Evaluation of the Dose Response of Gafchromic EBT Film for High Energy Photon Beams

Abstract

Purpose: Radiochromic film has gained increasing popularity in medical applications in recent years. It has several advantageous features, including high spatial resolution, relatively low spectral sensitivity, near tissue equivalence, and it requires no special development procedure. This study aimed to evaluate the response of new type Gafchromic EBT film for high energy photon beams in clinical use. Material and Methods: The ISP Gafchromic EBT film (prototype A) and ELEKTA Precise LINAC were used in this study. Gafchromic EBT films were exposed to radiation of difference energies, dose rates, fractionation and to the ambient light respectively. After completely being exposed, we scanned the film with Radlink Laserport 16 scanner and Epson Perfection 4990 photo scanner in red, blue, green channel and gray scale respectively. The post-exposure density growth with varied time was also evaluated. The calibration curves for high energy photon beams were established. Results: The results showed that the effect of dose fractionation was insignificant, and there is no significant energy dependence on 4MV, 6MV and 10MV photon beams. The response using red light source of Epson scanner against corresponding delivered dose is higher than that of the green, blue channel, gray scale and Radlink LaserPor scanner at doses below 4 Gy. The gray scale model showed the lowest response in all scanners. It is interesting that the green light source would have the highest response when the dose was greater than 4 Gy. The post-exposure density growth test showed a slight increase between 1st to 5th day, at sixth day, an obvious decrease occur, then became stable between 7th to 14th day. However, the net OD was nonlinear with time and dose, continuous exposure to the room light for 24 hours resulted in a density change of about 0.117, which is equivalent to a radiation exposure of about 10 cGy. Conclusion: Gafchromic EBT film is a new type of film for radiotherapy dosimetry. Its response to different dose rates and dose fractionations were insignificant, but theresults of the effect for room light exposure were not as good ad expected. However, it is still a reliable dosimeter for clinical dosimetry. Our suggestion is to read the film at least 24 hours after irradiation, and scan the films with color scanner in red channel to obtain better response. Furthermore, the film should be kept properly when not in use to avoid other confoundedly influences.