Pleomorphic adenoma

What is a pleomorphic adenoma?

Pleomorphic adenoma is the most common type of salivary gland tumour. Most tumours start in one of the major salivary glands (parotid gland or submandibular gland) although this type of tumour can also be found in the skin and lungs.

Pleomorphic adenomas are slow-growing tumours although they can reach a large size without treatment. Most pleomorphic adenomas are non-cancerous tumours. However, they can grow back again in the same location if not completely removed.

The salivary glands

When we chew food our body releases fluid into the mouth called saliva. Saliva is important because it contains chemicals that help with digestion. Saliva is produced by organs called salivary glands which are located throughout the head and neck.

Most people have three major salivary glands and numerous minor salivary glands. The major salivary glands are called the parotid gland, submandibular gland, and sublingual gland. The minor salivary glands are very small and there are so many of them that they are not given their own names. Most of the minor salivary glands are found inside the mouth.

The parotid gland is the largest salivary gland and it can be found on the side of the face just in front of the ear. The submandibular gland can be found just below the lower jaw near the top of the neck. The sublingual gland is the smallest of the major glands and it can be found below the tongue.

The salivary glands are made up of small groups of cells called glands which are connected to the inside of the mouth by long thin channels called ducts. The glands make the chemicals in the saliva which travels down the ducts into the mouth.

Carcinoma ex-pleomorphic adenoma

Rarely, cancer will develop from within a pleomorphic adenoma. These tumours are called carcinoma ex-pleomorphic adenoma which is just the medical way of saying ‘cancer arising from a pleomorphic adenoma’.  Tumours that are very large, have been present for many years, or have come back multiple times have a higher risk of developing cancer. Because pleomorphic adenoma can turn into cancer over time, it is recommended that all pleomorphic adenomas be removed completely.

How do pathologists make this diagnosis?

The diagnosis of pleomorphic adenoma is usually made after a small sample of tissue is removed from your body in a procedure called a biopsy. The tissue is then sent to a pathologist who examines it under a microscope. A test called immunohistochemistry may be performed to confirm the diagnosis.

Most patients with pleomorphic adenoma will be offered surgery to remove the tumour. The tumour will then be sent to a pathologist who will prepare another pathology report. This report will confirm or revise the original diagnosis and provide additional important information such as tumour size.

When examined under the microscope most pleomorphic adenomas are made up of a combination of cells found in the normal salivary gland. However, unlike the orderly arrangement of cells in the normal gland, the cells in the tumour are disorganized and do not form normal ducts or glands.

The tumour cells are surrounded by a type of connective tissue called stroma which in pleomorphic adenoma often looks blue or grey under the microscope. Pathologists describe this type of stroma as chondromyxoid or myxochondroid.  The tumour is often surrounded by a thin layer of tissue called a tumour capsule. The capsule separates the tumour from the normal surrounding salivary gland tissue.

pleomorphic adenoma

What to look for in your report after the tumour has been removed

Tumour size

This is the size of the tumour measured in centimetres (cm). The tumour is usually measured in three dimensions but only the largest dimension is described in your report. For example, if the tumour measures 4.0 cm by 2.0 cm by 1.5 cm, your report will describe the tumour as being 4.0 cm.

Margins

A margin is any tissue that was cut by the surgeon in order to remove the tumour from your body. The types of margins described in your report will depend on the organ involved and the type of surgery performed.

A negative margin means that no tumour cells were seen at any of the cut edges of tissue. A margin is called positive when there are tumour cells at the very edge of the cut tissue. A positive margin is associated with a higher risk that the tumour will recur in the same site after treatment.

Margins will only be described in your report after the entire tumour has been removed. However, many pathologists will not describe margins for pleomorphic adenoma in the pathology report if they are negative.

Margin

Lymph nodes

Lymph nodes are small organs, located in various locations throughout the body. They form part of the body’s immune surveillance system. When tumour cells travel to a lymph node it is called lymph node metastasis.

Lymph nodes around the tumour are often removed at the same time as the tumour. Your pathologist will examine each of these lymph nodes under the microscope to see if any contain tumour cells.

Tumour cells from a pleomorphic adenoma rarely travel to a lymph node. If tumour cells are found in a lymph node, your pathologist will examine them to see if they have changed to look more like cancer cells or if they still look similar to the cells in the pleomorphic adenoma.

Lymph node

by Jason Wasserman, MD PhD FRCPC (updated August 22, 2021)
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