by Jason Wasserman MD PhD FRCPC
July 26, 2024
Follicular thyroid carcinoma (FTC) is a type of thyroid cancer that originates from the follicular cells of the thyroid gland. These cells are responsible for producing and secreting thyroid hormones. Follicular thyroid carcinoma is the second most common type of thyroid cancer, following papillary thyroid carcinoma. It typically affects middle-aged adults and has a slightly higher prevalence in women.
The symptoms of follicular thyroid carcinoma can be subtle in the early stages and may include:
The exact cause of follicular thyroid carcinoma is not well understood, but several risk factors and genetic mutations have been identified:
The diagnosis of follicular thyroid carcinoma involves several steps:
Microscopically, follicular thyroid carcinoma is characterized by tumour cells arranged in follicular patterns resembling normal thyroid follicles. These cells are uniform, small to medium-sized with round to oval nuclei and have abundant colloid. A critical feature for diagnosis is capsular invasion, where tumour cells penetrate the fibrous capsule surrounding the nodule. Vascular invasion, where tumour cells invade blood vessels within or beyond the capsule, is also an important diagnostic criterion.
Most follicular thyroid carcinomas are surrounded by a layer of fibrous tissue called a tumour capsule. Capsular invasion refers to the penetration of this fibrous capsule by tumour cells. This invasion is a critical feature distinguishing follicular thyroid carcinoma from benign follicular adenoma.
After microscopic examination, follicular thyroid carcinoma can be categorized into different subtypes:
These subtypes are described in greater detail in the sections below.
In minimally invasive follicular thyroid carcinoma, the tumour is surrounded by a fibrous tumour capsule. However, microscopic examination shows tumour cells breaking through the capsule and spreading into the surrounding thyroid gland tissue. This subtype is usually associated with a better prognosis and a lower risk of metastasis compared to encapsulated angioinvasive and widely invasive follicular thyroid carcinoma.
Widely invasive follicular thyroid carcinoma exhibits extensive invasion into the surrounding thyroid tissue and may or may not show vascular invasion. This subtype is more aggressive and has a higher risk of distant metastasis and recurrence compared to minimally invasive follicular thyroid carcinoma.
Encapsulated angioinvasive follicular thyroid carcinoma is characterized by a well-defined capsule and vascular invasion without significant capsular invasion. This subtype falls between minimally invasive and widely invasive follicular thyroid carcinoma in terms of aggressiveness and prognosis.
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 it as being 4.0 cm. Tumour size is important for follicular thyroid carcinoma because it determines the pathologic tumour stage (pT) and because larger tumours are more likely to spread to other body parts, such as lymph nodes.
Extrathyroidal extension (ETE) refers to the spread of cancer cells beyond the thyroid gland into surrounding tissues. It is an important prognostic factor in thyroid cancer, as it can significantly influence both the staging and management of the disease.
Extrathyroidal extension is classified into two types based on the extent of the spread:
Extrathyroidal extension is important for the following reasons:
Vascular invasion, or angioinvasion, in the context of follicular thyroid carcinoma, means that the cancer cells have spread into the blood vessels in or around the tumour. This is an important sign because it can indicate that the cancer might spread to other body parts, such as the lungs or bones.
Pathologists use two terms to describe how much vascular invasion (angioinvasion) is present:
Extensive vascular invasion (4 or more blood vessels) usually means a higher risk of the cancer spreading, which can lead to a worse prognosis. If there is extensive angioinvasion, doctors often recommend more aggressive treatments to try to control the cancer better. This could include additional surgery, radioactive iodine therapy, or more frequent follow-up visits to monitor for any signs of cancer spreading.
Lymphatic invasion in the context of follicular thyroid carcinoma of the thyroid gland refers to the infiltration and spread of cancer cells into the lymphatic system. Cancer cells that enter the lymphatic system can travel to lymph nodes. It is relatively uncommon to find lymphatic invasion with follicular thyroid carcinoma, and unlike vascular invasion, the presence of lymphatic invasion is not necessarily associated with a more aggressive disease or a worse prognosis.
In pathology, a margin is the edge of tissue removed during tumour surgery. The margin status in a pathology report is important as it indicates whether the entire tumour was removed or if some was left behind. This information helps determine the need for further treatment.
Pathologists examine margins to check if tumour cells are present at the tissue’s cut edge. A positive margin, where tumour cells are found, suggests that some tumour cells may remain in the body. In contrast, a negative margin, with no tumour cells at the edge, suggests the tumour was fully removed. Some reports also measure the distance between the nearest tumour cells and the margin, even if all margins are negative.
Lymph nodes are small immune organs found throughout the body. Cancer cells can spread from a 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 cancer cells from the tumour to another part of the body, such as a lymph node, is called metastasis.
Cancer cells typically spread first to lymph nodes close to the tumour, although lymph nodes 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. 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 are removed from your body, they will be examined under the microscope by a pathologist, and the examination results will be described in your report. “Positive” means that cancer cells were found in the lymph node. “Negative” means that no cancer cells were found. If cancer cells are found in a lymph node, the size of the largest group of cancer 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.
The examination of lymph nodes is important for two reasons. First, this information determines 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 radioactive iodine, chemotherapy, radiation therapy, or immunotherapy, is required.
The pathologic stage for follicular thyroid carcinoma can only be determined after the entire tumour has been surgically removed and examined under the microscope by a pathologist. The stage is divided into three parts: tumour stage (pT) which describes the tumour, nodal stage (pN) which describes any lymph nodes examined, and metastatic stage (pM) which describes tumour cells that have spread to other parts of the body. Most pathology reports will include information about the tumour and nodal stages. The overall pathologic stage is important because it helps your doctor determine the best treatment plan and predict the outlook for recovery.