Abstract
The practical implementation of the International Commission on Radiological Protection’s (ICRP) system of radiological protection requires the availability of appropriate methodology and data. Over many years, ICRP Committee 2 has provided sets of dose coefficients to allow users to evaluate equivalent and effective doses for radiation exposures of workers and members of the public. The methodology being applied in the calculation of doses is state-of-the-art in terms of the biokinetic models used to describe the behaviour of inhaled and ingested radionuclides, and the dosimetric models used to model radiation transport for external and internal exposures. This overview provides an outline of recent work and future plans, including publications on dose coefficients for adults, children, and in-utero exposures, with new dosimetric phantoms in each case. For the first time, ICRP will publish dose coefficients for intakes of radon isotopes calculated using dosimetric models. Committee 2 is also working with Committee 3 on dose coefficients for radiopharmaceuticals, and leading a cross-committee initiative to provide advice on the use of effective dose. The remit of Committee 2 has now been widened to include all data requirements for the assessment of doses to humans and non-human biota.
1. Introduction
Committee 2 of the International Commission on Radiological Protection (ICRP) has a revised remit for the July 2017–June 2021 term of the Commission to reflect the change to encompass dosimetry for humans and non-human biota. The new remit is: Committee 2 develops dosimetric methodology for the assessment of internal and external radiation exposures, including reference biokinetic and dosimetric models and reference data and dose coefficients, for use in the protection of people and the environment.
The membership for the 2017–2021 term is John Harrison, Chairman (UK), François Paquet, Vice-Chair (France), Wesley Bolch, Secretary (USA), Eric Blanchardon (France), Volodymyr Berkovskyy (Ukraine), Augusto Giussani (Germany), Derek Jokisch (USA), Chan Hyeong Kim (Korea), Rich Leggett (USA), Maria Lopez (Spain), Nina Petoussi-Henss (Germany), Tatsuhiko Sato (Japan), Tracy Smith (UK), Alexander Ulanovsky (Germany), and Frank Wissmann (Germany). Keith Eckerman (USA) is an emeritus member. Thanks are due to the following members who left Committee 2 after the 2013–2017 term: Mike Bailey (UK), Luiz Bertelli (USA), Doug Chambers (Canada), Marina Degteva (Russian Federation), Akira Endo (Japan), John Hunt (Brazil), Jizeng Ma (China), and Dietmar Nosske (Germany).
Committee 2 currently has six task groups that are responsible for the production of reports: Computational Phantoms and Radiation Transport, chaired by Wesley Bolch; Mesh-type Reference Computational Phantoms, chaired by Chan Hyeong Kim; Internal Dose Coefficients, chaired by François Paquet; Age-dependent Dose Conversion Coefficients for External Exposures to Environmental Sources, chaired by Nina Petoussi-Henss; Use of Effective Dose as a Radiological Protection Quantity, chaired by John Harrison; and Radiation Dose to Patients in Diagnostic Nuclear Medicine, chaired by Augusto Giussani, with Sören Mattsson (Committee 3 emeritus member) as honorary co-chair.
Committee 2 works closely with the International Commission on Radiation Units and Measurements (ICRU). A joint ICRU/ICRP report is in preparation to update the operational quantities used in the measurement of external radiation exposures, as described by Endo (2016). Committee members also support the work of the other ICRP committees, currently providing members for task groups of Committees 1 and 3.
The following sections provide short summaries of the work of the task groups of Committee 2. Other papers in these proceedings provide more detailed discussions of work on development of computational phantoms (Kim et al., 2018; Zankl et al., 2018) and internal dosimetry (Paquet and Harrison, 2018).
