTDI phantom (ImPACT). A single phantom is 14 cm long with a 32-cm diameter. A double-length phantom (28 cm long) was constructed by fixing two phantoms together. Doses were measured in the centre of the phantom and at the eight outer chamber positions, 15 cm from the centre. The phantoms were placed at the centre of rotation of the tube such that the central chamber position was 60 cm from the imaging plate.

Validating CTDIW

Measurements of CTDIW were made for three combinations of two phantom lengths (14 and 28 cm) and two X-ray beam collimations (21 and 27 cm at the imaging plate). The phantom/collimation combinations measured were:

  • A: a single phantom with the beam collimated to the chamber length
  • B: a double-length phantom with the beam collimated to the chamber length
  • C: a double-length phantom with an uncollimated beam.

For all three combinations, the chamber was placed centrally within the phantom(s) in the craniocaudal direction, and doses were measured at the eight outer chamber positions and the centre chamber position. Collimation was set visually on the display monitor before each exposure. Reproduction of collimation was estimated to be 1 cm in 21 cm for the craniocaudal direction (5%).

Combination A is the preferred testing scenario for routine quality assurance measurements of radiation output usually only one phantom will be available for use, and it is convenient to transport only one phantom. Combination B is an approximation to an infinite length of PMMA and will include the effect of scatter from the craniocaudal directions. This combination has been shown to most closely represent a true measure of CTDIW [8]. Combination C more closely mimics patient exposure, including both the craniocaudal scatter caused by the patient body and the larger X-ray field.

The CTDIW is determined from doses measured from a 360° exposure assuming that there is a linear decrease in dose between the periphery and the centre of the phantom [12]. For a beam width W less than the length of the pencil chamber L, it is given by the empirical equation: ((1))

where Dcentre is the dose measured in the centre of the phantom and Dperiphery is the average of the doses measured at the outer chamber positions of the phantom. Dperiphery is usually calculated using four dose measurements, typically taken at the north, east, south and west positions in the phantom.

When the beam width is greater than the length of the chamber, W is given the value of L [9] and Equation (1) simplifies to: ((2))

To investigate whether this relation still holds for a partial irradiation over 200°, this empirical CTDIW value was compared with an interpolated average dose within a slice. Assuming the same linear decrease between the periphery and the centre of the phantom, the phantom was divided into a polar grid of 1-cm radius intervals and π/8 angular intervals. Doses were then interpolated between the eight periphery dose values and the centre dose value to give doses at each grid position. The interpolated average dose in the slice (Dave) was found for each phantom/collimation combination and compared with two empirical CTDIW values. The first (CTDIW ON ) was calculated using the four ‘on-axis’ periphery doses measured at the north, east, south and west chamber positions in the phantom. CTDIW OFF was calculated using the four ‘off-axis’ periphery doses measured at the northeast, southeast, southwest and northwest chamber positions.

Effective dose calculations: PCXMC

The effective dose from a DynaCT 8sDR examination of the abdomen was modelled with the PCXMC 2.0 dose calculation software. The exposure was initially modelled as 41 separate projections at 5° intervals around the PCXMC phantom covering a 200° rotation. An anode angle of 12° and tube filtration of 2.5 mm Al were used as tube parameters and the focus–image distance was 120 cm for all projections.

Because the mathematical phantom in PCXMC was elliptical, it was necessary to calculate the distance from the entry surface to the image plate for each projection to determine the dimensions of the beam as it entered the phantom. The phantom was chosen to be the standard adult patient in PCXMC, weighing 73.2 kg and measuring 178.6 cm in height.

The PCXMC software requires a measure of exposure to calculate the effective dose from each projection. The dose–area product (DAP) of the entire acquisition was used, divided by the number of projections used to simulate the exposure. The final kV reported by the unit was used for all projections. Wielandts et al [11] used frame-by-frame kV and mAs values extracted from the X-ray unit. They demonstrated that tube power remains constant throughout the run, so assuming constant kV and DAP per frame was a reasonable approximation. Exposure conditions for phantom/collimation combinations B and C were modelled in PCXMC. Combination A was expected to give the same result as B because the kV and DAP values were similar and the different phantom lengths had no effect on the PCXMC calculations. The image dimensions, DAP values and kV values used are shown in Table 1.

Table 1 Exposure details used in PCXMC (STUK, Helsinki, Finland) to model a DynaCT (Siemens Medical Solutions, Erlangen, Germany) examination of the abdomen
Measurement scenarioImage dimensions (cm)kVDAP (mGy cm 2 )
Per projectionTotal
B. Double phantom, collimated beam21×361241301.253347
C. Double phantom, uncollimated beam27×361211721.970596

Because the effective dose varies with the organs irradiated, the examination was simulated in PCXMC at three locations within the abdomen (Figure 1). The upper model covered the liver, stomach and spine the middle covered the intestines and upper pelvic bones and the lower covered the lower intestines and the reproductive organs. PCXMC 2.0 calculates the effective dose using the updated tissue-weighting factors issued in International Commission on Radiological Protection (ICRP) publication 103 [13] as well as the weighting factors given in ICRP publication 60 [14].

