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By the way, doctor: How does radiation cause thyroid cancer?

By the way, doctor

How does radiation cause thyroid cancer?

Q. What kind of radiation causes thyroid cancer? What about microwave ovens and dental x-rays?

A. The thyroid is a butterfly-shaped gland located at the front of the neck beneath the larynx. Compared to many other organs, the thyroid gland is particularly sensitive to certain types of radiation. For example, radiation from x-ray and radiation therapy machines and radioactive atoms are linked to both benign and malignant thyroid tumors. The younger the age of exposure and the higher and more direct the dose, the more likely it is that a thyroid tumor will develop in adulthood.

With therapeutic radiation, the thyroid risk is greatest when the head and neck tissues are exposed directly to the radiation beam. With dental x-rays, which are diagnostic, the thyroid is usually protected with a lead apron (each dose is very small, but the effects can be cumulative). In the United States, radiation was used during the 1940s and '50s to treat conditions such as chronic cystic acne and fungal infections of the scalp in children and teenagers. Radiation-tipped rods were inserted through the nose to shrink tonsils and adenoids. People who received such treatments as children are at increased risk for thyroid cancer in adulthood and should have regular thyroid examinations. High radiation exposures to an adult carry less of a risk of thyroid cancer.

X-ray and nuclear fallout are examples of ionizing radiation, which produces enough energy to cause disruptions in atoms. Ionizing radiation can cause DNA damage within the cell, which can result in cell death or trigger cancer-causing mutations.

We're exposed to radiation from our environment every day — from the sun, outer space, and the earth's crust. We have radioactive atoms in our bodies. Roughly 80% of our annual exposure comes from such natural sources. When studying overall radiation exposure in people, scientists use a unit called the millisievert (mSv). The average American is exposed naturally to 3 mSv every year. By comparison, a person gets 0.06 mSv from dental x-rays, 0.08 mSv from a chest x-ray, and 0.10 mSv from a mammogram. For most adults, the benefits of an x-ray outweigh the risk of this small additional exposure.

Microwaves do not cause thyroid cancer. They're a form of non-ionizing radiation and thus cannot ionize tissue. Microwave ovens use low-frequency waves of electrical and magnetic energy to produce heat to cook food. They don't make food radioactive, nor do they trigger cancer-causing genetic mutations. At the moment, we have little information on the health effects of long-term exposure to such low-level microwaves.

Ultraviolet light, visible light, infrared radiation, and radio waves are other forms of non-ionizing radiation. Thyroid cancer is not associated with these types of radiation.

— Celeste Robb-Nicholson, M.D. Editor in Chief, Harvard Women's Health Watch

Last updated:	January 19, 2007