The use of dose quantities in radiological protection: ICRP publication 147 Ann ICRP 50(1) 2021

The International Commission on Radiological Protection has recently published a report (ICRP Publication 147;Ann. ICRP50, 2021) on the use of dose quantities in radiological protection, under the same authorship as this Memorandum. Here, we present a brief summary of the main elements of the report. ICRP Publication 147 consolidates and clarifies the explanations provided in the 2007 ICRP Recommendations (Publication 103) but reaches conclusions that go beyond those presented in Publication 103. Further guidance is provided on the scientific basis for the control of radiation risks using dose quantities in occupational, public and medical applications. It is emphasised that best estimates of risk to individuals will use organ/tissue absorbed doses, appropriate relative biological effectiveness factors and dose-risk models for specific health effects. However, bearing in mind uncertainties including those associated with risk projection to low doses or low dose rates, it is concluded that in the context of radiological protection, effective dose may be considered as an approximate indicator of possible risk of stochastic health effects following low-level exposure to ionising radiation. In this respect, it should also be recognised that lifetime cancer risks vary with age at exposure, sex and population group. The ICRP report also concludes that equivalent dose is not needed as a protection quantity. Dose limits for the avoidance of tissue reactions for the skin, hands and feet, and lens of the eye will be more appropriately set in terms of absorbed dose rather than equivalent dose.

Use of effective dose

International Commission on Radiological Protection (ICRP) Publication 103 provided a detailed explanation of the purpose and use of effective dose and equivalent dose to individual organs and tissues. Effective dose has proven to be a valuable and robust quantity for use in the implementation of protection principles. However, questions have arisen regarding practical applications, and a Task Group has been set up to consider issues of concern. This paper focusses on two key proposals developed by the Task Group that are under consideration by ICRP: (1) confusion will be avoided if equivalent dose is no longer used as a protection quantity, but regarded as an intermediate step in the calculation of effective dose. It would 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 the quantity, absorbed dose (Gy) rather than equivalent dose (Sv). (2) 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 may be considered reasonable to use effective dose as a rough indicator of possible risk, with the additional consideration of variation in risk with age, sex and population group.

Doses from radiation exposure

Practical implementation of the International Commission on Radiological Protection's (ICRP) system of protection requires the availability of appropriate methods and data. The work of Committee 2 is concerned with the development of reference data and methods for the assessment of internal and external radiation exposure of workers and members of the public. This involves the development of reference biokinetic and dosimetric models, reference anatomical models of the human body, and reference anatomical and physiological data. Following ICRP's 2007 Recommendations, Committee 2 has focused on the provision of new reference dose coefficients for external and internal exposure. As well as specifying changes to the radiation and tissue weighting factors used in the calculation of protection quantities, the 2007 Recommendations introduced the use of reference anatomical phantoms based on medical imaging data, requiring explicit sex averaging of male and female organ-equivalent doses in the calculation of effective dose. In preparation for the calculation of new dose coefficients, Committee 2 and its task groups have provided updated nuclear decay data (ICRP Publication 107) and adult reference computational phantoms (ICRP Publication 110). New dose coefficients for external exposures of workers are complete (ICRP Publication 116), and work is in progress on a series of reports on internal dose coefficients to workers from inhaled and ingested radionuclides. Reference phantoms for children will also be provided and used in the calculation of dose coefficients for public exposures. Committee 2 also has task groups on exposures to radiation in space and on the use of effective dose.

