MO‐F‐108‐08: A Revolving Collimator for Proton Stereotactic Radiosurgery (pSRS)

Abstract

Purpose: Stereotactic radiosurgery (SRS) is a non‐invasive treatment modality appropriate for a wide array of tumors. Particularly in patients with benign tumors receiving proton SRS (pSRS), the generation of secondary neutrons from brass apertures may be associated with higher risk of secondary radiation‐induced tumors. A specially designed revolving titanium alloy collimator system with multiple sized collimator openings in circular and elliptical shapes is proposed and investigated in this study. Methods: Simulations of the passage of a proton beam through the proposed collimator system was performed using the FLUKA Monte Carlo general purpose code with treatment nozzle model implemented. The set of materials potentially applicable for the collimator fabrication, i.e. brass, stainless steel (SS), tungsten alloy (WA) and titanium alloy (TiA) (Ti‐6A1‐4V Grade 5) were simulated. The neutron fluence, dose equivalent, activation of the collimator material, the cooling times and beam shaping properties are analyzed. Results: Utilization of TiA results in less neutron dose equivalent at the patient compared to brass, WA or SS. The neutron flux spectra for TiA is an order of magnitude lower than those for the other materials in the energy interval with the maximum neutron weighting factor. The activation of TiA is lower and the cooling time 10 times shorter compared to WA. Dose shaping with the pSRS collimator is similar to a brass aperture. Dose enhancement at shallow depths for small proton fields limits the practical collimator opening size. Conclusion: The specially designed revolving pSRS titanium alloy collimator proposed in this study may reduce secondary neutron dose to the patient compared to other collimator materials investigated. This may reduce the risk of secondary tumors and malignancies and reduce late effects to the patient. The pSRS collimator allows the establishment of a pSRS system and methodology without the need for a dedicated pSRS treatment vault.