Influence of voxel S factors on three-dimensional internal dosimetry calculations

Internal dosimetry is a fundamental instrument for the personalization of nuclear medicine therapies, to maximize the therapeutic effect while minimizing the radiation burden to other organs. Three-dimensional (3D) dosimetry can quantify the impact of heterogeneous radiopharmaceutical distributions in organs, lesions and tissues. We analysed the influence of radionuclide voxel S factors in 3D dosimetry of In-111, Lu-177 and Y-90, the most used radionuclides in Peptide Receptor Radionuclide Therapy (PRRT). Calculations were carried out for kidneys on a workstation equipped with a software for 3D dosimetry (Imalytics STRATOS, Philips AG), adopting a computational anthropomorphic phantom and, retrospectively, the SPECT-CT image series of a clinical case of PRRT. Two sets of voxel S factors were adopted: the pre-loaded Philips kernels, calculated by direct Monte Carlo simulation, and the ones calculated through a previously proposed analytical approach. Philips 111In kernel did not account for mono-energetic Auger or Conversion electrons. Results indicate a difference of about -32% in voxel S factors for In-111 in 4.42 mm voxel size and around -35% in 4.80 mm voxel size, particularly self-dose values; this lead to significant shift in dose histograms and average doses. For Lu-177 and Y-90, differences are about 2% and 12% for 4.42 mm voxels and about -8% and 9% for 4.80 mm voxels, respectively, attributable to the different calculation methods of the voxel S factors; this does not lead to significant discrepancies between the two dose histograms. Consequently, voxel S factors must account accurately for all radiations emitted by the nuclide. (C) 2016 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.