Effective dose: a radiation protection quantity

Modern radiation protection is based on the principles of justification, limitation, and optimisation. Assessment of radiation risks for individuals or groups of individuals is, however, not a primary objective of radiological protection. The implementation of the principles of limitation and optimisation requires an appropriate quantification of radiation exposure. The International Commission on Radiological Protection (ICRP) has introduced effective dose as the principal radiological protection quantity to be used for setting and controlling dose limits for stochastic effects in the regulatory context, and for the practical implementation of the optimisation principle. Effective dose is the tissue weighted sum of radiation weighted organ and tissue doses of a reference person from exposure to external irradiations and internal emitters. The specific normalised values of tissue weighting factors are defined by ICRP for individual tissues, and used as an approximate age- and sex-averaged representation of the relative contribution of each tissue to the radiation detriment of stochastic effects from whole-body low-linear energy transfer irradiations. The rounded values of tissue and radiation weighting factors are chosen by ICRP on the basis of available scientific data from radiation epidemiology and radiation biology, and they are therefore subject to adjustment as new scientific information becomes available. Effective dose is a single, risk-related dosimetric quantity, used prospectively for planning and optimisation purposes, and retrospectively for demonstrating compliance with dose limits and constraints. In practical radiation protection, it has proven to be extremely useful.

ICRP Publication 119: Compendium of dose coefficients based on ICRP Publication 60

This report is a compilation of dose coefficients for intakes of radionuclides by workers and members of the public, and conversion coefficients for use in occupational radiological protection against external radiation from Publications 68, 72, and 74 (ICRP, 1994b, 1996a,b). It serves as a comprehensive reference for dose coefficients based on the primary radiation protection guidance given in the Publication 60 recommendations (ICRP, 1991). The coefficients tabulated in this publication will be superseded in due course by values based on the Publication 103 recommendations (ICRP, 2007).

Isoeffective dose: a concept for biological weighting of absorbed dose in proton and heavier-ion therapies

When reporting radiation therapy procedures, International Commission on Radiation Units and Measurements (ICRU) recommends specifying absorbed dose at/in all clinically relevant points and/or volumes. In addition, treatment conditions should be reported as completely as possible in order to allow full understanding and interpretation of the treatment prescription. However, the clinical outcome does not only depend on absorbed dose but also on a number of other factors such as dose per fraction, overall treatment time and radiation quality radiation biology effectiveness (RBE). Therefore, weighting factors have to be applied when different types of treatments are to be compared or to be combined. This had led to the concept of 'isoeffective absorbed dose', introduced by ICRU and International Atomic Energy Agency (IAEA). The isoeffective dose D(IsoE) is the dose of a treatment carried out under reference conditions producing the same clinical effects on the target volume as those of the actual treatment. It is the product of the total absorbed dose (in gray) used and a weighting factor W(IsoE) (dimensionless): D(IsoE)=D×W(IsoE). In fractionated photon-beam therapy, the dose per fraction and the overall treatment time (in days) are the two main parameters that the radiation oncologist has the freedom to adjust. The weighting factor for an alteration of the dose per fraction is commonly evaluated using the linear-quadratic (α/β) model. For therapy with protons and heavier ions, radiation quality has to be taken into account. A 'generic proton RBE' of 1.1 for clinical applications is recommended in a joint ICRU-IAEA Report [ICRU (International Commission on Radiation Units and Measurements) and IAEA (International Atomic Energy Agency). Prescribing, recording and reporting proton-beam therapy. ICRU Report 78, jointly with the IAEA, JICRU, 7(2) Oxford University Press (2007)]. For heavier ions (e.g. carbon ions), the situation is more complex as the RBE values vary markedly with particle type, energy and depth in tissue.

Response of neutron detectors to high-energy mixed radiation fields

Radiation protection around CERN's high-energy accelerators represents a major challenge due to the presence of complex, mixed radiation fields. Behind thick shielding neutrons dominate and their energy ranges from fractions of eV to about 1 GeV. In this work the response of various portable detectors sensitive to neutrons was studied at CERN's High-Energy Reference Field Facility (CERF). The measurements were carried out with conventional rem counters, which usually cover neutron energies up to 20 MeV, the Thermo WENDI-2, which is specified to measure neutrons up to several GeV, and a tissue-equivalent proportional counter. The experimentally determined neutron dose equivalent results were compared with Monte Carlo (MC) simulations. Based on these studies field calibration factors can be determined, which result in a more reliable estimate of H*(10) in an unknown, but presumably similar high-energy field around an accelerator than a calibration factor determined in a radiation field of a reference neutron source.

