1 The foundation of radiological protection and the early years of ICRP

In 1895 Wilhelm Conrad Röntgen discovered a new type of radiation - x-rays that could blacken photographic film and produce images of bones in the hand. In 1896 Henri Becquerel identified radiation being emitted from salts of uranium linked to radioactivity, and following on from this in 1898 Marie and Pierre Curie isolated radium and began to promote the idea of its use in treating disease. Radiation had burst onto the scene. The public was inspired by x-ray images of the hand and the use of these ionising radiations for medical applications developed rapidly. Unaware of potential risks, professionals working with x-rays used their own hands to check the outputs of their x-ray tubes. However, over subsequent years reports began to appear of radiation dermatitis and later more serious deformities to the hands of pioneer physicians and technicians. The first x-ray tubes had no shielding and long exposures were required because of the low sensitivity of the photographic plates and this resulted in loss of hair at the site of the x-ray beam entrance for some patients. In response to growing concerns about the effects of ionising radiation being observed in researchers, equipment manufacturers, radiologists, and radiation therapists, the second International Congress of Radiology, held in Stockholm in 1928, established the International X-ray and Radium Protection Committee, which was later to become the International Commission on Radiological Protection (ICRP), under the chairmanship of Rolf Sievert. The first set of recommendations on a system for radiological protection was published later that year and focused on occupational protection in medicine. It included three basic rules that are still applied when protecting individuals against external exposure today.

  1. 1.

    Maintaining distance: For imaging, “an operator should place himself as remote as possible from the x-ray tube.” and “for x-ray treatment the operator is best stationed completely outside the x-ray room behind a protected wall”.

  2. 2.

    Using shielding: “the x-ray tube should be surrounded as completely as possible with protective material of adequate lead equivalent” and “the operator should be afforded protection from scattered x-rays by a screen of a minimum lead equivalent of 1 mm”.

  3. 3.

    Limiting working time: through restrictions on working hours of “seven working hours per day” and “five working days per week”, and “screening examinations should be conducted as rapidly as possible”.

Papers following up and expanding the initial recommendations on radiation protection were published in various scientific journals in the fields of medicine and physics in 1928, 1931, 1934, 1937, and in 1950 maximum permissible doses were recommended for workers together with periodic monitoring instead of restrictions on working hours. By this time the field of radiological protection had widened from protection in medical radiology to embrace all other aspects of protection against ionising radiation, and the organization was renamed the International Commission on Radiological Protection (ICRP). Six committees were established under the ICRP umbrella: two dealing with permissible dose, one each for external and internal radiation, three concerned with protection against different types of radiation, low and high energy x-rays, and heavy particles; and one committee for disposal of radioactive wastes and handling of radioisotopes.

The rapid development of interest in nuclear energy during the 1950s resulted in further evaluation and development of radiological protection principles, and in 1958 ICRP published the first booklet of its own through Pergamon Press, (ICRP Publication 1) entitled “Recommendations of the International Commission on Radiological Protection” [1]. The report focussed primarily on occupational exposure and stated that “medical exposure be considered separately” and “be kept as low as is consistent with the necessary requirements of modern medical practice”. No recommendations were made about doses to individuals from medical exposures, although it was noted that “there are indications that the highest levels of medical exposure reported could be reduced significantly by careful attention to techniques”. One of the main concerns at this time was about possible genetic effects being passed to subsequent generations and the next review of the fundamental recommendations published in 1964 [2] stated that radiological examinations of the lower abdomen and pelvis of women of reproductive age should, where practicable, be limited to the 10-day interval following the onset of menstruation, when it is virtually certain that women of such age are not pregnant.

2 The ICRP organisation

ICRP is an independent, international, non-governmental organization, with the mission to provide recommendations and guidance on radiological protection. At the present time it has about 60 Committee members, 13 Main Commission members, and 6 Scientific Secretariat members who provide administrative support. The committees establish task groups to work on and develop recommendations and guidance on specific topics, and the total membership of committees and task groups comprises nearly 280 members from 30 countries spread over 6 continents. Members represent leading scientists and policy makers in the field of radiological protection. Committee membership is reviewed every four years, while task groups exist for the duration of the assigned task and usually result in preparation of an ICRP publication. ICRP is registered as a UK charity, funded by contributions from organisations with an interest in radiological protection. ICRP became formally affiliated with the World Health Organization (WHO) as a ‘participating non-governmental organisation’ in 1956, and has formal relationships with 29 international and regional organisations (Table 1).

Table 1 Organizations with which ICRP has formal relationships
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Members of ICRP prepare comprehensive reports that set out fundamental recommendations, describing the overall system of radiological protection. This is used world-wide as the basis for radiological protection standards, legislation, guidelines, programmes, and practice. The recommendations are based on:

  1. 1.

