Statistical Tolerances On Daily Quality Assurance for Proton Spot Width Using a Sphinx Compact Device
S St. James*, V Sarkar, R Price, A Paxton, F Su, B Salter, University of Utah - Huntsman Cancer Institute, Salt Lake City, UT
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PO-GePV-T-149 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: To evaluate the Sphinx Compact daily quality assurance device for pencil beam scanning proton therapy performance for identifying deviations in beam characteristics (spot sigma) that would otherwise be identified solely with monthly quality assurance.
Methods: Daily quality assurance results were compiled from 11 consecutive months of measurements and were compared to monthly quality assurance results, along with known interventions on a Mevion S250i with HYPERSCAN single room proton therapy system. Baselines for 4 energies (70 MeV, 125 MeV, 150 MeV and 175 MeV) were determined (mean sigma) and tolerances were defined as the mean plus or minus 2 standard deviations.
Results: Using the mean sigma plus or minus two standard deviations successfully identified deviations greater than 10% on spot sigma when compared to the monthly quality assurance tests. On the daily test, a comparison of an absolute threshold (10 percent of the spot width) alone would not have been sensitive enough to detect the known deviation.
Conclusion: The Sphinx Compact, when used with statistically reasonable tolerances, is sensitive enough to identify spot sigma deviations that exceed the TG-224 recommendation of 10%, and can be reliably used for daily quality assurance tests.
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