SU-G-TeP4-01: A High Resolution Pre-Treatment VMAT QA Technique Based On EPID for Single Isocenter Multiple Mets Stereotactic Radiosurgery.

PURPOSE: For multiple mets radiosurgery, it is advantageous to use VMAT with a single isocenter for the ease in planning and efficiency in delivery. Insightful pre-treatment QA for these cases is difficult to obtain because (1) target size is often small and the same order of magnitude as the detector size for commercial array based VMAT pre-treatment QA devices; and furthermore (2) targets are located away from central axis where the array detectors are located. In this study, we present and validate an in-house developed VMAT QA technique that is capable of addressing these challenges.

METHODS: The VMAT plan is delivered on a Varian Truebeam STX with HDMLC. The MV cine images are converted to portal dose (PD) after calibration. The gantry and MU index encoded in each image are used to extract a sub-arc from the plan to compute corresponding PD. A 3D matrix is constructed with the 3rd axis representing gantry angle for subsequent γ-analysis. The EPID has a resolution of 0.8×0.8 mm(2) and field size up to 40×30 cm(2) . Measurements were performed for 21 arcs from 4 patients. Average(range) number of mets is 7(4-9), with volume of 0.36(0.01-1.39)cc. Varian's integrated PD was compared.

RESULTS: In PD analysis with 3%, 3mm, and 5% in threshold, pass rate of γ>1.0 is (99.6±0.3)%. Our EPID 3D analysis has the pass rate of (98.8±0.9)%, with additional criteria of 3° in angle-to-agreement. Projecting the 3D matrix along each axis, our analysis is reduced to 3 2D analyses, with one mimicking PD analysis at (99.0±0.9)%. Results on other planes are (99.5±0.4)% and (99.5±0.4)%, respectively. Additional criteria can be chosen to diagnose potential treatment abnormalities.

CONCLUSION: We have developed an efficient EPID-based QA technique for VMAT that has sufficient spatial and gantry angular resolution for use with single isocenter VMAT radiosurgery of multiple mets.