18F-FLT PET performs better than 18F-FDG PET in differentiating malignant uterine corpus tumors from benign leiomyoma

The aim of this study is to test the hypothesis that positron emission tomography (PET) with 3'-deoxy-3'-[F-18]-fluorothymidine (F-18-FLT) can differentiate malignancy from benign leiomyoma better than PET with 2-deoxy-2-[F-18]fluoro-d-glucose (F-18-FDG), and to evaluate whether F-18-FLT and F-18-FDG uptake correlate with immunohistochemical index of cell proliferation. The protocol of this prospective study was approved by the institutional ethics committee, and all patients gave written informed consent. Fifteen patients (aged 26-65 years, median 44 years) with uterine corpus tumor which has the possibility of being leiomyosarcoma underwent F-18-FLT and F-18-FDG PET scans. Maximum standard uptake value (SUVmax) of PET scans and Ki-67 labeling index of surgical specimens were evaluated. Mann-Whitney's U test was used for comparing uptakes between benign and malignant, and linear regression analysis was used for evaluating the correlation between Ki-67 labeling index and SUVmax. Five cases were diagnosed as malignant (leiomyosarcoma for 3 cases, and carcinoma for 2 cases), and the others were benign leiomyoma. Sensitivity and negative predictive value of both tracers for detecting malignancy was 100 %. Specificity, positive predictive value and accuracy of F-18-FLT PET were higher than those of F-18-FDG PET. Difference in SUVmax between malignant and benign was significant for F-18-FLT PET (P < 0.01), but not for F-18-FDG PET. While all the malignant cases showed positive uptake in both tracers, a case of leiomyosarcoma with huge necrosis showed relatively low uptake. Uptake of F-18-FLT showed better correlation with Ki-67 labeling index compared with F-18-FDG (R (2) = 0.91 vs. R (2) = 0.26). Negative findings on additional F-18-FDG or F-18-FLT PET may rule out the possibility of malignancy for the patients with suspected leiomyosarcoma diagnosed by conventional methods. F-18-FLT PET is superior to F-18-FDG PET in differentiating malignant from benign leiomyoma. Moreover, F-18-FLT uptake correlated well with the immunohistochemical index of cell proliferation.