An electronic portal imaging device as a physics tool.

An electronic portal imaging device (EPID) can be used not only to acquire megavoltage patient images but also to measure certain radiation beam parameters of the linear accelerator. EPID images can be used to verify field junctions, center of collimator rotation, or radiation vs. light field coincidence. If the EPID images are calibrated in terms of dose rate, an EPID can be applied to beam penumbra measurement, collimator transmission determination, or compensator verification. Beam parameters measured with EPIDs are in close agreement with those measured with film or ionization chamber, making EPIDs reliable physics tools for quality control of various beam parameters in radiotherapy.