The ICRU (International Commission on Radiation Units and Measurements): its contribution to dosimetry in diagnostic and interventional radiology

The ICRU (International Commission on Radiation Units and Measurements was created to develop a coherent system of quantities and units, universally accepted in all fields where ionizing radiation is used. Although the accuracy of dose or kerma may be low for most radiological applications, the quantity which is measured must be clearly specified. Radiological dosimetry instruments are generally calibrated free-in-air in terms of air kerma. However, to estimate the probability of harm at low dose, the mean absorbed dose for organs is used. In contrast, at high doses, the likelihood of harm is related to the absorbed dose at the site receiving the highest dose. Therefore, to assess the risk of deterministic and stochastic effects, a detailed knowledge of absorbed dose distribution, organ doses, patient age and gender is required. For interventional radiology, where the avoidance of deterministic effects becomes important, dose conversion coefficients are generally not yet developed.

Dose equivalent measurements in a strongly pulsed high-energy radiation field

The stray radiation field outside the shielding of high-energy accelerators comprises neutrons, photons and charged particles with a wide range of energies. Often, accelerators operate by accelerating and ejecting short pulses of particles, creating an analogue, pulsed radiation field. The pulses can be as short as 10 micros with high instantaneous fluence rates and dose rates. Measurements of average dose equivalent (rate) for radiation protection purposes in these fields present a challenge for instrumentation. The performance of three instruments (i.e. a recombination chamber, the Sievert Instrument and a HANDI-TEPC) measuring total dose equivalent is compared in a high-energy reference radiation field (CERF) and a strongly pulsed, high-energy radiation field at the CERN proton synchrotron (PS).

Dose quantities in radiation protection and their limitations

For more than 50 years the quantity absorbed dose has been the basic physical quantity in the medical applications of ionising radiation as well as radiological protection against harm from ionising radiation. In radiotherapy relatively high doses are applied (to a part of the human body) within a short period and the absorbed dose is mainly correlated with deterministic effects such as cell killing and tissue damage. In contrast, in radiological protection one is dealing with low doses and low dose rates and long-term stochastic effects in tissue such as cancer induction. The dose quantity (absorbed dose) is considered to be correlated with the probability of cancer incidence and thus risk induced by exposure. ICRP has developed specific dosimetric quantities for radiological protection that allow the extent of exposure to ionising radiation from whole and partial body external radiation as well as from intakes of radionuclides to be taken into account by one quantity. Moreover, radiological protection quantities are designed to provide a correlation with risk of radiation induced cancer. In addition, operational dose quantities have been defined for use in measurements of external radiation exposure and practical applications. The paper describes the concept and considerations underlying the actual system of dose quantities, and discusses the advantage as well as the limitations of applicability of such a system. For example, absorbed dose is a non-stochastic quantity defined at any point in matter. All dose quantities in use are based on an averaging procedure. Stochastic effects and microscopic biological and energy deposition structures are not considered in the definition. Absorbed dose is correlated to the initial very short phase of the radiation interaction with tissue while the radiation induced biological reactions of the tissue may last for minutes or hours or even longer. There are many parameters other than absorbed dose that influence the process of cancer induction, which may influence the consideration of cells and/or tissues at risk which are most important for radiological protection.

Biological weighting of absorbed dose in radiation therapy

Absorbed dose is a quantity which is scientifically rigorously defined and used to quantify the exposure of biological objects, including humans, to ionising radiation. There is, however, no unique relationship between absorbed dose and induced biological effects. The effects induced by a given absorbed dose to a given biological object depend also on radiation quality and temporal distribution of the irradiation. In radiation therapy, empirical approaches are still used today to account for these dependencies in practice. In hadron therapy (neutrons, protons, ions), radiation quality is accounted for with a diversity of (almost hospital specific) methods. The necessity to account for temporal aspects is well known in external beam therapy and in high dose rate brachytherapy. The paper reviews the approaches for weighting the absorbed dose in radiation therapy, and focusses on the clinical aspects of these approaches, in particular the accuracy requirements.

