Information for healthcare providers

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The primary aim of radiological protection is to provide an appropriate standard of protection for people and the environment without unduly limiting the beneficial practices giving rise to radiation exposure. ICRP Publication 103[1] has formulated a set of fundamental principles of radiological protection that apply to radiation sources and to the individuals being exposed. These principles are applicable to radiological protection in medicine. ICRP has provided recommendations for protection in medicine through ICRP Publication 105 Radiological Protection in Medicine[2], ICRP Supporting Guidance 2 Radiation and your patient - A Guide for Medical Practitioners[3] and ICRP Publication 73 Radiological Protection and Safety in Medicine[4].

Radiation health effects

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Radiation exposure can lead to either tissue reactions or stochastic effects.[1] Tissue reactions[5] can occur in the application of ionizing radiation in radiation therapy, and in interventional procedures, particularly when fluoroscopically guided interventional procedures are complex and require longer fluoroscopy time or acquisition of numerous images. Tissue reactions occur when many cells in an organ or tissue are killed, the effect will only be clinically observable if the radiation dose is above some threshold. The magnitude of this threshold will depend on the dose rate (i.e. dose per unit time) and linear energy transfer of the radiation, the organ or tissue irradiated, the volume of the irradiated part of the organ or tissue, and the clinical effect of interest.

Stochastic effect (somatic or heritable) increases with radiation dose and is probably proportional to dose at low doses and low dose rates.[6] At higher doses and dose rates, the probability often increases with dose more markedly than simple proportion. At even higher doses, close to the thresholds of tissue reactions, the probability increases more slowly, and may begin to decrease as a result of the competing effect of cell killing. It is not feasible to determine on epidemiological grounds alone that there is, or is not, an increased risk of cancer for members of the public associated with absorbed doses of the order of 100 mGy or below. The linear non-threshold model remains a prudent basis for the practical purposes of radiological protection at low doses and low dose rates.

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Radiological protection of patients


Medical exposure of patients has unique considerations that affect how the fundamental principles of radiological protection are applied. Such uniqueness is reflected in the application of all three principles in the protection of patients.[1][2]

Justification in protection of patients

Justification in radiological protection of patients is different from justification of other radiation applications, in that generally the very same person enjoys the benefits and suffers the risks associated with a procedure. Dose limits are not directly relevant, since ionising radiation, used for medically indicated purpose and at the appropriate level of dose, is an essential tool that will cause more good than harm.[2]

There are three levels of justification of a radiological practice in medicine: (1) At the first and most general level, the proper use of radiation in medicine is accepted as doing more good than harm to the society. This general level of justification is now taken for granted; (2) At the second level, a specified procedure with a specified objective is defined and justified (e.g. chest x rays for patients showing relevant symptoms). The aim of the second level of justification is to judge whether the radiological procedure will improve the diagnosis or treatment, or will provide necessary information about the exposed individuals. The total benefits from a medical procedure include not only the direct health benefits to the patient, but also the benefits to the patient’s family and to the society; (3) At the third level, the application of the procedure to an individual patient should be justified (i.e. the particular application should be judged to do more good than harm to the individual patient).

Hence all individual medical exposures should be justified in advance, taking into account the specific objectives of the exposure and the characteristics of the individual involved. Usually, no additional justification is needed for the application of a simple diagnostic procedure to an individual patient with the symptoms or indications for which the procedure has already been justified in general. For high-dose examinations, such as complex diagnostic and interventional procedures, individual justification by the practitioner is particularly important and should take account of all the available information. This includes the details of the proposed procedure, the characteristics of the individual patient, the expected dose to the patient, and the availability of information on previous or expected examinations or treatment. In addition, alternative procedures should always be considered regardless an examination involves high-dose or low-dose exposure.[7][8][9]

Optimisation of protection for patients

Optimisation of protection for patients is also unique.[7] [10][11][9] In the first place, radiation therapy is entirely different from anything else in that the dose to a human being is intentional and its potentially cell-killing properties the very purpose of the treatment. In such cases, optimisation becomes an exercise in minimising doses (and/or their deleterious effects) to surrounding tissues without compromising the pre-determined and intentionally lethal dose and effect to the target volume.

