Identification and characterization of an optimal three -dimensional dosimetry system for remote auditing by the RPC
Advanced treatment techniques currently used in radiation treatment require the use of a dosimeter capable of performing measurements with high spatial resolution in three-dimensions. The Radiological Physics Center (RPC) evaluates an institution's ability to deliver advance treatments through the use of anthropomorphic phantoms. The phantoms currently use film dosimetry to provide two-dimensional measurements in selected planes. Therefore, the RPC has a need for a three-dimensional dosimeter. The purpose of this project was to characterize several three-dimensional dosimeters and identify a formulation best suited for use by the RPC in an anthropomorphic phantom. Two polymer gel formulations, PAGAT and BANG®kit, and a radiochromic dosimeter, PRESAGE™, were investigated in this project. An MR imaging protocol to perform spin-spin relaxation time (T2) measurements of the polymer gels was developed. Results showed that a multi-spin echo pulse sequence with at least 32 echoes and an echo time of 9 ms should be used to minimize the dose resolution of both gel formulations. A 64-echo multi-spin echo pulse sequence with a 9 ms echo time was used to perform T2 measurements of polymer gels in this project. Temperature measurements were made in a polymer gel during imaging to ensure the gel was not being heated thereby affecting the accuracy of the T2 measurements. A maximum temperature increase of 1.2 °C was measured and found to have no effect on the T2 measurements. Each dosimetry formulation was characterized to determine the dose response, linearity, reproducibility, spatial stability, and dose resolution. PRESAGE dosimeters were imaged with optical computed tomography (OCT) after irradiation. Both MRI and OCT were used to identify the best-suited imaging modality for each polymer gel formulation. Established criteria for each parameter were used to evaluate each dosimeter's characteristics and to identify the dosimeter best-suited for use by the RPC. PAGAT gel imaged with OCT was identified as the dosimetry system to be used in an anthropomorphic phantom. A conventional and IMRT treatment plan were developed for the RPC's head and neck phantom. An insert was manufactured to accommodate a polymer gel dosimeter. PAGAT polymer gel was used to measure both treatment plans. Film measurements were also made of both treatment plans. Film was able to achieve a pass rate of at least 93% for both the conventional and IMRT treatments. Pass rates no larger than 88% were achieved with gel. The gel irradiated with the IMRT treatment measured 35% more dose than calculated by the treatment plan. The gel irradiated with the conventional treatment also over-responded in certain regions. The three-dimensional dosimeter is capable of providing measurements of the entire dose distribution from a single irradiation. Any plane within the distribution could be extracted from the gel measurements and compared to the treatment plan. However, PAGAT gel imaged with OCT was unable to provide comparisons with the treatment plan that are equivalent or better than those performed with film.
Heard, Malcolm P, "Identification and characterization of an optimal three -dimensional dosimetry system for remote auditing by the RPC" (2009). Texas Medical Center Dissertations (via ProQuest). AAI3358128.