Giant Cell Tumor of the Tendon Sheath With Discordant Metabolism as a False Positive on Staging of Mantle Cell Lymphoma.

A baseline F-FDG PET/CT scan in a patient with mantle cell lymphoma showed diffuse minimally FDG-avid lymphadenopathy and splenomegaly. There was also a focus of uptake in the left subscapularis muscle without a CT correlate. A post-chemotherapy scan showed interval decrease in size, and resolution of FDG uptake, of the lymph nodes and spleen. Persistent activity was seen in the subscapularis muscle. Posttreatment biopsy of the FDG-avid lesion showed a benign giant cell tumor of tendon sheath. This case illustrates that a lesion with a markedly discordant SUV should raise suspicion for a second process.