Density response characteristics of GafChromic EBT dosimetry film using ultraviolet light exposure

In order to evaluate the performance of radiochromic films, irradiation of the known dosage of the high energy x-rays to films by the radiotherapy system is essential. The evaluation examination must be performed in addition to consultation hours, an operation engineer's participation is required, and the problem of costs, such as electricity cost and overtime pay, also occurs. In this study, we examined whether the performance of radiochromic film could be evaluated easily in a laboratory by ultraviolet light exposure to radiochromic film and measuring the density, without using a radiation therapy system. The radiochromic film model, GAFCHROMIC EBT dosimetry film (International Specialty Products) was used. As a source of ultraviolet light, the fluorescent light lamp (Black light, FL20 S-BLB-A, Toshiba) with a peak wavelength of 352 nm was used. In order to connect the data of high energy x-rays irradiation and ultraviolet light exposure, the conversion factor to the absorbed dose of the ultraviolet light exposure time is required. The data of the absorbed dose - density by high energy x-rays irradiation was prepared. The data of the exposure time - optical density by ultraviolet light exposure was also prepared. Curve fitting to them was performed by a fitting model. The four-parameter logistic-log model was employed as a curve fitter. The conversion factor from exposure time to absorbed dose was calculated from comparison of two fitting curves, and the value was about 925 seconds/Gy. The ultraviolet light exposure data of the calculated absorbed dose by the conversion factor - optical density showed good conformity with the data of high energy x-rays irradiation. This result shows that the density response curve of EBT equivalent to high energy x-rays irradiation was able to be obtained in a laboratory without radiological protection.