Track: Case Reports

Abstract:

Bone metastases from melanoma are rare, occurring in 5–17% of advanced cases, typically involving the axial skeleton. Distal appendicular involvement, including the phalanges, is very uncommon, and clinical and radiologic presentations mimicking chronic osteomyelitis are rarely reported. The literature notes that melanoma bone metastases often present as lytic lesions, which can mimic infection, particularly with superimposed bacteria, underscoring the need for histopathologic confirmation. We report a case illustrating these diagnostic challenges. A 72-year-old woman with no significant medical history presented with pain, swelling, and purulent discharge from the right thumb after trauma eight months earlier. MRI findings were consistent with osteomyelitis of the distal phalanx. Incision and drainage cultures grew Staphylococcus aureus (MRSA) and Streptococcus viridans, and targeted antibiotics were initiated. Despite therapy, symptoms progressed, necessitating partial amputation. Histologic examination revealed sheets of atypical epithelioid cells in an inflammatory, necrotic background. Immunohistochemistry was positive for Melan-A, HMB-45, S-100 and INI-1, and negative for CD31, CD34, CK5/6, CK7, CK20, Calretinin, CEA, BerEP4, EMA, and GCDFP-15 with Ki-67 labeling index 5-10%, confirming metastatic melanoma. No cutaneous primary was identified. This case highlights how amelanotic metastatic melanoma can mimic chronic osteomyelitis clinically and radiologically. Superimposed bacterial infection and prolonged inflammation may obscure malignancy. Given overlapping histology and inflammation, morphology alone can mislead diagnosis. Comprehensive immunohistochemical evaluation is critical in atypical or refractory bone lesions to avoid diagnostic delay and guide treatment. Amelanotic Metastatic melanoma should be considered in destructive bone lesions with inflammatory changes, even when imaging and cultures suggest infection.

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Shelcie Fabre