Room: AAPM ePoster Library
Purpose: energy transfer (LET) is essential to understanding biological effects of proton irradiation, and the goal of this work was to report the first complete Varian ProBeam model using TOPAS MC simulation toolkit, calculate LET, and apply quenching correction to scintillation imaging of dose and LET distributions. Experimental imaging of proton induced radioluminescence can be easily achieved in water doped with quinine sulfate (QS), but the imaged signal exhibits LET-dependent quenching. We used the characterization of this quenching experimentally with combined MC simulations, to generate a way for estimation of dose and LET quantification.
Methods: The Varian ProBeam model was based on its commissioning data, which included integral depth dose (IDD’s) and spot profiles measured at varying depths (for energies of 70MeV, 80MeV, …240MeV and 242MeV). Emittances were defined based on Courant-Snyder’s particle transport theory. Energy spectrums were defined as Gaussian distributions that minimized range and dose-to-peak difference in comparison to commissioned IDD’s. LET projection was scored in crossline(x) and depth(z) directions, matching the imaging setup. An intensified CMOS camera imaged 2g/L QS in an acrylic tank, while irradiated. Birks’ Law provided quenching parameters (A=1.212±0.001;kB=0.0706±0.0003mm/MeV) from simulated LET and corrected images for ionization quenching.
Results: size and IDD range of the simulated and commissioned beams agreed within 0.1mm. Dose-to-peak difference was generally within 0.5%. Quenching corrected optical images showed strong agreement with MC simulated dose (>99% pass rate of 2%/2mm gamma criterion).
Conclusion: simulated model is exceptionally accurate, computes LET, and may improve dose accuracy due to inline and crossline freedom in beam shaping (treatment planning system assumes cylindrical symmetry). Quenching corrected optical images produce accurate projected dose maps for clinical proton beams and can potentially be extended for accurate 3D dose and patient plan validation. The quenching-LET dependency can be explored for LET measurement by imaging proton radioluminescence.
Funding Support, Disclosures, and Conflict of Interest: Authors acknowledge the Irradiation Shared Resource at the Norris Cotton Cancer Center with NCI Cancer Center Support Grant 5P30 CA023108-41 and the NIH Grant R01 EB023909. Brian Pogue, Rongxiao Zhang, and David Gladstone report financial interest in DoseOptics LLC, a company that manufactures Cherenkov cameras used to monitor radiation therapy.