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Daily QA for Spot-Scanning Proton Radiotherapy Using Single Field Delivery

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J Younkin

J Younkin1*, M Bues1 , W Liu1 , D Mundy2 , D Robertson1 , J Shen1 , J Stoker1 , X Ding1 , (1) Mayo Clinic Arizona, Phoenix, AZ, (2) Mayo Clinic, Rochester, MN


SU-I-GPD-T-141 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Although there is currently no commercial daily QA solution for intensity modulated proton therapy (IMPT), the Sun Nuclear DQA-3, which is designed for x-ray daily QA, had previously been adapted for this purpose. However, published procedures for the DQA-3 required the delivery of multiple uniform fields, which creates competition for beam time in proton facilities with a single accelerator and multiple gantry rooms. We have developed a single field approach that utilizes a non-uniform, carefully modulated field to perform all dosimetric testing in parallel. This approach minimizes expenditure of beam-on time while still meeting established guidelines for daily IMPT QA.

Methods: A customized acrylic block was fabricated to provide two water equivalent thicknesses for the simultaneous measurement of SOBPs in low- and high-energy ranges. Film measurements were used to verify non-interaction between the simultaneous tests, while sensor depth and lateral position were carefully adjusted to ensure maximum test sensitivity. A calibration procedure was developed for each of the three dosimetric tests - beam output, beam energy/range, and spot position. Time usage reported by therapists and beam delivery log files were used to estimate potential time savings.

Results: Film measurements verified that field components for each test were dosimetrically independent. These tests were sensitive to our tolerances of ±1% output constancy, ±1mm distal range, and 1mm spot position deviation. Our technique more than doubled signal relative to the previous procedure while reducing background variability. Single field daily QA has been implemented at our clinic with an anticipated time savings of 150 beam-on hours per year.

Conclusion: The single field daily IMPT QA approach can perform multiple tests simultaneously, which maintains QA standards while reducing beam-on time usage. Further development of single field daily IMPT QA could potentially lead to an IMPT-specific QA device.

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