Thyroid cancer

Definition

Thyroid cancer is a disease in which the thyroid cells become abnormal, grow uncontrollably, and form tumors. Thyroid cancers are grouped into four types, based on how the cell appears under the microscope. If left untreated, the cancer can spread to other parts of the body.

Description

The thyroid is a butterfly-shaped gland, located at the base of the throat. It has two lobes, the left and the right. The thyroid gland makes hormones that regulate heart rate, blood pressure, body temperature, and metabolism. The hormones produced by the thyroid also affect the nervous system, muscles, and other organs, and play an important role in regulating childhood growth and development. The thyroid uses iodine, a mineral found in some foods, to make several of its hormones.

Diseases of the thyroid gland occur frequently, affecting millions of Americans. The most common diseases are an overactive or an underactive thyroid gland. These conditions are called hyperthyroidism (Grave's disease) or hypothyroidism. Sometimes, lumps or masses may develop in the thyroid glands. Ninety-five percent of these lumps or nodules are non-cancerous (benign), but all thyroid lumps should be taken seriously.

According to the estimates of the American Cancer Society, approximately 17,200 new cases of thyroid cancer will occur in the United States in 1998. This disease accounts for 1% of all cancers.

A woman's risk of developing thyroid cancer is three times greater than a man's. Most people who develop thyroid cancer are 50 years of age or older, but the disease can affect teenagers and young adults.

Thyroid cancers are grouped into four types, depending on how the cells look under the microscope. The types are papillary, follicular, medullary, and anaplastic thyroid cancers. The cancers grow at different rates, so the aggressiveness of each cancer is different.

Papillary cancer develops in the cells that produce thyroid hormones containing iodine. It is a slow-growing cancer and can be treated successfully. About 60-80% of all thyroid cancers are papillary cancers.

Follicular cancers also develop in the cells that produce iodine-containing hormones. Many of the follicular cancers have a good cure rate, but if the cancer invades blood vessels or grows into nearby structures in the neck, it may be difficult to control. About 30-50% of thyroid cancers are follicular cancers.

Medullary cancers develop in the parafollicular cells (also known as the C cells). These cells produce a hormone called calcitonin, which does not contain iodine. These cancers are more difficult to control because they have a tendency to spread to other parts of the body. About 5-7% of all thyroid cancers are medullary cancers. Approximately 7% of medullary cancers are caused by the alteration (mutation) of a gene called the RET gene; these cancers can be passed on in families.

Anaplastic cancer is the fastest growing of all thyroid cancers. The cells rapidly spread to the different parts of the body. About 2% of all thyroid cancers are anaplastic cancers.

Causes & symptoms

Although the exact cause of thyroid cancer has not yet been determined, it has been observed that thyroid cancer affects women three times as often as it affects men. The rate of thyroid cancer is also higher in whites than in African Americans.

Exposure to radiation during childhood is a known risk factor for thyroid cancer. In the 1950s and 1960s, radiation was used to treat acne and to reduce swelling and infection of organs in the neck, such as the tonsils, adenoids, and lymph nodes. Recent studies have proved that people who received radiation to the head and neck during their childhood have a higher than average chance of developing thyroid cancer.

In areas of the world where people's diets are low in iodine, papillary and follicular cancers occur more frequently. In the United States, dietary iodine is plentiful because it is added to table salt and other foods.

The most frequent symptom of thyroid cancer is a lump or nodule that can be felt in the neck. Other symptoms are rare. The lump usually is not painful, but some patients experience a tight or full feeling in the neck and have some difficulty breathing or swallowing. The lymph nodes may be swollen and the voice may become hoarse because the tumor presses on the nerves leading to the voice box.

Diagnosis

The doctor may use several tests to confirm a diagnosis of thyroid cancer. The size and location of the lump has to be identified and it has to be determined whether the lump is non-cancerous (benign) or cancerous (malignant). Blood tests, such as the thyroid stimulating hormone (TSH) test, may be ordered to check how well the patient's thyroid is functioning.

A test known as the calcitonin test may be ordered if medullary thyroid cancer is suspected. Calcitonin is a hormone produced by the C cells (also called the parafollicular cells) of the thyroid gland, in response to rising levels of calcium in the blood. This hormone lowers calcium levels in blood serum by slowing the rate at which the bones absorb it and increasing the rate at which the kidneys discharge it. When the parafollicular cells of the thyroid become cancerous, they produce too much calcitonin. Because increased serum levels of the calcitonin hormone are characteristic of medullary thyroid cancer, the calcitonin test, which measures serum levels of calcitonin, can be used to confirm a diagnosis of medullary thyroid cancer.

Imaging tests such, as a computed tomography scan (CT scan) or an ultrasound scan, may be used to produce a picture of the thyroid. In ultrasonography, high-frequency sound waves are bounced off the thyroid. The pattern of echoes that is produced by these waves is converted into an image by a computer. This test can determine whether the lumps found in the thyroid are fluid-filled cysts or solid malignant tumors.

A radioactive scan can be used to identify any abnormal areas in the thyroid. The patient is given a very small amount of radioactive iodine, which can either be taken by mouth or injected into the thyroid. Since the thyroid is the only gland in the body that absorbs iodine, the radioactive iodine accumulates there. A x-ray image can then be taken or an instrument called a "scanner" can be used to identify areas in the thyroid that do not absorb iodine normally. These abnormal spots are called "cold spots" and further tests are performed to check whether the cold spots are benign or malignant tumors. If a significant amount of radioactive iodine is concentrated in the nodule, then it is termed "hot" and is usually benign.

