Silent Invasion of the Rectum: Aggressive Anorectal Melanoma With Pelvic Mass Effect, Venous Compression, and Hepatic Metastases.

Anorectal melanoma is a rare and aggressive malignancy arising from melanocytes within the mucosal epithelium of the anorectal region. Because presenting symptoms such as rectal bleeding frequently mimic benign anorectal conditions, diagnosis is often delayed until advanced stages of the disease. Mucosal melanoma is characterized by aggressive local invasion and early metastatic spread. We present the case of a 54-year-old female with metastatic rectal melanoma whose disease course was complicated by extensive pelvic tumor burden, malignant venous compression leading to deep venous thrombosis, obstructive uropathy, and tumor-associated hemorrhage. Cross-sectional imaging demonstrated a massive posterior pelvic mass occupying the rectouterine space with displacement of surrounding structures, additional rectal tumor extension, metastatic lesions in the liver, and locally invasive disease adjacent to the patient's colostomy. Diagnosis was confirmed through sequential biopsy, molecular analysis, and definitive surgical pathology, demonstrating a poorly differentiated malignant neoplasm with melanocytic differentiation and immunohistochemical positivity for S100, SOX10, Melan-A, and HMB45. Despite thrombectomy with iliac vein stenting, percutaneous nephrostomy tube placement, arterial embolization, and multidisciplinary oncologic management, the patient experienced progressive metastatic disease and ultimately transitioned to hospice care. This case highlights the aggressive nature of anorectal melanoma and illustrates how advanced pelvic malignancy may produce complex multisystem complications requiring multidisciplinary management.