Room: Exhibit Hall | Forum 7
Purpose: To assess the use of methacrylic-acid based polymer gels for quality assurance of patient-specific treatment plans delivered by MR-image guided radiotherapy machines. This data will be used to support the implementation of volumetric dosimeters in MR-IGRT.
Methods: Methacrylic-acid based polymer gel dosimeters were manufactured off-site. Dose response, fractionation dependence, and dose-rate dependence were initially evaluated in a 1 T magnetic field using an electromagnet installed in a linear accelerator vault. All dosimeters were imaged with an MR scanner 24 hours post-irradiation. Spin-spin relaxation rates (R2) were determined from MR images. A large, spherical dosimeter (16 cm diameter) filled with the same gel was used for end-to-end phantom testing on a 7 MV/1.5 T MR-IGRT treatment unit. CT images of the dosimeter were used to create a reference plan. The CT images were fused with MR images acquired on the MR-IGRT unit pre-irradiation to generate an adapted plan. The TG-119 head and neck treatment plan was delivered to the dosimeter which was MR imaged post-irradiation for dose analysis. A 3D gamma analysis was performed on dose maps.
Results: A linear dose response was observed up to 10 Gy. An 84 % reduction in dose rate resulted in a 26 % increase in R2 measurements compared to delivering the same dose at 100 % dose rate. R2 values were 8 % higher when delivering the same dose in three fractions compared to a single fraction. A gamma pass rate of 85.6 % was determined using 7 %/4 mm gamma criteria.
Conclusion: This study showed that despite the linear dose response of the gel, the strong dose-rate dependence and the non-negligible fractionation dependence rendered the methacrylic-acid based polymer gel unsuitable for clinical dosimetry. An adjustment of the current chemical composition might decrease both the dose-rate and the fractionation dependence.