A 46-year-old woman presented with severe acute pain in the right iliac fossa and periumbilical region.

Ultrasound and a computed tomography scan revealed a mucocele in the vermiform appendix, with a well defined lesion located at the mid zone of the appendix.

The patient underwent right hemicolectomy, and her postoperative clinical course was uneventful.

Gross examination of the surgical specimen showed an enlarged appendix, which was filled with thick mucinous material.

A distinct lesion which involved the appendiceal wall, and measured 1.5 cm maximally, was identified in the mid-portion of the appendix.

There was no evidence of perforation, extravasation of mucin into the periappendiceal tissue, or pseudomyxoma peritonei during surgery.

Histopathological examination showed combined GCC and MCA of the vermiform appendix (Figure ​1A).

The appendiceal lumen was dilated and lined by mucin-containing columnar epithelial cells (Figure 1B).

There was no significant cytologic atypia, and no mitotic figures were identified.

Focal papillary configurations of the lining epithelium, and mild epithelial pseudostratification were present.

In addition, the appendiceal wall was infiltrated by glandular structures of various sizes which were arranged in nests and tubules.

These glandular structures comprised 2 distinct types of cells: (1) small to intermediate sized monotonous neuroendocrine cells with a small amount of finely granular eosinophilic cytoplasm, and mild cytonuclear atypia (Figure ​(Figure1C);1C); (2) mucin-filled intermediate sized cells (goblet cells), with peripherally located small, crescent-like hyperchromatic nuclei, and indistinct nucleoli (Figure ​(Figure1D).1D).

Scattered infiltrating single goblet neoplastic cells were focally present.

As previously described[1] the tumor nests appeared to arise from the basiglandular region of the intestinal crypts in close proximity to the MCA (Figure ​(Figure1E).1E).

There was no lymphovascular invasion, although perineural and intraneural invasion was present.

The tumor infiltrated the full thickness of the appendiceal wall and extended to the mesoappendix.

Ten lymph nodes were histologically identified, of which all were negative for malignancy.

Immunohistochemically (Table ​1), the tumor cells of the GCC were positive for chromogranin, synaptophysin, and serotonin, which are neuroendocrine markers.

Diffuse staining for cytokeratin (CK) 20 (Figure ​2), CK19, and CD99 was also present.

The Ki67 proliferating index revealed nuclear staining in approximately 15% of the tumor cells.

There was no staining for CK7.