A Pilot Study of the Value of 18F-Fluoro-Deoxy-Thymidine PET/CT in Predicting Viable Lymphoma in Residual 18F-FDG Avid Masses After Completion of Therapy

Background: Despite its success in diagnosing and staging lymphoma, F-18-FDG PET/CT can be falsely positive in areas of posttreatment inflammation. 3'-F-18-fluoro-3'-deoxy-l-thymidine (F-18-FLT) is a structural analog of the DNA constituent thymidine; its uptake correlates with cellular proliferation. This pilot study evaluates the ability of F-18-FLT PET/CT to distinguish viable lymphoma from posttreatment inflammatory changes in F-18-FDG avid residual masses. Methods: Twenty-one patients with lymphoma with at least 1 F-18-FDG avid residual mass after therapy underwent F-18-FLT PET/CT imaging. F-18-FDG and F-18-FLT uptake values were compared, including quantitative pharmacokinetic parameters extracted from the F-18-FLT time activity curves generated from dynamic data using graphical and nonlinear compartmental modeling. Results: The true nature of the residual mass was confirmed by biopsy in 12 patients (8 positive and 4 negative for viable lymphoma and by follow-up CT and/or repeat F-18-FDG PET/CT imaging over 1 year); among the remaining 9 patients, 7 lesions resolved or decreased and 2 showed growth indicative of lymphoma. F-18-FLT PET SUVest.max was significantly higher in tumors than in benign lesions (5.5 [2.2] vs 1.7 [0.6]; P < 0.0001), whereas the difference in F-18-FDG SUVs was not significant (malignant, 7.8 [3.8] vs benign, 5.4 [2.4]; P = 0.11). All of the benign lesions had an F-18-FLT SUVest.max of less than 3.0. Conclusions: F-18-FLT shows improved specificity over F-18-FDG in distinguishing residual lymphoma from posttreatment inflammation and may be useful in the evaluation of patients with residual F-18-FDG-positive masses after completing therapy.