Prediction of the presence of cupping artifacts for gel dosimeter based on considerations of scattered light in optical computed tomography measurements

摘要

Measurements using three-dimensional (3D) gel dosimeters and optical computed tomography (CT) have exhibited tremendous potential for fast and accurate 3D dose verifications in radiation therapy. However, accurate measurements may not be possible owing to cupping artifacts. It has been reported that the Fricke and the polymer gel dosimeters induce considerable and extensive cupping artifact effects, whereas the micellar gel dosimeter exhibits a smaller effect. In this study, we estimated that the effect of cupping artifacts in the micellar dye gel dosimeter can be predicted by considering the absorption in the dye and the scattering induced by the gelling agent. Solutions with various absorbances were prepared and measured by optical CT. In parallel, changes in absorbance owing to the gelling agent were investigated using cells with different optical path lengths. Spectroscopic analyses demonstrated that the Lambert Beer law holds for the light attenuation induced by the dyes and the gelling agent when measured with optical CT. We conclude that the generation of the cupping artifacts was attributed to the fact that the detection limit of the detector was exceeded owing to excessive light attenuation. In conclusion, we demonstrated the effectiveness of optical CT measurements based on the conversion of the measured value of the optical CT to absorbance and the prediction of the presence of cupping artifacts.