Comparison of SAF calculations using GATE/Geant4 and MCNPX: impact of respiratory and cardiac motion in a 4D Voxelized Phantom for internal dosimetry

This study investigates Specific Absorbed Fractions (SAFs) for internal dosimetry using a dynamic 4D XCAT phantom to quantify the influence of physiological motion. The model incorporates 13 respiratory phases over a normal 5.2 s cycle, with voxel-based material definition, and additionally includes cardiac beating within the same framework. Respiratory motion was found to significantly affect lung dosimetry. The self-absorbed fraction in the lungs showed the largest variation, reaching 13.74% at 2.018 MeV. Cross-irradiation of nearby organs was also sensitive to breathing motion, with relative SAF differences of 11.90% for the liver and 12.77% for the stomach between the start and end of inhalation. In contrast, dense abdominal organs exhibited limited variation in self-irradiation scenarios. The liver showed a maximum change of 0.79% for monoenergetic photons and 0.25% for electrons. Electron self-irradiation in the lungs displayed moderate sensitivity to motion, with variations up to 5.16%, consistent with the short penetration range of electrons. Comparison between GATE/Geant4 and MCNPX simulations demonstrated good agreement, confirming the robustness of the Monte Carlo framework. A novel aspect of this work is the inclusion of cardiac motion. Although its dosimetric impact was secondary, the maximum variation induced by heart motion reached 0.314%, representing a previously unquantified physiological contribution. Overall, these findings demonstrate that motion-resolved modeling improves the realism of SAF calculations and reduces uncertainties associated with static phantoms, providing a methodological basis for more accurate and individualized internal dose assessment. Clinically, these results can enhance personalized radionuclide therapy planning, optimize organ-specific dose estimation in PET/SPECT imaging, and support safer and more effective nuclear medicine procedures.