Exhibit Hall | Forum 3
Purpose: Collimation of individual proton pencil beamlets results in a sharp penumbra at the collimated edge, requiring correction for the analytical dose calculation to be consistent with measurements, and empirical corrections are necessary. In this work those corrections are presented and applied to an FDA-cleared treatment planning system (.decimal, Sanford, FL).
Methods: Measurement-validated, calibrated, uncollimated proton beamlet dose calculations were performed with the TOPAS Monte Carlo (MC) program for pencil beam energies from 70 – 160 MeV, and the analytical dose calculation model was demonstrated to have excellent agreement with the MC model for uncollimated beamlets. Collimated dose distributions were generated with MC for clinically relevant collimator offsets of 1 mm, 5 mm, and 55 mm from the spot centroid across the entire range of expected energies. The analytical dose calculation model was tuned to fit the MC dose distributions with the following modifications: the lateral beam spatial standard deviation (sigmaAir) was treated as a fitting parameter, and a Gaussian lateral penumbra blurring function (sigmaBlur) was applied to the collimated edge in each direction.
Results: The change in fitted sigmaAir values over measurement ranged from [-2.79%, +3.61%], and the range in sigmaBlur values were [0.43mm, 0.51mm]. The final collimated analytically-calculated pencil beam lateral profiles had an average 1%/1mm gamma pass rate (10% dose threshold) of 98.6%, 99.02%, and 99.94% at the surface, zref, and Bragg peak depth, respectively, over all beam energies and collimator position combinations when compared with measurement-validated MC.
Conclusion: Collimated penumbra have been tuned such that the Astroid analytical proton pencil beam model matches the measurement-validated MC model with sufficient accuracy to enable clinical treatment planning.
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.