Comparison of the Obtained Results from Two Calibration Methods in MRI Brachytherapy Normoxic Polymer Gel Dosimetry

Introduction: Despite the fact that the clinical implementation of polymer gel dosimetry has been facilitated after the introduction of normoxic gels, there are still complications in its clinical routine use that are under investigation. In the current work, the feasibility of using a normoxic polymer gel dosimeter named MAGICA has been investigated for use in our clinical brachytherapy applications at ImamHospital and the deviations between the results obtained from two different calibration methods were investigated. Materials and Methods: The gel fabrication was performed at Novin Medical Radiation Institute and 4 Perspex walled phantoms designed especially for brachytherapy irradiations were filled with the same gel, together with 15 plastic calibration test tubes. The test tubes were irradiated with a range of known doses using a cobalt-60 teletherapy unit and the phantoms were irradiated with two brachytherapy remote-after-loaders using Cs137 and Co60 brachytherapy sources according to the treatment planning data. Imaging was performed with a Multi-spin-echo protocol using a Siemens 1.5T MRI machine. Image processing was performed in MATLAB environment to extract R2 maps of the dose distributions. Gel fabrication, irradiation and imaging were performed at three different centers, so the dosimeters and test tubes were transferred from one center to another under non-uniform conditions. R2 values of the first dosimeter which was irradiated with a single cesium source were extracted from different distances from the source along a horizontal profile and correlated to the dose values at the same distances in treatment planning data. This was used as the second calibration method to extract the absolute measured values of the other three dosimeters. Diagrams of the absorbed dose versus distance were plotted for each dosimeter and the results obtained from the two calibration methods were compared. Results: Results of the measured values that were calibrated with the test tubes show a 3 to 6 Gy dose difference in plateaus and more than 2 mm distance to agreement gap in steep dose gradients. However, the results of using the second calibration method show at most a 0.36 Gy dose difference in plateaus and less than 2 mm distance to agreement gap in steep dose gradients. Discussion and Conclusion: As the gel fabrication, irradiation and imaging conditions were the same for all the gel dosimeters and the calibration tubes, the unexpected deviations between the results of the two calibration methods deserves more investigations. It seems plausible to attribute this deviation to the difference in the size of calibration test tubes and original dosimeters. From a practical point of view, as the origin of this effect has not yet been investigated, it would be best to use a calibration phantom with the same size and characteristics as the original dosimeters.