In 2016, the International Commission on Radiological Protection and Measurements (ICRP), published the results of Monte Carlo simulations performed using updated and anatomically realistic voxelized phantoms. The resulting absorbed fractions are substantially more accurate than calculations based on the Cristy-Eckerman (CE) stylized (or mathematical) phantoms. Despite this development, the ICRP absorbed fractions have not been widely adopted for radiopharmaceutical dosimetry. To help make the transition, we have established a correspondence between tissues defined in the CE phantom and those defined in the ICRP phantoms. Using pre-clinical data from biodistribution studies performed, we have calculated absorbed doses for Th-227 labeled HER2 targeted antibody. We compare the CE phantom-based calculations as implemented in the OLINDA v1 software with those obtained using ICRP absorbed fractions as implemented in 3D-RD-S, a newly developed software package that implements the MIRD S-value methodology. We also compare ICRP values with a hybrid set of calculations in which alpha-particle energy was assumed completely absorbed in activity containing tissues.
We observed a non-negligible difference in the absorbed dose calculated using each of the methods for each radiation type. This can be attributed to a combination of greater accuracy in absorbed fraction calculations and differences in the time integrated activity coefficient values due to difference in representation of anatomy by the phantoms. The total absorbed dose for Thorium-227 was dominated by alpha particles, hence the differences in beta and photon absorbed doses were inconsequential in terms of total dose.
The results obtained by comparing these different implementations of the MIRD S value methodology provide the data needed to help the field transition to the more anatomically accurate ICRP phantom-based dosimetry. Key words : ICRP phantom, radiopharmaceutical dosimetry, Cristy-Eckerman phantom