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Independent Validation of Machine Performance Check (MPC) for a Prototype Linac

Y Li

Y Li12, T Netherton12 , P Nitsch1 , P Balter1 , S Gao1 , A Klopp1 , L Court1* , (1) The University of Texas MD Anderson Cancer Center, Houston, TX, (2) The University of Texas MD Anderson Cancer Center UTHealth Graduate School of Biomedical Sciences, Houston, TX


TH-EF-FS1-9 (Thursday, August 3, 2017) 1:00 PM - 3:00 PM Room: Four Seasons 1

Purpose: In this study we independently evaluated the capability and accuracy of MPC to detect potential changes of beam and mechanical deviation of a prototype linac.

Methods: The redesigned MPC is an automated set of tests which uses a specific phantom and MV-imaging to verify beam and mechanic performance of a prototype linac. The following MPC tests were independently evaluated: (1) Beam output and uniformity: Solid water slabs totally or partially inserted between the beam source and the MV imager. IC Profiler measurements were used as “gold-standard”. (2) Absolute gantry angle: Linac’s gantry angle was intentionally mis-calibrated for MPC evaluation. (3) Relative gantry rotation: A rotating stage was used to rotate the MPC phantom during MPC tests. (4) Absolute couch positions: MPC phantom was intentionally displaced in different directions. (5) Relative couch shifts: A linear motion stage was used to move the MPC phantom to simulate incorrect relative shifts. (6) MLC positions: One MLC was intentionally mis-calibrated to simulate MLC mis-position.

Results: The average difference in measured output between IC profiler and MPC was 2.25% (0.66-7.38%) when the beam output was intentionally changed by 0.6-15.4%. The changes in beam symmetry were able to be detected by MPC uniformity test, which failed when changes in symmetry exceeded 1.4%. MPC was able to detect an absolute gantry angle deviation as small as 0.3⁰, and a relative gantry shift as small as 0.15⁰. The accuracy of MPC absolute and relative couch position were measured within 0.22mm and 0.45mm respectively. The average difference between known MLC mis-calibrated position value and measurement was 0.05mm.

Conclusion: We independently showed that MPC has sufficient accuracy and sensitivity in testing both dosimetric and mechanical parameters. It has potential to be used as daily QA for this prototype linac.

Funding Support, Disclosures, and Conflict of Interest: This study was funded by Varian Medical Systems.

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