Evaluation of an Epson flatbed scanner to read Gafchromic EBT films for radiation dosimetry.

Gafchromic EBT (EBT) films are becoming increasingly popular due to their advantageous properties. When flatbed colour scanners are used for film dosimetry, a good quality control of the scanning device is a crucial step for accurate results. The proposal of this work was to fully assess the performance of the scanner Epson Expression 10000XL in order to quantify all parameters and needed corrections to minimize dose uncertainties. A standard step tablet, with 32 steps and optical densities from 0.06 to 3.8, was used to check the scanner linearity. The scanner warming-up effect and reproducibility were evaluated by performing 30 consecutive scans plus 20 scans in 15 min intervals. The scanning colour modes: 24 and 48 bits and scanning resolutions from 50 to 300 dpi were tested. A Wiener filter with different pixels regions was applied with the purpose of reducing the film noise. All scans were made in transmission mode with a constant film orientation. The red colour channel was posteriorly extracted from the images to maximize readout sensitivity. Two EBT films were irradiated, perpendicularly and parallel to beam incidence, with a 6 MV photon beam with doses that ranged from 0.2 to 3 Gy. A polynomial expression was used to convert optical density into dose. Dose uncertainty was quantified applying error propagation analysis. A correction for the non-uniform response of the scanner was determined using five films irradiated with a uniform dose. The scanner response was linear until an optical density of approximately 1 which corresponds to doses higher than those of clinical interest for EBT films. The scanner signal stabilized after seven readings. Scanner reproducibility around 0.2% was obtained either with the scanner warm or cold. However, reproducibility was significantly reduced when comparing images digitized with the scanner at different temperatures. Neither the colour depth mode, the scanning resolution, the multiscan option nor the Wiener filter had a significant effect on the shape of the calibration curve. However, a reduction in dose uncertainty was possible by selecting appropriate reading parameters. These are a 48 bit colour depth, a 75 dpi resolution and repeating the scan four times. Finally, the two dimensional Wiener filter applied to a 3 x 3 pixel region to the red component of the image reduced the experimental scan uncertainty to about 0.5% for doses higher than 0.5 Gy. Total scan uncertainty was less than 2% for a perpendicular calibration and reduced to less than 1% for a parallel calibration. A dose over-estimation of around 5% for clinical doses may be made if the image acquired is not corrected for the non-uniform response of the scanner. A protocol to read EBT films using the Epson Expression 10000XL scanner was established for IMRT verification. The contribution for the overall uncertainty in film dosimetry coming from the scanning process was estimated to be around 0.5% for doses higher than 0.5 Gy when reading parameters are optimized. Total scan uncertainty achieved is about 2% when using a perpendicular calibration. It can further be reduced if a parallel calibration is used.