SU‐E‐T‐138: Evaluation of CAFCHROMIC® EBT2 Films as An Absolute and Relative Dosimeters in Small Field Dosimetry

Purpose: The accuracy of second generation EBT film (EBT2) in commissioning of small fields for stereotactic radiosurgery (SRS) has yet to be demonstrated. We evaluate EBT2 films against a diode in a set of small circular fields collimated by cones. We investigate the challenges of EBT2 films for small field dosimetry and evaluate its accuracy as an absolute or relative dosimeter against diode measurements. Methods: Output factors (OFs), central‐axis percentage depth dose curves (PDDs) and cross‐beam profiles from a set of 9 Brainlab® SRS cones, with diameters ranging from 5 to 30 mm, were measured with a Scanditronix stereostatic diode(SFD) on a Varian Trilogy Linac. Measurements were repeated using EBT2 films in a Solid Water phantom. Blue‐color channel correction, as recommended by the vendor, was also investigated for reducing the effect of film inhomogeneity. Results: The OFs with EBT2 and diode agree within 1.1%, on average, for cone sizes 10 mm and above. SFD measures higher OFs than EBT2 for cones smaller than 10mm, likely due to the water nonequivalence of the silicon material in the diode detector which effectively reduces lateral electronic disequilibrium in small fields. Profiles and PDDs between EBT2 and diode match closely with each other. Film measurements were sometimes irregular due to non‐homogeneous film response. Blue‐color channel correction did not mitigate the film inhomogeneity. Conclusions: This study demonstrates that EBT2 films can be applied as absolute and relative dosimeters for measuring dose in small fields. Because EBT2 are more water‐equivalent than diode, it is likely that OFs measured by EBT2 in cones smaller than 10 mm are closer to truth. Inhomogeneity of the EBT2 films needs to be carefully evaluated before clinical use and the blue‐color channel correction is not recommended.