Pleomorphic adenoma

by Jason Wasserman MD PhD FRCPC
November 28, 2024

A pleomorphic adenoma is a benign (non-cancerous) tumour that typically arises in the salivary glands. It is called “pleomorphic” because it contains a mix of different types of cells, including epithelial and myoepithelial cells. In the skin, these tumours are sometimes called “benign mixed tumours.” Pleomorphic adenomas are the most common type of salivary gland tumour and usually develop in the parotid gland, the largest salivary gland located near the jaw.

Is pleomorphic adenoma a type of cancer?

No, pleomorphic adenoma is not a type of cancer. It is a benign (non-cancerous) tumour, meaning it does not invade nearby tissues or spread to other parts of the body. However, it can grow larger over time, and in rare cases, it can transform into a cancerous tumour. Regular follow-up and, if needed, surgical removal are important to prevent complications.

What are the symptoms of a pleomorphic adenoma?

The symptoms of a pleomorphic adenoma depend on its size and location.

Common symptoms include:

• A painless, slow-growing lump in the cheek, jaw, or neck area.
• Swelling or fullness in the area of the salivary glands.
• Difficulty chewing, speaking, or swallowing if the tumour becomes large.
• Rarely, numbness or weakness in the face if the tumour presses on a nerve.

What causes a pleomorphic adenoma?

The exact cause of pleomorphic adenoma is not fully understood. These tumours are thought to arise due to changes in the genetic material (DNA) of salivary gland cells, leading to abnormal growth. While these changes are not inherited, environmental factors or random genetic mutations may play a role.

What genetic changes are commonly found in pleomorphic adenoma?

Pleomorphic adenomas often show specific genetic changes that drive their growth. One common genetic abnormality is a rearrangement involving the PLAG1 gene. When it becomes altered, this gene is thought to promote tumour growth. Another gene, HMGA2, can also be involved in some cases.

Can pleomorphic adenoma turn into cancer over time?

In rare cases, pleomorphic adenoma can transform into a malignant (cancerous) tumour called carcinoma ex pleomorphic adenoma. This transformation is more likely to occur if the tumour has been present for many years or grows very large. For this reason, doctors often recommend removing pleomorphic adenomas, even though they are benign.

How is this diagnosis made?

The diagnosis of pleomorphic adenoma is made by examining a tissue sample from the tumour. This sample is usually obtained through a biopsy, where a small piece of the tumour is removed and examined under a microscope by a pathologist. Imaging studies, such as ultrasound, CT, or MRI, may also be performed to evaluate the size and location of the tumour.

Microscopic features of this tumour

Under the microscope, pleomorphic adenomas show a mix of different types of cells and tissues. Key features include:

• Epithelial and myoepithelial cells: These cells are arranged in clusters, cords, or duct-like structures.
• Background stromal tissue: The stromal tissue is often described as chondromyxoid, which means it resembles a mix of cartilage and mucous-like material. This chondromyxoid matrix is highly characteristic of pleomorphic adenoma and helps pathologists distinguish it from other tumours.
• Encapsulation: Pleomorphic adenomas in the major salivary glands, such as the parotid gland, are usually surrounded by a well-defined capsule. However, tumours in the oral cavity and upper airway are often not encapsulated, making them more challenging to remove altogether.

These features provide important diagnostic clues and are used to confirm the diagnosis of pleomorphic adenoma.

Margins

In pathology, a margin is the edge of tissue removed during tumour surgery. The margin status in a pathology report is important as it indicates whether the entire tumour was removed or if some was left behind. This information helps determine the need for further treatment.

Pathologists typically assess margins following a surgical procedure, like an excision or resection, that removes the entire tumour. Margins aren’t usually evaluated after a biopsy, which removes only part of the tumour. The number of margins reported and their size—how much normal tissue is between the tumour and the cut edge—vary based on the tissue type and tumour location.

Pathologists examine margins to check if tumour cells are at the tissue’s cut edge. A positive margin, where tumour cells are found, suggests that some cancer may remain in the body. In contrast, a negative margin, with no tumour cells at the edge, suggests the tumour was fully removed. Some reports also measure the distance between the nearest tumour cells and the margin, even if all margins are negative.

Does a pleomorphic adenoma need to be removed?

Yes, in most cases, pleomorphic adenomas are surgically removed. Although these tumours are benign, they can continue to grow and cause discomfort or cosmetic concerns. Removing the tumour also eliminates the small risk of transformation into cancer over time. Surgery is typically curative, and patients usually recover well after the procedure. Your doctor can explain the benefits and risks of surgery based on your specific situation.