A Generalized Strategy for 3D Dose Verification of IMRT/VMAT Using EPID-measured Transit Images

The aim in this study is to develop a generalized strategy for 3D dose verification of IMRT and VMAT planes using EPID transit images in combination with Monte Carlo (MC) simulations. An EPID-based dosimetric verification procedure was developed to convert EPID-measured transit images into 2D exit photon fluence by de-convoluting with the MC-simulated EPID response kernels. The present scatter from the phantom to the EPID was iteratively corrected by using a series of pencil beam scatter kernels derived from MC simulations. The primary fluence is therefore yielded by subtracting the corrected scatter from the total reconstructed exit fluence and used to reconstruct the dose distribution in multiple 2D planes parallel to the EPID by convoluting with the pencil beam deposition kernels. After summing up all the reconstructed 2D dose planes, the 3D dose distribution is obtained. The EPID-based dosimetric system was validated using 6 MV photon beam available from Varian TrueBeam STXTM. The results show that the EPID-based dosimetry system developed in this study is an accurate and robust tool for dose verification of IMRT/VMAT plans.