A free database of radionuclide voxel S values for the dosimetry of nonuniform activity distributions

The increasing availability of SPECT/CT devices with advanced technology offers the opportunity for the accurate assessment of the radiation dose to the biological target volume during radionuclide therapy. Voxel dosimetry can be performed employing direct Monte Carlo radiation transport simulations, based on both morphological and functional images of the patient. On the other hand, for voxel dosimetry calculations the voxel S value method can be considered an easier approach than patient-specific Monte Carlo simulations, ensuring a good dosimetric accuracy at least for anatomic regions which are characterized by uniform density tissue. However, this approach has been limited because of the lack of tabulated S values for different voxel dimensions and radionuclides. The aim of this work is to provide a free dataset of values which can be used for voxel dosimetry in targeted radionuclide studies. Seven different radionuclides (Sr-89, Y-90, I-131, Sm-153, Lu-177, Re-186, Re-188), and 13 different voxel sizes (2.21, 2.33, 2.4, 3, 3.59, 3.9, 4, 4.42, 4.8, 5, 6, 6.8 and 9.28 mm) are considered. Voxel S values are calculated performing simulations of monochromatic photon and electron sources in two different homogeneous tissues (soft tissue and bone) with DOSXYZnrc code, and weighting the contributions on the basis of the radionuclide emission spectra. The outcomes are validated by comparison with Monte Carlo simulations obtained with other codes (PENELOPE and MCNP4c) performing direct simulation of the radionuclide emission spectra. The differences among the different Monte Carlo codes are of the order of a few per cent when considering the source voxel and the bremsstrahlung tail, whereas the highest differences are observed at a distance close to the maximum continuous slowing down approximation range of electrons. These discrepancies would negligibly affect dosimetric assessments. The dataset of voxel S values can be freely downloaded from the website www.medphys.it.