Analysis of criteria applied to Tomotherapy DQA using statistical process control (SPC)

Abstract

This study aims to analyze the suitability of the criteria applied to the analysis of Tomotherapy delivery quality assurance (DQA) at our institution using statistical process control (SPC). DQA data were retrospectively analyzed for 36 head and neck (H&N), 57 abdomen, and 115 pelvic cases, measured over a 1-year period. ArcCHECK and A1SL ionization chambers were used for DQA measurement for the evaluation of gamma index and point dose error, respectively. Depending on the treatment site, the analysis parameters include the gamma pass rate (%GP) of 95% with 3%/3 mm gamma index and 10% threshold, and point dose error of 3%. To analyze the characteristics of the process over time, a process behavior chart of the measured data for each treatment site was plotted. It has a center line (CL), and lower and upper control limit (LCL and UCL) lines. Process capability analysis was performed using the process capability index (Cp ) and the process acceptability index (Cpk ) from the specification. For the CL and LCL of the pelvic case, the %GPs observed were 99.01 and 95.48. The point dose errors were 0.79 (CL), − 2.34 (LCL), and 3.93 (UCL), respectively. The %GP results for the abdomen case were CL 97.97 and LCL 90.90. The point dose error showed a CL of 1.34, LCL of 1.28, and UCL of 3.96. For H&N, the %GPs observed were 98.41 (CL) and 93.73 (LCL). The CL, LCL and UCL of point error were 1.80, − 0.87, and 4.47, respectively. The Cp values of %GP for the pelvis, abdomen, and H&N were 1.417, 0.708, and 1.069, and the corresponding Cpk values were 0.281, 0.288, and 0.341, respectively. The appropriate pass rate was different for each treatment site, and the distribution pattern was also different for each parameter, affecting the DQA results. Therefore, it was concluded that different evaluation criteria should be applied according to the results of the DQA depending on the site and the applied parameters.