The
focus of the work of Committee 2 of the International Commission on Radiological Protection (ICRP) is the computation of dose coefficients compliant with Publication 103. A set of reference computational phantoms is being developed, based on medical imaging data, and used for radiation transport calculations. Biokinetic models used to describe the behaviour of radionuclides in body tissues are being updated, also leading to changes in organ doses and effective dose coefficients. Dose coefficients for external radiation exposure of adults calculated using the new reference phantoms were issued as Publication 116, jointly with the International Commission on Radiation Units and Measurements. Forthcoming reports will provide internal dose coefficients for radionuclide inhalation and ingestion by workers, and associated bioassay data. Work is in progress to revise internal dose coefficients for members of the public, and, for the first time, to provide reference values for external exposures of the public. Committee 2 is also working with Committee 3 on dose coefficients for radiopharmaceuticals, and leading a cross-Committee initiative to give advice on the use of effective dose.
1. INTRODUCTION
The remit of the International Commission on Radiological Protection (ICRP) Committee 2 is: the development of dose coefficients for the assessment of internal and external radiation exposure; the development of reference biokinetic and dosimetric models; and the development of reference data for workers and members of the public (www.ICRP.org). The current membership (2013–2017) is John Harrison (Chair), François Paquet (Vice-Chair), Wes Bolch (Secretary), Mike Bailey, Vladimir Berkovski, Luiz Bertelli, Doug Chambers, Marina Degteva, Akira Endo, John Hunt, Chan Hyeong Kim, Rich Leggett, Jizeng Ma, Dietmar Nosske, Nina Petoussi-Henss, and Frank Wissmann.
Currently, Committee 2 has five Task Groups that are responsible for the production of reports: (1) computational phantoms and radiation transport; (2) internal dose coefficients; (3) age-dependent dose conversion coefficients for external exposures to environmental sources; (4) the use of effective dose as a risk-related dosimetric quantity; and (5) radiopharmaceuticals.
Committee 2 works closely with the International Commission on Radiation Units and Measurements (ICRU); the Chairman of ICRU, Hans-Georg Menzel, is a member of the ICRP Main Commission. Joint work with ICRU is in progress to update the operational quantities used in the measurement of external radiation exposures. Committee members also support the work of the other ICRP Committees, currently providing members for Task Groups of Committees 1, 3 and 5.
Revisions of ICRP recommendations invariably require recalculation of dose coefficients because changes are made to the radiation weighting factors and tissue weighting factors used in the calculation of equivalent and effective dose. In addition, improvements to the models used to calculate doses also lead to revised values. Work is currently in progress to provide replacement dose coefficients based on the 2007 Recommendations (ICRP, 2007), in which a number of important methodological improvements will be incorporated. The following Chapters will provide short summaries of the work programmes of each Task Group.
2. COMPUTATIONAL PHANTOMS AND RADIATION TRANSPORT
The remit of Task Group 96 is to develop reference anatomical phantoms and associated radiation transport calculations for use in the revision of dose coefficients as defined in Publication 103 (ICRP, 2007) for both external and internal sources. This Task Group is chaired by Wes Bolch (USA) and includes other members of Committee 2: John Hunt (Brazil), Chan-Hyeong Kim (South Korea), Akira Endo (Japan), and Nina Petoussi-Henss (Germany). Additional full members are Maria Zankl (Germany) and Choonsik Lee (USA); and the corresponding members are Derek Jokisch (USA), Kwang Pyo Kim (South Korea), Helmut Schlattl (Germany), and Junli Li (China).
Publication 110 (ICRP, 2009), a joint report with ICRU, provided reference phantoms for the adult male and female derived from imaging data for individuals, replacing the use of stylised hermaphrodite phantoms as developed by the Medical Internal Radiation Dose Committee of the Society of Nuclear Medicine (e.g. Cristy, 1980; Cristy and Eckerman, 1987). In future calculations, equivalent dose will be calculated separately for males and females, and averaged in the calculation of effective dose (ICRP, 2007).
