What is thyroid cancer?

Related conditions:
Autoimmune thyroiditis, sarcoma of the thyroid, Gardner syndrome, Cowden syndrome

Definition:
Thyroid cancer is a malignant tumor in the thyroid gland, a butterfly-shaped gland in the neck that makes hormones essential for normal body function. The thyroid contains cells that grow and divide to form new ones as needed; however, when this process goes awry, tissue masses called nodules develop; these may be benign or malignant (cancerous). There are several types of thyroid cancer, with varying degrees of incidence according to the American Cancer Society. Papillary, the most common type (about 80 percent of all thyroid cancers), has a very high cure rate. It usually appears in people thirty to fifty years old. About 10 to 15 percent of thyroid tumors are follicular carcinoma. This cancer is more aggressive, occurs in a slightly older age group, and rarely occurs after radiation exposure but is more common in an iodine-deficient environment. Hurthle cell carcinoma is a rare variant that makes up about 3 percent of thyroid cancers. Medullary thyroid cancer (about 4 percent) is not associated with radiation exposure and is inherited. Anaplastic carcinoma is the rarest type (about 2 percent) but the most deadly; it may appear many years following radiation exposure. Thyroid lymphomas account for a very small percentage of thyroid cancers.

Risk factors: Increased risk of papillary carcinoma is associated with external-beam irradiation to the head and neck areas, especially during childhood. Exposure to ingested radioactive isotopes and radioactive fallout also poses a cancer risk. Having a parent with multiple endocrine neoplasia (MEN) types 2A or 2B or familial medullary cancer increases the chance of having the genetic mutation that causes thyroid cancer by as much as 50 percent. Women are two to three times as likely as men to develop thyroid cancer; women whose last pregnancy occurs at or later than age thirty are also at greater risk. Whites and Asians are more susceptible than are blacks. Other risk factors include dietary iodine deficiency; ingestion of goitrogenic (goiter-causing) or cruciferous vegetables (such as cabbage) and seafood and shellfish, especially when fished from sites near active volcanoes, as in Hawaii and Iceland; and chronic elevation of thyroid-stimulating hormone (TSH).

Etiology and the disease process: In many cases, the etiology, or cause, of thyroid cancer is unknown. Thyroid cancers are either differentiated or undifferentiated. Differentiated cells look and act like normal ones and actually assist in making thyroxine. They reproduce more slowly than undifferentiated ones do. Papillary and follicular carcinoma are two types of differentiated cancer. Around 85 percent of papillary thyroid cancers are caused by radiation exposure. Follicular cancer is more prevalent in countries where people are iodine deficient.

Undifferentiated cancer is made up of very primitive cells that do nothing but reproduce; this produces the rare anaplastic cancer that is not effectively treatable and therefore has a high mortality rate. Even rarer is medullary thyroid carcinoma (MTC), which is inherited. A specific cell, the C cell, is involved. It makes the hormone calcitonin, which helps regulate calcium in the body, but when it overproduces, medullary carcinoma must be suspected.

Incidence: Compared with other types of cancer, thyroid cancer is uncommon; only about 1.1 percent of men and women in the United States are diagnosed with it, and the annual number of deaths is just 0.5 per one hundred thousand people. The overall incidence of thyroid cancers, especially in women, has been on the rise due in part to improved diagnosis methods, but earlier detection, improved treatment, and a decline in the very aggressive anaplastic type have resulted in fewer deaths.

Symptoms: In its early stages, thyroid cancer often does not exhibit symptoms. As the disease progresses, however, the affected person may develop a nodule (lump) in the front of the neck; hoarseness or changes in the normal speaking voice; swollen lymph nodes, especially in the neck; swallowing or breathing difficulty; and pain in the throat or neck.

Screening and diagnosis: More often than not, a nodule felt in the thyroid during a routine physical examination or found incidentally during an imaging test for some other condition signals a tumor’s presence. When symptoms suggest thyroid cancer, a number of tests may be performed: An ultrasound scan outlines a growth but does not rule out malignancy. That determination is made with biopsy. Biopsy may be fine needle aspiration (FNA) biopsy, in which a needle is inserted in different parts of a nodule to remove cell samples that are analyzed in a laboratory, or surgical biopsy to remove the nodule and check the tissue for cancer cells.

Blood tests may also be done to detect abnormal levels of thyroid-stimulating hormone in the blood. If a physician suspects medullary cancer, blood tests are run to check for abnormally high levels of calcitonin in the blood or to detect an altered gene (RET gene), which aids diagnosis.

A radionuclide scan involves administering a small amount of radioactive material to make thyroid nodules show up on a picture. When thyroid cancer is diagnosed, tests are done to determine whether the cancer has spread; this process is called staging. Tests such as ultrasonography, computed tomography (CT) scanning, and magnetic resonance imaging (MRI) can help the doctor determine if the cancer has spread to the lymph nodes or to other areas. A special scan may be used to check for the spread of medullary cancer. Staging is essential to choose treatment options and predict odds for cure and long-term survival. A common system is that of the American Joint Committee on Cancer (AJCC), called TNM staging (tumor/lymph node/metastasis). This system looks at the size of the tumor, whether it has spread to the lymph nodes, and whether it has metastasized to distant organs, ranking their progress with Roman numerals from I to IV. In the case of papillary or follicular thyroid carcinoma, age is also taken into account. All anaplastic thyroid cancers are considered Stage IV to reflect the poor prognosis of this particular kind of thyroid cancer.

Treatment and therapy: Treatment for thyroid cancer includes surgery, thyroid hormone therapy, radiation, and chemotherapy. Total or near-total surgical removal is the treatment of choice in most cases. Following surgery, thyroid hormone medication is required for life to supply the hormone the thyroid would normally produce. Frequent blood tests are done until the proper dosage can be established. Radioactive iodine may be used for follow-up screening to detect remaining normal or abnormal tissues. Moderate doses can eliminate the normal tissue, and larger doses destroy any cancerous cells. External beam radiation also destroys cancer cells. Chemotherapy may be used when the cancer has metastasized, especially for the medullary type, which does not respond to radioiodine therapy. Other follow-up care includes blood tests for thyroglobulin levels that would indicate recurrence, and imaging such as ultrasonography.

Prognosis, prevention, and outcomes: The type of thyroid cancer affects the prognosis to a large extent. In papillary and follicular cancer, Stages I and II have a near 100 percent five-year relative survival rate. Stage III decreases to 93 percent for papillary and 71 percent for follicular types. Stage IV drops to 51 percent in papillary cancer and 50 percent for follicular. Stage I medullary thyroid cancer also has a near 100 percent five-year survival rate, then drops to 98 percent and 81 percent, respectively, for Stages II and III; however, the rate falls to only 28 percent for Stage IV. The five-year survival rate for anaplastic thyroid cancer is around 7 percent.

Thyroid cancer prevention measures are limited. However, certain steps can be taken in situations in which there is heightened risk. A genetic test can determine if there is increased risk for medullary thyroid cancer. If the results are positive, thyroid gland removal (thyroidectomy) may prevent development of cancer later in life. Also, government guidelines recommend that people living within ten miles of a nuclear power plant take potassium iodide tablets just before or immediately after exposure to fallout. Anyone who has received radiation to the head and neck during childhood should be examined carefully every year or two. More general measures include diets high in fruit and vegetables and low in animal fats and consumption of unsaturated fats, which contain omega-3 fatty acids. Maintaining a healthy weight may also help.

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