Primary Malignant Melanoma of the Colon: A Rare and Challenging Diagnosis.

Abstract

We describe a unique clinical case of primary melanoma of the ascending colon presenting with intussusception and profuse hemorrhage. A 72-year-old man on anticoagulant therapy was admitted with massive rectal bleeding and hypotension. Colonoscopy revealed an 8-cm obstructing exophytic tumor. CT scan confirmed enteroenteric intussusception via the pathognomonic pseudokidney sign. Diagnosis was established by immunohistochemical analysis, demonstrating positive expression of S100 protein and melanoma-associated antigens, while negative for melanocyte antigen A and BRAF mutations. The patient underwent emergency right-sided hemicolectomy with lymphadenectomy. Pathomorphological evaluation confirmed stage T4aN0M0 (R0 resection). Following an accelerated recovery protocol, the patient started adjuvant immunotherapy with PD-1 inhibitors. This case underscores the need for a multidisciplinary approach and extended immunohistochemical profiling to diagnose rare mucosal pathologies. It highlights the effectiveness of radical surgery combined with modern systemic immunotherapy in emergency settings. Given the aggressive nature of mucosal melanoma, adhering to oncological guidelines and considering early immunotherapy are crucial for optimizing clinical outcomes.