Novel EBT3 film calibration using an integrated HSV color space analysis

Purpose: Radiochromic EBT3 films are commonly used as dosimeter for clinical practice and research on radiotherapy. In principle, they are associated with a flatbed scanner to determine the optical density change, which can be correlated to the absorbed dose after calibration. Several approaches have been proposed to reduce the uncertainties during acquisition and to compensate the lighting inhomogeneities, thus improving the dose measurement. Those works have shown that good accuracy can be achieved for absorbed dose using EBT3 films, at the expense of complex data processing and time-consuming acquisition protocols.

Method: We introduce the new method to determine the calibration curve based on the HSV color space analysis, which provides robustness and invariance to illumination changes.

Results: With this new approach, it allows to compute the calibration curve by performing only a single scan of film pieces regardless either the lateral positions or control points on the scanner bed. Using the hue channel in HSV color space, we prove that the dose can be accurately reconstructed with a much simpler protocol than when using RGB channels with blank scans rectification. Our HSV approach includes comparative gamma index for conventional film analysis. It achieves a gamma index (3%/3mm) over 99% when comparing measurement and AAA computation for a modulated beam.

Conclusion: Compared to most existing approaches, our approach does not rely on complex mathematical reconstructions or additional scans. Instead, it uses another color model representation to rectify the scanner response, coping the dose measurement homogeneity problem over the scanner window. It facilitates the overall scan calibration to be much simpler, save time, and less manipulations, which also decreases the risk of human error.