Room: Stars at Night Ballroom 1
Purpose: Proton therapy is an effective treatment modality for ocular tumors, however additional beam collimation hardware is required to minimize dose to adjacent normal tissues. This work aims to develop and test an aperture system for the treatment of ocular targets with a gantry-based spot scanning proton beam.
Methods: The aperture system is made of acetal and polycarbonate plastic and inserts into the nozzle head. It consists of an intermediate scraper layer and a polycarbonate 3D printed patient-specific aperture positioned 5.7 cm proximal to the eye for final beam shaping. Plans were optimized in Eclipse without an aperture to deliver a homogeneous dose to 1 cm and 3 cm diameter targets in solid water. The aperture system was modeled in TOPAS and Monte Carlo (MC) simulations were run using a previously benchmarked beam line. Dose was also calculated with the clinically commissioned MC second check workflow. Film was used to experimentally measure the dose distributions at 1.5 cm depth.
Results: The central axis dose in TOPAS simulations and film measurements agreed within 3.5% and 1.6% for the 1 cm and 3 cm apertures, respectively. For both target sizes the lateral dose penumbra (80%-20%) was measured and simulated to be 3 mm. The central axis dose simulated by TOPAS and the clinical MC second check agreed within 1% for both apertures. TOPAS simulations indicated that neutron dose comprises 0.02% of the total dose. Film measurements showed that the maximum proton leakage dose outside of the field is less than 0.3% of the central axis dose.
Conclusion: Initial testing and modeling of the aperture system indicates that it can be used to deliver conformal dose distributions to representative ocular targets. Film measurements agreed well with TOPAS simulations, and the lateral dose fall-off, neutron contamination and leakage dose were determined to be acceptable.