Mucinous lesions of the appendix
• Mucocele is a cystically dilated appendiceal lumen containing mucus. It can be nonneoplastic or neoplastic. Any obstruction of the lumen can give rise to mucocele.
• In low-grade mucinous adenocarcinomas, with pseudomyxoma peritonei, the mucin/tumor cells dissect through the wall of the appendix into the peritoneum. Most cases of synchronous tumors in the ovary and appendix are now considered metastases from the appendiceal tumor. Acellular pools of mucin pose a diagnostic problem. A diagnosis of adenoma (or cystadenoma) should be rendered only if the entire muscularis mucosae is intact. A diagnosis of “low-grade appendiceal mucinous neoplasm or LAMN” previously known as “uncertain malignant potential” is favored in the cases in which intact muscularis mucosae cannot be seen.
• Mucinous adenocarcinomas with mucinous carcinomatosis include signet ring cell carcinomas, invasive well-differentiated carcinomas, and cystadenocarcinoma.