Purpose: To characterize changes in entrance dose resulting from low-energy scatter induced from the Dynamic Collimation System (DCS), an energy layer-specific collimator designed for low-energy pencil beam scanning proton therapy using a commercially available multilayer ionization chamber (MLIC).
Methods: Uncollimated integral depth dose (IDD) curves were acquired with an IBA Stingray parallel plate ionization chamber scanned in a water tank and the IBA Giraffe multilayer ionization chamber (MLIC). IDDs for beamlets collimated by various collimator orientations were then measured with the Giraffe MLIC. Results were compared to identical simulations performed using a benchmarked Monte Carlo model of the DCS. Simulated IDDs consisted of tallying the dose to water in a 0.1mm mesh with a diameter identical to the 12-cm diameters of Giraffe MLIC and Stingray chambers. Simulated and measured collimated IDDs were compared using 1D overall 0.5%/0.5mm gamma tests.
Results: For the uncollimated scenarios, the MLIC measurements exhibited a significant decrease in dose at the Bragg peak when compared to the uncollimated Stingray measurements that is most likely due to rise a rapid change in the mass stopping power ratios of water to Giraffe-equivalent material near the end of range. To account for this, uncollimated Stingray and MLIC measurements were then used to derive energy- and depth-dependent correction functions that were then applied to the collimated MLIC measurements. For the pristine collimated beams, 0.5%/0.5mm gamma pass rates increased from 17.5% to 89.6% with the use of the correction. For the range shifted collimated beams, pass rates increased from 39.0% to 95.4%.
Conclusion: We have demonstrated a need and implemented a method to correct IDDs acquired with an MLIC for the purpose of characterizing changes in entrance dose resulting from collimation.
Funding Support, Disclosures, and Conflict of Interest: Research reported in this abstract was supported by the National Cancer Institute of the National Institutes of Health under award number R37CA226518. Hyer, Flynn, and Hill are co-inventors on a patent that has been licensed to IBA.