Appendiceal Collision Tumors: An Institutional Case Series and Systematic Review of the Histologic Spectrum, Clinical Outcomes, and Management Strategies.

Background/Objectives: Appendiceal collision tumors (ACTs), defined by the coexistence of two or more histologically distinct neoplastic components within the appendix, are rare entities. We aimed to characterize their clinicopathologic features, management strategies, and outcomes by integrating an institutional case series with a systematic review of the literature. Methods: We retrospectively identified ACTs diagnosed at our institution and performed a PRISMA 2020-guided search of PubMed, Scopus, and Web of Science databases through May 2025 for case reports and case series. Two reviewers screened studies and extracted data on presentation, histologic composition, treatment, approaches and outcomes. Results: ACTs accounted for 4% of appendiceal tumors in our institution, all combining a neuroendocrine neoplasm (NEN) with a low-grade appendiceal mucinous neoplasm. The literature search identified 69 ACTs from 33 studies; pooled with our cases, 74 patients were evaluated. The most common pairings were NEN-appendiceal mucinous neoplasm (53%) and NEN-adenocarcinoma (26%), while three-component tumors were rare (n = 2). Early-stage tumors (pTis-pT1) were uniformly managed with appendectomy or limited resection, in line with established stage-based management algorithms for appendiceal neoplasms. Advanced-stage tumors (pT3-pT4) were treated according to the biologically dominant component, frequently with colectomy and, in high-risk mucinous disease, cytoreductive approaches. Across stages, outcomes appeared to be driven by the non-neuroendocrine component; a coexisting low-grade NEN did not independently confer worse prognosis. In ACTs with an adenocarcinoma component, goblet cell morphology was common, and outcomes appeared similar to those reported for non-collision appendiceal adenocarcinoma. Conclusions: ACTs represent a heterogeneous group in which prognosis is dictated by the non-neuroendocrine component and tumor stage. Low-grade NEN components appear biologically indolent, whereas adenocarcinoma and high-risk mucinous components have been observed to exhibit behavior similar to their solitary counterparts.