Encapsulated angioinvasive follicular thyroid carcinoma

by Jason Wasserman MD PhD FRCPC
March 26, 2024


Encapsulated angioinvasive follicular thyroid carcinoma is a type of thyroid cancer. This type of cancer is called “encapsulated” because it is separated from the normal thyroid gland by a thin band of tissue called a tumour capsule and “angioinvasive” because, upon microscopic examination, cancer cells were found inside at least one blood vessel outside the tumour.

Anatomy thyroid gland

What are the symptoms of encapsulated angioinvasion follicular thyroid carcinoma?

Symptoms of encapsulated angioinvasive follicular thyroid carcinoma may include:

  • A lump or swelling in your neck that you can see or feel.
  • Voice changes, like hoarseness.
  • Trouble with swallowing or breathing.

What causes encapsulated angioinvasion follicular thyroid carcinoma?

What causes encapsulated angioinvasion follicular thyroid carcinoma isn’t fully understood. However, it seems to involve a combination of both genetic changes and environmental risk factors, such as exposure to ionizing radiation and dietary influences. This type of cancer is also much more common in young women.

Why is this type of cancer called encapsulated?

Follicular thyroid carcinoma is called “encapsulated” because the tumour is separated from the normal thyroid gland by a thin band of tissue called a tumour capsule. The presence of a tumour capsule is important because it helps distinguish this type of cancer from a related entity called widely invasive follicular thyroid carcinoma which has very little or no tumour capsule and most of the cancer cells have spread into the surrounding normal thyroid gland.

Why is this type of cancer called angioinvasive?

Follicular thyroid carcinoma is called “angioinvasive” when cancer cells are seen inside a blood vessel. Angioinvasion is important because it increases the risk that cancer cells will travel to other parts of the body, such as the lungs or bones.

Your pathologist will carefully examine the tumour to determine the number of blood vessels containing 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.

How is this diagnosis made?

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).

Your pathology report for encapsulated angioinvasive follicular thyroid carcinoma

Your pathology report for encapsulated angioinvasion follicular thyroid carcinoma will information such as the tumour size, the number of blood vessels involved by the tumour, the presence or absence of extrathyroidal extension, and the assessment of margins. The results of any lymph nodes examined should also be included in the report. These topics are described in more detail in the sections below.

encapsulated angioinvasive follicular thyroid carcinoma
Encapsulated angioinvasive follicular thyroid carcinoma. In this picture, cancer cells can be seen inside a blood vessel outside of the tumour.

Tumour size

After the tumour is removed completely, it will be measured. 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. The size of the tumour is important because it is used to determine the pathologic nodal stage (pN) and because larger tumours are more likely to spread to other parts of the body, such as lymph nodes.

Lymphatic invasion

Lymphatic invasion is the spread of tumour cells into a lymphatic channel (small tubes that move fluid and immune cells around the body). Once tumour cells are inside a lymphatic channel, they can spread to small immune organs called lymph nodes. For this reason, lymphatic invasion increases the risk of developing lymph node metastasis. However, lymphatic invasion is uncommon in encapsulated angioinvasive follicular thyroid carcinoma.

Extrathyroidal extension

​Extrathyroidal extension is the spread of tumour cells from the thyroid gland and into the surrounding tissues. Tumour cells that spread far enough out of the thyroid gland may contact 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. Your surgeon may see this type of extrathyroidal extension at the time of surgery or by the pathologist’s 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. Microscopic extrathyroidal extension does not change the tumour stage.

Margins

A margin is the 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 tumour cells at the very edge of the cut tissue. A negative margin means there were no tumour cells seen at the cut edge of the tissue.

Margin

Lymph nodes

Lymph nodes are small immune organs found throughout the body. Tumour cells can spread from the primary tumour to lymph nodes through small lymphatic vessels. For this reason, lymph nodes are commonly removed and examined under a microscope to look for cancer cells. The movement of tumour cells from the tumour to another part of the body such as a lymph node is called a metastasis.

Lymph node

Tumour cells typically spread first to lymph nodes close to the tumour, although lymph nodes that are far away from the tumour can also be involved. For this reason, the first lymph nodes removed are usually close to the tumour. Lymph nodes further away from the tumour are only typically removed if they are enlarged and there is a high clinical suspicion that there may be cancer cells in the lymph node.

A neck dissection is a surgical procedure performed to remove lymph nodes from the neck. The lymph nodes removed usually come from different neck areas, 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.

If any lymph nodes were removed from your body, they will be examined under the microscope by a pathologist, and the results of this examination will be described in your report. “Positive” means that tumour cells were found in the lymph node. “Negative” means that no tumour cells were found. If tumour cells are found in a lymph node, the size of the largest group of tumour cells (often described as “focus” or “deposit”) may also be included in your report. Extranodal extension means that the tumour cells have broken through the capsule on the outside of the lymph node and have spread into the surrounding tissue.

extranodal extension

The examination of lymph nodes is important for two reasons. First, this information determines the pathologic nodal stage (pN). Second, finding tumour cells in a lymph node increases the risk that cancer cells will be found in other parts of the body in the future. As a result, your doctor will use this information when deciding if additional treatment, such as radioactive iodine, chemotherapy, radiation therapy, or immunotherapy, is required.

Pathologic stage (pTNM)

​​The pathologic stage for encapsulated angioinvasive follicular thyroid carcinoma is based on the TNM staging system, an internationally recognized system 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)

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)

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.

About this article

Doctors wrote this article to help you read and understand your pathology report for encapsulated angioinvasive follicular thyroid carcinoma. The sections above describe the results found in most pathology reports, however, all reports are different and results may vary. Importantly, some of this information will only be described in your report after the entire tumour has been surgically removed and examined by a pathologist. Contact us if you have any questions about this article or your pathology report. Read this article for a more general introduction to the parts of a typical pathology report.

Other helpful resources

American Thyroid Association (ATA)
American Cancer Society

Learn more pathology

Atlas of Pathology
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