Poorly differentiated thyroid carcinoma

by Jason Wasserman MD PhD FRCPC
June 8, 2022

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About this article: This article was created by doctors to help you read and understand your pathology report for poorly differentiated thyroid carcinoma. If you have any questions about this article or your pathology report, please contact us.

What is poorly differentiated thyroid carcinoma?

Poorly differentiated thyroid carcinoma is a rare and aggressive form of thyroid cancer. Poorly differentiated thyroid carcinoma often develops from a well-differentiated type of thyroid cancer such as papillary thyroid carcinoma or follicular thyroid carcinoma.

What causes poorly differentiated thyroid carcinoma?

Current research suggests that a combination of genetic and environmental factors contribute to the development of poorly differentiated thyroid carcinoma. However, to date, no single factor has been identified that makes a person more likely to develop this tumour.

What are the symptoms of poorly differentiated thyroid carcinoma?

Poorly differentiated thyroid carcinoma is a fast-growing tumour that starts in the thyroid gland. As a result, people often notice a growth in the front of the neck. As the tumour grows it can put pressure on surrounding tissues such as the esophagus or trachea. This can result in difficulty breathing or swallowing food. A lump may be felt or seen in the side of the neck if tumour cells have spread to lymph nodes in this area.

How is the diagnosis of poorly differentiated thyroid carcinoma made?

The diagnosis of poorly differentiated thyroid carcinoma is usually made after part or all of the thyroid gland has been removed in a procedure called a resection. The thyroid gland is then sent to a pathologist for examination under the microscope. The diagnosis can also be made after examination of an enlarged lymph node.

What does poorly differentiated thyroid carcinoma look like under the microscope?

When examined under the microscope, the tumour is made up of follicular thyroid cells. The pattern of growth may be described as solid, trabecular, or insular. Mitotic figures (tumour cells dividing to create new tumour cells) are usually seen and the number of mitotic figures may be described in your report. A type of cell death called necrosis may also be seen.

The microscopic appearance of poorly differentiated thyroid carcinoma.

In some cases, poorly differentiated thyroid carcinoma arises from a well-differentiated type of thyroid cancer such as papillary thyroid carcinoma or follicular carcinoma. If a more well-differentiated tumour is seen, it will be described in your report.

Why is the size of the tumour important?

For poorly differentiated thyroid carcinoma, the size of the tumour is important because it is used to determine the pathologic tumour stage (pT). Larger tumours are also associated with a worse prognosis because the tumour cells are more likely to spread to lymph nodes or other parts of the body.

What is extrathyroidal extension?

​Extrathyroidal extension is the spread of tumour cells out of the thyroid gland and into the surrounding tissues. Tumour cells that spread far enough out of the thyroid gland may come into contact with other organs such as muscles, the esophagus, or the trachea.​

There are two types of extrathyroidal extension:

• Microscopic – The tumour cells outside of the thyroid gland were only found after the tumour was examined under the microscope.
• Macroscopic (gross) – The tumour can be seen growing into the surrounding tissues without the use of a microscope. This type of extrathyroidal extension may be seen by your surgeon at the time of surgery or by the pathologist assistant performing the gross examination of the tissue sent to pathology.

Macroscopic (gross) extrathyroidal extension is important because it increases the tumour stage (see Pathologic stage below) and is associated with a worse prognosis. Microscopic extrathyroidal extension does not change the tumour stage.

What is vascular invasion (angioinvasion)?

​Blood vessels carry blood around the body. Tumour cells that enter a blood vessel are able to spread to distant parts of the body such as the lungs and bones. The movement of tumour cells into a blood vessel is called vascular invasion or angioinvasion. If vascular invasion is seen, your pathologist will count the number of vessels that contain tumour cells.

Your report will describe vascular invasion as negative if no tumour cells are seen inside of a blood vessel, positive and focal if there are less than 4 blood vessels with tumour cells, and positive and diffuse if there are 4 or more blood vessels with tumour cells.

Vascular invasion is important because tumour cells that enter a blood vessel are more likely to spread to other parts of the body. The movement of tumour cells from the tumour to another part of the body is called metastasis.

What is lymphatic invasion?

​Lymphatics are small thin vessels that provide a way for fluids and cells to leave a tissue. Lymphatics are found all over the body. Tumour cells that enter a lymphatic vessel are able to spread to other parts of the body, in particular lymph nodes.

The movement of tumour cells into a lymphatic vessel is called lymphatic invasion. Your pathologist will carefully examine your tissue for lymphatic invasion. If lymphatic invasion is seen, it will be called positive. If no lymphatic invasion is seen, it will be called negative.

Lymphatic invasion is important because tumour cells that enter a lymphatic space are more likely to spread to lymph nodes or other parts of the body. The movement of tumour cells from the tumour to another part of the body is called metastasis.

What is a margin?

​A margin is tissue that has to be cut by the surgeon to remove the thyroid gland from your body.  A margin is considered positive when there are cancer cells at the very edge of the cut tissue. A negative margin means there were no cancer cells seen at the cut edge of the tissue.

What are lymph nodes?

Lymph nodes are small immune organs located throughout the body. Tumour cells can travel from the thyroid to a lymph node through lymphatic channels located in and around the tumour (see Lymphatic invasion above). The movement of tumour cells from the thyroid to a lymph node is called metastasis.

Lymph nodes from the neck are sometimes removed at the same time as the thyroid 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 are numbered 1 through 7. 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 tumour cells. Lymph nodes that contain tumour 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 that contain tumour cells.

What is a tumour deposit?

A group of tumour 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.

What does extranodal extension (ENE) mean?

All lymph nodes are surrounded by a thin layer of tissue called a capsule. Tumour cells that have spread to a lymph node can break through the capsule and into the tissue surrounding the lymph node. This is called extranodal extension (ENE). Extranodal extension does not change the pathologic stage but your doctors may use this information when deciding which treatment is best for you.

How do pathologists determine the pathologic stage (pTNM) for poorly differentiated thyroid carcinoma?

​​The pathologic stage for poorly differentiated thyroid 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 a worse prognosis.

Tumour stage (pT) for poorly differentiated thyroid carcinoma

Poorly differentiated thyroid carcinoma is given a tumour stage between 1 and 4 based on the size of the tumour and the presence of tumour cells outside of the thyroid (see Extrathyroidal extension above).

• T1 – The tumour is less than or equal to 2 cm and the tumour cells do not extend beyond the thyroid gland.
• T2 – The tumour is greater than 2 cm but less than or equal to 4 cm and the tumour cells do not extend beyond the thyroid gland.
• T3 – The tumour is greater than 4 cm OR the tumour cells extend into the muscles outside of the thyroid gland.
• T4 – The tumour cells extend to structures or organs outside of the thyroid gland including the trachea, larynx, or esophagus.

Nodal stage (pN) for poorly differentiated thyroid carcinoma

Anaplastic thyroid carcinoma is given a nodal stage of 0 or 1 based on the presence or absence of tumour cells in a lymph node and the location of the involved lymph nodes.

• N0 – No tumour cells were found in any of the lymph nodes examined.
• N1a – Tumour cells were found in one or more lymph nodes from levels 6 or 7.
• N1b – Tumour cells were found in one or more lymph nodes from levels 1 through 5.
• NX – No lymph nodes were sent to pathology for examination.

Metastatic stage (pM) for poorly differentiated thyroid carcinoma

Anaplastic thyroid carcinoma is given a metastatic stage of 0 or 1 based on the presence of tumour 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 sent for pathological examination. Because this tissue is rarely sent, the metastatic stage cannot be determined and is listed as MX.