Pleomorphic adenoma

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.

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Clinical Presentation

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 classified as a benign tumour, pleomorphic adenomas have the capacity to grow to large proportions and may undergo malignant transformation, to form carcinoma ex pleiomorphic adenoma, a risk that increases with time. Although it is "benign" the tumor is aneuploid, it can recurr after resection, it invades normal adjacent tissue and distant matasteses have been reported after long (+10 years) time intervals.

Histology

Histologically, it is highly variable in appearance, even within individual tumours. Classically it is biphasic and is characterized by an admixture of polygonal epithelial and spindle-shaped 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 pleiomorphism). Areas of squamous metaplasia and epithelial pearls may be present. The tumour is not enveloped, but it is surrounded by a fibrous pseudocapsule of varying thickness. The tumour extends through normal glandular parenchyma in the form of finger-like pseudopodia, but this is not a sign of malignant transformation.

The tumor often displays characteristic chromasomal translocations between chromasomes #3 and #8. This causes the PLAG gene to be juxtaposed to the gene for Beta-catenin. This activates the catenin pathway and leads to inappropriate cell division.

Treatment

Needle biopsy is highly recommended prior to surgery to confirm the diagnosis. Treatment is by extensive surgical excision with a wide margin of normal healthy tissue. Recurrences are rare if the tumor is appropriately resected at the initial operation. Recurrences occur if the tumor is treated too conservatively i.e. enucleation of the main tumor mass leaves microscopic foci of tumor cells behind that regrow to form recurrent tumor masses or carcinoma. This surgical treatment has many sequellae such as cranial nerve damage, Freys syndrome, cosmetic problems, etc.  

See also

• Warthin's tumor - monomorphic adenoma