Poster - Thur Eve - 01: Development of simple and fast EBT2 film calibration procedure using PDD table

Standard calibration procedure for EBT films is laborious and time-consuming. The objective of this work was to develop a simple and fast approach of EBT2 film calibration using PDD tables. EBT2 sheet is cut into 3 stripes of 5×25.5cm² . The strips were exposed to dose of 600, 200 and 70cGy at d_max each while placed horizontally in the middle of a 30×30×30cm³ solid water phantom. Varian 21EX 6MV 10×10cm² beam was used with the gantry rotated to 90° and SSD of 100cm to the phantom surface. After at least 24 hours, the films were digitized with flatbed scanner (Epson10000XL), according to a modified ISP scanning protocol. All images were analysed using an in-house Matlab code and ImageJ software. The net-optical densities against depths in the solid phantom were calibrated using PDD tables measured with ionization chamber for same machine. For verification, another calibration curve was generated for the same film batch following the same calibration protocol. Seven pieces of films were exposed to known doses and these doses were reconstructed using two derived calibration curves. The proposed approach was 3.6 times faster than the standard considering the number of films used in each methods, 3 stripes compared to11 pieces. The mean relative dose difference calculated for these films using the PDD calibration and the standard methods was 1.0±1.2% and 0.5±2.2% with maximum relative differences of 3.0% and 4.7% respectively. Our results show that PDD calibration approach is much easier, faster and predicts dose more reproducibly and accurately than the standard approach.