SU‐E‐T‐128: Characterizing the Uniformity of Gafchromic EBT2 Film Response in a Cobalt‐60 Field

Purpose: The goal of this work is to evaluate the uniformity of International Specialty Products Gafchromic EBT2 film with a high‐resolution metric. The effect on uniformity of the marker dye (blue‐channel) correction and time post‐exposure are investigated. Methods: Film uniformity was quantified by using EBT2 and Kodak EDR2 film to measure the flatness of a cobalt‐60 field. EBT2 films were digitized 24, 48, and 72 hours post‐exposure and analyzed using three Methods: the manufacturer‐recommended blue‐channel correction, an in‐house blue‐channel correction, and a net optical density (OD) measurement in the red color channel. The field flatness was calculated from the horizontal and vertical dose profiles taken through the center of the cobalt‐60 field for each analysis method and compared to the EDR2 results. Results: The blue‐channel correction methods improved the flatness measured with EBT2 film by up to 1% over the net OD method. Additionally, the percent standard deviations of the flatness measurements with the blue‐channel correction methods were up to 0.7% (1‐sigma) lower than the net OD method. The flatness measured with EDR2 film was 1.4% to 1.8% better than the flatness measured with EBT2 film using the net OD method. The flatness did not exhibit any unidirectional change with time and showed a maximum change of 0.4%. Conclusions: Using field flatness as a metric for response uniformity, EBT2 film analyzed with the net OD method demonstrates substantial non‐uniformities relative to EDR2 film. The blue‐channel correction considerably improves the uniformity of EBT2 response and reduces the film‐to‐film response variation relative to the net OD method. The uniformity of EBT2 is independent of post‐exposure time. The blue‐channel correction can be used to reduce uncertainty in relative dosimetric measurements with EBT2 film.