Pure muscle tenosynovial giant cell tumor mimicks a metastasis in patient with melanoma

Tenosynovial Giant Cell Tumor (TGCT) is a rare benign synovial tumor arising from the tendon sheath, bursae, synovium or adjacent soft tissue. TGCT typical occurs in intra-articular site but can be rarely extra- articular. A 50-years-old woman already affected by malignant melanoma was submitted to PET/CT scan for routine diagnostic examinations during follow up; an intense FDG focal uptake corresponding to peritrochanteric medial part of right iliopsoas muscle was detected. Corresponding MRI images demonstrated a lesion with well-defined margins, slightly hyperintense in T1, low to intermediate signal intensity in T2 weighted scans, no significant restriction in DWI, and intense enhancement in T1 after intravenous contrast medium administration. There wasn’t evidence of extra-lesional spread. MRI findings oriented for benign lesion, but according to intermediate signal intensity in T2, intense enhancement in T1 after intravenous contrast medium, the presence of intense FDG uptake as well as history of aggressive malignancy, a local excision was done; then the final diagnosis of TGCT has been proven. Our case shows TGCT lesions may reproduce malignant appearance on FDG-PET, while MRI may be useful tool to properly manage affected patients. Keywords: Tenosynovial giant cell tumor; Melanoma metastasis; Diffusion-MRI; FDG-PET/CT.