Necrotic Liver Metastasis From Anorectal Mucosal Melanoma Mimicking a Pyogenic Liver Abscess: A Case Report

Anorectal mucosal melanoma is a rare and aggressive malignancy that is often diagnosed after regional or distant spread. Liver lesions in patients with melanoma are usually approached with metastatic disease in mind; however, extensive necrosis can make metastases appear clinically and radiologically similar to pyogenic abscesses. We report a 76-year-old woman with a history of anorectal mucosal melanoma treated by abdominoperineal resection with microscopically negative (R0) margins and adjuvant radiotherapy 10 years earlier. She presented with right upper quadrant pain, elevated inflammatory markers, and ultrasound findings interpreted as a hepatic abscess. Empirical broad-spectrum antibiotics and computed tomography (CT)-guided drainage yielded sterile brown hemorrhagic fluid. Persistent symptoms, fever, black-pigmented catheter output, sterile cultures, and magnetic resonance imaging (MRI) showing a centrally necrotic lesion prompted multidisciplinary reassessment. Cytology demonstrated melanin pigment but no viable malignant cells. The patient underwent right hemihepatectomy with cholecystectomy. Histopathology confirmed metastatic melanoma with approximately 95% tumor necrosis and scattered viable cells positive for SRY-box transcription factor 10 (SOX10) and human melanoma black 45 (HMB-45). This case emphasizes that necrotic melanoma metastasis can masquerade as a liver abscess and that sterile cultures, pigmented drainage, prior melanoma history, and discordant imaging should prompt early oncologic reassessment and tissue confirmation.