Encapsulated angioinvasive oncocytic carcinoma of the thyroid gland

by Jason Wasserman MD PhD FRCPC
November 17, 2022

What is encapsulated angioinvasive oncocytic carcinoma?

Encapsulated angioinvasive oncocytic 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 cancer cells were found inside at least one blood vessel outside the tumour. Another name for this type of cancer is encapsulated angioinvasive Hurthle cell carcinoma.

How is encapsulated angioinvasive oncocytic carcinoma diagnosed?

The diagnosis of encapsulated angioinvasive oncocytic 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).

What does encapsulated angioinvasive oncocytic carcinoma look like under the microscope?

When examined under the microscope the tumour is made up of large pink oncocytic cells. Until very recently, these cells were called Hurthle cells. Oncocytic cells appear pink because the cytoplasm (body of the cell) is full of a cellular part called mitochondria. Oncocytic cells also have a large round nucleus (the part of the cell that holds the genetic material) and a prominent central nucleolus (a clump of genetic material in the middle of the nucleus). The oncocytic cells can connect together to form small round structures called follicles or they may be in large groups that pathologists describe as a ‘solid pattern’.

Why is the tumour called encapsulated?

Oncocytic 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 oncocytic carcinoma where small groups of cancer cells have broken through the tumour capsule or widely invasive oncocytic 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?

Oncocytic 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?

After the entire tumour is removed, it will be measured and the size of the tumour will be included in your pathology report. The size of the tumour is important because it is used to determine the pathologic tumour stage (pT) and because larger tumours are more likely to spread to other parts of the body.

What does lymphatic invasion mean?

​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 mean?

​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?

​A margin is any tissue that has to be cut by the surgeon in order to remove the tumour and any surrounding normal tissue (such as 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.

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 (see Lymphatic invasion above). The movement of cancer cells from the thyroid to a lymph node is called metastasis. Encapsulated angioinvasive oncocytic carcinoma is less likely than other types of thyroid cancer to spread to lymph nodes.

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 is a neck dissection?

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.

What is a tumour deposit?

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.

What does extranodal extension (ENE) mean?

All lymph nodes are surrounded by a thin layer of tissue called a capsule. Cancer 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.

What is the pathologic stage (pTNM) for encapsulated angioinvasive oncocytic carcinoma?

​​The pathologic stage for encapsulated angioinvasive oncocytic 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 encapsulated angioinvasive oncocytic carcinoma

The pathologic tumour stage for encapsulated angioinvasive oncocytic carcinoma is based on the size of the tumour and the presence of extrathyroidal extension (tumour growing into tissue surrounding the thyroid gland).

• 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 oncocytic carcinoma

The pathologic nodal stage for encapsulated angioinvasive oncocytic carcinoma is based on the number of lymph nodes that contain cancer cells and the location of those lymph nodes. The pathologic nodal stage ranges from pN0 to pN1b. If no lymph nodes were sent for examination the pathologic nodal stage is pNx.

• 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.

Metastatic stage (pM) for encapsulated angioinvasive oncocytic carcinoma

The pathologic stage for encapsulated angioinvasive oncocytic carcinoma is based on the presence or absence of cancer cells at a distant body site such as the lungs or bones. The metastatic pathologic 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.