The tumours are usually well circumscribed but not encapsulated. Numerous haphazardly arranged cysts are evident that are partially filled with mucin, vary in shape and size, and have limited intervening fibrous connective tissue. Small solid neoplastic islands or duct-like structures may occur between the cysts or at the advancing front of the tumour. In about 75% of the cases the lumens of the cysts exhibit varying degrees of papillary proliferation. In either case, cell types that comprise the lining epithelium include, most often, small and large cuboidal, and columnar cells, but mucous, clear and oncocytic cells are occasionally noted. The columnar-rich tumours often predominate in the intralu-minal papillary areas and account for their "gastrointestinal" appearance, but the cells usually fail to stain for neutral mucin. Although nucleoli are evident, the nuclei typically are uniformly bland and mitoses rare. However, a prerequisite for the diagnosis is that the cysts and smaller duct-like structures at least focally infiltrate the salivary parenchyma and surrounding connective tissue. The presence of ruptured cysts with haemorrhage and granulation tissue is common.
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