The most common salivary gland tumour. Fortunately it is benign; unfortunately it has a high potential for recurrence and may also transform to an invasive carcinoma. 'Pleomorphic' describes the variable architecture of the tumour rather than the cytological appearance.
Mean age of presentation is mid-forties but the tumour can occur at any age after 10. Women are more commonly affected. Most tumours arise in the parotid gland (80%).
The tumour is usually slowly growing. It is non-invasive but can compress adjacent structures, including nerves. Transformation can occur. Although benign, the tumour frequently recurs locally.
The mass formed by pleomorphic adenoma is usually slow growing and painless. If adjacent structures are involved more specific symptoms may develop (eg. pain if nerves are compressed).
Unlike the malignant salivary tumours, pleomorphic adenoma is encapsulated with a distinct margin. The classical description of the surface is bossellated; ie, containing numerous rounded protuberances.
Microscopically the tumour consists of the capsule, epithelial cells, myoepithelial cells and stroma. Epithelial cells can be of almost any appearance, including squamous or spindle cells. The stromal portion can be mucoid, myxoid or even form cartilagenous structures. This broad range of appearances gives the tumour its adjective.
About 40% of tumours possess a rearrangement of chromosome 8q12. The involved gene is PLAG1 (for 'PLeomorphic Adenoma Gene 1'). This is a zinc finger gene that is involved in regulation of other genes and is normally not expressed except during development. Activation of this gene is associated with the potential for uncontrolled proliferation and transformation, potentially explaining the broad range of tissues which can develop from this tumour.