by Jason Wasserman MD PhD FRCPC
March 14, 2023
Encapsulated angioinvasive Hurthle cell 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. Another name for this type of cancer is encapsulated angioinvasive oncocytic carcinoma.
The diagnosis of encapsulated angioinvasive Hurthle cell 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).
When examined under the microscope the tumour is made up of large pink cells that pathologists call Hurthle cells. This name is technically a misnomer as these were not the original “Hurthle cells” described by Karl Hurthle. The cells we today call Hurthle cells appear pink because the cytoplasm (body of the cell) is full of a cellular part called mitochondria. Hurthle 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 Hurthle 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’.
Hurthle cell 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 Hurthle cell carcinoma where small groups of cancer cells have broken through the tumour capsule or widely invasive Hurthle cell carcinoma where very little or no tumour capsule is seen and most of the cancer cells have spread into the surrounding normal thyroid gland.
Hurthle cell 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.
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.
Lymphatic invasion is the movement of tumour cells into a lymphatic vessel (small tubes that move fluid and immune cells around the body). Once tumour cells are inside a lymphatic vessel, they are able to spread to small immune organs called lymph nodes. For this reason, lymphatic invasion increases the risk of developing lymph node metastasis.
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:
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.
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.
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 Hurthle cell 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.
The pathologic stage for encapsulated angioinvasive Hurthle cell 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.
Encapsulated angioinvasive Hurthle cell 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.
Encapsulated angioinvasive Hurthle cell 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.
The pathologic stage for encapsulated angioinvasive Hurthle cell 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.