CARBON-11-THYMIDINE AND FDG TO MEASURE THERAPY RESPONSE

This study was performed to determine if PET imaging with C-11-thymidi ne could measure tumor response to chemotherapy early after the initia tion of treatment. Imaging of deoxyriboneucleic acid biosynthesis, qua ntitated with C-11-thymidine, was compared with measurements of tumor energetics, obtained by imaging with F-18-ftuorodeoxyglucose (FDG). Me thods: We imaged four patients with small cell lung cancer and two wit h high-grade sarcoma both before and approximately 1 wk after the star t of chemotherapy. Thymidine and FDG studies were done on the same day . Tumor uptake was quantified by standardized uptake values (SUVs) for both tracers by the metabolic rate of FDG and thymidine flux constant (K-TdR) using regions of interest placed on the most active part of t he tumor. Results: In the four patients with clinical response to trea tment, both thymidine and FDG uptake markedly declined 1 wk after ther apy. Thymidine measurements of SUV and K-TdR declined by 64% +/- 15% a nd 84% +/- 33%, respectively. FDG SUV and the metabolic rate of FDG de clined by 51% +/- 9% and 63% +/- 23%, respectively. In the patient wit h metastatic small cell lung cancer who had disease progression, the t hymidine SUV decreased by only 8% (FDG not done). In a patient with ab dominal sarcoma and progressive disease, thymidine SUV was essentially unchanged (declined by 3%), whereas FDG SUV increased by 69%, Conclus ion: Images show a decline in both cellular energetics and proliferati ve rate after successful chemotherapy, In the two patients with progre ssive disease, thymidine uptake was unchanged 1 wk after therapy. In o ur limited series, K-TdR measurements showed a complete shutdown in tu mor proliferation in patients in whom FDG showed a more limited decrea se in glucose metabolism.