Comparison of [(18)F]FIMP, [(11)C]MET, and [(18)F]FDG PET for early-phase assessment of radiotherapy response

Several limitations of [(18)F]FDG have been reported, such as nonspecific uptake of inflammation foci. Moreover, [(11)C]MET has been found to accumulate in normal and inflammatory tissues as well as tumors. To increase specificity to tumor tissues, PET probes with tumor-specific molecular targets have been actively developed. [(18)F]FIMP was found to be highly accumulated in LAT1-positive tumors but not in inflamed tissue. The aim of this study was to explore whether [(18)F]FIMP can be used for the early-phase evaluation of radiotherapy accompanied by inflammation, and compare its effectiveness with those of [(11)C]MET and [(18)F]FDG. Tumor uptake of [(18)F]FIMP decreased at day 1 after irradiation, and remained low until day 14. Comparatively, that of [(18)F]FDG initially decreased at day 3 but was transiently elevated at day 7 and then decreased again at day 10. Decreased tumor uptake of [(11)C]MET was observed at day 10. In line with the uptake of [(18)F]FIMP, the ratio of Ki-67 immuno-positive cells in tumor tissues significantly decreased at day 1, 7, and 10 as compared with that in the control. These findings suggest that [(18)F]FIMP may be a PET probe involved in the early detection and prediction of radiotherapy efficacy, although further clarification is needed.