European measurements of aircraft crew exposure to cosmic radiation

For more than 5 y, the European Commission has supported research into scientific and technical aspects of cosmic-ray dosimetry at flight altitudes in civil radiation. This has been in response to legislation to regard exposure of aircraft crew as occupational, following the recommendations of the International Commission on Radiological Protection in Publication 60. The response to increased public interest and concern, and in anticipation of European and national current work, within a total of three multi-national, multi-partner research contracts, is based on a comprehensive approach including measurements with dosimetric and spectrometric instruments during flights, at high-mountain altitudes, and in a high-energy radiation reference field at CERN, as well as cosmic-ray transport calculations. The work involves scientists in the fields of neutron physics, cosmic-ray physics, and general dosimetry. A detailed set of measurements has been obtained by employing a wide range of detectors on several routes, both on subsonic and supersonic aircraft. Many of the measurements were made simultaneously by several instruments allowing the intercomparison of results. This paper presents a brief overview of results obtained. It demonstrates that the knowledge about radiation fields and on exposure data has been substantially consolidated and that the available data provide an adequate basis for dose assessments of aircraft crew, which will be legally required in the European Union after 13 May 2000.

Risk assessment for cancer induction after low- and high-LET therapeutic irradiation

The risk of induction of a second primary cancer after a therapeutic irradiation with conventional photon beams is well recognized and documented. However, in general, it is totally overwhelmed by the benefit of the treatment. The same is true to a large extent for the combinations of radiation and drug therapy. After fast neutron therapy, the risk of induction of a second cancer is greater than after photon therapy. Neutron RBE increases with decreasing dose and there is a wide evidence that neutron RBE is greater for cancer induction (and for other late effects relevant in radiation protection) than for cell killing. Animal data on RBE for tumor induction are reviewed, as well as other biological effects such as life shortening, malignant cell transformation in vitro, chromosome aberrations, genetic effects. These effects can be related, directly or indirectly, to cancer induction to the extent that they express a "genomic" lesion. Almost no reliable human epidemiological data are available so far. For fission neutrons a RBE for cancer induction of about 20 relative to photons seems to be a reasonable assumption. For fast neutrons, due to the difference in energy spectrum, a RBE of 10 can be assumed. After proton beam therapy (low-LET radiation), the risk of secondary cancer induction, relative to photons, can be divided by a factor of 3, due to the reduction of integral dose (as an average). The RBE of heavy-ions for cancer induction can be assumed to be similar to fission neutrons, i.e. about 20 relative to photons. However, after heavy-ion beam therapy, the risk should be divided by 3, as after proton therapy due to the excellent physical selectivity of the irradiation. Therefore a risk 5 to 10 times higher than photons could be assumed. This range is probably a pessimistic estimate for carbon ions since most of the normal tissues, at the level of the initial plateau, are irradiated with low-LET radiation.

Specification of radiation quality in fast neutron therapy: microdosimetric and radiobiological approach

Specification of radiation quality is an important issue in fast neutron therapy since the biological effectiveness of the beams varies to a large extent with neutron energy. It must meet specific criteria, mainly derived from the accuracy requirement for absorbed dose delivery. A first approach to this problem consists in identifying physical parameters that can be related to Relative Biological Effectiveness (RBE) and which describe the beam production technique (e.g. neutron-producing reaction, p + Be or d + Be, energy of the incident particle). A second is based on microdosimetry, which provides a description of the secondary radiation components to which the biological consequences of irradiations are more directly correlated. A third approach consists in experimental RBE determinations in reference conditions: intestinal crypt regeneration in mice after irradiation to the whole body with single doses is proposed as a standard biological system for radiobiological calibrations of clinical fast neutron beams. Dosimetric, microdosimetric and radiobiological intercomparisons are encouraged since they provide a homogeneous set of data which facilitate the exchange of clinical information. They also constitute a basis for the clinical RBE approach and an overall check of the irradiation procedure. Therefore they should be recommended in every non-conventional radiation therapy facility.

