Pigmented villous nodular synovitis mimicking metastases on (18)F-FDG PET/CT in a patient with rectal mucosal melanoma: a case report

BACKGROUND: Mucosal melanomas are rare and have a high potential for metastasizing. Surgical resection is the treatment of choice for single distant metastases. Malignant melanoma usually shows the highest uptake of fluorine-18 fluorodeoxyglucose ((18)F-FDG). (18)F- FDG positron emission tomography /computed tomography (PET/CT) is usually used for melanoma staging. An extensive literature review revealed only 4 published case reports and an original paper involving 8 cases (12 cases in total) of patients with skin melanomas in whom pigmented villous nodular synovitis (PVNS) mimicked metastatic melanoma, however, none of the melanomas reported were of rectal mucosal origin. CASE PRESENTATION: A 60-year-old woman presented with recent diagnosis of rectal mucosal melanoma, two additional (18)F-FDG-avid lesions in the left ankle and left foot were detected on (18)F-FDG PET/CT. Metastases were initially suspected; however, the final diagnosis was PVNS. CONCLUSIONS: This is the first report of PVNS mimicking metastases on (18)F-FDG PET/CT in a patient with rectal mucosal melanoma. Although high (18)F-FDG-avid lesions in patients with rectal mucosal melanoma are highly suspected to be metastasis and warrant an meticulous examination, the present case is a reminder that in such patients, not all lesions with high (18)F-FDG uptake, especially those near a joint, are metastases and that more extensive resection is unnecessary.