Finally, I'm working on sharing some interesting cases that have come across my desk recently! We've seen some interesting appendix pathology of late, including this case.
Adenomas and other epithelial neoplasms that produce a "mucocele" (=cystic distension by mucous) must be ruled out. I suppose the main differential diagnoses include appendiceal cystadenoma and low-grade appendiceal mucinous neoplasm, followed by adenomas associated with appendiceal dilation and low-grade mucinous adenocarcinoma. Appendiceal mucoceles are usually considered to be a post-inflammatory sequela and not neoplastic. Synonyms include retention cyst and inflammatory mucocele. Obviously, the entities in the d/dx are indeed neoplastic. In low-grade appendiceal mucinous neoplasm, the appendix is usually enlarged and fusiform, weirdly deformed or completely destroyed. Microscopically, the lining typically shows short villiform tufts but the epithelium may be lost or flat in areas. One may also see the epithelium invaginating deeply into the wall of the lesion without definitive invasion. The appendix may be dilated or even normal with adenomas. Most seem to me to be villous, although the literature has recently identified serrated adenomas to also be common. When an adenoma is associated with dilatation, the term cystadenoma is frequently used. Definitely one needs to extensively sample the specimen when a cystic abnormality is identified grossly or when the above microscopic features are seen.