(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: To show that proton monoenergetic arc therapy (PMAT) plans are producible and deliverable, and that radiochromic films are an adequate dosimetry method as long as quenching effects are considered and accounted for.
Methods: A single-energy PMAT plan was delivered onto Gafchromic EBT3 film; to simulate gantry rotation, a fixed gantry room of the IBA Proteus Plus cyclotron delivered 4 Gy onto film placed in a homogenous 360-degree rotating plastic-water phantom. The energy was selected to place the Bragg peak deposition approximately at the films center. An in-house optimizer was used to produce a plan with homogenous dose to a circular target. The phantom’s rotation speed was coincided with the plan delivery time according to log files produced by our treatment unit. Independently, dose within the treatment planning system was adjusted to correct the quenching effect according to the method proposed by Anderson et al (2019) in regions of high LET throughout the film.
Results: Vertical profiles of the measured Gafchromic film’s dose level and those of the quenching-corrected and uncorrected dose distributions were created for analysis. The quenching effect present in the measured film contributed to an absolute dose reduction of about 15% compared to that of the uncorrected dose distribution. When compared to the quenching-corrected dose distribution, there was only a relative increase of 5% at the center of the target compared to the periphery.
Conclusion: PMAT optimized plans can be correctly planned and delivered with a fixed gantry to a homogenous cylindrical phantom. The method proposed in Anderson et al (2019) appears to be a valid technique to account for quenching effects. This could allow for satisfactory dosimetry of PMAT plans with Gafchromic EBT3 films.
Funding Support, Disclosures, and Conflict of Interest: This project is supported by Varian Medical Systems, Palo Alto, CA.
Keywords
Protons, Radiochromic Film, Dosimetry
Taxonomy
Not Applicable / None Entered.
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