New estimation of S-coefficients for radionuclides C-11, N-13, O-15, and F-18 in male and female computational...

Accurate estimation of absorbed doses in organs/tissues is essential for effective internal dosimetry. This is especially the case for positron-emission tomography-utilized radiopharmaceuticals that contain positron-emitting radionuclides. To achieve this, it is essential to calculate S-coefficients (S), basic coefficients representing the absorbed dose in the target organ per unit of nuclear transformation in the source organ. In addition, as the evolution of computational phantoms from stylized, voxelized, to mesh-type models continues, updating the S-coefficients to correspond with the new phantom generation becomes required. We employed the DoseCalcs Monte Carlo platform to estimate S-coefficients for four positron-emitting radionuclides, namely, C-11, N-13, O-15, and F-18. Based on decay and energy data for emitted positrons that were obtained from ICRP Publication 107, the simulations involved 24 regions as internal radiation sources in the male and female mesh-type phantoms of the International Commission on Radiological Protection (ICRP). We calculated the S-coefficients for 25 radiosensitive target regions. The graphs of S-coefficients for all target $\leftarrow$ source pairs exhibit similar trends for the four radionuclides. We compared the results with the OpenDose database, which calculated S-coefficients for voxelized phantoms. The comparison showed that the S-coefficients and the OpenDose voxelized values were very close for most target regions in the mesh-type phantoms. However, discrepancies were observed in specific cases, such as thyroid $\leftarrow$ UBCs and liver $\leftarrow$ HeW. These discrepancies arise primarily from the differences in organs/tissues locations and shapes, as well as the differences in material composition, which is distributed across the large inter-distance between the source and target, contributing to significant variations.