Application and Development of Algebraic Reconstruction Algorithms for High Dose Rate Brachytherapy Gel Dosimetry with Optical Computed Tomography Readout
Radiation therapy treatment techniques have become more complex to offer optimal tumour control while preserving patient quality of life by avoiding damage to healthy tissue. This increased complexity requires more thorough quality assurance (QA). External beam radiation therapy has seen an increase of QA of individual patients’ treatments for complex deliveries. This is known as patient specific quality assurance. High dose rate (HDR) brachytherapy treatments have become more complex, but patient specific QA has not been adopted. One reason for this is the lack of inexpensive, simple to use, accurate, and complete dosimeters for brachytherapy. Radiochromic gel dosimeters with optical computed tomography (CT) readout would provide such a dosimeter, but the insertion of brachytherapy catheters into the gel cause streak artifacts in the reconstructed dose distributions, ruining the dosimeter’s accuracy. The objective of this work was to investigate the feasibility of using radiochromic gel dosimeters for HDR brachytherapy dosimetry. This work focused on the initial step for making radiochromic gel dosimeters compatible with brachytherapy irradiations: the removal of the streak artifacts in the reconstructed dose distributions. First the cause of the artifacts was investigated. It was found that photon starvation caused the streak artifacts present when using the standard Feldkamp-Davis-Kress (FDK) reconstruction algorithm. This work then investigated using the Ordered Subsets Convex algorithm with Total Variation minimization regularization (OSC-TV) instead of the FDK algorithm to suppress the artifacts. Three variants of the OSC-TV algorithm were also developed. These variants introduced reconstructing using flood field images (OSC-TV-FF), rejecting data from rays that passed through the catheters (OSC-TV-RR), or both (OSC-TV-FF-RR). Using controlled external beam irradiations, the OSC-TV and OSC-TV-RR algorithms were both found to remove the streak artifacts. The OSC-TV and OSC-TV-RR algorithms were then used for a set of HDR brachytherapy dosimetry experiments. Both the OSC-TV and OSC-TV-RR algorithms produced artifact free dose distributions of gels with a single catheter inserted. The OSC-TV-RR algorithm also removed artifacts from gels with multiple catheters. Therefore it was concluded that the OSC-TV-RR algorithm could be used with radiochromic gels to provide a feasible method for patient specific HDR brachytherapy dosimetry.