Dosimetry for occupational exposure to cosmic radiation

In the course of their work, aircraft crew and frequent flyers are exposed to elevated levels of cosmic radiation of galactic and solar origin and secondary radiation produced in the atmosphere, aircraft structure, etc. This has been recognised for some time and estimates of the exposure of aircraft crew have been made previously and included in, for example, UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) publications. The recent increased interest has been brought about by several factors--the consideration that the relative biological effectiveness of the neutron component as being underestimated; the trend towards higher cruising altitudes for subsonic commercial aircraft and business jet aircraft; and, most importantly, the recommendations of the International Commission on Radiological Protection (ICRP) in Publication 60, and the revision of the Euratom Basic Safety Standards Directive (BSS). In 1992, the European Dosimetry Group (EURADOS) established a Working Group to consider the exposure to cosmic radiation of aircraft crew, and the scientific and technical problems associated with radiation protection dosimetry for this occupational group. The Working Group was composed of fifteen scientists (plus a corresponding member) involved in this field of study and with knowledge of radiation measurement at aviation altitudes. This paper is based on the findings of this Working Group. Where arrangements are made to take account of the exposure of aircraft crew to cosmic radiation, dose estimation procedures will not be necessary for persons for whom total annual doses are not liable to exceed 1 mSv, and therefore, in general, for crew on aircraft not routinely flying above 8 km. Where estimates of effective dose and, in the case of female staff who are pregnant, equivalent dose to the embryo or fetus, are required (for regulatory or other purposes), it was concluded that the preferred procedure was to determine route doses and fold these with data on staff rostering.

Present status, trends and needs in fast neutron therapy

Fast neutrons were introduced in cancer therapy, in the 1970s, on the basis of radiobiological arguments. After 30 years, these arguments are still valid and have not been refuted by more recent experimental data. Although neutron therapy has been applied routinely for about 25 years, there is still no general agreement concerning its value and its place among the other radiation therapy techniques. In order to be able to draw objective conclusions from the available clinical results and mainly to compare the results from the different centres in a relevant way, a consensus has to be reached on several issues: 1) a protocol for dose measurement in a homogeneous phantom in reference conditions; 2) dose computation at the level of the target volume(s) and the normal tissues at risk; 3) method of dose specification for reporting; and 4) specification of radiation quality in neutron therapy and the related RBE problems. The International Commission on Radiation Units and Measurements (ICRU) has published recommendations on Clinical Neutron Dosimetry (ICRU Report 45, 1989) which are now universally applied. As far as dose specification for reporting is concerned, ICRU recommendations for photons (ICRU Report 50, 1993) can be extended and adapted for fast neutrons. However, special care is required to take into account the tissue compositions. In fast neutron therapy, specification of radiation quality raises a particular problem because the neutron RBE, relative to gamma rays, is higher than unity (it ranges from about 2 to 5) and furthermore significantly varies with neutron beam energy. In addition, the RBE also varies with dose and biological system. In these conditions, besides the classical concept of RBE introduced jointly by the ICRP and the ICRU in 1963, the concepts of "reference RBE" and "clinical RBE" are proposed here for application in fast neutron therapy. Microdosimetry provides an adequate method to describe radiation quality, at the point of interest in the irradiated medium and in the actual irradiation conditions. From the analysis of microdosimetric spectra, a RBE value of a particular neutron beam, for a given biological system, can be predicted provided that the biological weighing function for that biological system has been obtained. In any case, correlation of the microdosimetric description of a given beam and the experimental RBE values for that beam improves the confidence in both sets of data.

The clinical RBE and microdosimetric characterization of radiation quality in fast neutron therapy

High-LET radiation therapy using fast neutrons is being applied regularly at several centres worldwide and in the future, other types of radiation qualities, such as protons and heavier ions and boron neutron capture therapy (BNCT) are likely to be used. The neutron beams used are of considerably varying energy and thus considerable variations in the relative biological effectiveness (RBE) have been found. At present, no generally accepted method exists for the quantitative specification of these differences in radiation quality for clinical purposes. This is in clear discrepancy with the accuracy requirements in clinical dosimetry. An approach is presented which is based on a single parameter radiation quality characterization determined in combined microdosimetric and radiobiological experiments. It is shown that the method can meet the accuracy requirements of clinical dosimetry and that it is applicable within a concept of formalized procedure of clinical practice and experience ('clinical RBE').

