What Is Thyroid Cancer?
Cancer starts when cells in the body begin to grow out of control. Cells in nearly any part of the body can become cancer, and can spread to other areas of the body.
Thyroid cancer starts in the thyroid gland.
Benign thyroid enlargement and nodules
Lumps or bumps in the thyroid gland are called thyroid nodules. Most thyroid nodules are benign, but about 2 or 3 in 20 are cancerous. Sometimes these nodules make too much thyroid hormone and cause hyperthyroidism. Nodules that produce increased thyroid hormone are almost always benign.
People can develop thyroid nodules at any age, but they occur most commonly in older adults. Fewer than 1 in 10 adults have thyroid nodules that can be felt by a doctor. But when the thyroid is looked at with ultrasound, many more people are found to have nodules that are too small to feel. Most evidence suggests that they are benign.
Most nodules are cysts filled with fluid or with a stored form of thyroid hormone called colloid. Solid nodules have little fluid or colloid. These nodules are more likely to be cancerous than the ones fluid-filled nodules. Still, most solid nodules are not cancer. Some types of solid nodules, such as hyperplastic nodules and adenomas, have too many cells, but the cells are not cancer cells.
Benign thyroid nodules sometimes can be left alone (not treated) as long as they’re not growing or causing symptoms. Others may require some form of treatment.
Types of malignant (cancerous) thyroid tumors
The main types of thyroid cancer are:
- Differentiated (including papillary, follicular and Hṻrthle cell)
- Anaplastic (an aggressive undifferentiated tumor)
Differentiated thyroid cancers
Most thyroid cancers are differentiated cancers. The cells in these cancers look a lot like normal thyroid tissue when seen with a microscope. These cancers develop from thyroid follicular cells. These are described below.
About 8 out of 10 thyroid cancers are papillary cancers (also called papillary carcinomas or papillary adenocarcinomas). Papillary cancers tend to grow very slowly and usually develop in only one lobe of the thyroid gland. Even though they grow slowly, papillary cancers often spread to the lymph nodes in the neck. Still, these cancers that have spread to the lymph nodes can often be treated successfully and are rarely fatal.
There are several subtypes of papillary cancers. Of these, the follicular subtype (also called mixed papillary-follicular variant) occurs most often. The usual form of papillary cancer and the follicular subtype have the same good outlook (prognosis) when found early, and they are treated the same way. Other subtypes of papillary carcinoma (columnar, tall cell, insular, and diffuse sclerosing) are not as common and tend to grow and spread more quickly.
Follicular cancer, also called follicular carcinoma or follicular adenocarcinoma, is the next most common type, making up about 1 out of 10 thyroid cancers. It is more common in countries where people don’t get enough iodine in their diet. These cancers usually do not spread to lymph nodes, but they can spread to other parts of the body, such as the lungs or bones. The outlook (prognosis) for follicular cancer is not quite as good as that of papillary cancer, although it is still very good in most cases.
Hṻrthle (Hurthle) cell cancer:
This type is also known as oxyphil cell carcinoma. About 3% of thyroid cancers are this type. It is harder to find and to treat.
Medullary thyroid carcinoma
Medullary thyroid cancer (MTC) accounts for about 4% of thyroid cancers. It develops from the C cells of the thyroid gland, which normally make calcitonin, a hormone that helps control the amount of calcium in blood. Sometimes this cancer can spread to lymph nodes, the lungs, or liver even before a thyroid nodule is discovered.
This type of thyroid cancer is more difficult to find and treat, there are 2 types of MTC:
- Sporadic MTC, which accounts for about 8 out of 10 cases of MTC, is not inherited (meaning it does not run in families). It occurs mostly in older adults and affects only one thyroid lobe.
- Familial MTC: is inherited and 20% to 25% can occur in each generation of a family. These cancers often develop during childhood or early adulthood and can spread early. Patients usually have cancer in several areas of both lobes. Familial MTC is often linked with an increased risk of other types of tumors
Anaplastic (undifferentiated) thyroid cancer
Anaplastic carcinoma (also called undifferentiated carcinoma) is a rare form of thyroid cancer, making up about 2% of all thyroid cancers. It is thought to sometimes develop from an existing papillary or follicular cancer. This cancer is called undifferentiated because the cancer cells do not look very much like normal thyroid cells under the microscope. This cancer often spreads quickly into the neck and to other parts of the body, and is very hard to treat.
Less Common Thyroid Cancers
Less than 4% of cancers found in the thyroid are thyroid lymphomas, thyroid sarcomas, or other rare tumors.
Most thyroid cancers can be treated successfully. But advanced cancers can be hard to treat, especially if they do not respond to radioactive iodine (RAI) therapy. Doctors and researchers are looking for new ways to treat thyroid cancer that are more effective and lead to fewer side effects.
Surgery is an effective treatment for most thyroid cancers, and it can usually be done without causing major side effects, especially when done by experienced surgeons.
Radioactive iodine (RAI) therapy
Doctors are looking for better ways to see which cancers are likely to come back after surgery. Patients with these cancers may be helped by getting RAI therapy after surgery. Recent studies have shown that patients with very low thyroglobulin levels 3 months after surgery have a very low risk of recurrence even without RAI. More research in this area is still needed.
Some studies are testing the value of chemotherapy drugs, as well as combined chemotherapy and radiation in treating anaplastic thyroid cancer.
In general, thyroid cancers do not respond well to chemotherapy. But exciting data are emerging about some newer targeted drugs. Unlike standard chemotherapy drugs, which work by attacking rapidly growing cells (including cancer cells), these drugs attack specific targets on cancer cells. Targeted drugs may work in some cases when standard chemotherapy drugs do not, and they often have different (and less severe) side effects.
The chance of being diagnosed with thyroid cancer has risen rapidly in the US in recent years. Much of this rise appears to be the result of the increased use of thyroid ultrasound, which can detect small thyroid nodules that might not otherwise have been found in the past.
Recent international studies have suggested that some of these newly found, very small thyroid cancers (known as micro-papillary thyroid cancers) may not need to be treated right away but instead can be safely observed. Ongoing clinical trials in the US are now looking to confirm the results of these international studies.