Serous Oligocystic Adenoma

Although SCTs have been classified as microcystic adenomas, recent advances in imaging techniques have revealed more cases of serous oligocystic adenoma [25]. Serous oligocystic adenoma is the second common subtype of SCT, accounting for 7-30% of all SCTs [14, 19, 20, 27, 30, 31]. Synonyms include macrocystic serous cystadenoma, and serous oligocystic and ill-demarcated adenoma [1, 8, 30-33].

Serous oligocystic adenoma occurs equally in both sexes. Tumors usually affect individuals with a mean age in the fifth decade (26-73 years old), younger than patients with serous microcystic adenoma with a mean age in the seventh decade [31]. Tumors are often identified incidentally during US or CT. Some patients are symptomatic, complaining of abdominal pain or dyspepsia. Patients do not have VHL disease. Serous oligocystic adenomas can develop

Fig. 7. Serous oligocystic adenoma. A few, relatively large cysts in the pancreatic tail.

anywhere in the pancreas, but they show a prediction for the pancreatic head [32]. Laboratory examination including tumor markers is within the normal range in all patients. Fine-needle aspiration is acellular and thus not informative for the cytological analysis [7]. Pre-operative diagnosis of serous oligocystic adenoma is difficult. Serous oligocystic adenoma exhibits radiologic and gross pathologic features distinct from those described for serous microcystic adenoma. It may be misdiagnosed as MCT that require resection, as a pseudocyst, or as a solitary true cyst. Recognition of an oligocystic variant of SCT is therefore important for physicians and pathologists alike [33]. At present, the diagnosis is still based on pathologic examination after surgical removal of the tumor [31]. These tumors show no malignant transformation [11].

The gross appearance of serous oligocystic adenoma is distinctly different from that of serous microscopic adenoma. The sizes of the tumors range from 1.5 to 15.0 cm. They are characterized by macrocystic (>2.0 cm in diameter) or oligocystic patterns (fig. 7) [30]. These are composed of fewer but larger cysts. Some tumors show a predominantly or exclusive unilocular pattern [33]. Minute microcysts are found surrounding the main cavity; some of these are not apparent on gross examination. Endoscopic US may be useful in detecting peripherally located millimeter-sized cysts in unilocular lesions [31]. These microscopic cysts support the idea that this tumor is a particular form of serous microscopic adenoma in which some cysts develop at the expense of the others

Fig. 8. Solid serous adenoma. Well-demarcated solid mass in the pancreatic body.

[30, 31]. Thick-walled cysts have a glistening inner surface and clear fluid content. Some cysts are filled with dark-brown fluid derived from old blood [33]. The cyst walls are smooth. No mural nodules, papillary projections, or calcifications are present [34]. A central stellate scar or network of fibrous septa is absent [30, 35]. These features are similar to those seen in mucinous cystic tumors. Serous oligocystic adenomas are often well-circumscribed from the surrounding pancreatic tissue, although some are poorly demarcated [32].

Microscopically, the serous oligocystic adenoma is easily distinguishable from mucinous cystic neoplasm, as the latter is lined by tall columnar mucin-rich cells with an ovarian-like stroma [36]. The histological features of serous oligocystic adenoma are quite similar to those of the serous microcystic adenoma. The cysts are lined with a single layer of cuboidal, PAS-positive epithelium. The lining epithelium shows neither mitoses nor cytological atypia. SCTs lack ovarian-like stroma. There is no communication between the cysts and the excretory duct system [30]. The histopathologic diagnosis is difficult when the epithelial lining is extensively denuded.

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