Detection of tumor response to radiation therapy by in vivo proton MR spectroscopy

Purpose: These studies were performed to investigate the effects of radiation on levels of metabolites such as lactate and choline compounds detected by 1H magnetic resonance spectroscopy (MRS). The purpose was to determine the ability of spatially localized 1H MRS to detect tumor response to radiation therapy. Methods and Materials: Proton spectroscopic images were obtained from RIF-1 tumors with voxel spatial resolutions of 8-16 mm3 before and at 24 and 48 h following 2, 4, and 20 Gy of γ-radiation. Results: Lactate levels decreased significantly for all doses by 48 h. Tumors irradiated with 2 and 4 Gy showed a significant decrease by 48 h, but not at 24 h. A group of sham-irradiated control animals demonstrated no significant changes in lactate over the period of observation. Conclusions: Changes in lactate observed in this study are consistent with an increased blood flow observed in previous studies in the same tumor model following 20 Gy X-irradiation. These studies point to the feasibility of detecting response to clinical doses of fractionated radiation therapy by 1H MRS