2. Computational phantoms
Committee 2 currently has two task groups responsible for the development of dosimetric phantoms. Task Group 96 is chaired by Wesley Bolch, and has responsibility for the reference anatomical phantoms and associated radiation transport calculations being used in the revision of dose coefficients as defined in Publication 103 (ICRP, 2007) for both external and internal sources. Members are John Hunt, Derek Jokisch, Chan Hyeong Kim, Choonsik Lee (USA), Nina Petoussi-Henss, Tatsuhiko Sato, and Maria Zankl (Germany). Corresponding members are KwangPyo Kim (Korea), Junli Li (China), and Helmut Schlattl (Germany). Task Group 103, chaired by Chan Hyeong Kim, is converting the voxel-type reference computational phantoms into high-quality mesh format to address some limitations of the voxel-type phantoms, and allow all calculations to be made with the reference phantoms. Members are Wesley Bolch, Choonsik Lee, Nina Petoussi-Henss, Yeon Soo Yeom (Korea), and Maria Zankl. Corresponding members are Chansoo Choi (Korea), Beom Sun Chung (Korea), Min Cheol Han (Korea), Keith Eckerman, Han Sung Kim (Korea), Thang Tat Nguyen (Vietnam), and Rui Qiu (China).
Bolch et al. (2016) and Zankl et al. (2018) discuss the development of Publication 110 (ICRP, 2009) reference phantoms for the adult male and female, and reference phantoms for children of different ages: newborn, 1 year, 5 years, 10 years, and 15 years. Models are also being developed for the fetus and pregnant female at various gestational ages (Bolch et al., 2016). These dosimetric models 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 in calculations of doses from radiopharmaceuticals (see below).
The calculation of SAFs involves radiation transport of photons, electrons, and neutrons for an extensive set of source/target organ pairs. The report providing radiation transport calculations for adult phantoms (adult SAFs) was issued as Publication 133 (ICRP, 2016a). It includes revisions to separate calculations for dosimetry of the skeleton, respiratory tract, and alimentary tract as improvements to the approaches and calculations of Publication 30 (ICRP, 1979), Publication 66 (ICRP, 1994a), and Publication 100 (ICRP, 2006), respectively.
Kim et al. (2016, 2018) discuss the conversion of the voxel-type Publication 110 (ICRP, 2009) phantoms into high-quality mesh format phantoms, including all source and target tissues, so that all calculations can be performed without recourse to separate models. Task Group 103, led by Chan Hyeong Kim, will also develop paediatric phantoms, and these models, once published by ICRP, will be used in planned Committee 2 work beyond the current round of publications of dose coefficients calculated using methodology compliant with Publication 103 (ICRP, 2007). It is envisaged that this will include, for example, considerations of dosimetry for emergencies for which evaluations of the possibility of tissue reactions will be important, as well as the use of effective dose in control of stochastic risks.
3. Dose coefficients for external environmental exposures
The remit of Task Group 90, chaired by Nina Petoussi-Henss, is to calculate dose coefficients for members of the public, including adults and children of different ages, for exposures to external sources of radiation. Members are Wesley Bolch, Keith Eckerman, Akira Endo, and Helmut Schlattl. Corresponding members are Daiki Satoh (Japan), Michael Bellamy (USA), Nolan Hertel (USA), John Hunt, Jan TM Jansen (UK), Choonsik Lee, Kimiaki Saito (Japan), Yeon Soo Yeom, and Song Jae Yoo (Korea).
The work to provide dose coefficients for members of the public followed from Publication 116 (ICRP, 2010b), a joint report with ICRU, which provided conversion coefficients for organ and effective doses for external exposures in occupational settings. Assessment of external exposures of members of the public, including infants and children, is particularly important in the context of accidental releases from nuclear facilities. Calculation of dose coefficients for external environmental exposures requires evaluation of the environmental fields. Work to calculate fields for contaminated soil and air, considering different radiation types, has been completed. Organ and effective dose calculations for mono-energetic sources have also been completed, including quality assurance, using the new reference ICRP adult and paediatric reference phantoms (see above). Calculation of dose coefficients for specific radionuclides is in progress. It is anticipated that the report will be ready for public consultation during 2018.
4. Internal dose coefficients
The remit of Task Group 95, chaired by François Paquet, is to provide revised dose coefficients and associated data for intakes of radionuclides for workers and members of the public. Members are Volodymyr Berkovskyy, Eric Blanchardon, Augusto Giussani, Rich Leggett, Tracy Smith, Michael Bailey, George Etherington (UK), and Tim Fell (UK). Corresponding members are Luiz Bertelli (USA), Estelle Davesne (France), Demetrio Gregoratto (UK), James Marsh (UK), Dunstana Melo (USA), Dietmar Nosske, and Genadij Ratia (Ukraine).
