Abstract
The phantoms used by the International Commission on Radiological Protection (ICRP) are mathematical models of the anatomy of the human body, required for dosimetric calculations. The improvements made to these phantoms, particularly more recently, reflect considerable advances in scientific methodology and computing power, together with changing expectations that the best science will be used and will be communicated openly. The highly sophisticated models in this publication should ‘future-proof’ the Commission in readiness for anticipated requirements for new calculations.
Little or no mention was made of phantoms in earlier ICRP publications, including the general recommendations of Publications 26 and 60 (ICRP, 1977, 1991). Despite their low visibility, they were central to the calculation of organ and effective dose coefficients published by the Commission to enable the implementation of its system of protection. The phantoms used by the Commission throughout this period were developed at Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee, USA, originally for the Medical Internal Radiation Dose (MIRD) Committee of the Society of Nuclear Medicine. The organs of the body were represented in idealised form using mathematical expressions for shapes, including cylinders, cones, ellipsoids, and spheres (Snyder et al., 1969, 1978; Cristy and Eckerman, 1987). From the original adult MIRD phantom, several paediatric phantoms were derived to represent infants and children of various ages (Cristy, 1980). While clearly distinguishable from human beings, these phantoms had the advantages for their intended purpose of being hermaphrodite and being modelled on the organ masses and volumes of Reference Man (ICRP, 1975). The Commission also used models of the pregnant female and fetus developed at ORNL (Stabin et al., 1995).
Although the ORNL/MIRD phantoms were used by the Commission, they had not been adopted formally and so, in principle, effective dose could be calculated using any available phantom. This situation was changed with publication of the 2007 Recommendations [Publication 103 (ICRP, 2007)] which outlined the intention to use reference computational models based on medical imaging data for all ICRP dose calculations. The best-available methodology would be used, defining the structure of organs and tissues using three-dimensional volume pixels – or voxels. As these voxel phantoms were based on high-resolution scans of real individuals, two issues arose: there would be male and female models, and they would need to be adjusted to conform to the organ dimensions and masses of Reference Male and Female (ICRP, 2002). Publication 103 set out the procedure for the calculation of equivalent dose separately for males and females, and sex-averaging in the calculation of effective dose.
The ICRP reference adult male and female models of Publication 110 (ICRP, 2009) were based on voxel phantoms developed at GSF Munich, now Helmholtz Zentrum (Zankl and Wittmann, 2001; Zankl et al., 2005). Individuals as close as possible to reference dimensions were chosen to minimise the requirement for modifications. The total number of voxels used approached 2 million for the male and 4 million for the female. However, even with the greater resolution of the female model, it was not possible to delineate all important tissues sufficiently well for all applications. While photon and neutron absorbed doses could be calculated from radiation transport simulations in the reference phantoms, additional stylised models were required for some tissues to calculate energy deposition from internally deposited radionuclides with short-range emissions of alpha and beta particles. For example, separate models were needed for calculation of doses to cell layers in the lining of the respiratory and alimentary tracts (ICRP, 2009).
The Commission now also has a series of reference paediatric models (ICRP, 2020) based on voxel phantoms developed at the University of Florida (UF) and later at the US National Cancer Institute (NCI) (Lee et al., 2010). These mirror the adult models, with males and females at ages: newborn, 1 year, 5 years, 10 years, and 15 years. As for the adults, they were based on medical imaging data and were adjusted for consistency with reference data on organ dimensions and masses (ICRP, 2002). Rather than adding or subtracting layers of voxels to the various organs as in the adult models, adjustments were made by modelling organ surfaces using non-uniform rational basis spline (NURBS) and polygon mesh techniques, followed by voxelisation of the NURBS/polygon mesh surfaces.
The ICRP reference voxel models are being used in all calculations of equivalent and effective dose coefficients based on Publication 103 (ICRP, 2007) methodology. Dose coefficients for occupational exposures of workers have been issued for external exposures in Publication 116 (ICRP, 2010) and for occupational intakes of radionuclides in Publications 130, 134, 137, and 141 (ICRP, 2015, 2016, 2017, 2019). The last part of the Occupational Intakes of Radionuclides series should be published in early 2021. Dose coefficients for public exposures to external sources are in press, and values for intakes of radionuclides are in preparation. Also in preparation are dose coefficients for radiopharmaceutical administrations to patients.
Although the reference voxel phantoms will continue to be used for the current round of calculations of dose coefficients, scientific methodology has advanced so that radiation transport calculations can now be done in organ and tissue volumes without voxelisation, and phantoms can include even the smallest source and target regions. The development of state-of-the-art phantoms for the Commission is being led from Hanyang University, Korea (Kim et al., 2018) using polygon mesh and tetrahedral mesh modelling. The mesh-type reference adult phantoms in this publication were constructed by converting the Publication 110 (ICRP, 2009) voxel phantoms into high-quality mesh format, extending their functionality by including all source and target regions needed for dose calculations. All dose calculations can now be performed using the phantoms without the need for supplemental models for the respiratory airways, walls of the alimentary tract, urinary bladder, lens of the eye, and epidermis of the skin. As discussed in this publication, the modelling involved was incredibly detailed and includes, for example, the branching networks of airways of diminishing diameter in the lungs, and the multiple micrometre source and target layers on their epithelial surfaces (Kim et al., 2017). Despite this complexity resulting from a high level of anatomical realism, the phantoms have good computational speeds for Monte Carlo codes used for radiation transport calculations. Comparisons show that photon and neutron doses calculated using the mesh phantoms are very similar to values obtained with the voxel phantoms, but that, as expected, the mesh phantoms provide better estimates of doses from short-range alpha and beta particles.
Work is now in progress to provide mesh reference paediatric phantoms. The full set of new phantoms will be used in all future dose calculations, including the recalculation of equivalent and effective dose coefficients as required when the methodology is next changed. In the short term, the mesh phantoms are being used in new work on emergency dosimetry, and data are included in this publication for examples of exposures to industrial radiography sources. In considering accidents that might cause tissue reactions and possible fatalities, it is informative to consider the effect of body size and mass, and also different postures. These calculations illustrate the versatility of the mesh phantoms, their construction making them readily deformable to model a variety of exposure situations for a range of individuals, and to reconstruct doses for specific circumstances of exposure. The phantoms are also well suited for use in the virtual calibration of whole-body counters to account for the body size of radiation workers in efficiency calibrations, and provide ideal templates for the construction of physical phantoms. One of the aims of this publication is to assist those who wish to implement the phantoms for their own applications; detailed data on the phantoms are provided as a supplementary electronic annex.
The reputation and authority of the Commission depends on the quality of its reports – its science and advice. Maintaining that quality means keeping up with scientific advances, and the evolution of dosimetric phantoms is an excellent example, made possible by the expertise of the scientists involved and their willingness to collaborate internationally in support of the Commission. Their hard work is gratefully acknowledged.