by Jason Wasserman MD PhD FRCPC
July 15, 2022
Minimally invasive follicular thyroid carcinoma is a type of thyroid cancer. The tumour is separated from the normal thyroid gland by a thin band of tissue called a tumour capsule. In a “minimally invasive” tumour, groups of cancer cells have broken through the tumour capsule and spread into the surrounding normal thyroid gland.
The diagnosis of minimally invasive 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. The tumour needs to be removed because the entire tumour capsule needs to be examined under the microscope to look for tumour capsule invasion. Tumour capsule invasion means that cancer cells have broken through the tumour capsule and have spread into the surrounding normal thyroid gland. A follicular adenoma is a type of non-cancerous thyroid tumour that looks very similar to minimally invasive follicular carcinoma. However, in contrast to minimally invasive follicular carcinoma, the tumour cells in a follicular adenoma have not broken through the capsule and spread into the surrounding thyroid gland.
Follicular thyroid carcinoma is called “minimally invasive” when only some of the cancer cells have broken through the tumour capsule and spread into the surrounding normal thyroid gland. This is different from a related type of cancer called 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.
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.
Angioinvasion (vascular invasion) means that cancer cells were seen inside a blood vessel. In order to make the diagnosis of minimally invasive follicular thyroid carcinoma, your pathologist should not see angioinvasion (vascular invasion). If angioinvasion (vascular invasion) is seen, the tumour should be diagnosed as encapsulated angioinvasive follicular thyroid carcinoma.
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 minimally invasive follicular thyroid carcinoma.
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.
A margin is any tissue that has to be cut by the surgeon in order 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.
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. Minimally invasive 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.
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 typically be described in your report.
All lymph nodes are surrounded by a thin layer of tissue called a capsule. Extranodal extension (ENE) means that cancer cells have broken through the capsule and spread into the tissue that surrounds the lymph node.
The pathologic stage for minimally invasive 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 more advanced disease and a worse prognosis.
Minimally invasive 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.
Minimally invasive 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.
Minimally invasive follicular 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.