Work is in progress to replace the Publication 30 series and Publication 68 (ICRP, 1979, 1980, 1981, 1988, 1994b), which provide biokinetic data and dose coefficients for occupational intakes of radionuclides by inhalation and ingestion, and Publications 54 and 78 (ICRP, 1989, 1997), which provide information for bioassay interpretation, with a single series of publications 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, Publication 130 (ICRP, 2015b), provides a general introduction and description of biokinetic and dosimetric methodology. Subsequent parts consist of element sections describing element-specific biokinetic models and providing dose coefficients and bioassay data. Detailed supporting data are provided electronically, including organ doses and additional bioassay data. OIR Part 2, Publication 134 (ICRP, 2016b), provides data for hydrogen (H), carbon (C), phosphorus (P), sulphur (S), calcium (Ca), iron (Fe), cobalt (Co), zinc (Zn), strontium (Sr), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), and technetium (Tc). OIR Part 3, Publication 137 (ICRP, 2017b), provides data for ruthenium (Ru), antimony (Sb), tellurium (Te), iodine (I), caesium (Cs), barium (Ba), iridium (Ir), lead (Pb), bismuth (Bi), polonium (Po), radon (Rn), radium (Ra), thorium (Th), and uranium (U). OIR Part 4 should be published in 2018 and will provide data for lanthanides and actinides.
Dose coefficients for inhalation and ingestion of radioisotopes of radon, including inhalation of 222Rn and its radioactive progeny, are included in OIR Part 3. Previously, dose coefficients for inhaled 222Rn and progeny have been derived by epidemiological comparison, using the Publication 65 dose conversion convention (ICRP, 1993; Harrison and Marsh, 2012). Inhalation of 222Rn 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 (Publication 115; ICRP, 2010a). Using the Publication 115 nominal risk coefficient of 8 × 10−3 per mJ h m−3 (5 × 10−4 per working level month), and the Publication 103 detriment values (ICRP, 2007), gives a dose conversion convention value for adults of 3.3 mSv effective dose per mJ h m−3 (12 mSv per WLM). Dosimetric data provided in OIR Part 3 include values of approximately 3 mSv per mJ h m−3 (12 mSv per WLM) for mines and 6 mSv per mJ h m−3 (21 mSv per WLM) for indoor workplaces and tourist caves. However, using more realistic breathing rates for sedentary occupations, such as office workers, gives a value closer to 3 mSv per mJ h m−3 (Harrison and Marsh, 2012). As discussed below, the Commission recommends the use of a dose coefficient of 3 mSv per mJ h m−3 for most circumstances of exposure, noting that a higher value of 6 mSv per mJ h m−3 may be appropriate under particular circumstances (see below). OIR Part 3 also provides data to allow site-specific calculations where aerosol characteristics are known and exposures warrant more detailed consideration.
5. Use of efffective dose
Task Group 79 on the use of effective dose, led by Committee 2, also has members from Committees 1, 3, and 4. The remit of the task group is to provide advice on the use of effective dose, including medical applications. The members are John Harrison (Chair), Richard Wakeford (Committee 1, UK), Pedro Ortiz Lopez (formerly Committee 3, Spain), Colin Martin (Committee 3, UK), Hans-Georg Menzel (formerly Main Commission, Germany), Jane Simmonds (formerly Committee 4, UK), and Rebecca Smith-Bindman (USA). Corresponding members are François Bochud (Switzerland), John Cooper (UK), and Christian Streffer (Germany).
The concept of effective dose was introduced originally in Publication 26 (ICRP, 1977); Publication 103 (ICRP, 2007) provided a detailed explanation of the purpose and use of effective dose and equivalent dose to individual organs and tissues. Effective dose has proved to be a valuable and robust quantity for use in the implementation of protection principles. However, questions have arisen regarding practical applications, and further guidance has been considered necessary to provide clarity and address key issues of concern (Harrison et al., 2016). It is anticipated that the report will be published in 2018 following public consultation. Important preliminary conclusions are as follows:
Equivalent dose should be regarded simply as an intermediate step in the calculation of effective dose for the control of stochastic risks, and not used as a protection quantity for the avoidance of deterministic effects. It will be more appropriate for limits for the avoidance of deterministic effects to the hands and feet, lens of the eye, and skin to be set in terms of absorbed dose (Gy) rather than equivalent dose (Sv). Effective dose is in widespread use in medical practice as a measure of risk, thereby going beyond its intended purpose. While doses incurred at low levels of exposure may be measured or assessed with reasonable reliability, health effects have not been demonstrated reliably at such levels but are inferred. However, bearing in mind the uncertainties associated with risk projection to low doses or low dose rates, it is considered reasonable to use effective dose as an approximate indicator of possible risk, with the additional consideration of variation in risk with age and sex.