Figure 1

Left, screenshot of the mathematical phantom in PCXMC (STUK, Helsinki, Finland) with the location of the three abdomen exposures shown. Right, organs irradiated in each of the three abdomen exposures. From the upper abdomen the primary organs shown are: liver, stomach and spine intestines and upper pelvic bones lower intestines and reproductive organs.

In addition to modelling the DynaCT acquisition with 41 projections, the uncollimated beam acquisition (combination C) was modelled with a decreasing number of projections. The angle between projections was increased in 5° steps, eventually modelling the acquisition as four projections 55° apart. All three locations within the abdomen were simulated with the reduced projections.

Because effective dose is calculated from organ doses, the effect of reducing the number of modelled projections on individual organ doses was examined in addition to the final effective dose value.

Effective dose calculations: European Guidelines for Multislice Computed Tomography

For the abdomen and pelvis region, the effective dose conversion coefficient provided by the 2004 European Guidelines for Multislice Computed Tomography is ED=0.017 mSv mGy –1 cm –1 .

The interpolated average doses from the three phantom/collimation combinations were used with this coefficient to provide an estimate of effective dose from the DynaCT acquisition: ((3))

The irradiated length was taken to be the length of the X-ray beam at the centre of the phantom. The effective dose conversion coefficient has been calculated on the basis of the ICRP 60 tissue-weighting factors so the doses were compared with the equivalent effective dose values calculated using PCXMC.


ENVIRONMENTAL HEALTH & SAFETY

A web module produced by Committee 3 of the International Commission on Radiological Protection (ICRP)

What is the purpose of this document?

In the past 100 years, diagnostic radiology, nuclear medicine and radiation therapy have evolved from the original crude practices to advanced techniques that form an essential tool for all branches and specialties of medicine. The inherent properties of ionising radiation provide many benefits but also may cause potential harm.

In the practice of medicine, there must be a judgement made concerning the benefit/risk ratio. This requires not only knowledge of medicine but also of the radiation risks. This document is designed to provide basic information on radiation mechanisms, the dose from various medical radiation sources, the magnitude and type of risk, as well as answers to commonly asked questions (e.g radiation and pregnancy). As a matter of ease in reading, the text is in a question and answer format.

Interventional cardiologists, radiologists, orthopaedic and vascular surgeons and others, who actually operate medical x-ray equipment or use radiation sources, should possess more information on proper technique and dose management than is contained here. However, this text may provide a useful starting point.

The most common ionising radiations used in medicine are X, gamma, beta rays and electrons. Ionising radiation is only one part of the electromagnetic spectrum. There are numerous other radiations (e.g. visible light, infrared waves, high frequency and radiofrequency electromagnetic waves) that do not posses the ability to ionize atoms of the absorbing matter. The present text deals only with the use of ionising radiation in medicine.

Is the use of ionising radiation in medicine beneficial to human health?

Yes. Benefit to patients from medical uses of radiation has been established beyond doubt.

Modern diagnostic radiology assures faster, more precise diagnosis and enables monitoring of a large proportion of diseases. It has been estimated that in about one half of cases, radiological procedures (plain film radiography, fluoroscopy, computed tomography) have a substantial impact on the speed of diagnosis and in a large fraction of cases they are of decisive importance. Furthermore, several screening procedures (such as mammography) have been developed which are beneficial for specific populations at relatively high risk of some diseases. In addition a number of interventional radiological procedures (e.g angioplasty), introduced in the last 10-20 years, contribute significantly to the effectiveness of treatment of very serious and life threatening diseases of the cardiovascular, central nervous system and other organ systems. These procedures are also cost-effective.

Nuclear medicine uses radioactive substances, called radiopharmaceuticals, in the diagnosis and treatment of a range of diseases. These substances are especially developed to be taken up predominantly by one organ, or type of cell in the body. Following their introduction into the body for diagnostic purposes they are followed either by external measurements, yielding images of their distribution (both in space and in time), or by activity measurements in blood, urine and other media. In all cases the information obtained is of functional character. This information is not obtainable, or obtainable with less accuracy, by other modalities. Nuclear medicine offers therefore unique diagnostic information in oncology (diagnosis and staging), cardiology, endocrinology, neurology, nephrology, urology and others. Most of the methods currently in use are those of choice in the diagnostic process, because they show high sensitivity, specificity and good reproducibility. Their cost-effectiveness is also high. In addition it should be emphasized that these procedures are non-invasive and present no risk of direct complications to the patient.
One has to remember that whereas electrical generators of ionising radiation (X-ray machines, electron accelerators) stop emitting radiation when switched off, radioactive sources do emit radiation, which cannot be modified in course of the radioactive decay. This means that some precautions may have to be taken with such patients given large therapeutic amounts of radionuclides when they are in a hospital and afterwards when they go home -to protect against exposure of the staff, relatives, friends and members of the public.