Reference phantoms are being developed for newborns; children aged 1, 5, 10, and 15 years; and the fetus and pregnant female at various gestational ages. These dosimetric models, and those for adults provided in Publication 110 (ICRP, 2009), are being used to provide reference radiation transport data in the form of specific absorbed fractions (SAFs) for radiations emitted from radionuclides retained in body organs and tissues. SAFs represent the deposition of energy in all important organs/tissues (target regions) following emissions from radionuclides retained in body organs and tissues (source regions). These data are used in the calculation of dose coefficients for the inhalation and ingestion of radionuclides by workers and members of the public, and also in calculations of doses from radiopharmaceuticals.
The calculation of SAFs involves radiation transport of photons, electrons, and neutrons for an extensive set of source/target organ pairs. Additional work has focussed on microcomputed-tomography-based models of electron and α particle dosimetry of skeletal tissues, and revisions to electron and α particle dosimetry in the Human Respiratory Tract Model (ICRP, 1994a) and the Human Alimentary Tract Model (ICRP, 2006). The report providing radiation transport calculations for adult phantoms (adult SAFs) completed public consultation in October 2015.
3. INTERNAL DOSE COEFFICIENTS
The remit of Task Group 95 is to provide revised dose coefficients and associated data for intakes of radionuclides for workers and members of the public. This Task Group is chaired by François Paquet (France) and includes other members of Committee 2: Michael Bailey (UK), Vladmir Berkovski (Ukraine), and Rich Leggett (USA). Additional full members are Eric Blanchardon (France), Tim Fell (UK), and George Etherington (UK); and the corresponding members are Eric Ansoborlo (France), Luiz Bertelli (USA), Estelle Davesne (France), Demetrio Gregoratto (UK), Augusto Guissani (Germany), James Marsh (UK), Dunstana Melo (USA), Dietmar Nosske (Germany), Matthew Puncher (UK), Genadij Ratia (Ukraine), and Tracy Smith (UK).
Work is in progress to replace the Publication 30 series and Publication 68 (ICRP, 1979, 1980, 1981, 1988, 1994b) that give biokinetic data and dose coefficients for occupational intakes of radionuclides by inhalation and ingestion, and Publications 54 and 78 (ICRP, 1989, 1997) that give information for bioassay interpretation, with a single series of reports on occupational intakes of radionuclides (OIR series). Work is also in progress to replace all currently available dose coefficients for ingestion and inhalation of radionuclides by members of the public.
OIR Part 1 is now available as Publication 130 (ICRP, 2015b), providing a description of biokinetic and dosimetric methodology, including a summary of the Human Alimentary Tract Model (ICRP, 2006), a detailed description of changes to the Human Respiratory Tract Model (ICRP, 1994a), and an outline of approaches taken to the development of systemic models for elements. The use of bioassay data is also discussed. Subsequent parts will consist of element sections describing element-specific biokinetic models, and providing dose coefficients and bioassay data. Detailed supporting data will be provided electronically, including organ doses and additional bioassay data. The elements and radioisotopes to be included in OIR Parts 2–5 are listed by Harrison (2015). Dose coefficients will be given for different exposure conditions, including ranges of inhaled particle sizes, and different chemical forms where information is available, with the advice that site-specific dose coefficients may be calculated in situations where more specific data are available, and estimated doses warrant more detailed consideration.
Dose coefficients for inhalation and ingestion of radioisotopes of radon, including inhalation of Rn-222 and its radioactive progeny, will be included in OIR Part 3. Dose coefficients for inhaled Rn-222 and its progeny have previously been derived by epidemiological comparison using the Publication 65 dose conversion convention (ICRP, 1993; Harrison and Marsh, 2012). Inhalation of Rn-222 represents a special case because there is good and consistent information on risks of radon-induced lung cancer derived from epidemiological studies of underground miners and from residential pooled analyses (ICRP, 2010a). Using the Publication 115 (ICRP, 2010a) nominal risk coefficient of 5 × 10−4 per working level month (WLM), and the Publication 103 (ICRP, 2007) detriment values, gives dose conversion convention values of 12 mSv effective dose WLM−1 for adults and 9 mSv WLM−1 for a population of all ages (Marsh et al., 2010). Dosimetric data to be published in OIR Part 3 will include values of approximately 11 mSv effective dose WLM−1 for mines and 20 mSv WLM−1 for indoor workplaces. However, using a more realistic breathing rate for sedentary occupations such as office workers gives a value of approximately 14 mSv WLM−1 (Harrison and Marsh, 2012). For dwellings, the dose coefficient was calculated to be 13 mSv WLM−1.
