Abstract
The first dose estimates were made for adult workers, as they were the population most likely to receive doses in their workplaces. Today, the need to do this for adults has not diminished, and the dosimetric phantoms have been updated: the voxel-type adult reference phantoms defined in Publication 110 (ICRP, 2009) now have their mesh counterparts in Publication 145 (ICRP, 2020c). In contrast to the voxel-type phantoms, the mesh-type phantoms have the great advantage of allowing Monte Carlo calculations using a single model, the resolution of which varies according to the size of the organs, and tissue sensitivity to ionising radiation. These phantoms have shown continuity with their predecessors for photon radiation, while improving the accuracy of calculations for short range radiation, such as electrons or alpha particles.
The value of mesh-type phantoms is not limited to improved accuracy of dose calculation. They also offer more flexibility to change their morphology and posture, which is particularly useful for retrospective calculations in emergency exposure situations. However, they also open up new possibilities for planned exposure situations, such as for complex intervention preparation in the presence of high radiation fields. Other uses include calibration of detectors (Ferreira et al., 2014), three-dimensional printing, education and training, etc.
It is widely accepted that children are different in many ways from adults, and that they are generally more sensitive to ionising radiation than adults: a child is not a small adult! It is therefore not surprising that this publication presents the mesh version of the not-so-old paediatric voxel-type reference phantoms described in Publication 143 (ICRP, 2020a). Like their adult counterparts, the paediatric phantoms have been designed to be as close as possible to the reference humans described in Publication 89 (ICRP, 2002), while ensuring the realism of organs and tissues through close collaboration with medical anatomists. From a numerical point of view, these phantoms have also been validated extensively. In addition to checking the continuity of the dimensions with those of the voxel phantoms, dose coefficients (DCs) for external irradiation and calculations of specific absorption fractions were also carried out.
The availability of mesh-type phantoms and the possibility of modifying their size and posture easily, together with increasingly fast calculators, will necessarily accentuate the pressure to individualise dose calculations. In addition to retrospective estimates for emergency exposure situations, the need to estimate the dose for a given individual is particularly noticeable in the medical field (Ria et al., 2021). This is a growing inclination in medicine, not only for diagnostics and therapies based on ionising radiation. This trend is in line with the ethical values of radiological protection defined in Publication 138 (ICRP, 2018). For example, doing more good than harm (beneficence and non-maleficence) requires, in some cases, consideration of the individual characteristics of the patient. Using a phantom that is closer to the person under consideration also reduces the uncertainties of the risk calculation indirectly, and thus implements the core ethical value of prudence.
Finally, it should be recalled that the voxel phantoms have been implemented in the context of the recommendations of Publication 103 (ICRP, 2007). Thus, all DCs for external exposure, whether for workers (ICRP, 2010; ICRU, 2010), the public (ICRP, 2020b), or – in reports in preparation – patients, have been or will be established on the basis of voxel phantoms (ICRP, 2009, 2020a). The situation is the same for internal exposure: the DCs for workers in the Occupational Intakes of Radionuclides series (ICRP, 2016, 2017, 2019, 2022) were calculated from the voxel phantoms, and this will also be the case for the DCs for the public and nuclear medicine patients. There will be one exception: the reference phantoms for the fetus and the pregnant woman, currently under development, will be produced directly and solely in mesh format. This is due to the size of the fetal tissues, which would require a prohibitive number of voxels, and because the Monte Carlo codes most commonly used to calculate DCs (e.g. Geant4, MCNP6, and PHITS) now support the mesh format. There will therefore be no DCs based on voxel phantoms of the fetus and the pregnant woman. For the next set of ICRP general recommendations, it is expected that all DCs will be based on mesh phantoms.