Radiation therapy uses ionising radiation for treatment. The incidence of cancer is about 40%, reflecting long life expectancy. Cancer also leads to

20-30 per cent cumulative mortality. Current medical practice uses radiotherapy in about 1/2 of newly diagnosed cancer cases. Therapeutic techniques can be highly complex, and place very high demands on the accuracy of irradiation. To be effective they must be approached on an interdisciplinary basis, requiring effective and harmonious cooperation between radiation oncologists, medical physicists and highly qualified technicians. However, it should be remembered that radiotherapy of cancer is often accompanied by adverse side effects of the treatment. Some adverse effects are unavoidable and often resolve spontaneously or with treatment. Serious adverse effects may occur and result from the proximity of sensitive normal tissues to the treatment field or rarely as a result of individual radiation sensitivity. They do not undermine the purpose of radiotherapy. Appropriate use of radiotherapy saves millions of lives every year overall. Even if only palliative treatment is possible the therapy substantially reduces suffering. There are also some non-malignant diseases whose treatment by radiation is a method of choice.

Radiotherapy using radiopharmaceuticals is generally non-invasive but limited to several well-established situations where killing hyperfunctioning or malignant cells is important (for example hyperthyroidism, cancer of the thyroid, degenerative and inflammatory diseases of joints, palliative treatment of metastases to the skeleton). In addition, there are many studies showing significant potential for radio-labelled antibodies and receptor -avid peptides to be used in the treatment of several malignancies. However, this mode of treatment is still in its early days.

Ionising radiation is, therefore, one of the basic tools of contemporary medicine, both in diagnosis and therapy. Practice of contemporary, advanced medicine, without use of ionizing radiation appears currently unthinkable.

Are there risks to the use of ionising radiation in medicine?

There obviously are some risks. The magnitude of risk from radiation is dose-related with higher amounts of radiation being associated with higher risks. The undisputed health benefits of diagnostic X-ray and nuclear medicine diagnostics may be accompanied by a generally small risk (probability) of deleterious effects. This fact has to be taken into account while using ionising radiation sources in diagnosis. Since large amounts of radiation are required in radiation therapy, the risk of radiation-related adverse effects is measurably higher.

The aim of managing radiation exposure is to minimise the putative risk without sacrificing, or unduly limiting, the obvious benefits in the prevention, diagnosis and also in effective cure of diseases (optimisation). It should be pointed out that when too little radiation is used for diagnosis or therapy there is an increase in risk although these risks are not due to adverse radiation effects per se. Too low an amount of radiation in diagnosis will result in either an image that does not have enough information to make a diagnosis and in radiation therapy, not delivering enough radiation will result in increased mortality because the cancer being treated will not be cured.

Experience has provided ample evidence that reasonable selection of conditions, under which ionising radiation is being used in medicine, can lead to health benefits substantially outweighing the estimated possible deleterious effects.

How do we quantify the amount of radiation?

The frequency or intensity of biological effects is dependant upon the total energy of radiation absorbed (in joules) per unit mass (in kg) of a sensitive tissues or organs. This quantity is called absorbed dose and is expressed in gray (Gy). Some X or gamma rays will pass through the body without any interaction, and they will produce no biological effect. On the other hand, the radiation which is absorbed, may produce effects. Absorbed doses of radiation can be measured and/or calculated and they form basis for evaluation of the probability of radiation-induced effects.

In evaluating biological effects of radiation after partial exposure of the body further factors such as the varying sensitivity of different tissues and absorbed doses to different organs have to be taken into consideration. To compare risks of partial and whole body irradiation at doses experienced in diagnostic radiology and nuclear medicine a quantity called effective dose is used. It is expressed in sievert (Sv). Effective dose is not applicable to radiation therapy, where very large absorbed doses affect individual tissues or organs.

What do we know about the nature (mechanism) of radiation-induced biological effects ?

Cells can be killed by radiation During cellular division chromosomal aberrations due to radiation may result in loss of part of the chromosomal DNA which results in cell death. The probability of chromosomal aberrations is proportional to dose and those cells free of critical damage to DNA retain their dividing potential.

Surviving cells may carry changes in the DNA at a molecular level (mutations). Elementary, primary damage to DNA results from chemical damage by free radicals, originating from radiolysis of water. DNA damage also can result from the direct interaction of ionising particles with the DNA double helix (rarely).

Important changes in DNA occur in the form of breaks in continuity of the DNA chains although other forms of damage also arise. These breaks may affect one strand of the helix (single strand breaks, SSB) or both strands in the same location (double strand breaks, DSB). SSB occur very frequently in the DNA without irradiation, and are easily and effectively repaired by specific enzyme systems. In contrast, many induced DSB are more complicated and less easily repaired. As a result, a significant proportion of the damage is repaired incorrectly (mis-repair). These mis-repaired breaks can lead to chromosomal aberrations and gene mutations. Some of the genes mutated in such a way form the first step (initiation) of the very long and complicated process of carcinogenesis, requiring also several subsequent mutations (most likely not induced by radiation) in the affected cells. Similar mutation mechanisms, when affecting germinative cells, may lead to hereditary mutations expressed in descendants of the irradiated persons. Of course, the essential point in considering these possible sequelae of irradiation is the frequency (or probability of occurrence) of undesired effects in persons irradiated with a given dose, or in their descendants.

How are effects of radiation classified?

