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An Improvement in IMRT QA Results and Beam Matching in LINACs Using Statistical Process Control


J Gagneur

J Gagneur*, G Ezzell, Mayo Clinic Arizona, Phoenix, AZ

WE-E-141-2 Wednesday 2:00PM - 3:50PM Room: 141

Purpose: he purpose of this study is to apply the principles of Statistical Process Control (SPC) in the context of patient specific IMRT QA to set clinic specific action limits and evaluate the impact of changes to the MLC calibrations on IMRT QA results.

Methods: Ten months of IMRT QA data with 247 unique QAs collected on three beam matched LINACS were retrospectively analyzed with a focus on the Gamma Pass Rate (GPR) and the average ratio between the measured and planned doses. Initial control charts and action limits were calculated. Based on this data, changes were made to the leaf gap parameter for the MLCs to improve the consistency between LINACS. This leaf gap parameter is tested monthly using a MLC sweep test. A follow up data set with 119 unique QAs was used to evaluate the impact of the leaf gap parameter change.

Results: The initial data average GPR was 98.6 % with an SPC action limit of 93.7%. The average ratio of doses was 1.003 with an upper action limit of 1.017 and a lower action limit of .989. The sweep test results for the LINACS were -1.8%, 0%, and +1.2% from nominal. After the adjustment of the leaf gap parameter all sweep test results were within .4% of nominal. Subsequently, the average GPR was 99.4 % with an SPC action limit of 97.3%. The average ratio of doses was 1.003 with an upper action limit of 1.017 and a lower action limit of .989.

Conclusion: Applying the principles of SPC to IMRT QA allowed small differences between closely matched LINACS to be identified and reduced. Ongoing analysis will monitor the process and be used to refine the clinical action limits for IMRT QA.

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