The voxel-level dosimetry is the most simple and common approach to internal dosimetry of non-uniform distributions of activity within the human body. Aim of this work was to obtain the voxel S values in soft or bone tissue for four radionuclides commonly used in nuclear medicine therapies.
We developed two Monte Carlo simulations, in Geant4 and in MCNPX,of tissue regions divided into 11x11x11 cubic voxels of 3 mm side. We considered different models for soft and bone tissues, in which the different physics models available were tested.
The decay of each radionuclide (90Y, 111In, 177Lu, 131I) was simulated homogeneously distributed within the central voxel (0,0,0), and the energy deposited in it and in the surrounding voxels was scored.
The results obtained with the two Monte Carlo codes were compared each others and with those available in the literature. Voxel S factors were represented as a function of the “normalized radius”, defined as the ratio between the source–target voxel distance and the voxel side. We found that voxel S factors are mainly affected by the tissue density, while the material composition gives only a slight contribution to the difference between data series, which is negligible for practical purposes.
These data can be useful to broaden the applicability of voxel-dosimetry thanks to the availability of S values in different tissues.