Advanced neuroendocrine tumor of the appendix with adnexal infiltration: case report and literature review

Introduction: Neuroendocrine tumors (NETs) of the appendix are rare lesions, typically discovered incidentally and characterized by an indolent clinical course. High-grade neuroendocrine carcinomas (NECs), however, are exceptional and associated with aggressive behavior, early dissemination, and poor prognosis. Local invasion of adjacent organs, such as the adnexa, is exceedingly uncommon.Case presentation: We report a case of a 71-year-old woman presenting with right-sided abdominal pain and altered bowel habits. Imaging revealed a cecal mass measuring 7.5 cm, associated with an appendiceal mucocele and enlarged para-aortic lymph nodes. Intraoperatively, a locally advanced tumor involving the cecum, retroperitoneum, and right adnexa was identified. A right hemicolectomy with adnexectomy and lymphadenectomy was performed. Histopathology demonstrated a high-grade large-cell neuroendocrine carcinoma of the appendix (pT4N1M1b, LV1, Pn+, R0) with vascular and perineural invasion and multiple nodal metastases. The postoperative course was uneventful.Discussion: Large-cell NEC of the appendix is exceptionally rare and may mimic other ileocecal or adnexal malignancies. Definitive diagnosis requires histopathologic and immunohistochemical confirmation. Radical surgical resection remains the cornerstone of therapy, while systemic platinum-based chemotherapy (cisplatin/carboplatin with etoposide) is recommended for advanced disease. Although prognosis is poor, R0 resection offers the best chance for prolonged survival.Conclusion: This case highlights an unusual manifestation of locally advanced large-cell NEC of the appendix with direct adnexal and retroperitoneal invasion, underscoring the importance of a high index of suspicion and a multidisciplinary approach to diagnosis and management.