Pleomorphic adenoma is usually arisen from

Pleomorphic adenomas present as painless, slow-growing, mobile discrete masses. They represent about 60% of tumors in the parotid, are less common in the submandibular glands, and are relatively rare in the minor salivary glands. Pleomorphic adenomas are benign tumors that consist of a mixture of ductal (epithelial) and myoepithelial cells, so they exhibit both epithelial and mesenchymal differentiation. Epithelial elements are dispersed throughout the matrix, which may contain variable mixtures of myxoid, hyaline, chondroid (cartilaginous), and even osseous tissue. In some pleomorphic adenomas, the epithelial elements predominate; in others, they are present only in widely dispersed foci. This histologic diversity has given rise to the alternative, albeit less preferred name mixed tumor. The tumors consistently overexpress the transcription factor PLAG1, often because of chromosomal rearrangements involving the PLAG1 gene, but how PLAG1 contributes to tumor development is unknown. Pleomorphic adenomas typically manifest as rounded, well-demarcated masses rarely exceeding 6 cm in the greatest dimension. Although they are encapsulated, in some locations (particularly the palate), the capsule is not fully developed, and expansile growth produces protrusions into the surrounding tissues. The cut surface is grey-white and typically contains myxoid and blue translucent chondroid (cartilage-like) areas. The most striking histologic feature is their characteristic heterogeneity. Epithelial elements resembling ductal or myoepithelial cells are arranged in ducts, acini, irregular tubules, strands, or even sheets. These typically are dispersed within a mesenchyme-like background of loose myxoid tissue containing islands of chondroid and, rarely, foci of bone (Fig. 14–6). Sometimes the epithelial cells form well-developed ducts lined by cuboidal to columnar cells with an underlying layer of deeply chromatic, small myoepithelial cells. In other instances, there may be strands or sheets of myoepithelial cells. Islands of well-differentiated squamous epithelium also may be present. In most cases, no epithelial dysplasia or mitotic activity is evident. No difference in biologic behaviour has been observed between the tumors composed largely of epithelial elements and those composed largely of mesenchymal elements.