Case Report: Synchronous primary high-grade appendiceal mucinous neoplasm and ovarian borderline mucinous cystadenoma: a rare case and diagnostic challenges

Background Appendiceal mucinous neoplasms (AMNs), particularly high-grade (HAMN) variants, are rare and pose significant diagnostic challenges. Their presentation can mimic other abdominal pathologies, and the occurrence of synchronous primary mucinous tumors of the appendix and ovary is an exceptionally rare phenomenon that is poorly described in the literature. Case presentation We report a case of a 59-year-old woman who presented with right lower quadrant pain and a palpable mass. Preoperative imaging identified a large, complex cystic mass suspicious for an ovarian primary. Tumor markers (CA19-9, CA125, CA15-3) were elevated. Colonoscopy revealed mucous discharge from the appendiceal orifice, shifting diagnostic suspicion. Exploratory laparotomy revealed a large appendiceal mass with cellular peritoneal mucinous deposits and involvement of the right adnexa. The patient underwent right hemicolectomy and bilateral salpingo-oophorectomy. Histopathological and immunohistochemical analysis confirmed a synchronous primary HAMN (CK20+, Villin+, CK7-) of the appendix and a primary borderline mucinous cystadenoma (CK7+, PAX-8+, CK20-) of the left ovary. Conclusion This case underscores the diagnostic difficulty in distinguishing synchronous primary mucinous neoplasms from metastatic disease. A multidisciplinary approach, meticulous histopathological examination, and adjunctive immunohistochemistry are critical for accurate diagnosis and appropriate surgical management. This rare coexistence suggests the possibility of shared oncogenic pathways, warranting further investigation. Long-term surveillance is essential for these patients.