The optimisation of radiological protection for patients in medicine is usually applied at two levels: (1) the design, appropriate selection, and construction of equipment and installations; and (2) the day-to-day methods of working (i.e. the working procedures). The basic aim of this optimisation of protection is to adjust the protection measures for a source of radiation in such a way that the net benefit is maximised. The optimisation of radiological protection means keeping the doses ‘as low as reasonably achievable, economic and societal factors being taken into account’, and is best described as management of the radiation dose to the patient to be commensurate with the medical purpose.

In optimisation of protection of the patient in diagnostic procedures, such as diagnostic radiology and interventional procedures, again the same person gets the benefit and suffers the risk, and again individual restrictions on patient dose could be counterproductive to the medical purpose of the procedure. Therefore, source-related individual dose constraints are not relevant. Instead, diagnostic reference levels (DRLs)[12] for a particular procedure, which apply to groups of similar patients rather than individuals, are used to ensure that doses do not deviate significantly from those achieved at peer departments for that procedure unless there is a known, relevant, and acceptable reason for the deviation. This is in contrast to the Commission’s usual balancing of utilitarian protection policies based on collective doses against deontological safeguards using dose constraints for the individual. The policy for radiological protection in medicine is that the radiation exposure be commensurate with the medical purpose.

In radiation therapy, the aim is to eradicate the neoplastic target tissue or to palliate the patient’s symptoms. Some tissue reactions to surrounding tissue and some risk of stochastic effects in exposed non-target tissues are inevitable, but the goal of all radiation therapy is to optimise the relationship between the probability of tumour control and normal tissue complications. It is necessary to differentiate between the dose to the target tissue and the dose to other parts of the body. If the dose to the target tissue is too low, the therapy will be ineffective. The exposures will not have been justified and the optimisation of protection does not arise. However, the protection of tissues outside the target volume is an integral part of dose planning, which can be regarded as including the same aims as the optimisation of protection.

Application of dose limits for protection of patients

Medical exposures of patients have been properly justified and that the associated doses are commensurate with the medical purpose, so it is not appropriate to apply dose limits or dose constraints to the medical exposure of patients; such limits or constraints would often do more harm than good.[2] Often, there are concurrent chronic, severe, or even life-threatening medical conditions that are more critical than the radiation exposure itself. The emphasis is then on justification of the medical procedures and on the optimisation of radiological protection.[3]

In most situations in healthcare, other than radiation therapy, it is not necessary to approach the thresholds for tissue reactions, even for the most part in fluoroscopically guided interventional procedures, if the staff are properly educated and trained. The Commission’s policy is therefore to limit exposures so as to keep doses below these thresholds. The possibility of stochastic effects cannot be eliminated totally, so the policy is to avoid unnecessary sources of exposure and to take all reasonable steps to reduce the doses from those sources of exposure that are necessary or cannot be avoided.

Protecting pregnant patients

Video courtesy of the International Atomic Energy Agency

Early pregnancy can go undetected, so it is prudent to ensure that patients of childbearing potential are not pregnant before undergoing diagnostic radiology or nuclear medicine studies that provides doses above which the risk of adverse fetal health effects is not considered negligible (1–10 mGy).[13] Before radiation therapy, and in the absence of a documented history of applicable gynaecological surgery (e.g. tubal ligation, hysterectomy) or an established postmenopausal state, serum or urine pregnancy tests should be obtained, ideally 24–72 h prior to treatment.[14]

There are radiation-related risks to the embryo/fetus during pregnancy that are related to the stage of pregnancy and the absorbed dose to the embryo/fetus. ICRP has evaluated the effects of prenatal irradiation in detail.[15] These effects include lethal effects, malformations, central nervous system effects, leukaemia and childhood cancer. Consideration of these effects is important when pregnant patients undergo diagnostic radiology, interventional procedures, or radiation therapy using ionising radiation. A balance must be attained between the health care of the patient and the potential for detrimental health effects to the embryo/fetus that accompanies the specific radiological procedure.[16]

Pregnant Patients Undergoing Diagnostic Radiology

For diagnostic radiology, referring physicians should have discussions with imaging specialists to help determine the best tests for their pregnant patients, taking into account non-ionising diagnostic imaging modalities, such as ultrasound or magnetic resonance imaging. If these modalities are not the most appropriate test based on the clinical scenario, or are not available, the most appropriate ionising radiation imaging modality should be utilised in such a way as to achieve the diagnosis required while keeping fetal and maternal doses as low as reasonably possible. Almost always, if a diagnostic radiology examination is medically indicated, the risk to the mother of not performing the procedure is greater than the risk of potential harm to the embryo/fetus.

