SU-E-T-185: Time-Dependent Calibration Correction for EBT Film Dosimetry.
PURPOSE
The aim of this study is to simplify EBT film-based dosimetry calibration. In addition, two different approaches are proposed for film and scanner non-uniformity reduction.


METHODS
EBT films, EBT3 and EBT2 were exposed under a calibration plan using 6X flattening-filter-free (FFF) photon beams. In method 1, pre-irradiation scans were carried out and subtracted from post-irradiation scans to calculate net-optical density (net OD), removing non-uniformity from both scanner and film. In method 2, two unexposed films were independently scanned for each irradiated film scan and subtracted from post-irradiation scans. Calibration curves were then generated using the calibration plan dose and the film net OD values. Finally, a set of time-dependent correction factors were derived from the calibration curves. The per-batch average correction factors were applied in order to test the feasibility of using a per-batch generic calibration curve. All converted film doses were verified against the planned dose. The scanner light effect on the film dose was estimated by measuring the changes in mean pixel value.


RESULTS
Up to 7% pixel value change was noted four days after exposure. A correction was necessary if a generic calibration curve was utilized. The method to remove non-uniformity using the pre-exposure film (method 1) was more stable than using two unexposed films (method 2). The batch average correction factors reduced calibration curve differences to less than 0.6%. The dose effect from each scan was minimal (∼0.1 cGy).


CONCLUSION
It is feasible to use one generic calibration curve per film batch with a time-dependent correction factor table without compromising overall EBT film dosimetry accuracy. This method may alleviate the current practice of the typical labor intensive film-based dosimetry for patient specific QA as well as physics commissioning tasks.