SU‐F‐T‐271: Comparing IMRT QA Pass Rates Before and After MLC Calibration

Abstract

Purpose: To compare IMRT QA pass rates before and after an in‐house MLC leaf calibration procedure. Methods: The MLC leaves and backup jaws on four Elekta linear accelerators with MLCi2 heads were calibrated using the EPID‐based RIT Hancock Test as the means for evaluation. The MLCs were considered to be successfully calibrated when they could pass the Hancock Test with criteria of 1 mm jaw position tolerance, and 1 mm leaf position tolerance. IMRT QA results were collected pre‐ and postcalibration and analyzed using gamma analysis with 3%/3mm DTA criteria. AAPM TG‐119 test plans were also compared pre‐ and post‐calibration, at both 2%/2mm DTA and 3%/3mm DTA. Results: A weighted average was performed on the results for all four linear accelerators. The pre‐calibration IMRT QA pass rate was 98.3 ± 0.1%, compared with the post‐calibration pass rate of 98.5 ± 0.1%. The TG‐119 test plan results showed more of an improvement, particularly at the 2%/2mm criteria. The averaged results were 89.1% pre and 96.1% post for the C‐shape plan, 94.8% pre and 97.1% post for the multi‐target plan, 98.6% pre and 99.7% post for the prostate plan, 94.7% pre and 94.8% post for the head/neck plan. Conclusion: The patient QA results did not show statistically significant improvement at the 3%/3mm DTA criteria after the MLC calibration procedure. However, the TG‐119 test cases did show significant improvement at the 2%/2mm level.