by Jason Wasserman, MD PhD FRCPC, updated on September 2, 2019
Normal salivary glands
When we chew food our body releases a fluid into the mouth called saliva. Saliva is important because it contains chemicals that aid in 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.
What is mucoepidermoid carcinoma?
Mucoepidermoid carcinoma is a cancer that develops from the cells in the salivary gland. Mucoepidermoid carcinoma is believed to start from a cell in the duct that is capable of turning into different types of cells.
Mucoepidermoid carcinoma is typically made up of three different types of cells: mucus cells, intermediate cells, and epidermoid cells. You may find these cells described in your report.
Mucoepidermoid carcinoma can start on its own or it can grow out of a non-cancerous (benign) called a pleomorphic adenoma. When mucoepidermoid carcinoma grows out of a pleomorphic adenoma, it is called carcinoma ex pleomorphic adenoma which is just the medical way of saying ‘a cancer growing out of a pleomorphic adenoma’.
The diagnosis of mucoepidermoid carcinoma is usually made after a small sample of tissue is removed in a procedure called a biopsy and the tumour is later completely removed and sent to pathology for examination.
This is the size of the tumour measured in centimeters (cm). Tumour size will only be described in your report after the entire tumour has been removed. 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.
Grade is a word pathologists use to describe the difference between the cancer cells and the cells normally found in the salivary glands. Higher grade tumours are more abnormal and look less like normal tissue.
The grade can only be determined after a sample of the tumour has been examined under the microscope. The following features are used to decide the grade:
Using the features described above, pathologists divide the grade into three levels:
Why is this important? Grade is important because higher grade tumours (intermediate and high grade) are associated with worse prognosis than low grade tumours.
Parenchyma is a word used to describe the normal tissue of an organ. Because mucoepidermoid carcinoma starts in a salivary gland, parenchyma is used to describe the normal salivary gland tissue.
Extraparenchymal extension means that the cancer cells have grown beyond the normal salivary gland tissue and are seen in the surrounding tissue.
Why is this important? Tumours that have grown beyond the normal salivary gland tissue are associated with worse prognosis. Extraparenchymal extension is also used to determine the tumour grade (see Grade above).
A margin is a rim of normal tissue that surrounds a tumour and is removed with the tumour at the time of surgery. A positive margin means that cancer cells were seen at the cut edge of the tissue. In contrast, a negative margin means that no cancer cells were seen at the cut edge of the tissue.
The margins will usually only be described in your report after the entire tumour has been removed.
Why is this important? A positive margin is associated with an increased risk that the tumour will grow back in the same location after treatment.
Blood moves around the body through long thin tubes called blood vessels. Another type of fluid called lymph which contains waste and immune cells moves around the body through lymphatic channels.
Cancer cells can use blood vessels and lymphatics to travel away from the tumour to other parts of the body. The movement of cancer cells from the tumour to another part of the body is called metastasis.
Before cancer cells can metastasize, they need to enter a blood vessel or lymphatic. This is called lymphovascular invasion.
Why is this important? Lymphovascular invasion increases the risk that cancer cells will be found in a lymph node or a distant part of the body such as the lungs.
Nerves are like long wires made up of groups of cells called neurons. Nerves send information (such as temperature, pressure, and pain) between your brain and your body. Perineural invasion means that cancer cells were seen attached to a nerve.
Why is this important? Cancer cells that have attached to a nerve can use the nerve to travel into tissue outside of the original tumour. This increases the risk that the tumour will come back in the same area of the body (recurrence) after treatment.
Lymph nodes are small immune organs located throughout the body. Cancer cells can travel from the tumour to a lymph node through lymphatic channels located in and around the tumour (see Lymphovascular invasion above). The movement of cancer cells from the tumour to a lymph node is called a metastasis.
Lymph nodes from the neck are sometimes removed at the same time as the main tumour in a procedure called a neck dissection. The lymph nodes removed usually come from different areas of the neck and each area is called a level. The levels in the neck include 1, 2, 3, 4, and 5. Your pathology report will often describe how many lymph nodes were seen in each level sent for examination.
Lymph nodes on the same side as the tumour are called ipsilateral while those on the opposite side of the tumour are called contralateral.
Your pathologist will carefully examine each lymph node for cancer cells. Lymph nodes that contain cancer cells are often called positive while those that do not contain any cancer cells are called negative. Most reports include the total number of lymph nodes examined and the number, if any, that contain cancer cells.
A group of cancer cells inside of a lymph node is called a tumour deposit. If a tumour deposit is found, your pathologist will measure the deposit and the largest tumour deposit found will be described in your report.
Why is this important? Larger tumour deposits are associated with worse prognosis. The size of the largest tumour deposit is also used to determine the nodal stage (see Pathologic stage below).
Extranodal extension (ENE)
All lymph nodes are surrounded by a capsule. Extranodal extension (ENE) means that cancer cells have broken through the capsule and into the tissue that surrounds the lymph node.
Why is this important? Extranodal extension is also associated with a higher risk of new tumours developing in the neck and is often used by your doctors to guide your treatment. Extranodal extension is also used to determine the nodal stage (see Pathologic stage below).
The pathologic stage for mucoepidermoid carcinoma is based on the TNM staging system, an internationally recognized system originally created by the American Joint Committee on Cancer.
This system uses information about the primary tumour (T), lymph nodes (N), and distant metastatic disease (M) to determine the complete pathologic stage (pTNM). Your pathologist will examine the tissue submitted and give each part a number. In general, a higher number means more advanced disease and worse prognosis.
Tumour stage (pT) for mucoepidermoid carcinoma:
Mucoepidermoid carcinoma is given a tumour stage from 1 to 4 based on the size of the tumour and the distance that the cancer cells have traveled outside of the salivary gland (extraparenchymal extension).
Nodal stage (pN) for mucoepidermoid carcinoma:
Mucoepidermoid carcinoma is given a nodal stage from 0 to 3. If no cancer cells are seen in any of the lymph nodes examined, the nodal stage is N0.
If cancer cells are seen in a lymph node,your pathologist will look for the following features to determine the nodal stage:
Metastatic stage (pM) for mucoepidermoid carcinoma:
Mucoepidermoid carcinoma is given a metastatic stage of 0 or 1 based on the finding of cancer cells at a distant site in the body (for example the lungs). The metastatic stage can only be determined if tissue from a distant site is submitted for pathological examination. Because this tissue is rarely present, the metastatic stage cannot be determined and is listed as MX.