Usefulness of GafChromic EBT film for dosimetry beyond 8.0 Gy

The objective of this study was to extend the usable dose measurement range of Gafchromic EBT film (EBT). EBT has the advantages of high sensitivity and improved film uniformity up to 8.0 Gy over Gafchromic MD-V2-55 film (MD-V2-55). EBT samples were exposed from 0.3 to 80 Gy. Optical densities of the samples were measured using ES-2200 (Seiko Epson Corporation, Nagano, Japan). EBT has an absorption spectrum with a maximum absorption band centered at 630 nm. The wavelength position of a maximum absorption does not shift with increasing dose within usable dose range. High sensitivity measurement of density can be performed using measurement light of 630 nm. We designed a flat-bed document scanner with single emission spectrum centered at 630 nm (IR-4000, iMeasure, Inc.) for EBT. Optical densities of the samples were also measured using IR-4000. The optical density response characteristics obtained by the combinations of EBT/ES-2200 and EBT/IR-4000 were compared with those of MD-V2-55/ES-2200. As a result, optical densities of EBT increased from 8.0 to 80 Gy. From the optical densities the calibration curves of net optical density versus delivered dose for dosimetry could be obtained. The combinations of EBT/ES- 2200 and EBT/IR-4000 had higher sensitivity of densitometry than MD-V2-55/ES-2200. The sensitivity of EBT/IR-4000 was highest. rror of dose measurements up to 80 Gy is within about ±5%, ±8%, and ±4% for MD-V2-55/ES-2200, EBT/ES- 2200, and EBT/IR-4000, respectively. The IR-4000 with single emission spectrum can bring out the capacity of high sensitivity and accuracy of EBT up to 80 Gy.