Site-Specific Assessment of Statistical Process Control to Set Tolerance and Action Limits for Patient-Specific Quality Assurance in RapidArc Treatment Delivery.

Background: Patient-Specific Quality Assurance (PSQA) is essential in radiotherapy to ensure accurate treatment delivery, particularly with advanced treatment planning techniques like RapidArc (RA). Objective: The present study aimed to assess the use of Statistical Process Control (SPC) to evaluate Tolerance Limits (TL) and Action Limits (AL) in PSQA for various gamma criteria across different anatomical sites. Material and Methods: In this analytical study, RA treatment verification plans for brain (25), head and neck (50), thorax (25), and pelvis (50) were analyzed using an EPID to establish the Lower Control Limit (LCL). Gamma criteria (3%/3 mm, 3%/2 mm, 2%/3 mm, and 2%/2 mm) were evaluated, with the first ten samples used to calculate Individual Moving Range (I-MR) charts for TL and AL. Exponentially Weighted Moving Average (EWMA) and Cumulative Sum (CUSUM) charts were employed to detect control drifts. Results: For the head and neck site, LCLs from I-MR charts for global gamma were from 96.82 (3%/3 mm) to 89.42 (2%/2 mm), and for local gamma, from 91.40 (3%/3 mm) to 83.06 (2%/2 mm). The brain site showed similar agreement, while the pelvis and thorax sites had LCLs of 94.84 and 94.73 for 3%/3 mm. EWMA and CUSUM charts revealed that most control charts stayed within TL, except for the stringent 2%/2 mm criterion. AL for 3%/3 mm were 96.35, 92.85, 95.77, and 92.34 for head and neck, pelvis, brain, and thorax, respectively. Conclusion: I-MR, EWMA, and CUSUM charts are effective for establishing and monitoring TL and AL for RA-based PSQA, with site-specific limits required based on gamma criteria and measurement device.