An Adaptive Method for Recalibrating Gafchromic EBT3 Film

Ashland Inc. EBT3 film has high 2D dosimetry resolution and is self-developing so it is widely used as a dosimetry quality assurance tool. However, the film must be calibrated every half year or three months because the unexposed film slowly develops due to the aging effect. A new calibration method is proposed that uses an adaptive power function and no dose need be delivered to allow recalibration. EBT3 film is calibrated separately on different dates over several months. All calibrations use the one red-channel after three-channel method (Chang et al., 2015). The dose-dependent optical density (OD) for the red channel is fitted to the delivered dose using a power function with a constant fitting parameter. Instead of a general recalibration, the film-dose is calculated using all of the fitting parameters for the first calibration and by changing the constant parameter using a fresh measured background OD. The film-dose that is calculated using the proposed adaptive method is compared with the administered dose and the percentage difference is compared with that for a new set of recalibrated fitting parameters and that for all of the first fitting parameters. The results demonstrate that the difference between the film-dose and the dose that is delivered using the adaptive calibration method is less than 3%. However, if the first calibration fitting parameters are used, the percentage difference is more than 50% for a delivered dose of around 50 cGy at a calibration date of eight months after the first calibration. The film-dose is calculated using the first calibrated equation, but using a new measured background OD, so the constant fitting parameter is changed, rather than recalibrating the film. To precisely calculate the film-dose, the equation must be adapted for each individual film. The proposed method is convenient, rapid and cost effective because recalibration does not require the exposure of film. This new technique is a practical method for clinical use.