Pleomorphic adenomas, the most common benign salivary gland tumours, typically occur in females aged 30-60, and are characterised by a mixed cellular composition of epithelial and myoepithelial cells, encapsulation, and classically appear as a well-defined, lobulated mass on imaging, most often located in the tail of the parotid gland.
Description
Pleomorphic adenoma, also known as benign mixed tumour, is the most common benign neoplasm of the salivary glands, typically affecting the parotid gland. It comprises a mix of epithelial and mesenchymal tissue elements.
Pathogenesis
The pathogenesis of pleomorphic adenoma involves neoplastic transformation of salivary gland cells, although the exact mechanism is still not entirely understood. Genetic mutations, particularly in the PLAG1 (pleomorphic adenoma gene 1) and HMGA2 genes, have been associated with these tumours.
Epidemiology, Risk Factors & Associations
- Pleomorphic adenomas account for about 60% of all salivary gland neoplasms.
- It is most common in every salivary gland except sublingual (rare)
- They are most frequently seen in adults in the 4th to 6th decades, with a slight female predominance.
- There are no specific identified risk factors or associations for pleomorphic adenomas.
Clinical Features
- Patients typically present with a slowly enlarging, painless mass.
- In case of parotid gland involvement, it is typically located in the tail of the parotid gland in the preauricular area.
- Facial nerve weakness and pain are unusual unless malignant transformation has occurred.
Complications
- The main complication of pleomorphic adenomas is malignant transformation, known as carcinoma ex pleomorphic adenoma. This can occur in up to 10% of untreated cases and usually happens after a long-standing history of the benign tumour, often 10-15 years. The malignant transformation can result in a variety of carcinomas, the most common being adenocarcinoma, followed by salivary duct carcinoma and mucoepidermoid carcinoma.
- Surgical complications include potential injury to the facial nerve during parotid gland surgery, which can lead to facial weakness or asymmetry. Parotid surgery may also potentially impact salivary production, potentially leading to dry mouth (xerostomia). Recurrence of the tumour can occur if complete surgical resection is not achieved.
Pathological Features
Histopathology
- Pleomorphic adenomas are characterised by a mix of epithelial and myoepithelial cells in a variable stromal background. The stroma can range from myxoid to chondroid or even osseous.
- A fibrous pseudocapsule is typically present.
Radiological Features
CT
- Pleomorphic adenomas typically appear as a well-circumscribed, lobulated mass.
- They are isodense or slightly hyperdense to muscle with enhancement after contrast administration.
MRI
- These tumors are typically isointense to muscle on T1-weighted images and hyperintense on T2-weighted images.
- Enhancement is noted post gadolinium administration.
Grading and Staging
Pleomorphic adenomas are benign tumours, so staging is generally not applicable.
Differential Diagnosis
- Warthin tumour: Second most common benign salivary gland lesion. Typically has a more heterogeneous appearance on imaging, with cystic and solid components.
- Mucoepidermoid carcinoma: Most common malignant salivary gland tumour which typically presents with more aggressive features such as bone invasion or lymphadenopathy.
- Salivary duct cyst: These typically present as well-defined, non-enhancing, fluid-filled masses on imaging.
Management
- The mainstay of treatment for pleomorphic adenoma is surgical removal with care taken to preserve facial nerve function.
- Radiotherapy is typically not used due to the risk of malignant transformation.