Differential diagnosis

The differential diagnosis includes pleo-morphic adenoma (PA) and adenoid cystic carcinoma (AdCC), especially in small biopsy specimens. Unlike PLGA, PA is nearly always circumscribed and is composed of proliferating stromal, epithelial, and myoepithelial cells. It lacks the infiltrative, noncircumscribed character of

PLGA. Although myxoid tissue is present in both tumours, the myxochondroid and chondroid areas present in PA are not evident in PLGA. Also, the typical benign plasmacytoid myoepithelial cells characteristic of palatal PA are seldom observed in PLGA. Staining with GFAP may be helpful in differentiating PA from PLGA.

The distinction between PLGA and AdCC is based primarily on cytologic features. Cells in PLGA are cuboidal or columnar. They have vesicular nuclei and often conspicuous eosinophilic cytoplasm without the basaloid features characteristic of AdCC. Papillary and fascicular growth patterns are extremely rare in AdCC. Furthermore, PLGA does not have large cribriform pseudocystic spaces that contain pools of haema-toxyphilic glycosaminoglycans. The solid cellular areas of PLGA lack nuclear pleo-morphism, necrosis, increased mitotic activity, and the numerous tubular structures characteristic of the solid variant of AdCC. The potential discriminating value of immunohistochemistry between cases of PLGA and AdCC remains controversial {547}, although some subtle differences may be apparent when series of these two neoplasms are studied {342,2006,2011,2391}. Proliferative cell marker rates in PLGA are usually less than 6.4% (mean values 1.6% and 2.4%) {2391}. However, a higher proliferative rate (average 7%) has been reported by others investigators {2011}.

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