Purpose: To develop and experimentally validate a TOPAS Monte Carlo model of the Dynamic Collimation System (DCS) and the IBA dedicated nozzle (DN). The DCS is an energy layer-specific collimator specifically designed for low energy pencil beam scanning proton therapy and consists of four nickel trimmer blades that translate and rotate as a function of beam position located downstream from a polyethylene range shifter.
Methods: Uncollimated integral depth dose (IDD) curves, in-air spot profiles, and absolute dose measurements were used to commission a source model of the IBA DN system in TOPAS. Collimated pristine and range shifted IDD curves and in-air and in-phantom 2D spot profiles were acquired to benchmark the model’s performance for a variety of collimator configurations. The water equivalent thickness (WET) of the range shifter was determined by the shift in the depth of the 80% dose point distal to the Bragg peak between the range shifted and pristine uncollimated beams.
Results: Uncollimated IDDs 1D gamma pass rates were 99.4% and 91.2% at the 1%/1mm and 0.5%/0.5mm evaluation criteria, respectively. For the pristine collimated beams, the average 1%/1mm gamma pass rates across all collimator configurations investigated were 99.8% for IDDs and 97.6% and 95.2% for in-air and in-phantom lateral profiles. All range shifted IDDs passed at 100% while in-air and in-phantom lateral profiles had average pass rates of 99.1% and 99.8%, respectively. The measured and simulated WETs of the polyethylene range shifter were determined to be 40.9 and 41.0mm, respectively.
Conclusion: A TOPAS Monte Carlo model of the DCS and IBA DN system was developed and validated against uncollimated and collimated IDD and lateral profile measurements. As the DCS moves towards clinical implementation, this model will be used to assess the accuracy of analytical algorithms used in commercial treatment planning systems and help guide future dosimetric investigations.
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.