While serous acinar cell differentiation defines acinic cell carcinoma, several cell types and histomorphologic growth patterns are recognized. These are acinar, intercalated ductal, vacuolated, clear, and non-specific glandular and solid/lobular, microcystic, papillary-cystic, and follicular growth patterns {161, 478,668,1492,2290,2304}. Acinar cells are large, polygonal cells with lightly basophilic, granular cytoplasm and round, eccentric nuclei. The cytoplasmic zymogen-like granules are PAS positive, resistant to diastase digestion, and weakly stained or non-stained with mucicarmine. However, the PAS positivity can sometimes be very patchy and not immediately obvious. Intercalated duct type cells are smaller, eosinophilic to amphophilic, cuboidal with central nuclei, and surround variably sized luminal spaces. Vacuolated cells contain clear, cytoplasmic vacuoles that vary in number and size. The vacuoles are PAS negative. Clear cells are similar in size and shape to acinar cells but have non-staining cytoplasm that is non-reactive with PAS staining. Non-specific glandular cells are round to polygonal, amphophilic to eosinophilic cells with round nuclei and poorly demarcated cell borders. They often develop in syncytial sheets.

Tumour cells are closely apposed to one another in sheets, nodules, or aggregates in the solid/lobular growth pattern. Numerous small spaces that vary from several microns to a millimetre or more in size characterize the microcystic pattern. Prominent cystic lumina, larger than the

Fig. 5.1 Acinic cell carcinoma. Vacuolated cells in acinic cell carcinoma are similar in size to intercalated duct-type cells but have clear, cytoplasmic vacuoles, which sometimes distend the cellular membranes.

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Fig. 5.2 Acinic cell carcinoma. A Clear cells in acinic cell carcinoma are similar in size and shape to acinar-type cells but have non-staining cytoplasm. Some cells have a variable amount of eosinophilic cytoplasm. B The cytoplasmic granules in serous acinar-type cells in acinic cell carcinoma stain with PAS and are resistant to diastase digestion. C Sheets of tumour cells, acinar-type cells in this case, with few or no cystic spaces characterizes a solid growth pattern in acinic cell carcinoma. D Often acinar type cells are scattered among nonspecific glandular cells. They are often inconspicuous with H&E-stain but highlighted with PAS stain (PAS stain).

microcystic spaces that are partially filled with papillary epithelial proliferations characterize the papillary-cystic pattern. This variant, in particular, may be very vascular and haemorrhagic and sometimes phagocytosis of haemosiderin by luminal tumour cells is a conspicuous feature. In the follicular pattern, multiple, epithelial-lined cystic spaces are filled with eosinophilic proteinaceous material, which produces a thyroid follicle-like appearance. Psammoma bodies are occasionally seen and are sometimes numerous. They are not restricted to the papillary-cystic variant and have been reported in FNA specimens. Although a single cell type and growth pattern often dominate, many tumours have combinations of cell types and growth patterns. Acinar cells and intercalated duct-like cells often dominate while the other cell types seldom do. Clear cells are seen in only 6% of all acinic cell carcinomas {670}. They are usually focal and only rarely cause diagnostic confu sion. The solid/lobular and microcystic patterns are most frequent, followed by the papillary-cystic and follicular patterns.

A prominent lymphoid infiltrate of the stroma is associated with many acinic cell carcinomas {83,1717}. Whereas a heavy lymphoid infiltrate by itself has no prognostic significance, some tumours are well-circumscribed masses arranged in a microfollicular growth pattern and with a low proliferation index. They are completely surrounded by the lymphoid infiltrate (with germinal centre formation) and a thin fibrous pseudocapsule. These tumours appear to constitute a subgroup that behaves far less aggressively than other acinic cell carcinomas {1717}.

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