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Pleomorphic adenoma is a benign neoplastic tumour of the salivary glands. It is the most common type of salivary gland tumour and the most common tumour of the parotid gland. It derives its name from the architectural pleomorphism (variable appearance) seen by light microscopy. It is also known as "Mixed tumor, salivary gland type", which describes its pleomorphic appearance as opposed to its dual origin from epithelial and myoepithelial elements.

The tumour is usually solitary and presents as a slow growing, painless, firm single nodular mass. Isolated nodules are generally outgrowths of the main nodule rather than a multinodular presentation. It is usually mobile unless found in the palate and can cause atrophy of the mandibular ramus when located in the parotid gland. When found in the parotid tail, it may present as an eversion of the ear lobe. Though it is a benign tumour, pleomorphic adenomas have the capacity to grow to large proportions.

Histologically, it is highly variable in appearance, even within individual tumours. It is characterized by an admixture of epithelial and myoepithelial elements in a variable background stroma that may be mucoid, myxoid, cartilaginous or hyaline. Epithelial elements may be arranged in duct-like structures, sheets, clumps and/or interlacing strands and consist of polygonal, spindle or stellate-shaped cells (hence pleomorphism). Areas of squamous metaplasia and epithelial pearls may be present. The tumour is usually enveloped by a fibrous capsule of varying thickness and often incomplete. The tumour often extends through these discontinuities but is not a sign of malignant transformation as it does not invade surrounding tissues.

Treatment is by surgical excision with a margin of normal healthy tissue. Recurrences are rare unless treated too conservatively i.e. enucleation can lead to spillage and seeding of tumour cells.

Pleomorphic adenoma consists of mixed epithelial (left) and mesenchymal cell components (right). The latter often exhibits myxofibrous appearance and in some instances shows chondromatous differention.

• Warthin's tumor - monomorphic adenoma

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• Harvard