Abstract
Purpose: Standard proton therapy for prostate cancer treatment uses two lateral beams and extra margins around the tumor volume to average out range uncertainties. The drawback is a large lateral penumbra (∼10mm, 95–50%) at the target—rectum interface. Anterior beams are promising to improve radiation toxicity, by exploiting the sharp distal penumbra (∼4mm,95−50%) and show an improved target conformity. The problem is a potential under — or overshoot due to variations in the patient from day to day. The purpose of the study is to commission an in-vivo range verification system (IRVS) for anterior prostate beam treatment in passive scattering. Methods: The IRVS was developed and tested with anterior proton beam angles. The detector of the system is a 12 Si-diode array, attached to the endorectal balloon. A software was developed to guide through the range verification workflow. The X-ray image, acquired for patient positioning, is used to yield the diode coordinates. They are used for extracting the expected water equivalent path length (WEPL) and dose from the treatment plan. A “scout” beam acquires the WEPL information and calibrates the diodes for dose monitoring directly before treatment. Results: The system was tested on a solid water and an anthropomorphic pelvic phantom. The extra dose delivered by the “scout” beam is within the dose calibration standard deviation (2%). The WEPL measurement accuracy was <0.43%(0.55mm) for solid water. For the pelvic phantom-30° field the deviation to the treatment planning system is 0.53%(0.9mm). The diode for range decision is chosen based on the index of range mixing. Dose measurement compared to Marcus Chamber showed a deviation of < 4 % in solid water at the Isocenter. Conclusion: The IRVS meets the clinical requirements of 2 mm and 2% accuracy. The implementation of the proposed system could make anterior beams feasible. PROSA LMU study-abroad scholarship
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