    The current understanding of the science of radiation exposures and effects.

  2. 2.

    Value judgements taking into account societal expectations, ethics, and experience gained in application of the system.

The fundamental recommendations of ICRP are published periodically (every 10–20 years) and contain:

  1. 1.

    Basic concepts and dose quantities

  2. 2.

    Radiobiological data on which recommendations are based

  3. 3.

    General principles of operational radiation protection

  4. 4.

    Application to different types of exposure.

The ICRP recommendations evolve as radiation practices and the knowledge of radiation effects develops. The last four were in 1966 [3], in which patient protection appeared for the first time, in 1977 [4] that included dose limits with the first attempt at the summation of radiation doses to sensitive organs and inclusion of reference levels, in 1990 [5] and in 2007 [6]. Publication 60 [5] contained a framework for radiological protection, including the three fundamental principles of justification, optimization, and dose limitation for practices using radiation, and systems for occupational, medical, and public exposures, and Publication 103 [6] identified different groups for planned, emergency and existing exposures. The recommendations include dose limits for workers and exposure of the public from activities using radiation sources. The dose limits and ideas about protection needs have been amended over the years as more information has become available on potential effects of radiation exposure and the range of applications has broadened (Table 2). In addition to the fundamental recommendations, ICRP publishes documents to provide guidance on radiological protection in the application of techniques using ionising radiations in different areas, and these now make up the majority of the output. These were published in ICRP’s own journal, the Annals of the ICRP from 1977 onwards.

Table 2 Dose limits for whole body exposure
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3 Radiological protection dose quantities

The effects of ionising radiation on humans are divided into two classes:

  1. 1.

    Tissue reactions (deterministic effects) that will occur only above a relatively high threshold dose, but for which the severity increases with dose above that level. These take the form of skin and tissue injury, and radiation syndromes linked to acute exposure.

  2. 2.

    Stochastic effects that have a low probability of occurrence in an exposed population, which is a function of dose, but may not have a minimum threshold dose. These are initiation of cancer and heritable effects passed to future generations.

In the early days of radiation use, the primary concern was to limit the risk of tissue reactions, although there was an awareness of the potential for cancer induction and genetic effects, and limits were recommended for both whole body and skin dose. Concerns were raised during the 1950s, following observations of genetic effects in biological research and a maximum permissible genetic dose of 100 mrem (1 mSv) per week from all sources other than medical exposure and natural background was proposed for the gonads in 1959. A Life Span Study (LSS) had been set up in 1950 to assess the health of Japanese survivors of Hiroshima and Nagasaki atomic bombs after World War II. This demonstrated initially an increased incidence of leukaemia, and during the 1960s and 1970s it became apparent that incidences of several types of cancer, namely leukaemia, thyroid, breast and lung, were higher than expected.

Until this time, dose limits had been set for individual organs and tissues and the whole body. However, almost every exposure of the body involves the irradiation of more than one tissue and few exposures are uniform over the whole body. Therefore the dose quantities in use at the time could not readily be compared in relation to risk. A quality factor reflecting the biological effectiveness of different radiations in damaging tissue, due to the microscopic distribution of energy absorption, was introduced for calculating the dose equivalent to tissue. The concept of weighting doses to organs according to their sensitivity to radiation damage had been proposed by Jacobi in 1975 [7], and in 1977 the commission recommended derivation of a dose-equivalent limit based on the total risk to all tissues irradiated, linked to stochastic effects derived from the population in the LSS [4]. This was achieved by summing doses to individual tissues, each modified by a tissue weighting factor based on an assessment of the risk derived from the epidemiological data, to form an effective dose equivalent. The tissues included were those known to be at risk from cancer induction, the gonads for genetic effects, and a remainder consisting of an average dose for other tissues thought to be potentially at risk. This approach was based on the principle that the risk associated with the effective dose equivalent should be equal to that from a similar uniform whole body dose, and be independent of the parts of the body that had been irradiated or the uniformity of the exposure. This quantity could then provide a method for judging the acceptability of the level of risk in radiation work by comparing the risk with that for other occupations, as well as planning of operations and optimisation of procedures to keep dose levels to radiation workers and the public at acceptable levels. The dosimetric quantities were refined in Publication 60 in 1990, in which the Commission defined the equivalent dose as the absorbed dose averaged over a tissue or organ weighted for the radiation quality, and introduced the effective dose as the sum of equivalent doses to tissues weighted according to average risk over the population. This replaced the effective dose equivalent used earlier and included more organs and tissues. The changes in tissues included in the effective dose and the weighting factors were necessary because of the growing evidence of links between cancers in other tissues and radiation exposure identified through the LSS. The aim is to provide a radiation protection dose quantity that is easy to use so the weighting factors are grossly rounded to facilitate calculation. Effective dose is acknowledged to be an approximation with inherent uncertainties which should not be forgotten when it is used [8]. There have since been some more minor modifications in the reformulation of effective dose reported in Publication 103 [6], based on refinements of analysis of epidemiological data, and the changes over the three publications are summarised in Table 3.

