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Statistical Process Control and Process Capability Analysis for Non-Normal VMAT Patient-Specific QA Processes

Guangjun Li, Qing Xiao, Sen Bai*. Department of Radiation Oncology, Cancer Center and State Key Laboratory of Biotherapy, West China Hospital, Sichuan University, Chengdu, Sichuan 610041, China.


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

The application of statistical process control (SPC) to IMRT/VMAT patient-specific delivery quality assurance (DQA) programs was recommended by the American Association of Physics in Medicine, but a comprehensive analysis of DQA processes with non-normal distributions is lacking. This study investigates SPC and process capability analysis (PCA) methods for non-normal IMRT/VMAT DQA.

1119 VMAT DQAs were performed on three beam-matched linear accelerators (linacs), using gamma analysis. The DQA process distributions was tested for normality using Anderson-Darling statistic. The control charts for each VMAT DQA process were obtained using three non-normal-based methods and compared with the conventional normal-based method. The C_pk indices of each DQA process were calculated using two transformation methods and compared with the conventional method. The performances of the three linacs were assessed using SPC and PCA.

All three processes were non-normal (p-values < 0.005). For each linac, the obtained lower control limit (LCL) values were very close, with a maximum difference of 0.9%. The false alarm rates of the DQA process from the conventional Shewhart control chart were 0.83%, 3.77%, and 4.95%, for linac1, linac2, and linac3, respectively. For each linac, the C_pk indices obtained from two transformation methods were almost equal (maximum difference 0.11). However, the C_pk index calculated by the conventional method was considerably higher, with minimum differences of 0.59, 0.87, and 1.49 for linac1, linac2, and linac3, respectively. The process capabilities of the three beam-matched linacs were at different levels.

Applying the conventional Shewhart control charts for SPC to non-normal VMAT patient-specific DQA processes overstates the false alarm rate. The C_pk index calculated using the conventional method overestimates the process capability. Instead, non-normal-based SPC and PCA methods are more reliable and accurate in non-normal DQA processes. SPC and PCA are useful tools for assessing the performance of beam-matched linacs.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by the National Natural Science Foundation of China (Grant No. 81472807).


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