Towards implementation of independent calculation-based dose verification in proton therapy

Background: Proton therapy offers precision in targeting tumors while minimizing damage to surrounding healthy tissues. Independent dose calculation is a secondary dose validation tool for patient-specific quality assurance (PSQA) using Monte Carlo simulations. Purpose: To implement independent dose calculation for PSQA in proton therapy and establish confidence limits, tolerance limits, and action limits. Methods and Materials: The point dose validation of independent dose calculation was performed by ionization chamber. The dose distributions between independent dose calculation and treatment planning calculation were compared in single and multiple energy plans. Fifty clinical plans covering five regions (head and neck, breast, chest, abdomen, and pelvis) were evaluated using the gamma analysis criteria of 3%, 3 mm, and 5%, 3 mm. Confidence limits, tolerance limits, and action limits were determined. Results: The dose differences between measurement and independent dose calculation for each energy level were within 1.0%. The results showing gamma passing rates of 99.4 ± 0.6% for single energy plans and over 98.0% for multiple energy plans. The average gamma passing rates for all treatment sites was 95.9 ± 2.7% with 3%/3 mm criteria, which increased to 97.9 ± 1.8% with 5%/3 mm criteria. The confidence limits, tolerance limits, and action limits showed 90.7%, 89.1%, and 85.4% with 3%/3 mm criteria and 94.3%, 93.2%, and 91.6% for 5%/3 mm criteria, respectively. Conclusions: The independent dose calculation could potentially be implemented in PSQA. It is a secondary dose validation method for busy clinic. Particular attention should be paid to a confidence limit, tolerance limit, and action limit for reliable treatments.