Table 3 Tissue weighting factors for calculation of effective dose from ICRP Publications 26, 60, and 107
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4 Development of ICRP recommendations for medical exposures

The position of ICRP with regard to medical exposures had been evolving during this time and ICRP Publication 16 entitled “Protection of the Patient in X-ray Diagnosis” was published in 1970 [9], which stated that “the establishment of efficient measures for patient protection will in no way impede the continuing development of radiological diagnosis.” It contained advice on properties and size of the x-ray beams, organ shielding, anti-scatter grids, sensitivity of the recording system, processing control/recording of radiation exposure, and reduction in number of retakes. Report 17 published in 1971 [10] provided guidance on “Protection of the Patient in Radionuclide Investigations” as the number of radiopharmaceuticals available with short-lived radionuclides for clinical investigations expanded. It acknowledged that some clinicians are unfamiliar with the radiation doses received, particularly from the new nuclides, and of the dose commitment to organs in which the highest concentrations occurred, and that there were quite large gaps in metabolic information available, making accurate estimates of dose difficult. Following on from this there was a drive to develop models for the uptake and movement of radiopharmaceuticals through the body to allow doses to the organs of patients to be determined.

As awareness of the importance of the applications of radiation in medicine increased with the development of new techniques. ICRP Committee 3 devoted to Radiation Protection in Medicine was formed in 1977, including both patient and staff exposure. Other changes have since occurred in the ICRP Committees and the structure as of 2019 is shown in Fig. 1. The current remit of Committee 3 is concerned with the protection of persons and unborn children when ionising radiation is used in medical diagnosis, therapy, and biomedical research, and protection in veterinary medicine was added to this in 2018. Each committee meets annually to discuss current projects, tasks for the future, and areas in which research is needed, and sets up task groups comprising committee members and others to take forward the development of reports on different topics. The Main Commission oversees the work of ICRP as a whole and provides approval for establishment of task groups as considered appropriate and publication of reports.

Fig. 1
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ICRP Committee structure in 2019. Each committee sponsors or co-sponsors task groups made up of committee members, supplemented with individuals from different countries acknowledged to be experts in the particular application

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During the 1980s ICRP reports were published relating to protection of the patient in diagnostic radiology, radiotherapy and nuclear medicine. The reports were used by radiation protection specialists in hospitals, but not by the medical community who did not have ready access, so their impact was limited. It was clear that techniques being applied in medicine were becoming increasingly complex requiring more detailed guidance, and a decision was made in 1997 to identify and address demands by specific audiences within the medical community. After this, concise reports on specific topics such as pregnancy and medical radiation, and guidance for physicians on radiation and your patient were issued (Table 4). Attempts were made to increase the availability of reports by allowing translation by professional societies into local languages that could then be made available free of charge, examples being translations into Spanish and Korean. Reports were produced during succeeding years on radiotherapy with a particular focus on reducing the risk of accidents, and on radiology for avoiding injuries in interventional procedures. There has been a particular focus on diagnostic and interventional radiology, with issues relating to management of patient dose in the transition from film/screen to digital radiography, the expanding use of computed tomography, and in the use of fluoroscopy with application of the technique by a wider variety of medical specialties. Other publications have dealt with education and training for diagnostic and interventional procedures, setting out recommended training requirements in radiation protection for clinical staff, and paediatric radiology, where particular expertise is required to deal with the range in size of children being imaged and in addition the greater sensitivity of children to radiation (Table 4). ICRP has promoted methods for measurement of patient dose in radiology with dosimetry results now displayed on the majority of new radiology equipment that have been used for patient dose surveys and setting of diagnostic reference levels for use as a tool for optimisation of radiological protection. Guidance has also been published on protection of staff carrying out interventional procedures, as members of this group are more likely to receive higher doses, particularly to the eyes and so be affected by the reduction in dose limit for the lens of the eye [11]. Moreover the techniques are now widely employed by cardiologists and other clinicians outside the radiology department.

