A mucinous ovarian tumor, especially when it is bilateral or comes with mucinous ascites, is appendiceal in origin until proven otherwise. Immunohistochemistry helps: appendiceal metastases are typically CK20 and CDX2 positive with SATB2 expression and are often CK7 negative, the reverse of most primary ovarian mucinous tumors. When the pattern points away from the ovary, the appendix is the prime suspect.

At surgery for a mucinous ovarian mass, inspect the appendix and consider removing it, and make sure it reaches pathology. An appendix that looks normal can still be the primary source. Documenting its appearance and sending it for review can settle the origin question that changes the entire treatment plan.

Pseudomyxoma peritonei often arrives looking like advanced ovarian cancer: abdominal distension, gelatinous ascites, and raised CA125 or CA19-9. It is usually appendiceal. Recognizing it as mucinous peritoneal disease, rather than a gynecologic primary, points toward the right surgical team.

If an appendiceal origin is suspected, the surgical approach differs from gynecologic debulking. The disease is best managed with cytoreductive surgery and HIPEC at a peritoneal surface malignancy center. Referring before a large gynecologic operation protects the option of a complete cytoreduction later, which is the strongest driver of outcome. The reference for management is the 2025 appendiceal consensus guideline, not colorectal or ovarian protocols.