There are two basic categories of the biological effects that may be observed in irradiated persons. These are 1) due largely to cell killing (deterministic) and 2 mutations which may result in cancer and hereditary effects (stochastic or probabilistic ). Effects due to cell killing (such as skin necrosis) have a practical threshold dose below which the effect is not evident but in general when the effect is present its severity increases with the radiation dose. The threshold dose is not an absolute number and vary somewhat by individual. Effects due to mutations (such as cancer) have a probability of occurrence that increases with dose, it is currently judged that there is not a threshold below which the effect will not occur and finally the severity of the effects is independent of the dose. Thus a cancer caused by a small amount of radiation can be just as malignant as one caused by a high dose.

Deterministic effects. These effects are observed after large absorbed doses of radiation and are mainly a consequence of radiation induced cellular death. They occur only if a large proportion of cells in an irradiated tissue have been killed by radiation, and the loss cannot be compensated by increased cellular proliferation. The ensuing tissue loss is further complicated by inflammatory processes and, if the damage is sufficiently extensive, also by secondary phenomena at the systemic level (e.g. fever, dehydration, bacteraemia etc.). In addition, eventual effects of healing processes, e.g. fibrosis, may contribute to additional damage and loss of function of a tissue or an organ. Clinical examples of such effects are: necrotic changes in skin, necrosis and fibrotic changes in internal organs, acute radiation sickness after whole body irradiation, cataract, and sterility (table 1).

Doses required to produce deterministic changes are in most cases large (usually in excess of 1-2 Gy). Some of those occur in a small proportion of patients as side effects of radiotherapy. They can also be found after complex interventional investigations (such as vascular stenting) when long fluoroscopy times have been used.

The relationship between the frequency of a given deterministic effect and the absorbed dose has a general form presented in fig. 1. It can be seen that the essential feature of this dose-response relationship is the presence of a threshold dose. Below this dose no effect may be diagnosed but with increasing dose the intensity of the induced damage increases markedly, in some situations, dramatically. An example of the deterministic damage to the skin is presented in fig. 2.

Malformations induced by radiation in the conceptus in the period of organogenesis (3-8 week of pregnancy), are also due to cell killing and are classified as deterministic effects. The same applies to malformations of the forebrain -leading to mental retardation -induced by the exposure between 8 and 15 weeks (and to some extent up to 25 week) after conception. The threshold doses are however, substantially lower than those found for deterministic effects after irradiation in extrauterine life: thus, 100-200 mGy form a threshold-range for malformations induced between the 3d and 8th week, and

200 mGy for the aforementioned brain damage (8-25 week).

Stochastic effects. As mentioned above, irradiated and surviving cells may become modified by induced mutations (somatic, hereditary). These modifications may lead to two clinically significant effects: malignant neoplasms (cancer) and hereditary mutations.

Cancer: Ionising radiation is a carcinogen although a relatively weak one. Careful follow-up of over 80,000 atom bomb survivors in Hiroshima and Nagasaki over the last 50 years indicates that there have been 12,000 cancer cases of which less than 700 excess deaths were due to radiation. Expressed another way, only about 6% of the cancer occurring in these survivors is radiation-related.

These observations allow estimation of the probability with which a given dose may lead to diagnosis (incidence) and death (mortality) from various cancers. Among the latter there are several forms of leukaemia and solid tumours of different organs, mostly carcinomas of the lung, thyroid, breast, skin and gastrointestinal tract. Radiation-induced cancers do not appear immediately after radiation exposure but require time to become clinically apparent (latent period). Examples of minimum latent periods are non-CLL leukemia 2 years, about 5 years for thyroid and bone cancer and 10 years for most other cancers. Mean latent periods are 7 years for non-CLL leukemia and more than 20 years for most other cancers. It is important to note that some tumours do not appear to be radiation-induced or only weakly so. These include carcinomas of the prostate, cervix, uterus, lymphomas and chronic lymphatic leukemia.

Hereditary effects: The risk of hereditary effects of ionising radiation has been estimated on basis of experiments on various animal species, because there are no demonstrated effects in humans (the likely values of probability per unit dose are given later).

From careful analysis of the experimental studies and epidemiological surveys it may be concluded that dose-response relationships for these two categories of stochastic effects have a distinctly different form from those characterising deterministic sequelae. A general dose-response relationship for cancer is presented in fig. 3. The principal features of the relationship may be summarised as follows: not be interpreted as the presence of a dose threshold. It is assumed that at the low doses (< 0.2 Gy), probability of the effect (frequency) increases most likely proportionally with the dose.

c. There is always a spontaneous frequency of the effect (mutations, cancer) in non-irradiated populations (F0 in Fig 2), which cannot be differentiated qualitatively from that induced by radiation. In fact, mutations or cancers induced by irradiation have the same morphological, biochemical, and clinical etc. characteristics as the cases occurring in non-irradiated individuals

What is magnitude of the risk for cancer and hereditary effects?

1. Analysis of the epidemiological data of irradiated populations has allowed derivation of the approximate risk of radiation-induced cancer. The lifetime value for the average person is roughly a 5% increase in fatal cancer after a whole body dose of 1 Sv (which is much higher than would found in most medical procedures). A statistically significant increase in cancer has not been detected in populations exposed to doses of less than 0.05 Sv.

It appears that the risk in fetal life, in children and adolescents exceeds somewhat this average level (by a factor of 2 or 3) and in persons above the age of 60 it should be lower roughly by a factor of

5 (due to limited life expectancy and therefore less time available for manifestation of a cancer, which is a late appearing effect of the exposure).

