Abstract
This publication provides data on the specific absorbed fractions (SAFs) for reference males and females at 0 (newborn), 1, 5, 10, and 15 years of age. The SAF represents the fraction of energy emitted within a source region which is absorbed in a target region per mass of the target region, and is essential for calculating absorbed doses in organs or tissues for internal exposure. Combining the data in this publication with the SAF data for reference adult male and female, published in Publication 133 (ICRP, 2016), has resulted in a SAF dataset for the calculation of age-dependent dose coefficients for environmental intakes of radionuclides for members of the public.
The Commission’s development of age-dependent dose coefficients for environmental intakes of radionuclides by inhalation and ingestion for members of the public dates back to Publication 56 (ICRP, 1990). ICRP developed dosimetric models and biokinetic data in Publication 30 (ICRP, 1979, 1980, 1981, 1988) to establish secondary limits (annual limits on intake) for the control of intakes of radionuclides by workers. This work was led by Task Group 4 on Dose Calculations (DOCAL), which was established in June 1974 to support Committee 2. The models and data were developed for occupationally exposed adults, and were therefore not recommended for use by members of the public. In order to provide dose coefficients for members of the public, it is necessary to understand the effect of age on the biokinetics of radionuclides, and on anatomical and physiological data. In its statement of the 1983 meeting in Washington, DC, USA, the Main Commission of ICRP assessed those factors influencing the dose coefficients from the viewpoint of age dependence.
In the course of these considerations, the Chernobyl accident in 1986 highlighted the need for internationally accepted dose coefficients for members of the public. In March 1987, at the request of Committee 2, ICRP established Task Group on Age-dependent Dosimetry (AGDOS) for developing age-dependent dose coefficients for members of the public. AGDOS, together with DOCAL, incorporated age-dependent physical models and appropriate biokinetic information. Its first publication (Publication 56) was published in 1990, providing dose coefficients for members of the public from intake of radionuclides of 12 elements (ICRP, 1990). Subsequently, dose coefficients for ingestion and inhalation of radionuclides were published successively as Publications 67 (ICRP, 1993), 69 (ICRP, 1995a), and 71 (ICRP, 1995b), and Publication 72 (ICRP, 1995c) compiled the dose coefficients for radioisotopes of the 91 elements for members of the public. These data have been used in various aspects of internal exposure of members of the public.
In developing the dose coefficients in Publications 56, 67, 69, 71, and 72 (ICRP, 1990, 1993, 1995a,b,c), AGDOS, renamed Task Group 21 on Internal Dosimetry (INDOS) in 1993, used the age-specific SAFs calculated at Oak Ridge National Laboratory (TN, USA). The calculation of SAFs for photons is reported in a series of seven reports (Cristy and Eckerman, 1987), which indicate that a great deal of effort was expended, including the development of age-specific stylised phantoms and extensive calculations.
More than 30 years have passed since the publication of Publication 56 (ICRP, 1990), but the importance of dose coefficients for members of the public remains unchanged. The need for these coefficients has become even greater, especially since the accident at Fukushima Daiichi nuclear power plant. Dose coefficients for various ages are also essential for the use of radiopharmaceuticals. An important ongoing task of Committee 2 is the revision of dose coefficients for workers and members of the public following the 2007 Recommendations [Publication 103 (ICRP, 2007)]. The SAFs presented in this publication are an essential part of this task.
Since Publication 56 (ICRP, 1990), there have been advances in the techniques used for SAF calculations. Voxel phantoms constructed from medical imaging data of real people provide a more realistic description of the human body than afforded by the stylised phantoms. Using the voxel techniques, ICRP developed adult reference phantoms in Publication 110 (ICRP, 2009) and paediatric reference phantoms in Publication 143 (ICRP, 2020). In addition, driven by advancing technology, the use of sophisticated three-dimensional Monte Carlo radiation transport codes has become mainstream, allowing voxel phantoms representing complex human anatomy to be constructed on the computer to calculate SAFs.
However, the development of the SAF dataset still requires enormous effort. It involves performing calculations for numerous combinations of source and target organs, with a large number of source energy points, resulting in a huge amount of data. These data have to be evaluated and, if necessary, smoothed. In addition, special models and calculation methods are required for tissues and organs that cannot be represented geometrically in the voxel phantoms. Task Group 96 on Computational Phantoms and Radiation Transport has successfully completed this challenging work, which required careful consideration and professional judgement. This, together with the revised biokinetic models and nuclear decay data [Publication 107 (ICRP, 2008)], means that the key building blocks for the calculation of new dose coefficients are now in place. The outcome will soon be available in a series of publications of ‘Dose Coefficients for Intakes of Radionuclides by Members of the Public’.
The SAF data may only be used by experts in internal dosimetry. However, it is important for users of dose coefficients and people involved in radiation protection to acknowledge that the ICRP System of Radiological Protection relies on such meticulous work.