Localized Nodular Synovitis Mimicking Metastatic Melanoma in a Patient With Metastatic Melanoma on Whole-Body F-18 FDG PET/CT With MRI and Pathological Correlation

Abstract

A 47-year-old man with a history of malignant melanoma, status post wide local excision of the primary lesion of the left chest wall and postoperative interferon therapy for 1 week, presented with an indeterminate left upper lung lesion on CT. The patient was referred for an F-18 FDG-PET study to evaluate the extent of disease. PET/CT showed an intensely FDG avid nodule in the left upper lobe with a maximum standardized uptake value (SUVmax) of 6.9, and an incidental FDG avid lesion in the right mid leg, behind the right patella, with a SUVmax of 10.2, suggesting metastatic disease from malignant melanoma. Patient was referred for further characterization of this lesion with MRI. T1 and T2 MRI images demonstrated a hypointense mass in Hoffa's fat pad, caudal to the lower pole of the patella, posteriorly in contact with a minimal joint effusion suggesting localized pigmented villonodular synovitis (PVNS). This finding was subsequently confirmed on biopsy. The intense FDG activity in the subpatella lesion on FDG-PET in our case reminds us of the possibility of false-positive findings on FDG-PET in patients with malignant melanoma, and the usefulness of MRI in the evaluation of musculoskeletal lesions.