The higher dose diagnostic medical procedures (such a CT scan of the abdomen or pelvis) yield an effective dose of about 10 mSv. If there were a large population in which every person had 1 such scan, the theoretical lifetime risk of radiation induced fatal cancer would be about 1 in 2,000 (0.05%). This can be compared to the normal spontaneous risk of fatal cancer which is about 1 in 4 (25%).

Individual risk may vary from theoretical calculations. The cumulative radiation dose from medical procedures is very small in many individuals, however in some patients the cumulative doses exceed 50 mSv and the cancer risk should be carefully considered. Many relatively high dose diagnostic procedures (such as CT) should be clearly justified and when this is done, benefit will far outweigh risk. Unjustified procedures at any dose level should be avoided. In radiotherapy there is a risk of second cancers but the risk is small compared to the imperative to treat the current malignancy.

Hereditary effects as a consequence of radiation exposure have not been observed in humans. No hereditary effects have been found in studies of the offspring and grandchildren of the atomic bomb survivors. However, as based on animal models and knowledge of human genetics, the risk of hereditary deleterious effects have been estimated to not be greater than 10% of the radiation induced carcinogenic risk.

Is ionizing radiation from medical sources the only one to which people are exposed?

No. All living organisms on this planet, including humans, are exposed to radiation from natural sources. An average yearly effective dose from this so-called natural background, amounts to about 2.5 mSv. This exposure varies substantially geographically (from 1.5 to several tens of mSv in limited geographical areas). Artificial sources - except medicine - add very minute doses to population at large.

What are typical doses from medical diagnostic procedures?

Various diagnostic radiology and nuclear medicine procedures cover a wide dose range based upon the procedure. Doses can be expressed either as absorbed dose to a single tissue or as effective dose to the entire body which facilitates comparison of doses to other radiation sources (such as natural background radiation. Typical values of effective dose for some procedures are presented in Table 2. The doses are a function of a number of factors such as tissue composition, density and thickness of the body. For example, it takes less radiation to penetrate the air in the lungs for a chest radiograph than to penetrate the tissues of the abdomen.

One should also be aware that even for a given procedure there may be a wide variation in the dose given cfor the same procedure on a specific individual when performed at different facilities. This variation may be up to a factor of ten and is often due to differences in the technical factors for the procedure such as film/screen speed, film processing, and voltage. IN addition there often are even wider variations in and among facilities for a given type of procedure due to less than satisfactory conduct of the procedure in some facilities.

Can radiation doses in diagnosis be managed without affecting the diagnostic benefit?

Yes. There are several ways to reduce the risks to very, very low levels while obtaining very beneficial health effects of radiological procedures, far exceeding the health impact of a possible detriment. In this context it should be mentioned also that high ratio of benefit vs. radiological risk depends to large extent on a good methodology of procedures and high quality of their performance. Therefore quality assurance and quality control in diagnostic radiology and nuclear medicine play also a fundamental role in the provision of appropriate, sound radiological protection of the patient.

There are several ways that will minimise the risk without sacrificing the valuable information that can be obtained for patients' benefit. Among the possible measures it is necessary to justify the examination before referring a patient to the radiologist or nuclear medicine physician.

Repetition should be avoided of investigations made recently at another clinic or hospital. Results of the investigations should be recorded in sufficient detail in patients' documentation, and carried over to another health-care unit. This rule could result in avoidance of a significant fraction of unnecessary examinations.

Failure to provide adequate clinical information at referral may result in a wrong procedure or technique being chosen by radiologist or nuclear medicine specialist. The result may be a useless test, with the investigation contributing only to patients' exposure.

An investigation may be seen as a useful one if its outcome - positive or negative - influences management of the patient. Another factor, which potentially adds to usefulness of the investigation, is strengthening confidence in the diagnosis.

To fulfill these criteria, indications for specific investigation both in the general clinical situation, and in a given individual patient, must be made by the referring physician on basis of medical knowledge. Difficulties may arise in the referral procedure mainly due to the dynamic development of the field of medical imaging. Technical progress in medical radiology and nuclear medicine has been enormous over the last 30 years in addition two new modalities have entered the field: ultrasound and magnetic resonance imaging. It is not surprising therefore, that following the technical developments may be difficult both to a general practitioner and even specialists in many fields of medicine. There are, however, several published guidelines (see suggested readings), which may help in making an appropriate referral, using well-founded criteria, based on clinical experience and epidemiology.

The most important circumstances that should be taken into account to avoid inappropriate referrals can be broadly categorised as follows: possibility of obtaining similar information without using ionising radiation, i.e. by means of ultrasound (US) or magnetic resonance imaging (MRI). Their use is indicated where these modalities are available, and when the cost (this applies mostly to MRI), waiting times and organisational difficulties are not prohibitive. The guidelines mentioned above provide also information when these modalities are preferable as a starting and sometimes the only investigation to be performed.

Are there situations when diagnostic radiological investigations should be avoided?

Yes. There are well-established views -not always respected - which indicate that in some circumstances radiography or fluoroscopy does not contribute anything to patients' management. This applies to situations when a disease could not have progressed or resolved since the previous investigation, or the data obtained could not influence patients' treatment.

