Evaluation of Gafchromic XRQA2 Film for Varian OBI KV Scanner Dose Measurements
A Turner*, J Little, University of Arizona, TUCSON, AZMO-B-Salon EF-11 Monday 10:00:00 AM - 12:00:00 PM Room: Salon EF
Purpose:: To assess the magnitude of energy dependence of Gafchromic XRQA2 for dose measurements performed using the Varian On-Board Imager (OBI) kV imaging system.
Methods:All measurements were obtained with the OBI kV source at 120kVp with a source to isocenter distance of 100cm, 50cm squared field, and the full bowtie filter. Reference measurements were obtained in air with an approximately energy-independent CC13 ionization chamber. Exposure profiles were measured across the bowtie filter over the entire isocenter plane, with increments of 2.5 to 5cm, for longitudinal positions ranging from negative 20cm to positive 20cm. Film was exposed with a source to film distance of 40.6cm, digitized, and projected to the isocenter plane. Film measurements were corrected for background and noise and the red channel was extracted. Both film and ionization chamber measurements were normalized to the respective isocenter value. The normalized film and exposure profiles were compared for each longitudinal position measured with the ionization chamber.
Results:Examination of the normalized profiles indicates that there are considerable differences between the film and ionization chamber measurements across the bowtie filter plane regardless of longitudinal position. The average normalized difference across all measurements was 33.4% plus minus 13.6%. In general, differences increased with radial distance from the isocenter until approximately 7.5 to 10cm and then gradually decreased.
Conclusion:Since ionization chambers are approximately energy independent, the differences illustrate that Gafchromic XRQA2 film may exhibit substantial energy dependence for the range of beam qualities across the isocenter plane for Varian OBI kV scanners. The variation in beam qualities is due primarily to the varying thickness of the bowtie filter, resulting in varying levels of beam hardening. It may be possible to correct film measurements for controlled measurement setups. Future work will be done to evaluate different tube voltage levels, bowtie filter settings, and film processing protocols.