Optimization study of range-compensating bolus for particle therapy

A novel design method has been developed to obtain a single-material bolus (SM bolus) for a proton beam to form an optimized dose distribution in and around a target. We applied this method to a RTOG phantom simulating a paraspinal tumor and obtained a bolus with which a better dose distribution was obtained in and around the target than with a bolus designed based on a ray-tracing model and bolus smearing. We measured 3D dose distributions in water formed by protons passing through SM- and Bi-material (BM)-boluses comprised of segments each base size of which is 5 mm x 5 mm designed for a RTOG phantom to evaluate accuracy of dose calculations used for the bolus optimization. We have developed a ripple filter to increase the distal dose falloff of the 290 MeV/u carbon beam from 0.6 mm to 2.5 mm to facilitate manufacturing a ridge filter with better control of the shape based on the Bragg curve with the enlarged distal dose falloff. We also developed a new dose calculation model for designing the ridge filter taking into account the shape change of Bragg curves formed by carbons transmitting through the different thickness of the ridge filter. (author)