PET/CT with 18F-FLT: Does It Improve the Therapeutic Management of Metastatic Germ Cell Tumors?

Because F-18-FDG PET alone has only limited value in metastatic germ cell tumors (GCTs), we investigated the addition of 3'-deoxy-3'-F-18-fluorothymidine (FLT) to F-18-FDG for early response monitoring and prediction of the histology of residual tumor masses in patients with metastatic GCT. Methods: Eleven patients with metastatic GCT were examined with both F-18-FDG PET/CT and F-18-FLT PET/CT before chemotherapy, after the first cycle of chemotherapy (early response), and 3 wk after completion of chemotherapy. In 1 patient with negative F-18-FLT PET/CT results before chemotherapy, no further F-18-FLT scanning was performed. PET images were analyzed visually and, using standardized uptake values (SUVs), semiquantitatively. The results were compared with the findings of CT and tumor marker levels and validated by histopathologic examination of resected residual masses, including Ki-67 immunostaining (7 patients), or by clinicoradiologic follow-up for at least 6 mo (4 patients). A responder was defined as a patient showing the presence of necrosis, a complete remission, or a marker-negative partial remission within a minimum progression-free interval of 6 mo. Early treatment response was judged according to the criteria of the European Organization for Research and Treatment of Cancer. Results: Before chemotherapy, reference lesions showed increased F-18-FDG uptake (mean SUV, 8.8; range, 2.9-15.0) in all patients and moderate F-18-FLT uptake (mean SUV, 3.7; range, 1.7-9.7) in 10 of 11 patients. After 1 cycle of chemotherapy, mean SUV decreased in responders and nonresponders by 64% and 60%, respectively, for F-18-FDG (P = 0.8) and by 58% and 48%, respectively, for F-18-FLT (P = 0.5). After the end of chemotherapy, mean SUV decreased in responders and nonresponders by 85% and 73%, respectively, for F-18-FDG (P = 0.1) and by 68% and 65%, respectively, for F-18-FLT (P 5 0.8). The results of early and final PET were inconsistent in 6 of 11 patients for F-18-FDG and in 4 of 10 patients for F-18-FLT. Both patients with teratoma had false-negative results on both F-18-FDG and F-18-FLT. The sensitivity, specificity, positive predictive value, and negative predictive value for detection of viable tumor after 1 cycle of chemotherapy were 60%, 33%, 43%, and 50%, respectively, for F-18-FDG and 60%, 80%, 75%, and 67%, respectively, for F-18-FLT PET/CT. The respective values after the end of chemotherapy were 20%, 100%, 100%, and 60% for F-18-FDG and 0%, 100%, 0%, and 50% for F-18-FLT PET/CT. Conclusion: PET-negative residual masses after chemotherapy of metastatic GCT still require resection, since the low negative predictive value of F-18-FDG PET for viable tumor cannot be improved by application of F-18-FLT.