Amorphous silicon electronic portal imaging devices (EPIDs) have been investigated and used for dosimetry in radiotherapy for several years. The presence of a phosphor scintillator layer in the structure of these EPIDs has made them sensitive to low-energy scattered and backscattered radiation. In this study, the backscattered radiation from the walls, ceiling, and floor of a linac bunker has been investigated as a possible source of inaccuracy in EPID dosimetry. EPID images acquired in integrated mode at discrete gantry angles and cine images taken during arcs were used with different field setups (18 × 18 and 10 × 10 cm2 open square fields at 150 and 105 cm source-to-detector distances) to compare the EPID response at different gantry angles. A sliding gap and a dynamic head-and-neck IMRT field and a square field with a 15 cm thick cylindrical phantom in the beam were also investigated using integrated EPID images at several gantry angles. The contribution of linac output variations at different angles was evaluated using a 2D array of ion chambers. In addition, a portable brick wall was moved to different distances from the EPID to check the effect at a single angle. The results showed an agreement of within 0.1% between the arc mode and gantry-static mode measurements, and the variation of EPID response during gantry rotation was about 1% in all measurement conditions.