If the examinations are justified and their performance is optimised, they should not be withheld from pregnant patients. Fetal dose reduction measures will vary depending on the specific test being administered, but may include reducing the dose of injected radiopharmaceutical, limiting the number of images performed, beam collimation, patient shielding, and ensuring that the maternal pelvis (and fetus) is not in the beam path during fluoroscopic procedures unless it is absolutely necessary.

A pregnant patient has a right to know the magnitude and type of potential radiation effects that may result from in-utero exposure. Benefits and risks of the examination should be discussed with the patient, including developmental and cancer risks to the unborn baby. Central nervous system malformations and intellectual deficits have only been reported at fetal doses >100 mSv. Fetal doses from routine medical imaging are generally well below 100 mSv, and are generally below 20 mSv[17]

Pregnant Patients Undergoing Radiation Therapy

Radiation therapy (whether with external beam, brachytherapy, or nuclear medicine) can involve high radiation doses and may cause harm to the fetus. The risk to the fetus is dependent on the stage of fetal development. Lethal risk occurs in the pre-implantation phase, malformation occurs during major fetal organogenesis 3–7 weeks post implantation, mental retardation occurs at 8–15 weeks post implantation, and future cancer risk follows a stochastic model. Although there is variability in threshold radiation doses, it can be generalised that risks become notable at fetal doses of 100 mGy (10 rad) or above. So, it is essential to ascertain whether a female patient is pregnant prior to radiation therapy. In pregnant patients, cancers that are remote from the pelvis can usually be treated with radiation therapy. However, this requires careful planning. Cancers in the pelvis cannot be treated adequately during pregnancy without severe or lethal consequences for the embryo/fetus. radiation therapy in pregnant patients requires pre-therapy fetal dosimetry estimation followed by a comprehensive discussion of the benefits and risks of the procedure, with the patient included as part of the informed consent process.

It should also be noted that 131I used for diagnostic or therapeutic purposes and 32P used for therapeutic purposes, should be avoided in pregnant patients because iodine and phosphor can cross the placental barrier readily. The fetal thyroid is sufficiently mature to concentrate iodine at approximately 10 weeks post implantation, and there is a risk of causing fetal hypothyroidism.[16] As a rule, a pregnant patient should not be treated with a radioactive substance unless the radionuclide therapy is required to save her life: in that extremely rare event, the potential absorbed dose and risk to the fetus should be estimated and conveyed to the patient and the referring Physician. Considerations may include terminating the pregnancy.[16]

Termination of pregnancy is an individual decision affected by many factors. Absorbed doses below 100 mGy to the developing embryo/fetus should not be considered a reason for terminating a pregnancy. At doses to the embryo/fetus above this level, informed decisions should be made based upon individual circumstances, including the magnitude of the estimated dose to the embryo/fetus, and the consequent risks of harm to the developing embryo/fetus and risks of cancer in later life.

Protecting pediatric patients

Video courtesy of the International Atomic Energy Agency

Children are more sensitive to radiation exposure than adults. Depending on their age, organ, and tumour type, children are reported to be, on average, two to three times more sensitive to radiation than adults, and the younger the infants or children, the more radiosensitive they are at high doses. So, the potential risks of ionising radiation in paediatric patients need to be considered. Physicians should exercise caution when using ionising radiation to image or treat children. In nuclear medicine, a lower administered activity than that would be used for an adult may be used; acceptable images could still be obtained as the size of a child is typically smaller than that of an adult.   For diagnostic radiology, the following should be taken into consideration: (1) select the most optimised imaging protocol based on the patient’s age and size; (2) repeat imaging, or phases (e.g. CT), need to be justified relative to the importance of the additional information being gained vs the additional radiation dose; and (3) only image the indicated area. ICRP Publication 121 Radiological Protection in Paediatric Diagnostic and Interventional Radiology[9] and Image Gently provide more details. It is worthwhile to note that where applicable, non-ionizing radiation imaging modalities may be considered.

