Evaluation of D-18F-FMT, 18F-FDG, L-11C-MET, and 18F-FLT for Monitoring the Response of Tumors to Radiotherapy in Mice

O-F-18-fluoromethyl-D-tyrosine (D-F-18-FMT) is a promising novel agent for tumor imaging by PET. The aim of this study was to evaluate the potential of D-F-18-FMT and the other conventional ligands used for tumor imaging, namely, F-18-FDG, L-C-11-methionine (L-C-11-MET), and 3'-deoxy-3'-F-18-fluorothymidine (F-18-FLT), as a PET ligand for monitoring early responses to radiotherapy in tumor-bearing mice. Methods: C3H/HeN mice inoculated with murine squamous cell carcinomas were treated with a single dose of x-ray irradiation at 2, 6,20, or 60 Gy. Tumor uptake of each ligand was examined 1, 3, and 7 d after the irradiation. Results: Tumor uptake of D-F-18-FMT was decreased on day 1 after irradiation at 6, 20, or 60 Gy, and the decrease persisted until day 7. Tumor uptake of F-18-FDG was elevated on days 1 and 3 after irradiation at 2, 6, or 20 Gy, followed by a decrease in uptake on day 7 in mice irradiated at 20 or 60 Gy. Decreased tumor uptake of L-C-11-MET was observed only on day 3 after the irradiation. Decreased tumor uptake of F-18-FLT was detected on day 1 after irradiation at 2, 6, 20, or 60 Gy; thereafter, the dose-dependent decrease in uptake was no longer seen. Only for D-F-18-FMT were significant positive correlations found between ligand uptake at all the time points examined and tumor volume on day 14 after various doses of irradiation. Conclusion: The findings suggest that D-F-18-FMT is a promising PET ligand for early-phase detection and prediction of the effects of radiation therapy.