6. Radiopharmaceuticals
The remit of Joint Committee 2/Committee 3 Task Group 36 is to develop dose coefficients to assess doses to patients from administered radiopharmaceuticals. Task Group 36 is chaired jointly by Augusto Giussani (Committee 2, Germany) and Sören Mattsson (Committee 3 emeritus member, Sweden). Members are Martin Andersson (Sweden), Lennart Johansson (Sweden), Keon Kang (Committee 3, Korea), Sigrid Leide-Svegborn (Sweden), and Dietmar Nosske (Germany). Corresponding members are Wesley Bolch (Committee 2, USA), Katrine Åhlström Riklund (Committee 3, Sweden), Lars Söderberg (Sweden), Michael Stabin (USA), and Marie Sydoff (Sweden). Thanks are due to Dietmar Nosske, former member of Committee 2, who chaired this task group until October 2017.
Publication 128 (ICRP, 2015a) provided a compilation of Publication-60-based (ICRP, 1991) dose coefficients for radiopharmaceuticals considered in Publications 53, 80, and 106 (ICRP, 1987, 1998, 2008a). This publication also includes new information for 82Rb-chloride, (123I, 124I, 125I, and 131I)-iodide, and 123I-labelled 2.-carbomethoxy 3.-(4-iodophenyl)-N-(3-fluoropropyl) nortropane. The main future work for Task Group 36 is to update Publication 128 (ICRP, 2015a) with values calculated using 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 model improvements are needed. The task group has recently prepared sets of educational slides explaining the use of Publication 128 (ICRP, 2015a), and providing practical guidance on the protection of the hands during the preparation and administration of radiopharmaceuticals; these slides are available on the ICRP website (www.icrp.org).
7. Discussion
The replacement of all reference dose coefficients with values compliant with Publication 103 (ICRP, 2007) is a substantial scientific effort that will continue over the next few years. At the request of the International Atomic Energy Agency and the European Commission, a compilation of dose coefficients was provided as Publication 119 (ICRP, 2012), based on ICRP’s 1990 Recommendations (Publication 60; ICRP, 1991). These values are referred to in International and European Basic Safety Standards for use while new values based on ICRP’s 2007 Recommendations (ICRP, 2007) are being calculated (EC, 2014; IAEA, 2014). Similarly, Publication 128 (ICRP, 2015a) provides a compilation of Publication-60-based (ICRP, 1991) dose coefficients for administered radiopharmaceuticals.
For the first time, Committee 2 has provided dose coefficients for radon and its radioisotopes calculated using biokinetic and dosimetric models (ICRP, 2017b). However, as discussed in Section 4 above, inhalation of 222Rn and progeny represents a special case because effective dose coefficients can also be derived by epidemiological comparison, as was done in Publication 65 (ICRP, 1993). Taking account of the epidemiology and dosimetric calculations, the Commission recommends the use of a single dose conversion coefficient of 3 mSv per mJ h m−3 (approximately 10 mSv per WLM) for the calculation of doses following inhalation of radon and progeny in mines and all indoor workplaces (ICRP, 2017b). This reference dose coefficient is considered to be applicable to the majority of circumstances with no adjustment for aerosol characteristics. However, for exceptional cases of indoor workplaces where workers are engaged in substantial physical activity, and for workers in tourist caves, the Commission recommends the use of a higher dose coefficient of 6 mSv per mJ h m−3 (approximately 20 mSv per WLM). In addition, 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 provided in OIR Part 3 (ICRP, 2017b) and the accompanying electronic annex.
