Radioactivity evaluation induced by beamline components in the ocular proton treatment

Abstract

Using Geant4/TOPAS and FISPACT-II simulations, we built an ocular proton treatment system and performed activation analysis. We evaluated two scenarios: a simple one assuming 15 years of operation and a detailed one based on 10 patients per day, each receiving a 1-min beam irradiation with a 30-min cooling period. To assess if the radionuclide mixture was below the clearance level, we calculated the relative activity ratio as the sum of C_i/C_L,i, where C_i is the concentration and C_L,i is the clearance concentration level of radionuclide i, based on two national reports. In the simple scenario, the inner ring of ion chamber, the metallic component closest to the proton source, showed a relative activity ratio exceeding 7.8 × 10⁴ at the end of the treatment. The detailed scenario resulted in a relative activity ratio of 5.6 × 10³ at the end of the treatment, which is about 10 times lower than the simple scenario. Main long-lived radionuclides, such as ¹⁹⁵Au (1.07 × 10² Bq/g), ¹⁹⁴Hg (0.73 Bq/g), and ¹⁹³Pt (1.66 × 10 Bq/g), were identified as crucial in long-term disposal planning. This study shows that the simulations can identify activation-sensitive components, aiding in disposal planning.