The Effect of Nucleus Size on the Cell Dose in Targeted Radionuclide Therapy – A Monte Carlo Study

Background: Nowadays, the use of radiopharmaceuticals in medicine is unavoidable. Depending on the distribution of the radiopharmaceutical in the cells, the nucleus absorbed dose changes by the variations in their geometry size. Therefore, this study aims to investigate the S‑value by the variation of nucleus size using Geant4 toolkit. Methods: Two spherical cells with a variety of nucleus size have been considered as the cancerous cell. Monoenergetic electrons ranging from 5 to 300 keV are distributed uniformly. The S‑value for four target‑source components (including Nucleus←Cytoplasm, Nucleus←Cell surface, Nucleus←Nucleus, and Nucleus←Nucleus surface) is computed and plotted. Then, the obtained data are compared with analytical Medical Internal Radiation Dose (MIRD) data. Results: In Nucleus←Cytoplasm compartment for electrons below 10 keV, obtained S‑values show a slight decrease for the nucleus in the radii of around half of the cell radius and then S‑values increase with the increase in the nucleus radii. In the S‑value of Nucleus←Cell surface, for all electron energy levels, a slight decrease observed with the increase of nucleus radii. For Nucleus←Nucleus and Nucleus←Nucleus surface cases, with an increase in the size of the cell nucleus, a sharp reduction in the S‑values is detected. Conclusion: It can be concluded that for the beta emitters with low‑energy radiation (<40 keV), the S‑value is heavily dependent on the nucleus size which may affect the treatment of small tumors. While for the beta emitters with higher‑energy radiation (>100 keV), the size of the nucleus is not very noticeable in the induced S‑value.