What is pleomorphic adenoma?

What is Pleomorphic Adenoma?

Pleomorphic adenoma is the most common benign tumor of the salivary glands, accounting for approximately 60-70% of all benign salivary gland neoplasms and nearly 50% of all salivary gland tumors overall. 1, 2, 3

Anatomical Distribution

• The parotid gland is the primary site, harboring approximately 80-85% of all pleomorphic adenomas 2, 3, 4
• The submandibular gland accounts for 5-10% of cases 3
• Minor salivary glands (particularly the palate, followed by lips and cheek) represent 7-10% of pleomorphic adenomas 3, 4
• The sublingual gland is the least common location 3

Clinical Presentation

• Patients typically present with a slow-growing, painless mass that may be present for years (mean duration of benign symptoms ranges from 2 to 40 years, with an average of 17.8 years) 2, 5, 6
• The mean age at presentation is approximately 44 years, though it can occur across a wide age range 2
• There is a female predilection with a male-to-female ratio of approximately 13:8 2
• The tumor most commonly presents in the facial region (42.85% of cases) 2

Histopathology and Molecular Features

• Pleomorphic adenoma is also known as "mixed tumor" or "tumor mixtus" because it contains both epithelial and mesenchymal (myoepithelial) components 6, 4
• Cytogenetically, approximately 30% show a normal diploid stemline, while the remainder demonstrate clonal chromosomal abnormalities 3
• The most common cytogenetic abnormality involves chromosome 8, particularly translocations at 8q12, with t(3;8)(p21;q12) being the most frequently encountered aberration 3
• Other cytogenetic subgroups include translocations involving 12q15 and various non-recurrent clonal abnormalities 3

Malignant Transformation Risk

• The rate of malignant transformation is 2-5% if left untreated, typically occurring after 15-20 years 5, 3
• Warning symptoms of malignant transformation include: rapid tumor enlargement, pain, and facial nerve palsy 5
• Three subtypes of malignant pleomorphic adenoma exist: carcinoma ex pleomorphic adenoma (CXPA, the most common), carcinosarcoma (true malignant mixed tumor), and metastasizing pleomorphic adenoma 5
• Carcinoma ex pleomorphic adenoma is classified as a high-grade salivary malignancy with >20% rate of nodal involvement 7

Diagnostic Approach

• Fine-needle aspiration biopsy (FNAB) has limited sensitivity (60%) and accuracy (46%) in detecting malignant transformation within pleomorphic adenoma 5
• MRI with and without intravenous contrast is the preferred imaging modality when the lesion shows suspicious features, demonstrates growth, or when detailed surgical planning is required, as it provides superior soft-tissue delineation of tumor extent and relationship to the facial nerve 8
• CT scanning may be used when MRI is contraindicated but provides inferior soft-tissue detail 8

Treatment Principles

• Complete surgical excision is the definitive treatment, as enucleation (simple removal without surrounding tissue) results in recurrence 2
• For benign pleomorphic adenoma of the parotid gland, partial superficial parotidectomy with facial nerve preservation is appropriate 8
• The facial nerve must be preserved when performing parotidectomy for parotid pleomorphic adenoma 2
• Early and radical removal of all major salivary gland tumors is the best method of prevention of malignant transformation 5

Common Pitfalls

• Do not perform simple enucleation, as this leads to tumor recurrence due to incomplete excision 2
• Do not delay treatment of long-standing salivary gland masses, as the risk of malignant transformation increases with time, particularly after 15-20 years 5
• Large tumors can cause life-threatening complications due to their size and location, emphasizing the importance of early intervention 6