The amorphous-silicon based design of electronic portal imaging device (a-Si EPID) is commonly available on medical linear accelerators, and thus presents enormous potential as a radiotherapy dosimetry tool. One of the recognized technical challenges with using the device is it’s lack of water equivalency when measuring dose. In this work a radiation transport model, previously used to predict dose to the phosphor of an a-Si EPID, was modified to predict dose to a water-equivalent planar detector as well as to a 3D water-tank. Initial testing was performed for a 6MV beam using a variety of simple square fields and clinically relevant intensity modulated fields. Using a 2% criterion, the predicted versus measured image comparisons had pass rates between 95.9-98.2% for the square fields, and 89.2-96.3% for the modulated fields. The predicted 3D dose distribution showed a percentage depth dose agreement within 1% of that measured in a water tank (beyond 5 mm depth). These initial validation results provide confidence that the radiation transport model could be used for water-equivalent dosimetric applications in clinical radiotherapy.