Assessment of cancer recurrence in residual tumors after fractionated radiotherapy: 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 alter fractionated radiotherapy to assess cancer recurrence in residual tumors. Methods: AH109A tumor-burdened rats were treated with one to eight doses of 5Gy ⁶⁰Co radiation. Tissue distribution study with ¹⁸F-FDG, ³H-thymidine and ¹⁴C- methionine, double-tracer autoradiography with ¹⁸F-FDG and ¹⁴C- methionine, and single-tracer autoradiography with ¹⁴C-labeled deoxyglucose, thymidine and methionine were performed 6 days after the end of therapy. Results: Dose response study shows a significant decrease of tumor uptake of all tracers after two and more doses, even in the ease of later recurrence. Whereas ³H-Thd and ¹⁴C-Met tumor uptake was similar to that of normal muscle, ¹⁸F-FDG tumor uptake remains higher than that of muscle, even in the case of complete tumor cure. The irradiated muscle shows a higher ¹⁸F-FDG uptake than the nonirradiated muscle. Autoradiography after eight doses (100% tumor cure) reveals elevated ¹⁴C-DG tumor uptake to be ascribable to nonmalignant cellular elements, in particular to a macrophage layer at the rim of necrotic areas. Autoradiography after four and six doses (33% and 57% tumor cure) shows the highest methionine and thymidine uptake in viable cancer cells, whereas deoxyglucose uptake did not differ between viable cancer cells and macrophages. Conclusion: To detect and differentiate viable cancer cells in a residual tumor mass after radiotherapy, PET using ¹¹C-methionine or ¹¹C-thymidine may have some advantages over ¹⁸F-FDG, especially if the residual tumor includes larger areas of necrosis.