Synchronous Low-Grade Appendiceal Mucinous Neoplasm and Neuroendocrine Tumor Presenting as Pseudomyxoma Peritonei: A Case Report.

Synchronous primary neoplasms of the appendix are exceptionally rare. We report a low-grade appendiceal mucinous neoplasm (LAMN) and a well-differentiated neuroendocrine tumor (NET) arising within the same appendix and presenting as pseudomyxoma peritonei (PMP). A 44-year-old man undergoing staging for a solitary plasmacytoma of the left acetabulum was incidentally found on computed tomography (CT) to have extensive peritoneal mucinosis. Diagnostic laparoscopy confirmed diffuse PMP, and appendectomy revealed two synchronous appendiceal primaries: a perforated LAMN (pT4a pNX) and a 6 mm, grade 1 NET with lymphovascular invasion (pT3 pNX). After multidisciplinary review, the patient completed radiation therapy for the plasmacytoma and subsequently underwent cytoreductive surgery with hyperthermic intraperitoneal chemotherapy (CRS/HIPEC), achieving complete cytoreduction with 33 lymph nodes negative for malignancy. This case highlights the importance of intraoperative suspicion for an appendiceal origin of PMP, the definitive role of histopathology in identifying synchronous pathologies, and the value of coordinated multidisciplinary care when multiple malignancies coexist.