Dual-tracer 18F-FDG and 18F-FLT PET/CT for differentiating metastatic lymph nodes from sarcoid involvement in a patient with vulvar squamous cell carcinoma and coexisting pulmonary sarcoidosis

Abstract

Differentiating sarcoidosis lesions from metastatic lymph nodes is a well-known clinical challenge, particularly in patients with malignancy in whom sarcoid lesions may mimic nodal metastases on 18 F-fluorodeoxyglucose positron emission tomography/computed tomography ( 18 F-FDG PET/CT). We report a 75-year-old woman with concurrent sarcoidosis and vulvar squamous cell carcinoma in whom 18 F-FDG PET/CT demonstrated avid uptake in enlarged lymph nodes throughout the mediastinum, pulmonary hilum, upper abdomen, and right groin. 18 F-fluorothymidine ( 18 F-FLT) PET/CT performed 7 days later showed FLT uptake comparable to FDG uptake in the right groin lymph node (FDG SUV max 5.5, FLT SUV max 5.8), which was subsequently confirmed as a metastatic node at histopathology, whereas the mediastinal lymph nodes showed markedly weaker FLT uptake relative to FDG (FDG SUV max 11.5, FLT SUV max 4.5) and were considered compatible with sarcoidosis on the basis of the patient's known sarcoid involvement and stability on follow-up imaging over 385 days. This case illustrates that the combination of 18 F-FDG and 18 F-FLT PET/CT may aid in distinguishing sarcoid lesions from metastatic lymph nodes in patients with concurrent sarcoidosis and malignancy.