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Purpose: This study was performed to know whether [18F]Fluorothymidine (FLT) positron emission tomography (PET) can be used to monitor early response to radiotherapy in comparison with [18F]Fluorodeoxyglucose (FDG) PET, and to establish the optimal imaging time for prediction of therapy response. Materials and Methods: Two patients with nasopharyngeal cancer underwent serial FLT PET and FDG PET before and during radiotherapy. Three on-treatment FLT and FDG PET scans were performed on 1 week, 2 weeks and 3 weeks (at each time of 10 Gy, 20 Gy and 30 Gy delivered). The peak standardized uptake values (SUVpeak) of primary tumors were measured on FLT and FDG PET. Then, percent changes of SUVpeak after therapy were calculated. Results: In two patients, baseline values of SUVpeak on FDT PET were higher than those on FLT PET (FLT vs FDG; 3.7 vs 5.0, and 5.7 vs 15.0). In patient 1, FLT SUVpeak showed 78%, 78% and 84% of decrease on 1 week, 2 and 3 weeks after treatment, whereas FDG SUVpeak showed 18%, 52% and 66% of decrease, respectively. In patient 2, FLT SUVpeak showed 75%, 75% and 68% of decrease, whereas FDG SUVpeak showed 51%, 49% and 58% of decrease, respectively. Both patients reached to complete remission after radiotherapy. Conclusion: After radiotherapy, the decrease of FLT tumor uptake preceded the decrease of FDG tumor uptake in patients with nasopharyngeal cancer, and 1 week after therapy may be appropriate time for the assessment of early response. FLT PET might be more useful than FDG PET for monitoring early response to radiotherapy.