Full Monte Carlo internal dosimetry in nuclear medicine by means of GAMOS

Three-dimensional internal dosimetry is increasingly used in diagnostic and therapeutic nuclear medicine. Even if simplified calculation approaches are currently available, the full Monte Carlo simulation of the radiation spectra emitted by nuclides and its interaction in living matter remains the gold standard in terms of accuracy of results. We developed a procedure exploiting the GAMOS (GEANT4-based Architecture for Medicine-Oriented Simulations) interface to GEANT4 (Geometry and Tracking 4) Monte Carlo, together with ad-hoc implemented ancillary codes for managing I/O. SPECT and PET images were employed to model the 3D distribution of radionuclide emissions in a voxelized volume whose density and composition was obtained from CT scan. All pre-treatments of SPECT, PET and CT images, including registration, segmentation of Volumes Of Interest (VOIs) and calculation of Time-Integrated Activity (TIA) maps for multiple time-points imaging, were carried out through either the commercial Philips Imalytics or the free 3D-Slicer software. We applied our procedure to SPECT-CT pre-therapeutic imaging with 99mTc-MAA of Trans-Arterial Radio-Embolization of hepatocellular carcinoma and to 18F-Choline PET-CT scans. Three-dimensional absorbed dose maps, dose profiles and Dose-Volume Histograms (DVHs) were produced and compared with results coming from Medical Internal Radiation Dose (MIRD) approach at organ level. Discrepancies were mainly found where photon contribution to absorbed dose is dominant as well as in presence of relevant tissue inhomogeneities.