Room: Exhibit Hall
Purpose: To report and analyze observations of irradiation leakage outside of PTV in patient VMAT-specific-QA.
Methods: VMAT patient QA plans were verified for dosimetric comparison between planning in Eclipse-v13.7 and measurement on Varian (21-iX) with two-dimensional (2D) array device. The measurement was accomplished by a-single-field delivery utilizing the MapCheck-2 mounted to the gantry of linac. Gamma analysis with SNC-Patient-software was employed to analyze measurements in absolute-dose with a criterion of 3%-3mm and threshold of 10% for each verification field.
Results: The planned and measured dose distribution agreed with each other over 99% for gamma analysis. However, the doses deposited outside of PTV was observed visually for both planned and measured doses. The irradiation leakage to normal tissue was found to be as high as 33% of the maximum dose in the cross profile for QA plan, as well as for the original patient plan. The investigation revealed leakage caused from the rounded end of MLC with the leaf pair parked inside the jaw field, but outside the PTV. The leaf pairs appeared open outside PTV took part in the irradiation of the target. The contribution to leakage leaf pairs were not automatically closed and remained behind the secondary jaws. All patient plans that observed leakage were re-planed and re-measured with successful improvements. Methods were developed to overcome the leakage by tracking jaw closer to PTV boundaries or re-optimize the treatment planning distribution dependent upon magnitude of the leakage.
Conclusion: This work suggests the high pass rates in VMAT-QA dose gamma analysis is necessary, but it is not sufficient to detect potential leakage due to MLC nature and performance. A pre-patient measurement using a radiation detector device is necessary to ensure planning accuracy. The modeling in leaf positions can result in inaccurate dose distribution during optimization in TPS.