SU‐E‐T‐537: A Dosimetric Study of Gafchromic EBT2 Film for Small Field Size Stereotactic Radiosurgery QA
Purpose: To perform a dosimetric study of Gafchromic EBT2 film for small field size SRS/SBRT QA. Methods: patient‐specific QA was performed using the Lucy phantom and doses were measured with EBT2 films and a PinPoint ion chamber. The films were scanned using Epson perfection V700 flatbed scanner. Open source program (ePin) software was employed to read the films using the red channel that converted the image pixel values into OD values. Changes in HD curves corresponding to different films and film orientations were investigated. Results: For small field size dose measurements, percent differences between calculated and measured doses for the ion chamber ranged from 3.83% to 0.2% for cone sizes 30 mm down to 10 mm, respectively. For the two smallest cones, 7.5 and 5 mm, the percent difference was beyond clinical tolerance (≥ 5%). For EBT2 films, the differences were less than 3% for cone sizes down to 10 mm, and the difference was less than 4% for cone sizes less than 10 mm. The variation in the HD curves for different films ranged from 1 % to 6%, and for different film orientations it ranged from 1% to 5%. Conclusions: EBT2 film gave better results for the small field sizes between 30 mm down to 10 mm than the ion chamber. For the field sizes less than 10mm, the current market available PinPoint ion chamber can not measure it while the EBT2 film has its advantage over ion chambers to measure absolute dose of these very small field sizes. The dose calibration curves were difference for different films and different film orientations. EBT2 film can measure the absolute small field size dose very accurately as long as proper calibration curves are used and proper quality control is performed.