Encapsulated angioinvasive follicular thyroid carcinoma

by Jason Wasserman MD PhD FRCPC
March 13, 2023

What is encapsulated angioinvasive follicular thyroid carcinoma?

Encapsulated angioinvasive follicular thyroid carcinoma is a type of thyroid cancer. The tumour is called “encapsulated” because it is separated from the normal thyroid gland by a thin band of tissue called a tumour capsule. “Angioinvasive” means that cancer cells were found inside at least one blood vessel outside of the tumour.

How is encapsulated angioinvasive follicular thyroid carcinoma diagnosed?

The diagnosis of encapsulated angioinvasive follicular thyroid carcinoma can only be made after the entire tumour is removed and sent to a pathologist for examination. This usually involves surgically removing one lobe of the thyroid gland although sometimes the entire thyroid gland is removed. This diagnosis cannot be made after a less invasive procedure called a fine needle aspiration biopsy (FNAB).

Why is the tumour called encapsulated?

Follicular thyroid carcinoma is called “encapsulated” when the entire tumour is separated from the normal thyroid gland by a thin band of tissue called a tumour capsule. This is different from a related type of cancer called minimally invasive follicular thyroid carcinoma where small groups of cancer cells have broken through the tumour capsule or widely invasive follicular thyroid carcinoma where very little or no tumour capsule is seen and most of the cancer cells have spread into the surrounding normal thyroid gland.

Why is the tumour called angioinvasive?

Follicular thyroid carcinoma is called “angioinvasive” when cancer cells are seen inside a blood vessel. Pathologists use the word invasion to describe the movement of cancer cells from one area to another and angioinvasion means that the cancer cells have spread from the tumour into a blood vessel. Angioinvasion is important because it increases the risk that cancer cells will travel to another part of the body such as the lungs or bones.

Your pathologist will carefully examine the tumour to determine the number of blood vessels that contain cancer cells. When less than four vessels contain cancer cells it is called focal. When four or more blood vessels contain cancer cells it is called extensive. The number of blood vessels involved is important because the risk of developing metastatic disease is higher when more than four blood vessels are involved.

Why is the tumour size important?

The size of the tumour is important because larger tumours are more likely to spread to other parts of the body. The tumour size is also used to determine the pathologic tumour stage (pT).

What is lymphatic invasion and why is it important?

​Lymphatic invasion means that cancer cells were seen inside a lymphatic vessel. Lymphatic vessels are small thin channels that allow waste, extra fluid, and cells to leave a tissue. Lymphatics are found all over the body. Lymphatic invasion is important because it increases the risk that cancer cells will be found in a lymph node. Lymphatic invasion is not commonly seen in encapsulated angioinvasive follicular thyroid carcinoma.

What does extrathyroidal extension means and why is it important?

​Extrathyroidal extension means that cancer cells have spread beyond the thyroid gland and into the surrounding tissues. Cancer cells that move 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 cancer 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 pathologic tumour stage (pT) and is associated with a worse prognosis. In contrast, microscopic extrathyroidal extension does not change the tumour stage and is not associated with a worse prognosis.

What is a margin and why are margins important?

In pathology, a margin is the edge of a tissue that is cut when removing a tumour from the body. The margins described in a pathology report are very important because they tell you if the entire tumour was removed or if some of the tumour was left behind. The margin status will determine what (if any) additional treatment you may require.

Pathologists carefully examine the margins to look for tumour cells at the cut edge of the tissue. If tumour cells are seen at the cut edge of the tissue, the margin will be described as positive. If no tumour cells are seen at the cut edge of the tissue, a margin will be described as negative. Even if all of the margins are negative, some pathology reports will also provide a measurement of the closest tumour cells to the cut edge of the tissue.

Were lymph nodes examined and did any contain cancer cells?

Lymph nodes are small immune organs located throughout the body. Cancer cells can travel from the thyroid to a lymph node through lymphatic channels located in and around the tumour. The movement of cancer cells from the thyroid to a lymph node is called metastasis. Encapsulated angioinvasive follicular carcinoma is less likely than other types of thyroid cancer to spread to lymph nodes.

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 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.

What stage is encapsulated angioinvasive follicular thyroid carcinoma?

​​The pathologic stage for encapsulated angioinvasive follicular 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 a more advanced disease and a worse prognosis.

Tumour stage (pT) for encapsulated angioinvasive follicular thyroid carcinoma

Encapsulated angioinvasive follicular thyroid carcinoma is given a tumour stage between 1 and 4 based on the size of the tumour and the presence of cancer cells outside of the thyroid.

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

Nodal stage (pN) for encapsulated angioinvasive follicular thyroid carcinoma

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

• N0 – No cancer cells were found in any of the lymph nodes examined.
• N1a – Cancer cells were found in one or more lymph nodes from levels 6 or 7.
• N1b – Cancer 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 encapsulated angioinvasive follicular thyroid carcinoma

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