The most accurate diagnostic tool for thyroid cancer is a biopsy. In this process, a sample of thyroid tissue is withdrawn and examined under a microscope. The tissue samples can be obtained either by drawing out a sample of tissue through a needle (needle biopsy) or by surgical removal of the nodule (surgical biopsy). If thyroid cancer is diagnosed, further tests may be done to learn about the stage of the disease and help the doctors plan appropriate treatment.

Treatment

Treatment for thyroid cancer depends on the type of cancer and its stage. Cancer staging considers the size of the tumor, whether it has grown into surrounding lymph nodes, and whether it has spread to distant parts of the body (metastasized). The patient's age and general health status are also taken into account. Four types of treatment are used: surgical removal, radiation therapy, hormone therapy, and chemotherapy.

If the cancer has not spread to distant parts of the body, surgical removal is the usual treatment. The surgeon may remove the side or lobe of the thyroid where the cancer is found (lobectomy) or all of it (total thyroidectomy). If the adjoining lymph nodes are affected, they may also be removed during surgery.

Radiation therapy uses high-energy x rays to kill cancer cells and shrink tumors. The radiation may come from a machine outside the body (external beam radiation) or the patient may be asked to swallow a drink containing radioactive iodine. Because the thyroid cells take up iodine, the radioactive iodine collects in any thyroid tissue remaining in the body and kills the cancer cells.

Hormone therapy uses hormones to stop the cancer cells from growing. When the thyroid gland is removed and levels of thyroid hormones fall, the pituitary gland starts producing a hormone called "thyroid stimulating hormone" (TSH). TSH stimulates the thyroid cells to grow. This stimulation would also induce growth of the cancerous thyroid cells. To prevent cancerous growth, the natural hormones that are produced by the thyroid are taken in the form of pills. Thus, their levels remain normal and inhibit the pituitary from making TSH.

If the cancer has spread to other parts of the body and surgery is not possible, the treatment is aimed at killing or slowing the growth of cancer cells throughout the body. Chemotherapy may be used, either in a pill or an injection through a vein in the arm.

Prognosis

Like most cancers, cancer of the thyroid is best treated when it is found early. More than 90% of patients who are treated for papillary or follicular cancer will live for 15 years or longer after having had thyroid cancer. Eighty percent of patients with medullary thyroid cancer will live for at least 10 years after surgery. The fourth type of thyroid cancer, anaplastic, is usually fatal. Only 3-17% of patients with this cancer survive for 5 years.

Prevention

Because most people with thyroid cancer have no known risk factor, it is not possible to completely prevent this disease. However, inherited cases of medullary thyroid cancer can be prevented. If a family member has had this disease, the rest of the family can be tested.

The National Cancer Institute recommends that a doctor examine anyone who has received radiation to the head and neck during childhood at intervals of one or two years. The neck and the thyroid should be carefully examined for any lumps or enlargement of the nearby lymph nodes. Ultrasonography may be used for people at risk for thyroid cancer.

Key Terms

Biopsy

The surgical removal and microscopic examination of living tissue for diagnostic purposes.

Calcitonin

A hormone produced by the parafollicular cells (C cells) of the thyroid. The main function of the hormone is to regulate calcium levels in body serum.

Chemotherapy

Treatment of cancer with synthetic drugs that destroy the tumor either by inhibiting the growth of the cancerous cells or by killing them.

Hormone therapy

Treatment of cancer by inhibiting the production of hormones such as testosterone and estrogen.

Hyperthyroidism

A condition in which the thyroid is overactive due to overstimulation of the thyroid cells.

Hypothyroidism

A condition in which the thyroid gland is underactive.

Lobectomy

A surgical procedure that removes one lobe of the thyroid.

Radiation therapy

Treatment with high-energy radiation from x-ray machines, cobalt, radium, or other sources.

Total thyroidectomy

A surgical procedure that removes the entire thyroid gland.

Further Reading

For Your Information

Books

• Dollinger, Malin, Ernest H. Rosenbaum, and Greg Cable. Everyone's Guide to Cancer Therapy. Kansas City: Somerville House Books Limited, 1994.
• Morra, Marion and Eve Potts. Choices. New York: Avon Books, 1994.
• Murphy, Gerald P., Lois B. Morris, and Diane Lange. Informed Decisions: The Complete Book of Cancer Diagnosis, Treatment, and Recovery. New York: Viking, 1997.

  Organizations

• American Cancer Society (National Headquarters). 1599 Clifton Road N.E., Atlanta, Georgia 30329. (800) 227-2345. http://www.cancer.org.
• American Thyroid Association, Inc. Montefiore Medical Center, 111 East 210th Street, Bronx, New York 10467. http://thyroid.org.
• Cancer Research Institute (National Headquarters). 681 Fifth Avenue, New York, New York 10022. (800) 992-2623. http:://www.cancerresearch.org.
• National Cancer Institute. 9000 Rockville Pike, Building 31, Room 10A16, Bethesda, Maryland 20892. (800) 422-6237. http://www.nci.nih.gov.
• Thyroid Foundation of America, Inc. Ruth Sleeper Hall, RSL 350, 40 Parkman Street, Boston, MA 02114-2698. (800) 832-8321.

  Other

• 1998 Cancer Facts and Figures. American Cancer Society.
• NCI/PDQ Patient Statement, "Thyroid Cancer." National Cancer Institute.
• ONCOLINK. University of Pennsylvania Cancer Center. http://cancer.med.upenn.edu.

Gale Encyclopedia of Medicine. Gale Research, 1999.