SU‐E‐T‐678: Response Calibration Using Electron Depth‐Dose Data for MRI‐Based 3D Polymer Gel Dosimetry

Abstract

Purpose:To evaluate a calibration method using the depth‐dose data of an electron beam for MRI‐based polymer gel dosimetry.Methods:MAGAT was manufactured in‐house to fill two 400mL‐cylindrical phantoms and nine 22mL‐glass vials. Phantom‐A was irradiated along the cylinder axis with a 9MeV electron beam of 6 cm × 6 cm field size (FS). Phantom‐B was irradiated with a 6MV photon beam of 3 cm × 3 cm FS by a 360‐degree arc technique. Eight vials were irradiated in a water‐bath to various doses with a 20 cm × 20 cm FS 6MV photon beam. All irradiated phantoms and one un‐irradiated vial were scanned with a 3T MRI scanner to obtain the spin‐spin relaxation rate (R2) distributions. By comparing the measured R2‐to‐depth data with the known depth‐dose data for Phantom‐A, R2‐to‐dose calibration data were obtained (e‐beam method). Another calibration data were obtained from the 9 vials data (9‐vial method). We tested two regression equations, i.e., third‐order polynomial and tangent functions, and two dose normalization methods, i.e., one‐point and two‐point methods. Then, these two calibration methods were used to obtain the 3D dose distribution of Phantom‐B and evaluated by comparing the measured data with the dose distribution from a treatment planning system. The comparison was made with gamma passing rate (2%/2mm criteria).Results:We did not observe a clear advantage of the e‐beam method over the 9‐vial method for the 3D dose comparison with the test case. Nevertheless, we found that the e‐beam method required a smaller dose scaling for the dose comparison. Furthermore, the tangent function showed better data fitting than the polynomial function with smaller uncertainty of the estimated coefficients.Conclusions:Considering the overall superior performance, we recommend the e‐beam method with the tangent function as the regression equation and one‐point dose normalization for the MRI‐based polymer gel dosimetry.