Accurate dosimetry with GafChromic™ EBT film of a 6MV photon beam in water: What level is achievable?

Abstract

This paper focuses on the accuracy, in absolute dose measurements, with GafChromicTM EBT film achievable in water for a 6 MV photon beam up to a dose of 2.3 Gy. Motivation is to get an absolute dose detection system to measure up dose distributions in a (water) phantom, to check dose calculations. An Epson 1680 color (red green blue) transmission flatbed scanner has been used as film scanning system, where the response in the red color channel has been extracted and used for the analyses. The influence of the flatbed film scanner on the film based dose detection process was investigated. The scan procedure has been optimized; i.e. for instance a lateral correction curve was derived to correct the scan value, up to 10%, as a function of optical density and lateral position. Sensitometric curves of different film batches were evaluated in portrait and landscape scan mode. Between various batches important variations in sensitometric curve were observed. Energy dependence of the film is negligible, while a slight variation in dose response is observed for very large angles between film surface and incident photon beam. Improved accuracy in absolute dose detection can be obtained by repetition of a film measurement to tackle at least the inherent presence of film inhomogeneous construction. We state that the overall uncertainty is random in absolute EBT film dose detection and of the order of 1.3% (1 SD) under the condition that the film is scanned in a limited centered area on the scanner and at least two films have been applied. At last we advise to check a new film batch on its characteristics compared to available information, before using that batch for absolute dose measurements.

Dosimetric characterization and use of GAFCHROMIC EBT3 film for IMRT dose verification

“…The overall accuracy of EBT3 film measurements was derived using the method proposed by van Battum et al ( 23 ) that takes into account the most pronounced sources of uncertainties in dose determination (scanner, lateral correction, fit accuracy, intrabatch variation, background, intrinsic film inhomogeneity) and using error propagation analysis. An overall uncertainty of 1.7% was observed.…”

Errors introduced by dose scaling for relative dosimetry

“…Our results showed that the inter‐batch variation of the sensitivity can lead to easily 5% or larger error when the dose scaling is applied to the measured doses. On the other hand, the intra‐batch (or film‐to‐film) variation of the film sensitivity is smaller than 1%, and the uncertainty in the optical density is about 1% 6 , 9 . Hence, the errors with dose scaling procedure are smaller than those expected for the inter‐batch variation, but a potentially large error should be anticipated.…”

Dosimetric evaluation of a novel high dose rate (HDR) intraluminal / interstitial brachytherapy applicator for gastrointestinal and bladder cancers

“…GAFCHROMIC EBT shows a nonlinear relation between the net optical density and dose. ( 24 ) There are several studies which fit a third order polynomial to the calibration data to convert NOD to dose. ( 21 , 25 , 26 ) Using this method, an increasing incline was obtained in our experiment in higher doses, which is in contrast with the saturation effect in the films.…”