1. A method of electronic portal imaging (EPID) for radiation dosimetric measurement and quality assurance (QA) in radiation therapy, comprising: a. imaging a calibration photon beam and a treatment beam using an EPID imager, wherein said EPID imager is disposed between a plastic water build-up phantom and a plastic water back-scatter phantom, wherein said plastic water back-scatter phantom is thicker than said plastic water build-up phantom; b. correcting said EPID images using an off-axis correction map, wherein said off-axis correction takes into account a horn-shape of a photon fluence profile; c. deconvolving said corrected EPID images using a Monte Carlo simulated EPID response kernel, wherein said deconvolved corrected EPID images provide a primary photon fluence map; d. reconstructing a relative dose map using said primary photon fluence map convolving with a Monte Carlo simulated pencil-beam dose distribution kernel; and e. generating an absolute dose map using cross calibration to a 10×10 cm 2 square field, wherein said absolute dose map is compared with a planned dose distribution for QA analysis.