Evaluation of electronic portal imaging device for missing tissue compensator design and verification

The purpose of this investigation is to determine if electronic portal imaging devices (EPIDs) can be used for the design and verification of compensating filters. In order to do this, we investigated the operating characteristics of a commercially available EPID and the variation in transmitted dose for various measurement situations. We performed four initial tests to determine the EPID response specific to compensator situations. The tests determined EPID response to variable patient SSDs, different gantry angles, positions of an inhomogeneity within a phantom, and the sensitivity variation of different parts of the imager. After these tests, we determined the attenuation functions relating EPID response to phantom thickness for various phantom materials. With these functions, we tested simple compensation situations to demonstrate that missing tissue compensators can both be designed and verified using EPIDs.