Sci‐Thur PM Therapy‐02: Accounting for the off‐axis detector response for improved a‐Si EPID dosimetry

In portal imaging applications using an a‐Si electronic portal imaging device (EPID), a calibration procedure must be performed before clinical images may be acquired. This calibration procedure incorporates a dark‐field image acquisition and a flood‐field image acquisition. Every clinical image is corrected by subtraction of the dark field image and then division by the flood‐field image. The flood‐field image exhibits two main features, the pixel‐to‐pixel sensitivity (present due to inherent limitations in the manufacturing process), and the beam profile shape as measured by the detector (a function of off‐axis distance, ie. energy spectra and incident fluence). While it is necessary to correct for the pixel‐to‐pixel sensitivity, the beam profile shape as measured by the detector is also removed by this process, which is undesirable for dosimetric applications. This work develops an approach which allows for the pixel‐to‐pixel sensitivity correction to be made, while still preserving the inherent response of the detector to the incident beam.The approach consists of directly measuring the beam profile shape independently from the pixel‐to‐pixel sensitivity. This profile information may then be reintroduced into the clinical image. The beam profile measurement is achieved by building a small “point” detector using materials that are analogous to the clinical EPID. The point detector is scanned in two dimensions across the open radiation flood‐field at the same source‐to‐detector distance used for clinical flood‐field acquisition. Clinical images are then multiplied by this acquired 2D profile (normalized to central axis) to reintroduce the beam profile shape.