Most common examples of unjustified examinations include: routine chest radiography at admission to a hospital or before surgery in absence of symptoms indicating cardiac or pulmonary involvement (or insufficiency) skull radiography in asymptomatic subjects of accidents lower sacro-lumbal radiography in stable degenerative condition of the spine in the 5th or later decade of life, but there are of course many others.

Screening of asymptomatic patients for presence of a disease may be done only if national health authorities made a decision that high incidence in a given age bracket, high efficacy of early disease detection, low exposure of screened individuals, and easily available and effective treatment may result in high benefit vs. risk ratio. Positive examples include fluoroscopy or radiography for detection of tuberculosis in societies or groups with high prevalence of the disease, mammography for early detection of breast cancer in women after 50 y of age, or screening for gastric carcinoma by specialised contrast fluoroscopy in countries with high incidence of this disease. All factors involved in screening must be periodically reviewed and reassessed. If positive criteria cease to be satisfied the screening should be discontinued.

Irradiation for legal reasons and for purposes of insurance should be carefully limited or excluded. Generally irradiation of individuals for legal reasons is without medical benefit. One of common examples is that insurance companies may require various X-ray investigations to satisfy the expectation that a person to be insured is in good health. In numerous cases such demands, particularly in asymptomatic individuals, should be treated with caution and often appear unjustified when they are medically not in the direct interest of the person concerned.

Are there special diagnostic procedures that should have special justification?

While all medical uses of radiation should be justified, it stands to reason that the higher the dose and risk of a procedure, the more the medical practitioner should consider whether there is a greater benefit to be obtained. There are radiological procedures that deliver doses at the upper end of the scale, presented in table 2.

Among these special position is occupied by computed tomography (CT), and particularly its most advanced variants like spiral or multi slice CT. Usefulness and efficacy of this great technical achievement is beyond doubt in particular clinical situations, however the ease of obtaining results by this mode and temptation to monitor frequently the course of a disease or perform screening should be tempered by the fact that repeated examinations may deliver an effective dose of the order of 100 mSv, a dose for which there is direct epidemiological evidence of carcinogenicity.

Do children and pregnant women require special consideration in diagnostic procedures?

Yes. Both the fetus and children are thought to be more radiosensitive than adults. Diagnostic radiology and diagnostic nuclear medicine procedures (even in combination) are extremely unlikely to result in doses that cause malformations or a decrease in intellectual function. The main issue following in-utero or childhood exposure at typical diagnostic levels (<50 mGy) is cancer induction.

Before a diagnostic procedure is performed it should be determined whether a patient is, or may be, pregnant, whether the fetus is in the primary radiation area and whether the procedure is relatively high dose (e.g. barium enema or pelvic CT scan). Medically indicated diagnostic studies remote from the fetus (e.g. radiographs of the chest or extremities, ventilation/perfusion lung scan) can be safely done at any time of pregnancy if the equipment is in proper working order. Commonly the risk of not making the diagnosis is greater than the radiation risk.

If an examination is typically at the high end of the diagnostic dose range and the fetus is in or near the radiation beam or source, care should be taken to minimize the dose to the fetus while still making the diagnosis. This can be done by tailoring the examination and examining each radiograph as it is taken until the diagnosis is achieved and then terminating the procedure. In nuclear medicine many radiopharmaceuticals are excreted by the urinary tract/ In these cases maternal hydration and encouraging voiding will reduce bladder residence time of the radiopharmaceutical and therefore will reduce fetal dose.

For children, dose reduction in achieved by using technical factors specific for children and not using routine adult factors. In diagnostic radiology care should be taken to minimize the radiation beam to only the area of interest. Because children are small, in nuclear medicine the use of administered activity lower than that used for an adult will still result in acceptable images and reduced dose to the child.

What can be done to reduce radiation risk during the performance of a diagnostic procedure?

The most powerful tool for minimising the risk is appropriate performance of the test and optimisation of radiological protection of the patient. These are the responsibility of the radiologist or nuclear medicine physician and medical physicist

The basic principle of patients' protection in radiological X-ray investigations and nuclear medicine diagnostics is that necessary diagnostic information of clinically satisfactory quality should be obtained at the expense of a dose as low as reasonably achievable, taking into account social and financial factors.

Evidence obtained in numerous countries indicates that the range of entrance doses (i.e. doses measured at surface of the body at the site where X-ray beam is entering the body) for a given type of radiographic examination is very wide. Sometimes the lowest and highest doses, measured in individual radiological installations, vary by a factor of

100. As most measured doses tend to group at the lower end of the distribution (fig. 4 Need to add figure and legend of dose distribution for a specific examination) it is clear that the largest doses, above, for instance, 70-80 percentile of the distribution, cannot be reasonably justified. By establishing the so called diagnostic reference levels for each of principal investigations at such a percentile, one can identify the places (institutions, X-ray machines) in need of corrective actions, which will easily and substantially reduce the average dose to patients on a country-wide scale.

This goal may be reached by co-operation of radiologists with medical physicists and auditing persons or teams. There are numerous technical factors, which when systematically applied, reduce exposure significantly. The effort to optimise protection requires good organisation as well as permanent willingness and vigilance to keep the doses as low as reasonably achievable. It may be easily shown that the risk, even if it is quite small, can still be reduced several - fold compared with the situation prevailing in previous decades.

