Portal dosimetry correction method for validation of single isocenter VMAT plans for multiple brain metastases

Portal dosimetry is one option for verification of volumetric-modulated arc therapy (VMAT) planning for multiple brain metastases. However, due to the changing response of the portal imager with photon beam energy, the dose transmitted through closed multileaf collimator (MLC) leaves or narrow MLC gaps may be underestimated by the imager. We present a simple method for correcting for these effects that may be implemented within the Eclipse treatment planning system. We recalculated the predicted portal dose with and without this correction for 20 multiple brain met VMAT plans. Before the correction, 3/20 composite plan fields passed our standard quality assurance (QA) criteria (54/80 individual fields); the average gamma passing rate for the composite plans was 76.9 ± 16.6%, and the average gamma value across the composite plans was 0.67 ± 0.23. After correction, 20/20 composite plan fields passed the QA criteria (80/80 individual fields); the average gamma passing rate for composite plans was 99.2 ± 1.4%, the average gamma value across the composite plans was 0.33 ± 0.90. A measure of plan complexity, the average leaf pair opening could be correlated to the gamma analysis results for the uncorrected plans but not for the corrected plans.