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Feasibility Study of Quality Assurance of Proton Range Compensator Using the Proximal Side of Spread-Out Bragg Peak

H Jin

H Jin*, S Ahmad , Oklahoma Univ. Health Science Ctr., Oklahoma City, OK


SU-I-GPD-T-142 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To assess quality assurance (QA) of proton range compensator (RC) for passively scattered proton therapy using the proximal side of the spread-out Bragg peak (SOBP).

Methods: The dose gradient in the proximal side of the narrow SOBP was tested to perform RC QA. A beam of 8.5 g/cm2 range and 2.0 g/cm2 modulation (500 MU) from the Mevion S250 proton therapy system was delivered to RC on Gafchromic EBT3 film. The RC was overlaid with a 2-cm solid water block to eliminate the flat entrance dose region. A calibration curve was generated using an acrylic phantom whose thickness varied from 0 to 40 mm in steps of 2 mm to convert the film images to thickness maps. Three RCs from a brain plan which passed conventional RC QA were tested. The thickness maps were compared to those from the Eclipse treatment planning system using an in-house Matlab software. The thickness difference and a modified gamma test using 2 mm thickness difference and 2 mm DTA criteria were employed to evaluate the QA results.

Results: The RC thickness maps between measurements and plans agreed well when the thickness was greater than ~5 mm (≤2 mm difference considering relatively high noise in film dosimetry). The mean gamma passing rates for the three RCs were 85.3% for all pixels within aperture and 90.6% with thickness ≥5 mm. Areas of thin RC (<~5 mm) were not clearly shown in the film images due to low dose gradient in entrance region and low sensitivity of EBT3 film.

Conclusion: The dose gradient of the proximal side of the spread-out Bragg peak has a potential to be used for range compensator QA. If the film is sensitive enough to discern the thin RC thickness, it can competently be used for two-dimensional RC QA.

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