Among the procedures that should be avoided are: 1) fluoroscopy and photofluorography for screening for tuberculosis in children and adolescents (only normal radiographs should be made instead at this age). 2) Fluoroscopy without electronic image intensification. In most developed countries such procedure - which gives quite high doses to the patient - is now legally forbidden.

It should be emphasised, that radiological interventional procedures lead to higher doses to patients than normal diagnostic investigations. However, indications for such procedures in most cases result from a high risk from conventional surgery. Appropriate modern equipment and training of personnel allow the patients' exposure to be limited to an acceptable level, securing a very high benefit/risk ratio.

In nuclear medicine the magnitude of dose to the patient results principally from the activity 1/ of the administered radiopharmaceutical. The range of activity of the latter, administered for a given purpose, varies among different departments by a small factor -usually a factor of three spans the highest and the lowest values. In several countries there are established respective reference or recommended levels and exceeding those should be usually avoided in examination of an individual of standard size. There are also accepted rules (formulae) for changing the activity as a function of body mass and for reducing the activity given to children relative to that administered to adults. Typical effective doses to patients in diagnostic nuclear medicine are in a similar range as those observed in X-ray diagnostics (Table 2). Good procedures and adherence to the principles of quality assurance and quality control secure a high benefit: risk ratio for the properly justified examinations. During pregnancy, investigations using radiopharmaceutical should be treated in a similar way as normal radiographic procedures. Accordingly, they should be performed only if no alternative diagnostic methods are available and if the investigations cannot be delayed until after delivery. To avoid serious damage to the foetal thyroid any procedure employing free 131 I ions -even in small activities -is contraindicated starting with

10-12 weeks of the pregnancy (when foetal thyroid becomes functional).

Lactating women may be investigated with radiopharmaceuticals. There are some radiopharmaceuticals that are relatively long lived and which are excreted in breast milk (such as iodine-131). After administration of such radiopharmaceuticals, breast-feeding must be discontinued to avoid transfer to the child. There are, however other radionuclides that are short lived (such as most technetium-99m compounds) that may not require discontinuation of breast-feeding or only for a few hours or a day.

1/ - Activity -number of nuclear disintegration per second (dps) in a given sample. Used as a measure of quantity of radioactive substances, here radiopharmaceuticals administered to patients. The unit is the becquerel which is 1 dps. A megabequerel (MBq) is 1 million dps.

The optimisation of patients' protection is based on a principle that the dose to the irradiated target (tumour) must be as high as it is necessary for effective treatment while protecting the healthy tissues to the maximum extent possible.

What can be done to reduce radiation risk during conduct of radiation therapy?

Radiotherapy based on proper indications is frequently a successful way of prolonging the life of a patient or of reducing suffering when only palliation is possible, thus improving the patient&rsquossible, thus improving the paties quality of life. To achieve this success requires the highest standards of performance (accuracy of delivered dose), both when planning irradiation for an individual patient and in actual delivery of the dose.

The decision to undertake a radiotherapy course is optimally made through a multidisciplinary team including surgeons, medical and radiation oncologists. This discussion should confirm the justification of the procedure, absence of more beneficial alternative treatments and commonly the optimal way of combining different techniques (radiotherapy, surgery and chemotherapy). When such a multidisiplinary approach is not possible, the radiation oncologist making the decision alone should keep in mind the alternative treatments or combine treatment strategies.

Actually, while the generic justification of radiotherapy cannot be questioned in the vast majority of cases. Increasing efforts are being made, in some cases, to decrease the delivered dose and to reduce the irradiated volumes. This is particularly true for some specific types of cancers, such as Hodgkins disease and for cancers of children, where the almost constant association with chemotherapy may allow the radiation oncologist to reduce dose and irradiated volume and a subsequent reduction in adverse side effects.

In a large number of cases, decreasing the dose to the target volume is not possible since it would unacceptably decrease the cure rate. In these cases present technological developments aim at optimising the patients protection, keeping the absorbed tumor dose as high as is necessary for effective treatment while protecting nearby healthy tissues. Conformal therapy has helped greatly in this regard.

It should be remembered that successful eradiaction of a malignant tumor by radiation therapy requires high-absorbed doses and there is a delayed (and usually low) risk of late complication. The above mention techniques are used to provide the best benefit/risk ratio.

Can pregnant women receive radiotherapy?

A malignant tumour in a pregnant woman may require radiotherapy in attempt to save life of the patient. If a tumour is located in a distant part of the body, the therapy - with individually tailored protection of the abdomen (screening) - may proceed. If the beam needs to be closer to the conceptus but still not irradiating the latter directly, special precautions need to be taken and an expert in dosimetry should make calculations of the dose to the fetus before the decision of starting the therapy is made. A dose to the conceptus (3-8 weeks post conception) from direct irradiation by the primary beam will reach values exceeding substantially thresholds for malformations of various organs, or of the brain (8 to 25 weeks) with resulting mental retardation in post-uterine life. It may also lead to stunting of foetal growth, even if the treatment took place in 3 rd trimester of pregnancy.

