S Thompson*, W Hsi, University of Florida Health Proton Therapy Institute, Jacksonville, FL

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  S Thompson

PO-GePV-T-140 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To investigate the utilization of beamline-model parameters in the TOPAS and MC-square Monte Carlo dose-simulation codes.

Methods: The TOPAS code allows for the simulation of beamline modifiers such as range-compensators to conform distal penetration depth for double-scattering protons, as well as apertures to sharpen the lateral profiles of double-scattering and pencil-beam-scanning (PBS) protons. Before studying the effects of any modifier, three major parameters of a PBS proton beamline-model should be assessed: (1) correlate proton energy with corresponding penetration depth, (2) simulate the spot divergence by a proton source with its spot-size/location, and (3) scale the absolute dose with the related proton flux. Using our existing MC-square model for one of our PBS proton beamlines, we validated the TOPAS code for the same beamline. We utilized the MC-square parameters to simulate integrated depth-dose (IDD) curves for nine proton energies and in-air spot-sizes at five locations to isocenter with five proton energies. All simulations were compared with corresponding measurements.

Results: Using the MC-square parameters for IDD, the TOPAS simulated IDDs for all nine energies are within 1.0 mm of each corresponding measured IDD on the distal 80% penetration depth. The distal penumbra for each energy is identical. However, TOPAS simulated in-air spot-size differed from measurement by about 1.0 mm for the 3.5 mm spot-size for 225 MeV protons. The spot-size difference is more than the clinical acceptance (maximum of 0.3 mm or 5% of spot-size).

Conclusion: The TOPAS simulations using the MC-square beamline-model parameters accurately reproduced the measured IDDs. The particle transport scattering parameters in TOPAS and MC-square codes can be different, as evidenced by the slight discrepancy in the spot-size parameters between the two codes. Once the accuracy of TOPAS is confirmed for PBS proton beamline, we will focus on simulating the effects of modifiers in our double-scattering beamline.

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