Simulation of beta-emitters for radiopharmaceutical dosimetry using voxel phantoms and Monte Carlo calculations

Abstract

Radiation dose estimates are needed for assessment of the risk to patients associated with the use of radiopharmaceuticals both for comparison with the possible benefit of an investigation and to help giving adequate information to the patient. With respect to the physical aspects, one of the most crucial requirements is to establish values of socalled Specific Absorbed Fractions (SAF) which specify the fraction of penetrating energy emitted by radioactivity in a given organ which is absorbed in the source organ itself and in other organs. Until recently, photon SAFs were calculated on the basis of MIRD-type mathematical anthropomorphic phantoms and a whole range of data exists covering all ages. For electrons, the radiation was assumed to be absorbed entirely in the source region, except when the source was part of the skeleton or when the source is in the contents of a walled organ. For this work, photon SAFs and electron SAFs were derived with the Monte Carlo code EGSnrc for the new reference male and female voxel-based phantoms adopted by the ICRP. The present results of electron SAFs show that the previously applied assumption of electrons being fully absorbed in the source organ itself presents an over-simplification at higher energies. For organs in close vicinity, such as liver and stomach wall, high-energetic electrons escaping from the source organ may result in cross-fire SAF values that can reach the same order of magnitude as those from photons. Examples of organ absorbed doses per nuclear transformation will be given for some radiopharmaceuticals. The impact of the new electron SAFs used for the calculation of the absorbed doses instead of the previously used assumption, will be discussed. © 2009 Springer-Verlag.