SU-E-T-69: A Novel Method Using Agfa-Kodak- Computer Radiography System for Routine Quality Assurance Tests on Linear Accelerators

Abstract

Purpose: Since conventional radiographic films are becoming obsolete, we have developed a novel technique utilizing Computer Radiography (CR) plates to perform routine Linac quality assurance checks. Methods: A photostimulable CR plate can detect both direct and indirect ionization radiation, and is even sensitive to photons in the visible range. A technique involving optical bleaching of radiation field on CR plate is being established as the calibration tool for performing routine quality checks such as: light field radiation congruence, symmetric/asymmetric jaw positioning. The CR plate is removed from its cassette with minimal background light and is exposed to a known field size of 6/18 MV photons in air at a fixedsource to surface distance. The jaws are then moved in to form a smaller field size, and light field is then shone on the pre exposed plate. This causes photo bleaching of radiation field proportional to the jaw settings and thereby providing a suitable surrogate to establish the light and radiation coincidence. The plate is then read utilizing a Kodak (ACR 2000i) scanner which reads the photo stimulated luminescence and converts the signal into a digital image pixel map. Analysis using a MATLAB-based algorithm was done to compute the dose gradient at the junction of the light and radiation field. The technique has been tested against conventional methods of usingfilm and graph paper. Results: All light and radiation field measurements using the CR plate were within 1 mm from measurements. The method has been verified for its reproducibility and estimated uncertainty involving software based analysis has been found to be within +/− 0.2%. Conclusions: A method to efficiently, accurately and reproducibly perform routine QA has been developed. The technique has yielded a reproducible method to quantify the congruence between the light and the radiation field.