Preventing accidents in radiation therapy

Accident prevention in radiation therapy should be an integral part of the design of equipment and premises, and of the working procedures. Radiation therapy equipment should be designed to reduce operator errors by automatically rejecting demands outside the design specification. Radiation therapy equipment should be calibrated after installation or any modification and should be routinely checked by a standard test procedure that will detect significant changes in performance. [10] Working procedures should require key decisions to be subject to independent confirmation.[10] In radiation therapy, the avoidance of accidents is the predominant issue. A review of such accidents and advice for accident prevention is found in ICRP Publication 86; specific advice for brachytherapy can be found in ICRP Publication 97 Prevention of High-dose-rate Brachytherapy Accidents[18] and ICRP Publication 98 Radiation Safety Aspects of Brachytherapy for Prostate Cancer using Permanently Implanted Sources[19], and that for ion beam therapy can be found in ICRP Publication 127 Radiological Protection in Ion Beam Radiotherapy[20] and ICRP Publication 112 Preventing Accidental Exposures from New External Beam Radiation Therapy Technologies.[11]

Special notes on the use of effective dose and dose measurements

Effective dose was originally introduced so that all radiation exposures, external and internal, could be treated together in the control of occupational and public exposures, but it is also applicable to exposures in healthcare. Effective dose is used to inform decisions on justification of patient diagnostic and interventional procedures, planning requirement for research studies, and evaluation of accidental exposures. In each case, effective dose provides a measure of detriment. Effective dose can be used to classify different types of medical procedure into broad risk categories for the purpose of communicating risk levels to clinicians and patients. These applications rely on the validity of the linear-non-threshold (LNT) dose-response relationship. In addition, effective dose should not be used for individual or population-based cancer risk assessment as it does not consider individualized information. ICRP TG79 is preparing further advice on the use of effective dose in medicine as part of a task group report.

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Radiological protection of family members, carers and the public

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When a patient is exposed to external sources of radiation during diagnostic radiology or radiation therapy, there is no residual radiation in the patient after the procedure, and they pose no radiation risk to people around them. However, when nuclear medicine is used for diagnostic radiology or radiation therapy, protection of family members or the others who provide care to the patient, and the protection of the public the patient may come into contact, should be considered.[21][2]

For diagnostic nuclear medicine procedures (e.g. bone or myocardial perfusion scans), where the source of radiation is inside the body, radiopharmaceuticals retained in these patients emit radiation but the level of radiation is sufficiently low that these patients do not pose a radiation risk to those around them. These patients are generally discharged immediately after the procedure and instructed that they can carry on their normal daily activities.

For specific therapeutic nuclear medicine (e.g. unsealed source therapy) or radiation oncology (e.g. brachytherapy) procedures, such as 131I therapy for thyroid cancer or some forms of hyperthyroidism, the patient has significant amounts of residual radioactivity in their body that they may pose a slight risk to the others. Thus, radiation safety counselling is required to reduce the exposure of other individuals. Depending on the quantity of radionuclide, the treatment facility may need to hold (e.g. admit) the patient until the quantity is sufficiently reduced through a combination of radioactive decay and biological elimination. In addition, these patients are generally given radiation protection instructions, such as avoiding prolonged close contact with children or pregnant women for a specific period of time post therapy. The radiation safety advice will depend on the burden of underlying disease being treated and the treatment dose of 131I administered.

Radiation safety advice related to implanted therapeutic sources (e.g. brachytherapy) will vary based on the specific radioactive source used. For some sources, such as 125I used for prostate cancer, present very low risk to others. For other sources, such as 192Ir, there may be a radiation risk to others and these patients are usually hospitalised with restricted close contact to others until the source is removed.

Family members and other carers

Exposure to family members or the others who provide care to the patient is defined as medical exposures as there is direct benefits to them, but dose constraints should be established for use in defining the protection policy for visitors to patient and family members at home when a nuclear medicine patient is discharged from hospital. Such groups may include children. ICRP has not previously recommended values for such constraints, but a value of 5 mSv per episode for an adult (i.e. for the duration of a given release of a patient after therapy) is reasonable.[22] The constraint needs to be used flexibly. For example, higher doses may well be appropriate for the parents of very sick children or for an elderly who would like to take care of his/her sick spouse. Young children, infants, and visitors not engaged in direct comforting or care should be treated as members of the public, who are subject to the public dose limit of 1 mSv/year.