In terms of measurement of 222Rn gas exposure, the reference effective dose coefficient of 3 mSv per mJ h m−3 (approximately 10 mSv per WLM) corresponds to 6.9 × 10−6 mSv per Bq 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 year−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 mSv. For the reference residential occupancy of 7000 h year−1, the corresponding value of effective dose is 14.5 mSv.
A report providing advice on the use of effective dose is scheduled for publication in 2018. It draws on and expands the approaches and advice provided in Publication 103 (ICRP, 2007). Important issues include a proposal for simplifying the system by discontinuing the use of equivalent dose to set limits to avoid deterministic effects, and the acceptance that effective dose can be used as an approximate indicator of possible risk at low doses.
Confusion can arise in the use of the quantities, equivalent and effective dose (both in Sv), when they are not sufficiently well distinguished, and between equivalent dose and the operational quantity, dose equivalent (Sv), used in measurements of exposures to external radiation for the assessment of effective dose (Gonzalez et al., 2013). It is essential to specify which quantity is being used under particular circumstances. Difficulties would be reduced or avoided if organ and tissue doses were referred to in terms of mean absorbed dose (Gy), specifying low- and high-linear energy transfer components as necessary. The unit ‘Sv’ would then apply to the protection quantity, effective dose, and the corresponding operational quantity, dose equivalent. Limits to prevent deterministic effects to the lens of the eye, skin, and hands and feet would more appropriately be set in absorbed dose rather than equivalent dose. The Commission’s response on this issue is to recognise that limits to prevent deterministic effects will more correctly be set in absorbed dose, but to recommend that the current limits set in equivalent dose should continue to be applied until new general recommendations are issued.
Effective dose is a risk-adjusted dosimetric quantity for use in the control of exposures to all sources of radiation. However, it is also commonly used as a measure of stochastic risk, particularly in medical applications. While doses can be measured or assessed with reasonable reliability down to very low levels, the associated risk is increasingly uncertain as doses decrease. Risks at doses below approximately 50–100 mGy are inferred on the basis of epidemiological observations relating to higher doses, usually assuming a linear dose–response relationship at lower doses or dose rates. As discussed in Publication 103 (ICRP, 2007), risks associated with medical procedures are best evaluated using appropriate risk values for the individual tissues at risk, and for the age and sex distribution of the individuals undergoing the medical procedures. However, the analyses of Wall et al. (2011) and Balonov and Shrimpton (2012) showed that the use of effective dose and nominal risk coefficients rather than best-available data might underestimate risk for most procedures by approximately a factor of two for young children, and overestimate risk for most procedures by a factor of two for individuals aged 60–69 years. It is concluded that effective dose to a reference person may be used judiciously as an approximate indicator of possible risk, with simple adjustments to take account of age and sex differences, without implying greater knowledge of risks at low doses than is justified (Harrison and Ortiz-Lopez, 2015; Harrison et al., 2016). The over-riding consideration in assessing doses received in diagnostic x-ray procedures is arguably the inference that risks demonstrated at higher doses will apply at lower doses. The Commission recognises this need for pragmatic and practical guidance.
A new responsibility for Committee 2 is dosimetry for non-human biota, previously within the remit of Committee 5. Joining Committee 2 for the 2017–2021 term is Alexander Ulanovsky, previously a member of Committee 5 and the chair of a task group on dosimetry for non-human biota. The diversity of non-human biota is a specific challenge when developing and applying dosimetric models for assessing exposures of flora and fauna from radioactive sources in the environment (Ulanovsky, 2016). Dosimetric models, adopted in Publication 108 (ICRP, 2008b), provided dose coefficients for the Reference Animals and Plants. These models assume simple body shapes with uniform composition and density, homogeneous internal contamination, limited sets of idealised sources of external exposure to ionising radiation for aquatic and terrestrial animals and plants, and truncated radioactive decay chains. This methodology has been further developed and systematically extended, and updated dose coefficients are provided in Publication 136 (ICRP, 2017a). This publication is accompanied by a software tool which enables the calculation of doses for internal and external exposures of organisms with user-defined masses, shapes, and locations in the environment. The challenge for Committee 2 is to determine future requirements and ICRP’s role in providing data and advice.