1161 - Novel Spatiotemporal Visual Verification Method of High-Precision IMRT: In Vivo Redox Imaging with Hyaluronic Acid-Gelled Probe Spacer
Presenter(s)
T. Mori1, F. Hyodo2, T. Mori3, N. Koyasu4, A. Kobori1, M. Ito1, and M. Matsuo1,5; 1Department of Radiology, Graduate School of Medicine, Gifu University, Gifu, Japan, 2Department of Pharmacology, Graduate School of Medicine, Gifu University, Gifu, Japan, 3Department of Veterinary Medicine, Faculty of Applied Biological Sciences, Gifu University, Gifu, Japan, 4NIH/NCI, Bethesda, MD, 5Innovation Research Center for Quantum Medicine, Graduate School of Medicine, Gifu University, Gifu, Japan
Purpose/Objective(s): Recent advancements in radiotherapy modalities, including intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT), and the development of cutting-edge technologies such as a 3-D CRT and IMRT system and MR-Linac, have significantly improved treatment precision. These advancements have enabled dose escalation per fraction while minimizing radiation exposure to organs at risk and shortening treatment durations, making hypofractionation as the standard for prostate cancer treatment. Additionally, hyaluronic acid gel spacers have emerged as effective tools for protecting organs at risk. Despite these advancements, no method currently exists to visually verify the actual dose distribution and irradiation site after X-ray treatments. Free radical visualization could enhance our understanding of treatment dose and site. Our research focuses on free radical imaging using Dynamic Nuclear Polarization (DNP)-MRI with nitroxyl radicals as redox probes. Previously, we demonstrated that a Tempo methacrylate (TempoMC) and glutathione (GSH) mixture effectively quantifies free radical reactions, enabling clear visualization of redox processes post-irradiation. In this study, we evaluated the feasibility of in vivo redox imaging using DNP-MRI with a hyaluronic acid-gelled probe spacer.
Materials/Methods: A hyaluronic acid-gelled probe spacer containing TempoMC/GSH was injected into the abdominal cavity of mice. IMRT plan was prepared using software routinely used in clinical practice, and each mouse was irradiated with X-rays at prescribed doses of 2.5 Gy, 5.0 Gy, and 10 Gy using a 3-D CRT and IMRT system. Free radical imaging was performed using a low-field DNP-MRI system, along with ESR signal measurements and MRI scans.
Results: Gel probe spacers were clearly visible on T1 and T2-weighted MRI and distinctly visualized by DNP-MRI. The DNP-MRI signal intensity of the gel decreased depending on the prescribed irradiation dose due to redox reaction of probes. Interestingly, a linear relationship was observed between the DNP-MRI signal and the irradiation dose. In addition, the EPR signal of the remaining TempoMC radicals in the gel decreased following irradiation and corresponded well with the in vivo imaging data.
Conclusion: In vivo redox imaging using DNP-MRI with a hyaluronic acid-gelled probe spacer enables the spatiotemporal visual verification of high-precision IMRT. As more data are collected, this approach is expected to provide quantitative visualization of radiation dose and site based on signal intensity. Our findings suggest potential applications with instruments such as the MR-Linac, using a hyaluronic acid-gelled probe spacer to achieve more precise irradiation assessment.