Induced radioactivity in a patient-specific collimator used in proton therapy

This paper discusses the activation of a patient-specific collimator, calculating dose rates, total activities and activities per unit mass of the mixture of radionuclides generated by proton irradiation in the energy range 100–250 MeV. Monte Carlo simulations were first performed for a generic case, using an approximate geometry and on the basis of assumptions on beam intensity and irradiation profile. A collimator used for a prostate cancer treatment was obtained from the MD Anderson Cancer Center (MDACC), Houston, USA, from which a number of samples were cut and analyzed by gamma spectrometry. The results of the gamma spectrometry are compared with the results of Monte Carlo simulations performed using geometrical and irradiation data specific to the unit. The assumptions made for the simulations and their impact on the results are discussed. Dose rate measurements performed in a low-background area at CERN and routine radiation protection measurements at the MDACC are also reported. It is shown that it should generally be possible to demonstrate that the material can be regarded as non-radioactive after allowing a sufficient decay-time, typically of the order of a few months.