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  4. Homogeneous and Heterogeneous 3D Dosimeters for IMRT End-To-End Testing in a Pre-Clinical MR-Linac

Homogeneous and Heterogeneous 3D Dosimeters for IMRT End-To-End Testing in a Pre-Clinical MR-Linac

B McDonald1,2*, H Lee1, Y Roed1,3, Muhammad Akram1,4, G Ibbott1, (1) Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, (2) The University of Texas MD Anderson Cancer Center UTHealth Graduate School of Biomedical Sciences, Houston, TX, (3) Department of Physics, University of Houston, Houston, TX, (4) Department of Physics, The Islamia University of Bahawalpur Pakistan

Presentations

(Sunday, 7/29/2018) 4:00 PM - 4:30 PM

Room: Exhibit Hall | Forum 4

Purpose: To demonstrate the quantitative abilities of MR-visible radiochromic and polymer gel dosimeters in homogeneous phantoms and in a heterogeneous anthropomorphic 3D-printed skull phantom for end-to-end testing in MR-IGRT.

Methods: Radiochromic Fricke-xylenol orange-gelatin (FXG) dosimeters were created in-house in a cylindrical 2-liter container (homogenous) and in a 3D-printed skull phantom (heterogeneous). A spherical dosimeter with methacrylic acid-based polymer gel was created off-site. All phantoms were irradiated with a TG119 IMRT Head/Neck plan in a pre-clinical 1.5T/7MV MR-linac. Each dosimeter was imaged with CT (reference plan), MR-imaged in the MR-linac immediately prior to irradiation (adapted plan), irradiated in the MR-linac, then immediately MR-imaged in the MR-linac post-irradiation (dose analysis). Radiochromic dosimeters were imaged with T1-weighted FFE sequences (TR/TE=11/4.6ms, flip angles (FA)=30,15). Polymer dosimeters were imaged with T2-weighted TSE sequences (TR/TE=1000/93ms). Dose maps were obtained from the MR-images, and a noise-reduction filter was applied. Gamma analysis was used to compare the planned and measured doses.

Results: The gamma pass rates using 7%/4mm criteria were 94.3% (homogeneous radiochromic), 96.8% (heterogeneous radiochromic, FA=30), 96.8% (heterogeneous radiochromic, FA=15), and 85.6% (homogeneous polymer). With tighter criteria of 3%/3mm, the pass rates were 27.6%, 25.8%, 34.4%, and 41.4%, respectively. The dose maps of the radiochromic dosimeters appeared noisy.

Conclusion: The calculated relative volumetric doses from the homogeneous and heterogeneous radiochromic phantoms were within 2.5% of each other. Improving the image quality of the acquired MRI improved the 7%/4mm gamma pass rates by 0.02% and the 3%/3mm by 8.57%, indicating the importance of optimizing the image acquisition parameters rather than applying noise-reduction filters. The polymer gel performed better than the radiochromic gel with 3%/3mm criteria, which is consistent with the lower noise in the polymer gel relative doses. These results demonstrate the capabilities of radiochromic and polymer gel dosimeters to be used for end-to-end testing on MR-IGRT systems.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by a Master Research Agreement grant from Elekta AB.

Keywords

Gel Dosimeter, Image-guided Therapy, Quality Assurance

Taxonomy

TH- Radiation dose measurement devices: 3D solid gel/plastic

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