Answer: Ruptured appendiceal diverticula mimicking low grade mucinous neoplasm

Histology: none provided

Discussion: Sections show appendix with diverticula and periappendiceal
extravasated mucin with associated fibroinflammatory reaction and scant detached fragments of free floating appendiceal mucosa with acute inflammation and reactive epithelial atypia. A small foreign body giant cell granuloma is also noted. The luminal mucosa shows areas of hyperplastic change. No direct communication between the luminal mucosa of the appendix including the diverticula and the serosal surface is identified. Benign, partially flattened colonic mucosa is seen on what appeares to be serosal surface. This area lacks muscularis propria and exhibits protrusion of submucosa and therefore we interpret it as an appendiceal diverticulum with eversion of the appendiceal lining onto the serosa. No adenomatous epithelium is identified in ether the luminal mucosa or within the extravasated mucin.

These findings are consistent with a ruptured appendiceal diverticula that mimics a low grade appendiceal mucinous neoplasm.

Appendiceal diverticula can rupture, resulting in mucin on the appendiceal serosa, which may raise concern for an underlying appendiceal mucinous neoplasm. In a recent study (Ruptured appendiceal diverticula mimicking low-grade appendiceal mucinous neoplasms by Maylee Hsu, M.D.; Robert H Young, M.D.; Joseph Misdraji, M.D.; AJSP, in press) the authors report 11 cases of ruptured appendiceal diverticula that were initially either misdiagnosed as appendiceal mucinous neoplasms, raised concern for a neoplasm, or were thought to exhibit localized pseudomyxoma peritonei. Two cases showed eversion of the appendiceal lining onto the serosa; 1 showed collision between the diverticulum and endosalpingiosis; and 3 had rare non-neoplastic epithelial cells in extra-appendiceal mucin. Most cases showed mucosal hyperplasia, mild crypt disarray, and variable reactive atypia. Eight cases had mucosal neuromas or other neural changes. None of the patients progressed to pseudomyxoma peritonei during the follow up interval (mean 23 months). The authors concluded that ruptured appendiceal diverticula may be associated with serosal mucin and even extra-appendiceal epithelium. These findings support our interpretation of the current material.