Table 4 ICRP Reports on Radiological Protection in Medicine published since the formation of Committee 3, Radiological Protection in Medicine
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5 Radiation doses to patients from radiopharmaceuticals

Measurements of radiation emitted from x-ray equipment are used in evaluating x-ray exposures, but the only quantity relating to doses for nuclear medicine patients is the amount of radioactivity administered. Committees 2 and 3 have worked together to quantify doses received by nuclear medicine patients in order to fill this gap. This has included methods for calculating absorbed doses to organs and tissues, assessment of effective dose (equivalent), and doses to the embryo and foetus. ICRP Publication 52 (Table 4) had acknowledged that the effective dose equivalent was a convenient measure of doses to patients that could facilitate comparisons between different types of radiological investigation, extending its use beyond occupational and public exposures. The reports use bio-kinetic models developed within a generic framework to evaluate amounts of radionuclides in different organs. They are based on practical information that is available, together with an assessment of data quality and biological realism. Radionuclide distributions and transit times through different organs are then evaluated and activity-time curves generated for all parts of the system. An example of a compartment model employed is shown in Fig. 2. The information is then used to evaluate mean absorbed doses to radiosensitive target organs from radionuclide uptake in adjacent organs. Publications giving the coefficients for assessment of doses to patients from radiopharmaceutical intakes are listed in Table 5.

Fig. 2
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Example of compartment models used in evaluating radionuclide transit through the body for the gastrointestinal tract

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Table 5 Reports prepared by ICRP Committees 2 and 3 giving coefficients for assessment of doses to patients from radiopharmaceutical intakes
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6 Present and future ICRP work on radiological protection in medicine

ICRP Committee 3 has 16 members from 12 countries, containing a mixture of professionals including medical physicists, radiologists, radiotherapists, nuclear medicine physicians, radiographers, public health physicians, and regulators, from countries with different cultures, and levels of expertise and professional development. At any one time members will be involved in a number of task groups (TGs), which will each be chaired by a member of committee 3 and include other acknowledged experts in the respective fields, who can contribute from their experience to the development of recommendations and guidance in the relevant field. At the present time TG 79 has just completed a report on “The Use of Dose Quantities in Radiological Protection”, including recommendations on the use of effective dose, which has been used widely in medicine, with the application often extending beyond what is appropriate, ignoring the many uncertainties in the derivation. Other current TGs, and Working Parties preparing the background and terms of reference for future TGs are listed in Table 6, with TG 36 on Radiation Dose to Patients from Radiopharmaceuticals being an established group that is working continually to improve bio-kinetic modelling and dose assessment as more data becomes available through research programmes.

Table 6 ICRP Task Groups and Working Parties currently developing guidance for future publication
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In order to maintain and continue to improve the system of fundamental recommendations on radiological protection that are used by countries throughout the world, ICRP needs to continually evaluate radiological protection needs related to emerging technologies, and identify and encourage research to support the development. ICRP also has a role in promoting education and training in radiological protection and related fields. Therefore ICRP needs to engage with professionals, policy makers, and the public to promote a balanced understanding of radiological hazards and protection methods, and to this end it aims to expand relations with other organisations. Since 2013 ICRP has engaged more actively through the holding of biennial ICRP symposia in different locations around the world, and other events to increase direct interactions with user groups. An important component of ICRP’s role is to promote awareness of radiation protection through ensuring that ICRP publications are user-friendly and contain plain-language summaries broadening access to the recommendations. The ICRP publications prepared in medicine in recent years have been steadily increasing and they have been targeted as specific audiences. The reports are written initially in English, but national societies can translate reports into their own languages and make these freely available. However, ICRP is a charity and in order to gain funds has to make charges for the original publications, in order to fund their preparation. This has created an obstacle to dissemination of the recommendations and guidance. ICRP has marked its 90th year by launching an appeal for funds in order to allow publications to be available free of charge, so from January 2020, all reports over 2 year old (Table 4) will be freely available from the ICRP website http://www.icrp.org/. It is hoped that through this approach, recommendations and guidance can be made readily available to all who require them and in this way they will be available to less developed countries as they install new equipment and technologies which is particularly important for the development of the safe application of radiation in medicine.

7 Summary

ICRP was founded in 1928 in response to growing concerns about the effects of ionising radiation and is now in its 90th year. ICRP prepares fundamental recommendations on the system of radiological protection that are adopted world-wide as the basis for radiological protection standards, legislation, guidelines, programmes, and practice. In 1977 ICRP Committee 3 was formed specifically for radiation protection in medicine, including patients, and since that time 27 reports have been published devoted to patient protection in radiology, nuclear medicine, and radiotherapy. ICRP aims to maintain and continually evaluate radiological protection needs related to emerging technologies in order to keep advice relevant and effective. It has engaged more actively with users in recent years through holding biennial ICRP symposia and participating in other events. The fact that ICRP has to charge for reports to fund their preparation has created an obstacle to their wider dissemination, but from January 2020, all reports over 2 year old will be available free of charge. It is hoped that this will aid the dissemination of information and experience, promote a high standard of radiological protection, and help medical and other healthcare professionals to adjust to challenges presented by new radiation techniques in medicine as they arise.