Does radiation from dental X-rays cause thyroid cancer?

Every weekday, a CNNHealth expert doctor answers a viewer question. On Wednesdays, it's Dr. Otis Brawley, chief medical officer at the American Cancer Society.

Asked by Cate, Kennesaw, Georgia
I have heard that there is an increase in the number of people getting thyroid cancer. Is this true and is radiation from dental X-rays a cause for the increase?
Expert answer:
Dear Cate,
The thyroid is a gland in the neck below and in front of the throat. It secretes hormones that control metabolism. People with low thyroid function gain weight and are lethargic. Those with an overactive thyroid have weight loss and can experience nervousness and other problems.
The American Cancer Society estimates that 44,670 new cases of thyroid cancer occurred in 2010 (33,930 in women, and 10,740 in men). Thyroid cancer caused about 1,690 deaths (960 women and 730 men). Thyroid cancer is different from many other adult cancers in that it is commonly diagnosed in younger people. Nearly two of three cases are found in people between the ages of 20 and 55.
The chance of getting thyroid cancer has risen in recent years. In the U.S., there has been a rise in the age-adjusted incidence rate of diagnosed thyroid cancer since 1980. The rate for women went from 6 cancers per 100,000 American women in 1980 to 17 per 100,000 American women in 2007. The comparable rates of diagnosis for men went from 2.5 per 100,000 in 1980 to 5.8 per 100,000 in 2007. Even with a near tripling of the female incidence rate in the past 30 years, it is small compared with the cancer rates for lung, colon, breast or prostate cancer.
Despite these changes in incidence rates, risk of death has remained stable over the past 30 years. It has been at about 0.5 deaths per 100,000 American women and at 0.4 to 0.5 per 100,000 for American men. Indeed, thyroid cancer is one of the most survivable of cancers. More than 97% of patients do very well.
Much of this increase is thought to be the result of the increased medical imaging of the thyroid. It is not so much that there are more thyroid cancers, as that more thyroid cancers are being discovered or diagnosed. Use of CAT scans, MRI scans and thyroid ultrasound has dramatically increased over the past 30 years. These tests can detect very small thyroid nodules that several decades ago would not have been found.
I know it is hard to imagine that there are some cancers that are not clinically significant, meaning cancers of no threat that do not need to be treated. This is the case in some thyroid cancers, some prostate cancers and in several other malignancies. One of medicine's great problems is we can diagnose a cancer, but we cannot accurately predict those who need therapy and those who do not. Therapy for thyroid cancer is most commonly surgical removal of half or all of the thyroid.
While small cancerous thyroid tumors of no threat to health are being found, a part of the rise in thyroid cancer incidence is due to finding an increased number of larger cancerous tumors that are clinically significant.
Radiation exposure does increase risk of thyroid cancer, and the risk increases with the amount of radiation exposure. The developing thyroid is especially susceptible to radiation. Children exposed to radiation are at increased risk of developing thyroid cancer later in life. The thyroid can absorb radiation through ingestion of radioactive material, especially radioactive iodine, or through external exposure such as from an X-ray machine.
The National Cancer Institute has done a study to see if the rise in thyroid cancer incidence is related to radioactive iodine released during aboveground nuclear weapons testing, which was common in the western U.S. from 1945 to 1963. The conclusion is that the rise is not related. Most of those affected by the increase in incidence were born years after nuclear testing stopped.
Dental X-rays give a very low dose of radiation to the mouth. There is some scatter of radiation and the potential for some radiation absorption by the nearby thyroid and other organs. The American Dental Association notes that a leaded apron placed over the torso minimizes radiation exposure to the chest and abdomen and should be used when any dental X-ray is taken. The group also notes that a leaded thyroid collar can protect the thyroid from radiation and should be used whenever possible.
While the experts that I consulted do recommend use of a thyroid collar, none of the experts was especially worried that dental X-rays are a significant cause of thyroid cancer. There is however significant concern that radiation from other forms of medical testing may be causing some thyroid cancers. CT scanning is the medical procedure that concerns experts the most. It uses higher levels of radiation than conventional X-rays and much higher levels of radiation than are used in dental X-rays.