It should be also remembered that irradiation of the fetus in all trimesters of the pregnancy carries an increased risk of cancer in the newborn in the first or second decade of life and at therapeutic doses -or their significant fraction -this risk can be substantial. Therefore, in view of all mentioned factors termination of pregnancy may be considered. The decision should be based on careful estimation of the entailed risk to the fetus, which in turn requires calculation of the dose to conceptus by a qualified expert. The decision itself should be made by the women to be treated in consultation with their physician, partner and counsellor. Particularly difficult problems arise when radiotherapy is performed in a woman with early, undiagnosed pregnancy. The result is sometimes a massive irradiation of the conceptus in a period when malformations are easily induced (at or after 3 weeks post conception). To avoid such unintentional irradiation it seems necessary to perform pregnancy tests to diagnose, or exclude the pregnancy before undertaking radiotherapy.

Therapy of hyperthyroidism with 131 I in a pregnant woman is strictly contraindicated due to possibility of external irradiation of the foetus but mostly due to radioactive iodide crossing the placenta into the foetal circulation with subsequent uptake by its thyroid. The gland may well be destroyed by beta radiation from the nuclide taken up ( 131 I). Therefore, other methods of treatment should be employed, if possible, until delivery.

When thyroid cancer with metastases is diagnosed in a pregnant woman, treatment with 131 I, if it cannot be delayed after delivery, is not compatible with continuation of the pregnancy.

Can patients treatment with radiation endanger other people?

Medical radiation can be delivered to the patient from a radiation source outside the patient (e.g. from an x-ray machine for diagnosis or linear accelerator for radiotherapy). Regardless of how much dose the patient received, they do not become radioactive or emit radiation. As a result they present absolutely no radiation hazard to family or others.

The other way that medical radiation is given is by placing radioactive materials in the patient. In these cases the patient will emit radiation. For diagnostic nuclear medicine studies (such as a bone or thyroid scan) the amount of radioactivity injected is small and such patents present no hazard to their family or to the public. Such patients are discharged immediately after the procedure.

Patients may undergo radiation therapy by having radioactivity injected or radioactive sources implanted in the tumor. Such patients may or may not present a hazard to others based upon the penetration capability of the radiation emitted by the radionuclide. Some are very poorly penetrating (such as iodine-125 prostate implants. Such patents are discharged. Others who receive iridium-192 or cesium implants must remain in the hospital until the sources are removed.. The radiation in penetrating enough that visitors will be restricted from visiting the patient

Patients treated with high activity of 131 I for cancer of the thyroid, in some cases for hyperthyroidism, or patients with permanent implants of radioactive sources (special category of brachytherapy), once released home from a clinic or hospital may present some - however slight - risk to their family members it they do not observe specific rules of behaviour in such situations. These patients must be informed orally to avoid close bodily contact with children and of other necessary precautions by specialists responsible for conduct of their therapy.

Deterministic effects after whole-body and localised irradiation by X and gamma rays approximate absorbed threshold doses for single (short-term) and fractionated or low dose-rate (long-term) exposures [5,6].

Typical effective doses from diagnostic medical exposures in the 1990s (U.K.).

Lung ventilation (Xe-133)
Lung perfusion (Tc-99m)
Kidney (Tc-99m)
Thyroid (Tc-99m)
Bone (Tc-99m)
Cardiac gated study (Tc-99m)
PET head (F-18 FDG)
-----------------
Annual natural background

Data from the National Radiation Protection Board in the U.K.

Table 2b. - alternative versions (from NRPB, modified).

Broad levels of risk for common x-ray examinations and isotope scans
X-ray examination (or nuclear medicine isotope scan) Effective doses (mSv) clustering around a value of: Equivalent period of natural background radiation Lifetime additional risk of cancer per examination*
Chest
Teeth
Arms and legs
Hands and feet
0.01 A few days Negligible risk
Skull
Head
Neck
0.1 A few weeks Minimal risk
1 in 1 000 000
to
1 in 100 000
Breast (mammography)
Hip
Spine
Abdomen
Pelvis
CT scan of head
(Lung isotope scan)
(Kidney isotope scan)
1.0 A few months to a year Very low risk
1 in 100 000
to
1 in 10 000
Kidneys and bladder
(IVU)
Stomach - barium meal
Colon - barium enema
CT scan of abdomen
(Bone isotope scan)
10 A few years Low risk
1 in 10 000
to
1 in 1 000
*These risk levels represent very small additions to the 1 in 3 chance we all have of getting cancer.

Suggested information sources

Web sites for:

ICRP
NRPB
American College or Radiology
European Community
ASTRO
ESTRO
US National Cancer Institute
BMJ Evidence Based Medicine


1 Answer 1

The Sievert is a derived measure of stochastic health risk. It's used only in cases of low dosage ionizing radiation. High dosages that produce deterministic health effects are measured in the Gray (Gy), a purely physical term which represents the actual deposit of one joule of energy in one kilogram of matter.

Unlike the Gray, the Sievert does not measure an actual deposit of energy into tissue. The Sievert is an equivalent dose which represents the likelihood of the effect of depositing one joule of energy in one kilogram of matter. The Sievert is used to calculate an equivalent dose, which is computed using the actual deposited dose multiplied by a weighting factor which depends on the type of ionizing radiation to which one has been exposed.



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