Purpose: Uncorrected analytical dose calculation algorithms do not accurately represent the integral depth dose (IDD) curves of individually collimated proton pencil beams generated using the dynamic collimation system (DCS), with ≥5% IDD disagreement at the surface and Bragg Peak depth being commonplace. A simple method is presented for efficiently correcting the analytically calculated beamlets using Monte Carlo (MC) generated IDDs for collimated scenarios, addressing this discrepancy. This method enables DCS-collimated proton pencil beam dose calculations using the FDA-cleared treatment planning system Astroid (.decimal, Sanford, FL).
Methods: Dose calibration for Astroid and MC (TOPAS) were conducted based on dose measurement of an uncollimated broad beam for energies from 70–160 MeV. This process yielded excellent agreement between Astroid, MC, and measurement for uncollimated beamlets in units of Gy per MU. Collimated IDD curves were generated with MC for clinically relevant collimator offsets at 1 mm, 5 mm, and 55 mm from the spot centroid across the entire range of expected energies. Collimated pencil beams calculated using uncollimated IDDs were then scaled (corrected) at all depths using a lookup table of collimated IDDs from MC to obtain the final collimated pencil beams – a more efficient approach than using collimated IDDs at the infinitesimal pencil beam calculation stage.
Results: After integrating the Monte Carlo generated corrections, analytically calculated collimated IDD curves matched TOPAS exactly, representing 0% disagreement between MC and analytical models for collimated beamlets. The final collimated analytically-calculated pencil beam lateral profiles had an average 1%/1mm gamma pass rate (10% dose threshold) of 98.56%, 99.02%, and 99.94% at the surface, z_ref, and Bragg peak depth, respectively, over all beam energies and collimator position combinations when compared with measurement-validated MC.
Conclusion: A computationally efficient method for accurately calculating dose due to collimation with the Astroid TPS pencil beam model has been developed.
Funding Support, Disclosures, and Conflict of Interest: Research reported in this publication was supported by the National Cancer Institute of the National Institutes of Health under Award Number R37CA226518. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.