Comparison of optical CT imaging versus NMR imaging for nPAG gel dosimetry

Radiotherapy has evolved towards a more accurate technique of delivering a prescribed dose to the target volume. As a consequence, steep dose gradients in three dimensions occur near the edges of the target volume. Polymer gel dosimeters are able to record the integrated absorbed dose in three dimensions (3D). To read out the dose information of a 3D gel dosimeter we need a dedicated imaging technique. The aim of this study was to compare magnetic resonance imaging (MRI), an extensively studied imaging modality, with a more user friendly technique of optical computed tomography (optical CT). An optical CT scanner was constructed on site which is able to measure 3D polymer gels. The first challenge involved the elimination of all imaging artifacts. Different components were characterized such as laser stability, laser spot size, detector stability and refractive index matching fluid (RIMF). Some imaging artifacts were compensated by post processing techniques. Finally polymer gel phantoms were fabricated and irradiated with square beams. The dose distribution measured with optical CT was compared with the dose distribution measured with MRI. In order to make a fair comparison of precision between both imaging modalities, the intrinsic dosimetric precision on readout (IP_D) was calculated. Laser stability and detector stability were accepted to be sufficient. RIMF was optimized using an automated procedure. Most artifacts were dealt with on a hardware level. The IPD value shows that optical CT scanning has a five times better intrinsic dose precision than MRI. In this study it is shown that optical CT is able to produce high quality images, but further quantitative evaluation is required to valorize the dosimetric value.