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Breastfed babies

In nuclear medicine, varying amounts of radiopharmaceuticals are retained in the patients for varying periods of time. Also, some radiopharmaceuticals can be transferred to breast milk and passed from mother to child during breast feeding. As a result, both the mother, and her breast milk, can be a source of radiation to the baby. Non-urgent tests should be postponed until the breastfeeding period is completed.[23]

However, most nuclear medicine agents can be handled by providing patient reassurance, educational materials, and instructions on breast milk pumping and storage during a prescribed period of time prior to nuclear medicine treatments. Breast milk can be pumped, frozen and used during a period of time following the treatment. Depending on the specific radiopharmaceutical administered to the mother, guidelines range from interruption in breast feeding for the majority of radiopharmaceuticals, to interruption for a prescribed period of time, to total cessation of breastfeeding[24] It is important to consult with a nuclear medicine specialist before ordering a diagnostic nuclear medicine procedure for breastfeeding patients. This will help to provide the information necessary to prepare the patient (e.g. the need to pump breasts, store pumped milk for prescribed decay periods etc.).

131I (half-life of 8 days) is one of the few radioactive agents that deserves special attention. 131I is used for both diagnostic and therapeutic thyroid procedures. Even small amounts of 131I as are used in thyroid uptake studies, are excreted into breast milk and are sufficient to have an adverse effect on the baby’s thyroid function. When 131I is used, breastfeeding should be terminated.

The public

Public access to hospitals and radiology rooms is restricted, but it is more open than is common in industrial and research laboratory operations. There are no radiological protection grounds for imposing restrictions on public access to non-designated areas. Due to the limited duration of public access, an access policy can be adopted for supervised areas if this is of benefit to patients or visitors and there are appropriate radiological protection safeguards. Public access to controlled areas with high-activity sources (e.g. brachytherapy and other therapy sources) should be limited to patients’ visitors, who should be advised of any restrictions on their behaviour.

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Radiological protection of volunteers in biomedical research

Volunteers acting in biomedical research makes a substantial contribution to medicine and human radiobiology. Some research studies are of direct value in the investigation of disease, and others provide information on the metabolism of pharmaceuticals and radionuclides that may be absorbed from contamination of the workplace or the environment. Not all of these studies take place in medical institutions, but ICRP includes the exposure of all these volunteers under the category of medical exposure.

The ethical and procedural aspects for volunteers in biomedical research have been addressed in ICRP Publication 62 Radiological Protection in Biomedical Research.[25] The key aspects include the need to guarantee a free and informed choice by the volunteers, the adoption of dose constraints linked to the societal benefits of the studies, and the use of an ethics committee that can influence the design and conduct of the studies. Involvement of children and the mentally ill or defective in biomedical research is also addressed in ICRP Publication 62 Radiological Protection in Biomedical Research.[25] It is important that the ethics committee should have easy access to radiological protection advice.

In many countries, radiation exposure of pregnant females in biomedical research is not specifically prohibited. However, their involvement in such research is very rare and should be discouraged unless pregnancy is an integral part of the research. In order to protect the embryo/fetus, strict controls should be placed on the use of radiation in these cases.

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Radiological protection of healthcare staff

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The principles for the protection of workers from ionising radiation are discussed fully in ICRP Publication 75 General Principles for the Radiation Protection of Workers.[26] These principles apply to healthcare staff working in diagnostic radiology, interventional procedures, nuclear medicine, and radiation therapy facilities.[2] ICRP has recently published recommendations on occupational radiological protection in interventional procedures.[27]

The control of occupational radiological exposure in healthcare can be simplified and made more effective by the designation of workplaces into two types: controlled areas and supervised areas. In a controlled area, normal working conditions, including the possible occurrence of minor mishaps, require workers to follow well-established procedures and practices aimed specifically at controlling radiation exposures. A supervised area is one in which the working conditions are kept under review, but special procedures are not normally needed.

Individual monitoring for external radiation is simple and does not require a heavy commitment of resources. In medicine, it should be used for all those who work in controlled areas.

In several areas of healthcare, the control of occupational exposure is of particular importance, including nursing of brachytherapy patients when the sources have been implanted, palpation of patients during procedures utilising fluoroscopy, fluoroscopically guided interventional procedures such as in heart catheterisation, or radiopharmaceutical preparation by staff in nuclear medicine. In all these procedures, careful shielding and time limits are needed. Individual monitoring with careful scrutiny of the results is also important. In brachytherapy, frequent and careful accounting of sources is essential.

