Poster — Thur Eve — 51: An Investigation of Geometry Issues for EPID Dosimetry during Rotational IMRT

INTRODUCTION: Amorphous‐silicon electronic portal imaging devices (EPIDs) have been established as useful tools for dosimetry. To accurately reconstruct the patient dose delivered during rotational IMRT, one must acquire time‐resolved EPID images as a function of gantry‐angle. Dose reconstruction accuracy is directly impacted by the accuracy of the geometry of the imaging system, including the gantry‐angle readout (i.e. source geometry) and the EPID support‐arm sag (i.e. imager geometry). This work investigates these two factors. METHODS: The EPID support‐arm sag was investigated through measurements performed on Varian E‐arm and R‐arm models at two institutes and employing two different analysis methods. One method imaged an isocentric ball‐bearing whose position was tracked over all gantry‐angles. The second method involved analysing field edges to obtain the field centre location of all images. Gantry‐angle accuracy was examined by comparing the gantry‐angle indicated at the treatment console readout to the gantry‐angle written to the EPID DICOM header. We developed a method of measuring gantry‐angle directly from the gantry‐angle potentiometer. RESULTS: The E‐arm showed maximum displacement of roughly 0.6mm (cross‐plane) and 0.8mm (in‐plane). R‐arm results were significantly worse, estimated at 8.5mm (cross‐plane) and 5.0mm (in‐plane). Gantry‐angle analysis demonstrated approximately 2 degrees of uncertainty in the gantry‐angle contained in the EPID image. A direct measurement of the gantry angle potentiometer was demonstrated. CONCLUSIONS: Two main factors affecting patient dose reconstruction using EPID dosimetry have been investigated. EPID support‐arm sag can be measured (and corrected). Near real‐time gantry‐angle measurement can be performed through directly monitoring the potentiometer signal.