Modeling photon output caused by backscattered radiation into the monitor chamber from collimator jaws using a Monte Carlo technique

A method for designing a variable-SOBP (spread-out Bragg peak) ridge filter has been proposed. First, ridge filter parameters are determined by using a Monte Carlo calculation followed by a fast two-step iterative optimization. Then, tilting the ridge filter results in continuous variation of the SOBP width. Monte Carlo calculations show that depth dose uniformity changes from ±1.3% to ± 1.6% for SOBP widths ranging from 10.3 cm to 14.5 cm. Advantages of the proposed tilting ridge filter include a capability of continuous SOBP variation and cost-effective installation for a given SOBP width range.