by Jason Wasserman MD PhD FRCPC
March 22, 2023
Widely invasive follicular thyroid carcinoma is a type of thyroid cancer. The tumour is called “widely invasive” because groups of cancer cells are found throughout the normal thyroid gland.
The diagnosis of widely 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 diagnosis cannot be made after a less invasive procedure called a fine needle aspiration biopsy (FNAB).
Follicular thyroid carcinoma is called “widely invasive” when the cancer cells have spread throughout the normal thyroid gland. In contrast, the cancer cells in a related type of cancer called minimally invasive follicular thyroid are mostly separated from the normal thyroid gland by a thin layer of tissue called a tumour capsule.
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 at least one blood vessel. It is very common to find angioinvasion in widely invasive follicular thyroid carcinoma. Angioinvasion is important because cancer cells that have entered a blood vessel are able to spread to other parts of the body such as the lungs or bones.
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 widely 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 found throughout the body. Cancer cells can spread from a tumour to lymph nodes through small vessels called lymphatics. For this reason, lymph nodes are commonly removed and examined under a microscope to look for cancer cells. The movement of cancer cells from the tumour to another part of the body such as a lymph node is called a metastasis.
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.
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. Most reports will include the total number of lymph nodes examined, where in the body the lymph nodes were found, and the number (if any) that contain cancer cells. If cancer cells were seen in a lymph node, the size of the largest group of cancer cells (often described as “focus” or “deposit”) will also be included.
The examination of lymph nodes is important for two reasons. First, this information is used to determine the pathologic nodal stage (pN). Second, finding cancer 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 chemotherapy, radiation therapy, or immunotherapy is required.
Pathologists often use the term “positive” to describe a lymph node that contains cancer cells. For example, a lymph node that contains cancer cells may be called “positive for malignancy” or “positive for metastatic carcinoma”.
Pathologists often use the term “negative” to describe a lymph node that does not contain any cancer cells. For example, a lymph node that does not contain cancer cells may be called “negative for malignancy” or “negative for metastatic carcinoma”.
The pathologic stage for widely 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 a more advanced disease and a worse prognosis.
Widely 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.
Widely 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.
Widely 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.