Melanoma of unknown primary origin with skeletal muscle metastasis: a case report

Melanoma is usually discovered from an irregular skin patch or a modification of a preexisting patch. Cutaneous and lymph node metastases are common. Muscle metastases are rare. We report a case of melanoma with infiltration of the gluteus maximus, which had normal dermatological examination.

A 43-year-old Malagasy man with no history of skin surgery was admitted with progressively worsening dyspnea. On admission, he presented with superior vena cava syndrome, painless cervical lymphadenopathy, and a painful swelling in the right buttock. Skin and mucous membrane examination did not reveal any abnormal or suspicious lesions. The biology was limited to a C-reactive protein of 40 mg/L, a white blood cell count of 23 G/L, and a lactate dehydrogenase level of 1705 U/L. The computed tomography scan showed several lymphadenopathies, compression of the superior vena cava, and a tissue mass at the expense of the gluteus maximus. Cervical lymph node biopsy and cytopuncture of the gluteus maximus were consistent with a secondary melanoma location. A stage IV melanoma of unknown primary origin, and with stage TxN3M1c associated with lymph node metastases and extension to the right gluteus maximus, was suggested.

Melanoma of unknown primary origin accounts for 3% of diagnosed melanomas. Diagnosis is difficult in the absence of a skin lesion. Patients are diagnosed with multiple metastases. Muscle involvement is unusual and may suggest a benign pathology. In this context, biopsy remains essential for diagnosis.