EP641 - Collision Tumor in the Rectum. Case report

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Purpose/Background: Collision tumors are rare entities in oncology, defined as the simultaneous coexistence of two independent neoplasms in the same anatomical site. Unlike composite tumors, collision tumors lack significant intermingling between their components, presenting unique diagnostic and therapeutic challenges. Their diagnosis primarily depends on histopathology, supported by immunohistochemistry and genetic analysis. Although few cases have been documented in the rectum, adenocarcinomas are the most frequent component, with neuroendocrine tumors, squamous cell carcinomas, and lymphomas also reported.

Methods/Interventions: A 44-year-old man with HIV infection under ART and virological suppression presented with a persistent anal mass and hematochezia. After an initial evaluation, well-differentiated squamous cell carcinoma in the anal region was diagnosed, along with a rapidly growing tumor compatible with Buschke-Löwenstein. The patient underwent chemotherapy and radiotherapy according to the NIGRO protocol (5-fluorouracil and mitomycin), achieving a complete clinical response at 12 weeks. However, 18 months later, the presence of a mixed anal carcinoma was confirmed, identifying small-cell neuroendocrine carcinoma (Ki67 index of 95%), with components of high-grade neuroendocrine carcinoma and non-keratinizing grade 2 squamous cell carcinoma. An abdominoperineal resection was performed, with the assistance of plastic surgery for primary closure of the surgical bed. The histopathological examination of the surgical specimen confirmed the presence of mixed anal carcinoma.

Results/Outcomes: Collision tumors in the rectum, though rare, are clinically significant due to their diagnostic and therapeutic complexity. The coexistence of squamous cell carcinomas and neuroendocrine tumors is particularly rare and poses challenges in both treatment planning and prognosis. Various theories on their pathogenesis suggest the possibility of separate histogenetic events or a common carcinogenic stimulus. In this case, the primary tumor was successfully managed, but the later emergence of aggressive neuroendocrine carcinoma underscores the need for long-term surveillance. Molecular studies can clarify whether these tumors arise from different clones or share a common genetic mutation.

This case highlights the importance of a multidisciplinary approach in managing collision tumors, combining surgery, chemotherapy, and radiotherapy to maximize tumor control. Despite the initial treatment success, the aggressive evolution of neuroendocrine carcinoma indicates that early identification and continuous follow-up are essential for improving prognosis. Collision tumors present a clinical challenge due to their low incidence and unpredictable biological behavior, justifying the need for multicenter collaborative studies to develop clearer treatment guidelines.

Conclusion/Discussion: Collision tumors in the rectum present a rare combination of neoplasms that require individualized therapeutic approaches. Surgery, combined with systemic treatments, remains the cornerstone of their management. However, the lack of specific guidelines and the complexity of these neoplasms highlight the need for further studies to better understand their biology and optimize therapeutic strategies. This case reinforces the relevance of molecular studies and advanced imaging for early detection and precise characterization of tumor components to improve long-term clinical outcomes.