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

Authors

  • Watanabe Y,

    1. University of Minnesota, Minneapolis, MN
    2. University of Minnesota, Minneapolis, MN
    3. All India Institute of Medical Sciences, Newdehli, India
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  • Warmington L,

    1. University of Minnesota, Minneapolis, MN
    2. University of Minnesota, Minneapolis, MN
    3. All India Institute of Medical Sciences, Newdehli, India
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  • Gopishankar N

    1. University of Minnesota, Minneapolis, MN
    2. University of Minnesota, Minneapolis, MN
    3. All India Institute of Medical Sciences, Newdehli, India
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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.

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