Pleomorphic adenoma

by Jason Wasserman MD PhD FRCPC
July 6, 2025

Pleomorphic adenoma is a non-cancerous tumour that most commonly arises in the salivary glands, especially the parotid gland, the largest salivary gland located near the ear and jaw. It is called “pleomorphic” because it contains a mix of different types of cells and tissue structures, including epithelial cells, myoepithelial cells, and stromal (connective tissue) elements.

Pleomorphic adenoma is the most common type of salivary gland tumour. It can also occur in other locations, such as the minor salivary glands inside the mouth or in the upper airway. In the skin, it is sometimes referred to as a benign mixed tumour.

Is pleomorphic adenoma a type of cancer?

No. Pleomorphic adenoma is a benign tumour, meaning it does not spread to other parts of the body and usually does not invade surrounding tissues. However, the tumour can continue to grow over time, and in rare cases, it can transform into a cancerous tumour if left untreated. For this reason, doctors usually recommend surgical removal and regular follow-up.

What are the symptoms of a pleomorphic adenoma?

The symptoms depend on the size and location of the tumour. Many people first notice a painless lump in the area near the jaw, cheek, or neck.

Other possible symptoms include:

• Swelling or a feeling of fullness in the area of the salivary glands.

• Difficulty chewing, speaking, or swallowing (if the tumour grows large).

• Numbness or facial weakness (if the tumour presses on nearby nerves – this is uncommon).

What causes a pleomorphic adenoma?

The exact cause is not fully understood. Most pleomorphic adenomas develop due to genetic changes in the cells of the salivary glands, leading to abnormal but non-cancerous growth. These changes are not inherited and usually occur by chance. In some cases, environmental factors or exposure to radiation may play a role, but the evidence is limited.

What genetic changes are commonly found?

Many pleomorphic adenomas contain specific genetic changes that are thought to drive tumour growth. The most common are:

• PLAG1 gene rearrangement: This genetic change, commonly found in many tumours, activates a gene that promotes cell growth.

• HMGA2 gene alterations: These are found in a smaller number of cases and may be seen in tumours without PLAG1 changes.

These changes help pathologists confirm the diagnosis and understand how the tumour developed, but they are not currently used to guide treatment.

Can a pleomorphic adenoma turn into cancer?

Yes, but this is rare. Over time, a pleomorphic adenoma can transform into a malignant (cancerous) tumour called carcinoma ex pleomorphic adenoma. This is more likely if:

• The tumour has been present for many years.

• It grows very large.

• It has not been completely removed.

Because of this small risk, doctors often recommend removing pleomorphic adenomas even though they are benign.

How is pleomorphic adenoma diagnosed?

Diagnosis usually involves a combination of imaging and biopsy:

• Imaging: Ultrasound, CT scan, or MRI may be used to evaluate the size and location of the tumour.

• Fine-needle aspiration (FNA) or core biopsy: A small sample of the tumour is removed and examined under the microscope by a pathologist.

In many cases, a definitive diagnosis is only made after the tumour is surgically removed and examined fully.

What does a pleomorphic adenoma look like under the microscope?

Under the microscope, pleomorphic adenomas have several characteristic features:

• A mix of epithelial and myoepithelial cells arranged in clusters, cords, or duct-like structures.

• A background of stromal tissue, often with a chondromyxoid appearance—a mix of cartilage-like and mucous-like material that is unique to this tumour.

• A fibrous capsule surrounds the tumour in most salivary gland locations (such as the parotid). However, tumours in the oral cavity or upper airway may not have a clear capsule, making them harder to remove completely.

• Low mitotic activity (few dividing cells) and no signs of invasion or spread.

These features help distinguish pleomorphic adenoma from other tumours.

What are margins in a pathology report?

A margin is the edge of tissue removed during surgery. After the tumour is removed, the pathologist examines the margins to determine whether the tumour was completely excised:

• A negative margin means no tumour cells are seen at the edge, suggesting the tumour was fully removed.

• A positive margin means tumour cells are found at the edge, suggesting that some tumour may remain.

• In some reports, the distance from the tumour to the closest margin may also be measured.

Margin status helps your doctor decide whether additional treatment or closer follow-up is needed. Margins are only assessed after complete removal (excision or resection), not after a small biopsy.

Should a pleomorphic adenoma be removed?

Yes, surgical removal is usually recommended. Although the tumour is benign, it can:

• Continue to grow and become more difficult to remove.

• Cause pressure on nearby structures.

• Carry a small risk of turning into cancer over time.

Surgery is usually curative, and patients recover well. Your surgeon will discuss the risks and benefits of the procedure based on your tumour’s size and location.

What is the prognosis for pleomorphic adenoma?

The prognosis is excellent. Most tumours are completely removed with surgery and do not come back.

However, recurrence can occur if:

• The tumour is not fully removed

• The tumour does not have a well-defined capsule

• It occurs in areas like the mouth or throat, where complete removal may be more challenging

In cases of recurrence, additional surgery is often recommended. Malignant transformation is rare but can occur if the tumour is left untreated for many years.

Questions to ask your doctor

• Was the tumour completely removed?

• Do I need any follow-up tests or imaging?

• What is the chance that the tumour will come back?

• Should I be concerned about cancer?

• What are the risks and benefits of surgery?

• How often should I be monitored after surgery?