Post-irradiation darkening model for EBT-3 films characterized using a single lot calibration approach.

Abstract

In this study, we present a model to correct the progressive post-irradiation darkening of EBT3 films. The model allows for a clinical use of EBT3 using application and calibration films scanned with different post-irradiation times.
Approach. The model is a post-irradiation time- and dose-dependent power-law function, projecting the scanned transmittance of application films to the transmittance matching the same post-irradiation time of calibration films. The model was characterized for two EBT3 production lots within the dose range 0.1-12.8 Gy. A first characterization was performed utilizing calibration films scanned repeatedly for 54 days post-irradiation (lot 1), while a fast re-characterization of a second lot used three post-irradiation times (lot 2). 
For a long-term validation validation of the model, 16 film strips were irradiated at 2 Gy on different time points starting from the day of film calibration up to 43 days afterwards (lot 1). For the multi-dose validation of the model, 8 strips were irradiated with dose levels ranging 0-12 Gy deposited 25 days after the calibration (lot 2). 
As a proof of principle, the model was applied to four clinical patient-specific quality assurance film measurements with prescribed dose/fraction ranging 2.66 Gy-8 Gy.
Main results. The post-irradiation transmittance decreased for higher doses up to -2.5% at 12.8 Gy, and 54 days post-irradiation. 
With a lot-specific model correction, the mean dose accuracy of validation strips that ranged from initial -3.4% (triple-channel) and -9.90% (blue-channel) reduced to within 3.0% (all colour channels) for doses above 1 Gy. 
The median dose difference with the planned dose improved from -3.5% to -1.1%, and the 3%/2 mm local gamma ranged from (48.5 - 92.5)% to (81.2 - 99.2)%.&#xD.