ASSESSMENT OF CANCER RECURRENCE IN RESIDUAL TUMORS AFTER FRACTIONATEDRADIOTHERAPY - A COMPARISON OF FLUORODEOXYGLUCOSE, L-METHIONINE AND THYMIDINE

This study evaluates the midterm follow-up of tumor and normal tissue uptake of deoxyglucose, thymidine and methionine after fractionated ra diotherapy to assess cancer recurrence in residual tumors, Methods: AH 109A tumor-burdened rats were treated with one to eight doses of 5Gy C o-60 radiation. Tissue distribution study with F-18-FDG, H-3-thymidine and C-14-methionine, double-tracer autoradiography with F-18-FDG and C-14-methionine, and single-tracer autoradiography with C-14-labeled d eoxyglucose, thymidine and methionine were performed 6 days after the end of therapy. Results: Dose response study shows a significant decre ase of tumor uptake of all tracers after two and more doses, even in t he case of later recurrence. Whereas H-3-Thd and C-14-Met tumor uptake was similar to that of normal muscle, F-18-FDG tumor uptake remains h igher than that of muscle, even in the case of complete tumor cure, Th e irradiated muscle shows a higher F-18-FDG uptake than the nonirradia ted muscle, Autoradiography after eight doses (100% tumor cure) reveal s elevated C-14-DG tumor uptake to be ascribable to nonmalignant cellu lar elements, in particular to a macrophage layer at the rim of necrot ic areas. Autoradiography after four and six doses (33% and 57% tumor cure) shows the highest methionine and thymidine uptake in viable canc er cells, whereas deoxyglucose uptake did not differ between viable ca ncer cells and macrophages. Conclusion: To detect and differentiate vi able cancer cells in a residual tumor mass after radiotherapy, PET usi ng C-11-methionine or C-11-thymidine may have some advantages over F-1 8-FDG, especially if the residual tumor includes larger areas of necro sis.