New calculations for internal dosimetry of beta-emitting radiopharmaceuticals

The calculation of absorbed dose from internally incorporated radionuclides is based on the so-called specific absorbed fractions (SAFs) which represent the fraction of energy emitted in a given source region that is absorbed per unit mass in a specific target organ. Until recently, photon SAFs were calculated using MIRD-type mathematical phantoms. For electrons, the energy released was assumed to be absorbed locally ('ICRP 30 approach'). For this work, photon and electron SAFs were derived with Monte Carlo simulations in the new male voxel-based reference computational phantom adopted by the ICRP and ICRU. The present results show that the assumption of electrons being locally absorbed is not always true at energies above 300-500 keV. For source/target organ pairs in close vicinity, high-energy electrons escaping from the source organ may result in cross-fire electron SAFs in the same order of magnitude as those from photons. Examples of organ absorbed doses per unit activity are given for (18)F-choline and (123)I-iodide. The impact of the new electron SAFs used for absorbed dose calculations compared with the previously used assumptions was found to be small. The organ dose coefficients for the two approaches differ by not more than 6 % for most organs. Only for irradiation of the urinary bladder wall by activity in the contents, the ICRP 30 approach presents an overestimation of approximately 40-50%.