Histopathology

MCNs show two distinct components: an inner epithelial layer and an outer densely cellular ovarian-type stromal layer. Large locules can be extensively denuded and many sections are often needed to demonstrate the epithelial lining. The epithelium may be flat or it may form papillary or polypoid projections, pseu-

Fig. 10.18 Mucinous cystic neoplasm presenting as multiloculated cystic mass in the tail of the pancreas.

dostratifications and crypt-like invaginations. The columnar cells are characterized by basally located nuclei and abundant intracellular mucin which is diastase-PAS and Alcian blue positive. Pseudopyloric, gastric foveolar, small and large intestinal, and squamous differentiation can also be found. About half of the tumours contain scattered argy-rophil and argentaffin endocrine cells at the bases of the columnar cells {33, 36, 328, 2151}.

Spectrum of differentiation

This ranges from histologically benign appearing columnar epithelium to severely atypical epithelium. According to the grade of intraepithelial neoplasia (dysplasia), tumours may be classified as adenoma, borderline (low-grade malignant) and non-invasive or invasive carcinoma {947}.

Mucinous cystadenomas show only a slight increase in the size of the basally located nuclei and the absence of mitosis. Mucinous cystic neoplasms of borderline malignant potential exhibit papillary projections or crypt-like invaginations, cellular pseudostratification with crowding of slightly enlarged nuclei, and mitoses. Mucinous cystadenocarcinomas may be invasive or non-invasive. They show changes of high-grade intraepithelial neoplasia which are usually focal and may be detected only after careful search of multiple sections from different regions. The epithelial cells, which often form papillae with irregular branching and budding, show nuclear stratification, severe nuclear atypia and frequent mitoses.

Invasive mucinous cystadenocarcinoma is characterized by invasion of the malignant epithelium into the stroma. The invasive component usually resembles the common ductal adenocarcinoma. How

Fig. 10.19 Mucinous cystadenocarcinoma. The neoplasm exhibits well differentiated and poorly differentiated mucinoius epithelium.

ever, mucinous cystadenocarcinomas with invasive adenosquamous carcinoma, osteoclast-like giant cell or chorio-carcinoma have been reported {328, 1530, 1571, 2194}. Invasive foci may be focal and require careful search.

Stroma

The ovarian-type stroma consists of densely packed spindle-shaped cells with round or elongated nuclei and sparse cytoplasm. It frequently displays a variable degree of luteinization, characterized by the presence of single or clusters of epithelioid cells with round to oval nuclei and abundant clear or eosinophilic cytoplasm. Occasionally, these cells, resembling ovarian hilar cells, can be found associated with (or present in) nerve trunks. Stromal luteini-zation is found in decreasing order of frequency from adenomatous to carcinomatous cases {2194}. The stroma of large MCNs may become fibrotic and hypocel-lular. Rare MCNs show mural nodules with a sarcomatous stroma or an associated sarcoma {1932, 2088, 2198}.

Immunohistochemistry

The epithelial component is immunoreac-tive with epithelial markers including EMA, CEA, cytokeratins 7, 8, 18 and 19 {2151}, and it may show gastroen-teropancreatic differentiation, as is also observed in ovarian and retroperitoneal MCN {1714, 1910}. With increasing degrees of epithelial atypia the character of mucin production changes from sul-phated to sialated or neutral mucin {1932}. The neoplastic cells express gas-

Fig. 10.20 Mucinous cystadenocarcinoma. The thick wall of this cystic neoplasm is invaded by mucinous carcinoma at upper left.

tric type mucin marker M1 and PGII, the intestinal mucin markers CAR-5 and M3SI, and the pancreatic type mucin marker DUPAN-2 and CA19-9 {119, 1714, 2151, 2190}. Furthermore, pancreatic, hepatobiliary, and retroperitoneal MCNs share the same types of intraep-ithelial endocrine cells {613, 1911, 1910}. p-53 nuclear positivity in more than 10% of neoplastic cells, found in 20% of MCN, strongly correlates with mucinous cys-tadenocarcinoma {2198}. The stromal component expresses vimentin, alpha smooth muscle actin, desmin and, in a high proportion, progesterone and estrogen receptors {2198}. The luteinized cells are labeled with antibodies against tyrosine hydroxylase, cal-retinin, which have been shown to recognize testicular Leydig cells and hilar ovarian cells, and the sex cord-stromal differentiation marker inhibin {2198, 2206}.

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