Abstract
The International Commission on Radiological Protection (ICRP) system of radiological protection aims to protect people and the environment from detrimental effects of exposure to ionising radiation. This relies on practical and effective management of radiation doses, which, in turn, relies on clear understanding and proper use of dose quantities.
The system of radiological protection has evolved over more than 90 years, adapting to progress in scientific knowledge about the effects of radiation exposure. Protection of people is based on two protection goals, implemented through application of the fundamental principles of justification, optimisation of protection, and individual dose limitation:
keeping doses to organs and tissues below thresholds to avoid harmful tissue reactions (deterministic effects); and management of effective dose to limit the probability of occurrence of stochastic effects.
The close relationship between dose and health risk is inherent to construction of the system of radiological protection, specifically:
the relationship between severity of an effect and dose above a threshold for tissue reactions; and the relationship between probability of occurrence of an effect and dose without a threshold for stochastic effects.
The system of radiological protection for humans currently uses three dose quantities: absorbed dose, equivalent dose, and effective dose. Absorbed dose to organs and tissues is the fundamental scientific quantity and starting point for calculation of the other risk-adjusted quantities. Equivalent dose to organs and tissues enables the summation of doses from different radiation types, and is currently used to set limits to prevent harmful tissue reactions. Effective dose combines equivalent doses for protection from stochastic effects.
One key point in the present publication is that the Commission now considers absorbed dose to be the most appropriate quantity to use when limiting doses to organs and tissues to avoid harmful tissue reactions. This draws a clear distinction between limits applying to tissue reactions, set in absorbed dose (Gy), and those applying to stochastic effects, set in effective dose (Sv).
This position is consistent with the approach taken by the US National Council on Radiation Protection and Measurements (NCRP, 2018) and proposals from the International Commission on Radiation Units and Measurements (ICRU, 2020) on changes to operational quantities.
However, ICRP does not recommend an immediate change to the system of radiological protection. Rather, this should be considered an important input into the next fundamental recommendations of the Commission. A review of the system of radiological protection is already underway, aiming to develop the next fundamental recommendations which will supersede Publication 103 (ICRP, 2007) in about a decade.
For stochastic effects, it is now possible to provide an individual risk assessment taking into account the characteristics of the individual (sex, age at exposure, country of residence) and the dose received (acute or chronic, organs exposed), and thus to make an ad-hoc estimate of the risk for a specific type of cancer. The present publication indicates that this is the approach that should be favoured for risk assessment, using ad-hoc risk models from the epidemiological literature. Variations in health risk with age and sex are illustrated in Tables 2.4 and 2.5 of this publication. It is clear, for example, that the lifetime risk of cancer incurred by a young girl with breast exposure is not of the same nature and magnitude as that of a 60-year-old man with abdominal exposure.
Keeping this in mind, another key point in the present publication is that, under a variety of exposure conditions, effective dose can be an indicator of the magnitude of the associated health risk. Taking into account the age at exposure, which introduces an approximate variation of a factor of three, effective dose can be considered an approximate indicator of the stochastic risk. For most situations of practical radiological protection, this approximation is sufficient to inform judgements and the assessment of possible risks, for example in the medical uses of radiation and communication with patients.
Effective dose continues to be key to practical and effective management of the risk of stochastic effects for the purposes of radiological protection. It provides single values that relate to stochastic risk averaged over all organs and tissues as sites of radiation-induced cancer for both sexes, all ages, and even different populations around the world.
Likewise, detriment is also central to the well-established system of radiological protection. As organ and tissue doses are calculated using dosimetric phantoms of the human body for males and females of various ages (newborn, 1 year, 5 years, 10 years, 15 years, and 20 years), it is possible to present cancer risks and detriment separately for each sex and for different age groups. It would then be clearer that the inferred risk associated with a given effective dose depends on the age and sex of the exposed individuals. The corollary is that optimisation is applied with a better understanding of possible risks. This evolution towards increased scientific precision and clarity could help facilitate the practical application of appropriate protective measures.
As noted previously, a review of the system of radiological protection is underway, aiming towards development of the next fundamental recommendations of ICRP. Considerations include improvement of the discrimination between tissue reactions and stochastic effects, updating of radiological risk models, and revision of the parameters involved in the construction of radiological damage, based on progress in scientific and health knowledge since Publication 103 (ICRP, 2007). Several task groups have been established or are being considered on related topics, such as the calculation of detriment, the determination of dose and dose-rate effectiveness factors, and the integration of heritable effects and cardiovascular disease risks. The conclusions of these task groups will form cornerstones of the new recommendations. The present publication is part of this dynamic, considering the ways in which the dose quantities used in the system of radiological protection relate to risks, and are used to prevent or optimise protection against these risks, paving the way for future changes.