4. DOSE COEFFICIENTS FOR EXTERNAL ENVIRONMENTAL EXPOSURES
The remit of Task Group 90 is to calculate dose coefficients for members of the public, including adults and children of different ages, for exposures to external sources of radiation. This Task Group is chaired by Nina Petoussi-Henss (Committee 2, Germany). The full members are Akira Endo (Committee 2, Japan), Wes Bolch (Committee 2, USA), Keith Eckerman (Committee 2 Emeritus, USA), and Helmut Schlattl (Germany); and the corresponding members are Daiki Satoh (Japan), Michael Bellamy (USA), Nolan Hertel (USA), John Hunt (Brazil), Jan Jansen (UK), Choonsik Lee (USA), Kimiaki Saito (Japan), and Song Jae Yoo (Korea).
The work to provide dose coefficients for members of the public follows on from the completion of Publication 116 (ICRP, 2010b), a joint report with ICRU, providing conversion coefficients for organ and effective doses for external exposures in occupational settings. Calculations were performed using the Publication 110 (ICRP, 2009) phantoms and in accordance with the 2007 Recommendations (ICRP, 2007). The radiations considered were external beams of mono-energetic photons, electrons and positrons, neutrons, protons, pions (negative/positive), muons (negative/positive), and helium ions. The organ dose conversion coefficients tabulated in the report represent ICRP/ICRU recommended values. Comparisons of the protection quantities, equivalent and effective dose, with corresponding operational quantities show the latter to provide conservative estimates of dose in the majority of cases. Annexes and a CD provide detailed supporting information, including equivalent dose coefficients for the lens of the eye and the skin.
Assessment of external exposures of members of the public, including infants and children, is particularly important in the context of accidental releases from nuclear installations. Calculation of dose coefficients for external environmental exposures requires the evaluation of the environmental fields. Work to calculate fields for contaminated soil, water, and air considering different radiation types is nearing completion. Organ and effective doses will be calculated using the adult and paediatric reference phantoms being developed by Task Group 96 on computational phantoms and radiation transport. Dose coefficients will be reported for several radionuclides, both in terms of environmental radionuclide concentration and measured ambient dose equivalent.
5. USE OF EFFECTIVE DOSE
Task Group 79 on the use of effective dose, led by Committee 2, also has members from Committees 1, 3, and 4. Its remit is to provide advice on the use of effective dose, including medical applications. This Task Group is chaired by John Harrison (UK). The full members are Richard Wakeford (Committee 1, UK), Pedro Ortiz López (Committee 3, Spain), Colin Martin (Committee 3, UK), Hans-Georg Menzel (Main Commission, Switzerland), Jane Simmonds (ex-Committee 4, UK), and Rebecca Smith-Bindman (USA); and the corresponding members are Wes Bolch (USA), John Cooper (UK), and Christian Streffer (Main Commission Emeritus, Germany).
Experience has shown that ‘effective dose’, which has been defined and introduced by ICRP for risk management purposes (i.e. for risk limitation and optimisation), is widely used in radiological protection and related fields beyond its original purpose, and is used incorrectly in some cases. Useful guidance on the use of the quantity is provided in Annex B to the 2007 Recommendations (ICRP, 2007). Further guidance is being formulated with an important focus on medical exposures (Balonov and Shrimpton, 2012; Harrison and Ortiz López, 2015). Task Group proposals are discussed by Harrison et al. (2016).
6. RADIOPHARMACEUTICALS
The remit of Task Group 36 (joint Committees 2 and 3 Task Group) is to develop dose coefficients for administered radiopharmaceuticals. This Task Group is chaired jointly by Dietmar Nosske (Committee 2, Germany) and Sören Mattsson (Committee 3 Emeritus, Sweden). The full members are Lennart Johansson (Sweden), Keon Kang (Committee 3, South Korea), Sigrid Leide-Svegborn (Sweden), and Michael Stabin (USA); and the corresponding members are Martin Andersson (Sweden), Wes Bolch (Committee 2, USA), Augusto Giussani (Germany), Julian Liniecki (Poland), Katrine Åhlström Riklund (Committee 3, Sweden), and Marie Sydoff (Sweden).
