Specific absorbed fractions for reference individuals for the current and next general recommendations

1.1. Brief review of ICRP internal dosimetry

An intermediate step in calculating an effective dose involves the determination of the equivalent dose rate, H ˙ ( r T , t ) , as a function of time to all target regions, r T , contributing to effective dose which is given in Eq. (1). It is the product of two terms – a source term, A i ( r S , t ) , which is the time-dependent activity of the radioactive material as it moves through source regions, r S of the body, and an energy absorption term, S w ( r T ← r S , t ) i , called the S-coefficient.

H ˙ ( r T , t ) = ∑ i = 1 n ∑ r S m A i ( r S , t ) S w ( r T ← r S , t ) i

(1)

The S-coefficient is obtained as described in Eq. (2). It gives, for a given radionuclide, the equivalent dose to a target region per nuclear transformation that takes place in a source region, or correspondingly, the equivalent dose rate per unit activity in a source region. The summations are over the radiation type, R , and the emission, j , of the given radionuclide. The quantity is the product of the radiation weighting factor, w R ; the energy, E R , j ; and yield, Y R , j , of the emission, and the specific absorbed fraction, Φ ( r T ← r S , E R , j , t ) .

S w ( r T ← r S , t ) = ∑ R w R ∑ j E R , j Y R , j Φ ( r T ← r S , E R , j , t )

(2)

The SAF is the fraction of energy emitted from a source region which is absorbed in a target region (the absorbed fraction) per mass of the target region, m T . As shown in Eq. (3), the SAF can be split into two terms: the absorbed fraction, ϕ ( r T ← r S , E R , j , t ) and the mass of the target region. Absorbed fractions are limited to having values between zero and one. They are obtained from models which are based on the definition of reference individuals.

Φ ( r T ← r S , E R , j , t ) = ϕ ( r T ← r S , E R , j , t ) m T

(3)

2.1. Reference individuals

The ICRP reference individuals consist of a set of idealised males and females with defined anatomical and physiological characteristics for the purpose of radiological protection (ICRP 2007). Thus, the reference individuals are theoretical constructions designed for the purpose of computing the effective dose. Reference individuals are not reference phantoms, although phantoms can be designed based on the parameters of the reference individuals. The SAF values provided in Publications 133 and 155 (ICRP 2016a, 2023) are based on the reference individuals defined in Publication 89 (ICRP 2002). Publications 133 and 155 provide additional reference mass data not included in Publication 89 as well as improvements for a few tissue regions.

For the first time, ICRP has modelled a whole-body blood source where that blood is distributed throughout tissues and vasculature in the body (ICRP 2002; Wayson et al. 2018). Understandable confusion has resulted in differences between the masses of organs in the voxel phantoms of Publications 110 and 143 (ICRP 2009, 2020b) and the target masses of the same organs in Publications 133 and 155. It was the intention of phantom developers to include appropriate blood content in the organs. Regrettably, the misinterpretation of Table 2.8 in Publication 89 led to the Publication 110 and 143 voxel phantoms not appropriately assigning the mass of systemic blood. Consequently, for geometries where the target and source regions are the same or overlapping (self-irradiation geometries), the SAF computed in the phantoms must be adjusted to account for the missing organ blood mass. Publication 155 describes approaches for making this adjustment and when it is or is not desirable.

The set of reference individuals is defined at six ages (newborn; 1, 5, 10, 15 years; and adult) with two sexes at each age. The biokinetic models define 79 source regions, and the definition of effective dose along with inclusion of other targets of interest (e.g. lens of the eye) results in 43 target regions. When considering four radiation types (alpha, electron, photon, and spontaneous fission neutrons) and the energy grids associated with each, the full set of age- and sex-dependent SAF values consists of more than three million data points.

2.2. Supporting models

2.3. Example SAF values

The full set of SAF values is provided as an electronic download accompanying Publication 155. Multiple example plots are provided in the text of Publication 155 as well as explanations of theoretical checkpoints and their basis. In this article, one such example is provided.

Fig. 1 displays a set of SAF values versus electron energy from Publications 133 and 155. Values for all six reference ages are provided for the left colon wall irradiating the left colon stem cell region. This geometry is an example of the source region fully encompassing the target region. For such regions a theoretical limiting value of the SAF can be computed for electrons of energy approaching zero. This limit is simply the volume fraction of the stem cell region to the wall divided by the mass of the target region. In this case the SAF does not fall away from this limiting value until electrons of about 200 keV or more are considered.

OPEN IN VIEWER

Fig. 1.

Specific absorbed fractions for electrons emitted from the left colon wall irradiating the left colon stem cell target region for females at all reference ages (ICRP 2023).

The SAF values in Publication 133 were used to support the computation of internal dose coefficients for the occupational intakes of radionuclides (OIR series) (ICRP 2015, 2016b, 2017, 2019, 2022). Similarly, the age-dependent SAF values in Publication 155, which includes the adult values from Publication 133, support the computation of internal dose coefficients for members of the public exposed to radionuclides in the environment (ICRP 2024a). The age-dependent SAF values are also supporting work in progress from ICRP Task Group 36 on radiation dose to patients in diagnostic nuclear medicine and ICRP Task Group 112 on emergency dosimetry.

3.1. New target and source geometries

The first improvement which may be needed to support dosimetry associated with the next system of radiological protection involves modelling new target regions. The science is still evolving on which tissues are sensitive to ionising radiation. For example, if cardiovascular disease is deemed to be a disease associated with exposure to ionising radiation (Zanzonico et al. 2016; Boice Jr et al. 2022; Little et al. 2023), the target tissue will need to be identified and modelled appropriately.

New target models may also be of benefit as new knowledge is gained about where within a volume the sensitive tissues or cells reside. For example, Eckerman and Veinot (2019) have developed a new model for the region where at-risk cells reside in the urinary bladder wall.

And in some cases, new knowledge on the non-uniformity of the distribution of material within an organ may drive a need for new models even if the target remains the same. For example, NCRP Commentary 31 proposes new biokinetic models for certain elements in subregions of the brain where data show non-uniformity of activity (Leggett et al. 2022; NCRP 2022).

3.2. Model improvements

Even where there are not new definitions of source or target regions, improvements to models can be made which are more representative of the specific body region. At the whole-body level, the recently developed ICRP mesh phantoms of Publications 145 and 156 (ICRP 2020a, 2024b) represent improvements to the surfaces of regions over their voxel ancestors. A future task will involve determining for which source–target combinations and radiation types the mesh phantoms will represent significant improvements to internal dosimetry applications. For many of the important geometries, the SAF value will not be sensitive to a mesh versus a voxel surface.