Pleomorphic Adenoma

Pleomorphic adenoma, also known as benign mixed tumour, is the most common benign salivary gland neoplasm, predominantly affecting the parotid gland. It is characterised by a neoplastic proliferation of epithelial (ductal) cells along with myoepithelial components, exhibiting a malignant potentiality.

The tumour typically presents as a slow-growing, painless, firm single nodular mass. Isolated nodules are generally outgrowths of the main nodule rather than a multinodular presentation. The tumour is usually mobile unless found in the palate and can cause atrophy of the mandibular ramus when located in the parotid gland. When situated in the parotid tail, it may present as an eversion of the ear lobe.

Though classified as benign, pleomorphic adenomas have the capacity to grow significantly and may undergo malignant transformation to form carcinoma ex-pleomorphic adenoma. This risk increases with time, with a 9.5% chance of conversion into malignancy over 15 years. Despite its benign nature, the tumour is aneuploid, can recur after resection, invade adjacent tissues, and even metastasise after long intervals. The tumour occurs more frequently in females than in males, with a ratio of approximately 6:4, and is predominantly found in patients in their fourth to sixth decades, with an average age of occurrence being around 43 years. However, it can also be seen in young adults and children.

Histology

Morphological diversity is the hallmark of pleomorphic adenomas. Histologically, these tumours are highly variable, even within individual cases. Typically biphasic, they feature an admixture of polygonal epithelial and spindle-shaped myoepithelial elements in a varying background stroma, which may be mucoid, myxoid, cartilaginous, or hyaline. The tumour is not enveloped but surrounded by a fibrous pseudocapsule of varying thickness. Finger-like pseudopodia extend through normal glandular parenchyma, though this is not indicative of malignant transformation. Characteristic chromosomal translocations between chromosomes #3 and #8 are often observed, leading to the inappropriate activation of the catenin pathway and resultant cell division.

Diagnosis

Diagnosis involves both tissue sampling and radiographic studies. Fine needle aspiration (FNA) and core needle biopsy are commonly used for tissue sampling, with the latter offering greater diagnostic accuracy. Imaging studies include ultrasound, CT, and MRI, each providing distinct advantages in characterising the tumour and guiding biopsies. Ultrasound is effective for superficial tumours and guiding FNA or core biopsies, while CT is excellent for demonstrating bony invasion. MRI provides superior soft tissue delineation, especially for perineural invasion.

Treatment

The mainstay treatment for pleomorphic adenoma is surgical resection, with needle biopsy recommended prior to confirm the diagnosis. The choice of surgical approach depends on the tumour's size, location, and mobility, with techniques ranging from extracapsular dissection to total parotidectomy. Early detection is very important for a favourable prognosis, as extensive surgery can lead to significant morbidity, including Frey's syndrome and salivary fistula. The experience of the surgeon is key to successful outcomes, and it is important to note that recurrence can occur many years post-surgery.

For benign tumours of the submandibular gland, a simple excision is typically sufficient, with care taken to preserve surrounding nerves. Malignant tumours often require wide local resection, potentially supplemented with radiotherapy to improve local control. Surgical treatment can lead to various sequelae such as cranial nerve damage and cosmetic issues, underscoring the importance of complete histologic removal for optimal survival rates.

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Self-assessment MCQs (single best answer)

What is the most common benign salivary gland neoplasm?

Which salivary gland is most frequently affected by pleomorphic adenoma?

What is the risk of pleomorphic adenoma undergoing malignant transformation over a period of 15 years?

Which histological feature is NOT typically seen in pleomorphic adenoma?

Which imaging modality provides superior soft tissue delineation, especially for perineural invasion, in diagnosing pleomorphic adenoma?

Which chromosomal translocation is commonly observed in pleomorphic adenoma?

What is the typical age range of patients affected by pleomorphic adenoma?

Which surgical approach is commonly used for extensive pleomorphic adenomas?

What is a common complication of extensive surgery for pleomorphic adenoma?

Which diagnostic method offers greater accuracy for pleomorphic adenoma, Fine Needle Aspiration (FNA) or Core Needle Biopsy?