SU‐E‐T‐360: A Novel Procedure to Determine the Dose Output Change with Spread Out Bragg Peak Width and Range Shift in Passively Scattered Proton Beams

Purpose: Two of the major factors affecting the change in output of a passively scattered proton beam are the spread‐out Bragg peak width factor (SOBPF) and range shifter factor (RSF), both of which are usually measured during clinical commissioning. The purpose of this study is to determine these two factors by numerically constructing the SOBP from experimentally measured pristine Bragg peak (PBP), which can serve as an independent monitor unit (MU) calculation procedure. Methods: SOBPs were constructed by superposition of PBPs that are shifted by the water‐equivalent thicknesses (WET) of the corresponding steps of the range modulation wheel. A least‐squares fit was used to determine the weightings of the shifted PBPs to form the required SOBPs. With a simplification in modeling that MU monitor in the nozzle receives dose proportional to the surface dose of the SOBP (higher by an inverse‐square factor), we can relate the MU to the dose at the center of SOBP and compute the SOBP factors for different SOBP widths. When the range shifter was added, the SOBP was shifted toward the surface by the WET of the range shifter and the RSF can also be calculated from the change in the surface dose.Results: The SOBP and range shifter factors were calculated for SOBPs with modulation widths between 4 and 16 cm of 250 and 200 MeV proton beams. The calculated SOBPF and RSF agreed within 2%, and 1.5% with the respective measured values. Conclusions: Starting with measured pristine Bragg peaks, SOBPs can be numerically constructed by simulating the range modulation process, and important dosimetry factors dependent on the modulation width and range shifters can be calculated within 2% accuracy.