Protecting pregnant workers

The basis for the control of occupational exposure of women who are not pregnant is the same as that for men. However, if a female worker declares to her employer that she is pregnant, additional controls have to be considered in order to attain a level of protection for the embryo/fetus broadly similar to that provided for members of the public. The working conditions of the pregnant worker, after the declaration of pregnancy, should be such as to make it unlikely that the additional equivalent dose to the embryo/fetus will exceed approximately 1 mSv during the remainder of the pregnancy. The part of a pregnancy prior to declaration of the pregnancy is covered by the normal protection of workers, which is essentially the same for females and males.

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  1. 1.0 1.1 1.2 ICRP Publication 103 The 2007 Recommendations of the International Commission on Radiological Protection. Ann. ICRP 37(2-4), 2007.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 ICRP Publication 105 Radiological Protection in Medicine. Ann. ICRP 37(6), 2007.
  3. 3.0 3.1 ICRP Supporting Guidance 2 Radiation and Your Patient A Guide for Medical Practitioners. Ann. ICRP 31(4), 2001.
  4. ICRP Publication 73 Radiological Protection and Safety in Medicine. Ann. ICRP 26(2), 1996.
  5. ICRP Publication 118 ICRP Statement on Tissue Reactions and Early and Late Effects of Radiation in Normal Tissues and Organs. Ann. ICRP 41(1-2), 2012.
  6. ICRP Publication 99 Low-dose Extrapolation of Radiation-related Cancer Risk. Ann. ICRP 35(4), 2005.
  7. 7.0 7.1 ICRP Publication 85 Avoidance of Radiation Injuries from Medical Interventional Procedures. Ann. ICRP 30(2), 2000.
  8. ICRP Publication 117 Radiological Protection in Fluoroscopically Guided Procedures Performed Outside the Imaging Department. Ann. ICRP 40(6), 2010.
  9. 9.0 9.1 9.2 ICRP Publication 121 Radiological Radiological Protection in Paediatric Diagnostic and Interventional Radiology. Ann. ICRP 42(2), 2013.
  10. 10.0 10.1 10.2 ICRP Publication 86 Prevention of Accidents to Patients Undergoing Radiation Therapy. Ann. ICRP 30(3), 2000.
  11. 11.0 11.1 ICRP Publication 112 Preventing Accidental Exposures from New External Beam Radiation Therapy Technologies. Ann. ICRP 39(4), 2009.
  12. ICRP Publication 135 Diagnostic Reference Levels in Medical Imaging. Ann. ICRP 46(1), 2017.
  14. The SNMMI Practice Guideline for Therapy of Thyroid Disease with 131I[2]
  15. ICRP Publication 90 Biological Effects after Prenatal Irradiation (Embryo and Fetus). Ann. ICRP 33(1-2), 2003.
  16. 16.0 16.1 16.2 ICRP Publication 84Pregnancy and Medical Radiation. Ann, ICRP 30(1), 2000.
  17. Radiation dose management: part 2, estimating fetal radiation risk from CT during pregnancy.[3]
  18. ICRP Publication 97 Prevention of High-dose-rate Brachytherapy Accidents. Ann. ICRP 35(2), 2005.
  19. ICRP Publication 98 Radiation Safety Aspects of Brachytherapy for Prostate Cancer using Permanently Implanted Sources. Ann. ICRP 35(3), 2005.
  20. ICRP Publication 127 Radiological Protection in Ion Beam Radiotherapy. Ann. ICRP 43(4), 2014.
  21. ICRP Publication 94 Release of patients after therapy with unsealed radionuclides. Ann. ICRP 34(2), 2004.
  22. Radiation Dose Limits for Individual Members of the Public[4]
  23. Radiation Protection of Patients[5]
  24. Breast milk excretion of radiopharmaceuticals: mechanisms, findings, and radiation dosimetry.[6]
  25. 25.0 25.1 ICRP Publication 62 Radiological Protection in Biomedical Research. Ann. ICRP 22(3), 1991.
  26. ICRP Publication 75 General Principles for the Radiation Protection of Workers. Ann. ICRP 27(1), 1997.
  27. ICRP Publication 139 Occupational Radiological Protection in Interventional Procedures. Ann. ICRP 47(2), 2018-1.

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