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Conference Proceeding

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Medical Physics


Purpose: I t has been previously shown that utilizing a collimator angle of ±45° has demonstrated a reduction in MU and VMAT beam complexity. Studies have demonstrated this for VMAT prostate plans with small sample size. In this study we evaluate multiple collimator angles with a larger sample size and different treatment sites. Methods: A total of 17273 VMAT beams were evaluated with beam collimator rotations of 0°, ±5°, ±10°, ±15°, ±30°, ±45°, and ±90°. QA Gamma pass rates for our institution use a 3%/2mm criteria 10% threshold with 95% limit pass rate utilizing Sun Nuclear ArcCheck (helical diode array). Philips Pinnacle plans were evaluated across 8 Elekta Agility machines with energies of 6 and 10 MV. Complexity metric modulation complexity score (MCS) was calculated and averages were determined from Pinnacle plan. Results: In total 179 out of 17273 VMAT beams failed our criteria of 95% pass rate for patient specific QA. We observe highest percentage failure rate (2.3%) for ±10°collimator rotation, followed by 1.5% failure rate for 0°. The lowest observe failure rate was for collimator rotation of ±5° and ±45 which was 0.7%. Average MCS values were also determined for different collimator angles where ±90° demonstrated the most complex plan (MCS = 0.2017±0.0808) and10° demonstrated the least complex plan where MCS 0.2753±0.1125. Conclusion: This study demonstrates that collimator rotation affects VMAT beam complexity. Utilizing a collimator rotation of 10° showed lower MCS values compared to other collimator angles but did not correlate to lower PSQA failure rates (10° had highest failure rates at 2.3%). Lowest PSQA failure rates were seen on ±5° and ±45. A further evaluation will need to performed by breaking up the data into different sites and evaluating based on plan quality.





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American Association of Physicists in Medicine 65th Annual Meting & Exhibition, July 23-27, 2023, Houston, TX