Method and Materials:Eight packets of films were exposed to 13.5cm ×13.5cm, 6MV radiation fields in a solid water phantom. Dose levels of 1.1, 3.2, 5.3, 7.4, and 9.5 Gy were delivered to five films in each packet. Films were scanned both before and after irradiation using an Epson flat‐bed scanner (24hr wait‐time for post‐irradiation coloration). Corresponding 2D dose distributions were measured with a detector‐array (MatriXX). Point dose comparisons were performed with an ion chamber. Digitized film images were registered to the 2D dose distribution to generate a correction map that compensated the scanner non‐uniform response as a function of dose. Optical density (OD) and net optical density (NetOD) values were calculated for all images. Dose response curves were established using mean values of a central 0.5cm × 0.5cm region‐of‐interest (ROI). Images were converted to dose, and error uncertainties (1SD) were measured in the central 8cm × 8cm ROI. Results: The overall dosimetric uncertainties (1SD) of the NetOD approach were 2.2%, 1.9%, and 3.5% for red, green, and blue channels, respectively. The corresponding uncertainties of OD were 2.7%, 3.1%, and 8.3%, respectively. For low dose range (<3 Gy), the green channel revealed higher uncertainty (SDgreen= 3.3%) than the red channel (SDred=2.6%). However, for high doses (3∼9 Gy), the green channel showed less variability (SDgreen=1.6%, SDred=2.9%). Minimum SDred and SDgreen were 1.6% at 5.3Gy and 1.3% at 7.4 Gy, respectively. Scanner non‐uniformity correction mitigated the irregular response of scanner detector elements observed initially. Conclusion: NetOD may be a more useful metric for benchmarking EBT2 than OD. We demonstrated that the lowest dose uncertainties were achieved using the red channel for low dose range, while the green channel was preferred for higher doses. Scanner non‐uniformity correction is necessary for higher precision dosimetry.

These findings have justified our patient localization tolerance levels of 2 mm translation and 1 degree rotation for spine SRS treatment.

ExacTrac system successfully corrected angular shifts using the virtual isocenter method in a rigid phantom setup. The sequence ofcouch correction did not influence the localization accuracy. Further patient study is warranted.

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.