Evaluation of Dose Distribution Accuracy in HDR Brachytherapy of Esophagus Cancer Based on MRI Normoxic Polymer Gel Dosimetry

The purpose of this work was to study the ability of MRI normoxic polymer gel dosimetry for evaluating the dose distribution in HDR brachytherapy of esophagial cancer at Imam Reza brachytherapy center (Mashhad, Iran). Initially, 2liters of normoxic gel (MAGIC) was fabricated and then poured into 12 calibration test tubes and placed in a perspex walled phantom. The gel phantom was irradiated with a brachytherapy remote-afterloader unit using a cobalt-60 brachytherapy source and the test tubes were irradiated with a range of known doses with a cobalt-60 teletherapy unit. Imaging was performed with a multi-spin-echo protocol and a T2 quantitative technique using a Siemens 1.5 T MRI machine. The MRI images were transferred to a computer and then image processing was performed in the MATLAB environment to extract R2 maps of the irradiated area. In this study and at the reference point, the dose deviation between the gel dosimetry and the calculated data was 4.5%. The distance to agreement (DTA) for dose profiles was 2.7 mm. Also, dose sensitivity of the MAGIC gel dosimeter was 0.693 S-1Gy-1 (R2 =0.9376). In this work, the data obtained from TPS calculations were found in very good agreement with the measured results provided by gel dosimetry. It was evaluated using a comparison of isodoses and dose at the reference point, and dose profile verification. It is also concluded that the gel dosimetry systems have proven to be a useful tool for dosimetry in clinical radiotherapy applications.