Sci—Thur AM: YIS ‐ 06: An EPID‐based 3D patient dose verification method for SBRT‐VMAT delivery

Abstract

Purpose:Stereotactic body radiation therapy (SBRT) delivered via volumetric modulated arc therapy (VMAT) can strongly benefit from an in vivo patient dose verification due to the large doses per fraction. Electronic portal imaging devices (EPIDs) can be utilized as a patient dose dosimeter. In this work we present a physics‐based model which utilizes on‐treatment EPID images to reconstruct the dose delivered to an anthropomorphic phantom during SBRT‐VMAT delivery.Methods:An SBRT linac beam was modeled using Monte Carlo methods and verified with measured data. Our dose reconstruction model back‐projects EPID measured focal fluence upstream of the patient and adds a predicted extra‐focal fluence component. This fluence is forward projected onto the patient's density matrix and convolved with dose kernels to calculate dose. The model was validated for two prostate, three lung, and two spine SBRT‐VMAT treatments. Results were compared to the treatment planning system's calculation.Results:2%/2 mm chi comparison calculations gave pass rates for the whole volume, infield, and high dose region respectively, and no lower than: 98%, 95%, 99% for the prostate plans, 99%, 92%, 85% for the lung plans, and 91%, 85%, 81% for the spine plans. A 3%/3mm calculation gave pass rates no lower than 99%, 94%, and 90% for all dose regions for the prostate, lung, and spine respectively.Conclusions:We have developed a physics‐based model which calculates delivered dose to phantom (or patient) for SBRT‐VMAT delivery using on treatment EPID images. The accuracy of the results has allowed us to test this model clinically.