Publication 128 (ICRP, 2015a) from the Task Group provided a compilation of Publication 60 (ICRP, 1991)-based dose coefficients for radiopharmaceuticals considered in Publications 53, 80 and 106 (ICRP, 1987, 1998, 2008). This report also includes new information for Rb-82-chloride, (I-123, I-124, I-125, I-131)-iodide, and I-123-labelled 2-carbomethoxy 3-(4-iodophenyl)-N-(3-fluoropropyl) nortropane. The main future work for the Task Group is to update Publication 128 by values calculated using the new ICRP adult and paediatric reference voxel phantoms, and Publication 103 (ICRP, 2007) methodology, as well as to develop biokinetic and dosimetric models for new substances, and to identify older substances where improvement models are needed. Biokinetic models for C-11-PIB, C-11-choline, and Ga-68-HA-DOTA-TATE are in an advanced stage of development, and new models for F-18-Flutemetamol, F-18-Florbetapir, F-18-Florbetaben (BAY94-9172; Neuraceq), F-18-FMISO, and other Ga-68-DOTA-peptides (-TOC, -NOC, -TATE, -NOT) are also being considered.
7. DISCUSSION
The replacement of all reference dose coefficients with values compliant with Publication 103 (ICRP, 2007) is a substantial scientific effort that will be completed over the next few years. At the request of the International Atomic Energy Agency and the European Commission, a compilation of dose coefficients has been provided as Publication 119 (ICRP, 2012), based on the 1990 Recommendations in Publication 60 (ICRP, 1991). These values are referred to in the International and European Basic Safety Standards for use while new values based on the 2007 Recommendations are being calculated (EC, 2014; IAEA, 2014). Similarly, Publication 128 (ICRP, 2015a) provides a compilation of dose coefficients for administered radiopharmaceuticals based on Publication 60.
As presaged in the ICRP Statement on Radon (ICRP, 2010a), for the first time, Committee 2 will provide dose coefficients for radon and its radioisotopes calculated using biokinetic and dosimetric models. However, as discussed in Chapter 3, inhalation of Rn-222 and its progeny represents a special case because effective dose coefficients can also be derived by epidemiological comparison, as done in Publication 65 (ICRP, 1993). Taking into account the epidemiology and dosimetric calculations, the Commission will recommend the use of a single dose conversion coefficient of 12 mSv WLM−1, equivalent to 3.4 mSv mJ−1 h m−3, for the calculation of doses following inhalation of radon and its progeny in workplaces (OIR Part 3). This reference dose coefficient is considered to be applicable to the majority of circumstances with no adjustment for aerosol characteristics. However, in cases where aerosol characteristics are significantly different from typical conditions, where sufficient, reliable aerosol data are available and estimated doses warrant more detailed consideration. It will be possible to calculate site-specific dose coefficients using the biokinetic and dosimetric data to be provided in OIR Part 3 and the accompanying electronic annexes.
In terms of measurement of Rn-222 gas exposure, the reference effective dose coefficient of 12 mSv WLM−1 (3.4 mSv mJ−1 h m−3) corresponds to 7.5 × 10−6 mSv Bq−1 h m−3, assuming an equilibrium factor, F, of 0.4 between radon and its short-lived progeny (Harrison and Marsh, 2012). With an occupancy of 2000 h y−1 for a worker (ICRP, 1993, 2010a) and F = 0.4, the effective dose corresponding to annual exposure at the upper reference level of 300 Bq m−3 recommended in Publication 126 (ICRP, 2014) is 4.5 mSv. For the reference residential occupancy of 7000 h y−1, the corresponding value of effective dose is 15.8 mSv.
The dosimetric methodology employed by Committee 2 aims to make best use of developing scientific approaches. As part of these continuing efforts, a working group has been set up, led by Chan Hyeong Kim, to develop polygon mesh (PM) phantoms to overcome some difficulties in the application of voxel phantoms (Kim et al., 2016). Proper segmentation of smaller tissues such as the lens of the eye, the skin, the walls of some organs, and skeletal tissues is not possible using voxel phantoms with resolution of the order of millimetres. Currently, separate stylised models are being used to represent these tissues. The first objective of the working group is to convert the Publication 110 (ICRP, 2009) phantoms to a high-quality PM format, including all source and target regions, including thin tissue layer targets (10–300 µm) in the alimentary and respiratory tracts. Such phantoms will be used in future ICRP calculations.
