Room: AAPM ePoster Library
Purpose: purpose of this study is to investigate low pass rates in the VMAT patient-specific QA and identify possible causes,such as machine performance and or errors in QA process.
Methods: ensure the treatment delivery is accurate and per approved plan,all parameters for modulated plans are QA’ed before patient treatment.Plans are computed in Pinnacle TPS and recomputed on Arccheck digital phantom for dose analysis.Measured and calculated doses are compared for gamma analysis using the SNC Patient software.Per department policy, 90% pass rate is required at 2%/2mm and 20% threshold with no measurement uncertainty criteria applied.The presented cases were investigated for passing rate below 90% using the above criteria.Possible changes in MLC leaf bank calibration, beam tuning and output fluctuations were investigated.
Results: clinical VMAT plans were evaluated.Initial pass rate average was 84.4% with a range of 77.3%-89.7%.Adjusting for daily output variation improved pass rates by an average of 1.7%(0%-3.2%) with a pass rate range of 78.4%-92.9%.MLC bank recalibration improved rates by 3.8%(-0.9%_+12.4%) with a pass rate range of 81.3%-91.5%,and adjusting for daily output variation improved these rates with a range of 82.1%-93.5%.The beam tuning improved pass rates another 2.6%(-0.2_+9.4%) with a pass rate range of 81.5%-95% and adjusting these results for daily output fluctuation improved these pass rates with a range of 85.4%-95.8%. As an example,one patient started with 77.3% passing, which rose to 78.4% with output correction,then 90.8% after MLC correction,and then 92.5% with beam tuning. Plan complexity calculations will also be carried out to investigate the impact on low pass rates.
Conclusion: the trend of QA pass rates in the clinic per AAPM 218 can help identify Linac performance issues when the trend does not follow set bounds.From our subset of studied plans we found that MLC and beam output calibration have a significant impact on QA pass rates.