Dose and absorption spectra response of EBT2 Gafchromic film to high energy x-rays.

With new advancements in radiochromic film designs and sensitivity to suit different niche applications, EBT2 is the latest offering for the megavoltage radiotherapy market. New construction specifications including different physical construction and the use of a yellow coloured dye has provided the next generation radiochromic film for therapy applications. The film utilises the same active chemical for radiation measurement as its predecessor, EBT Gafchromic. Measurements have been performed using photo spectrometers to analyse the absorption spectra properties of this new EBT2 Gafchromic, radiochromic film. Results have shown that whilst the physical coloration or absorption spectra of the film, which turns yellow to green as compared to EBT film, (clear to blue) is significantly different due to the added yellow dye, the net change in absorption spectra properties for EBT2 are similar to the original EBT film. Absorption peaks are still located at 636nm and 585nm positions. A net optical density change of 0.590 +/- 0.020 (2SD) for a 1 Gy radiation absorbed dose using 6 MV x-rays when measured at the 636nm absorption peak was found. This is compared to 0.602 +/- 0.025 (2SD) for the original EBT film (2005 Batch) and 0.557 +/- 0.027 (2009 Batch) at the same absorption peak. The yellow dye and the new coating material produce a significantly different visible absorption spectra results for the EBT2 film compared to EBT at wavelengths especially below approximately 550nm. At wavelengths above 550nm differences in absolute OD are seen however, when dose analysis is performed at wavelengths above 550nm using net optical density changes, no significant variations are seen. If comparing results of the late production EBT to new production EBT2 film, net optical density variations of approximately 10 % to 15 % are seen. As all new film batches should be calibrated for sensitivity upon arrival this should not be of concern.