RBE in fast neutron therapy and in boron neutron capture therapy. A useful concept or a misuse?

In high-LET radiation therapy, radiation quality and radiation quality differences have to be taken into account and specified. In fast neutron therapy, an operational approach has been adopted which is based on the concept of "clinical RBE". The paper discusses the quantities of RBE (relative biological effectiveness), reference RBE and clinical RBE and their relationship and significance in radiation therapy. In particular, the difference between the well defined RBE in radiation biology and the clinical RBE which is based on the judgement of radiotherapists is elucidated and emphasized. The clinical RBE is based on the reference RBE and implicitly includes differences in physical selectivity of the irradiation beams and clinical experience. The particular aspects of radiation quality in boron neutron capture therapy (BNCT) are due to the poor penetration of the primary beam, the inhomogeneity of the spatial distribution of 10B nuclides in the irradiated tissue and the short range of the alpha-particles emitted after neutron capture in 10B. The related problems in accounting for radiation quality in BNCT and in the applicability of the clinical RBE concept are discussed.

Microdosimetric specification of radiation quality in neutron radiation therapy

The neutron beams used by various radiotherapy centres are of widely differing energies, and differences of up to 50 per cent in the relative biological effectiveness (RBE) between different beams have been found in radiobiological experiments. Moreover, at some facilities RBE variations have been observed with increasing depth in a phantom. In spite of this evidence, there is no quantitative and uniquely accepted specification of radiation quality used in practice. The urgency of an adequate solution of this problem is illustrated by the fact that in radiation therapy the usual accuracy requirement for the quantity of radiation, i.e. the absorbed dose to be delivered to the tumour, is 3.5 per cent (1 SD). In this paper a pragmatic solution for the specification of radiation quality for fast neutron therapy is proposed. It is based on empirical RBE versus lineal energy response or weighting functions. These were established by using existing radiobiological data and microdosimetric spectra measured under identical irradiation conditions at several European neutron irradiation units.

Dosimetry of low-energy neutrons using low-pressure proportional counters

Measurements in nearly monoenergetic beams of 144, 24.5, and 2 keV neutrons and of thermal neutrons have been performed with low-pressure proportional counters. The suitability of a tissue-equivalent proportional counter (TEPC) for dosimetry of low-energy neutrons has been investigated. In contrast to higher neutron energies, the modification of the primary radiation field by the detector wall and the contribution of secondaries produced in the gas are significant. These effects have been investigated by additional measurements with a carbon-walled proportional counter. The various physical processes of neutron interaction with wall and gas of the TEPC have been analyzed, and absorbed dose, kerma, and kerma contributions from the various processes are presented. In addition, dose contributions from contaminating neutrons and photons have been obtained for the calibration fields used. The results have been related to neutron fluence. The comparison with tabulated kerma factors shows excellent agreement, indicating the suitability of the TEPC method for dosimetry of low-energy neutrons.

Proportional counter dosimetry and microdosimetry for radiotherapy with multiple pion beams

At the Swiss Institute for Nuclear Research (SIN) cancer patients are irradiated with negatively charged pi mesons using a 60-beam medical pion generator, the Piotron. A low-pressure tissue-equivalent proportional counter was used to measure absorbed dose and microdosimetric spectra. A method was developed to allow discrimination of events from different beam components, i.e., beam contamination (electrons and muons), pions in flight, and stopping pions. Measurements were performed along the axis and at lateral distances off one of these identical pion beams. The marked changes of total microdosimetric spectra with depth in phantom detected in earlier measurements are mainly due to large variations in the dose contributions of the beam components and much less to changes in the shapes of the individual microdosimetric spectra. The single beam measurements were used to calculate three-dimensional distributions of absorbed dose and of dose mean lineal energy, yD, for dynamic patient irradiations. Within the whole target volume yD remains nearly constant when irradiated with all 60 beams, whereas considerable changes were found for irradiations with 31 beams coming from a semicircle. Both size and shape of target volumes influence yD, the maximum values ranging from 30 to 45 keV/micron.