Radiation and Thyroid Cancer

Medical Treatments with Cancer Risks, Other Cancers
Sonia Nagda, MD, MPH, Cancer Prevention and Treatment Fund

Recent media reports have raised fears that radiation from dental x-rays and mammograms increase the risk of thyroid cancer.  Thyroid cancer rates have been increasing over the past 30 years, and this cancer affects women three times as often as men.[1] Is it true that dental x-rays and mammograms are to blame for the increase in thyroid cancer?  Can a simple thyroid shield (an optional extension of the lead apron that blocks x-rays from reaching the neck) reduce the risk and put fears to rest?

Radiation in many forms-including x-rays, CT scans, sunlight, nuclear fallout (from atomic warfare or nuclear accidents such as Chernobyl) and even the radiation therapy that is used to treat cancer-can harm the DNA in the body and cause cancer. For more on this, see:  Everything You Ever Wanted to Know About Radiation and Cancer but Were Afraid to Ask.

The thyroid gland is one of the organs most sensitive to the risk of radiation. Located in the neck directly over the trachea (the tube that brings air from the nose and mouth into the lungs)¸the thyroid gland makes two different hormones, known as T3 and T4, which are responsible for regulating energy and the body’s metabolism.

Background Radiation vs. X-Rays

We are all exposed to small amounts of radiation all the time. This is called “background” radiation. People who live in areas where radon gas is common or at high altitude have higher levels of background radiation.

For radiation to affect your thyroid and cause cancer, it has to reach your thyroid gland in sufficiently high doses-either from a big one-time dose (as from a nuclear disaster) or through many smaller doses. The closer the area being x-rayed is to the thyroid, the greater the amount of scattered rays that will reach it.  Since the mouth is closer to the thyroid than the breasts, it makes sense that dental x-rays are more likely to affect the thyroid gland than mammograms, which are x-rays of the breast. On the other hand, dental x-rays expose patients to much less radiation than a mammogram: 0.005 millisieverts (mSv) for a dental x-ray, which is comparable to one day of natural background radiation, as compared with 0.4 mSv for a mammogram, which is comparable to 7 weeks of background radiation.[2]

Besides the dose, the age of the person being x-rayed or scanned is important. The thyroid gland is particularly sensitive to radiation during childhood and adolescence-when the gland is most active and a person’s body grows the most. As we age, the thyroid gland doesn’t work as hard, and the amount of radiation that it takes in becomes much smaller.

Dental X-Rays: a Risk for Thyroid Cancer or Not?

A study by Sara Schonfield and colleagues at the National Cancer Institute, published in 2011, compared the number of dental x-rays received by a group of thyroid cancer patients prior to their diagnosis with the number received by a group of similar individuals without thyroid cancer. Overall, those who had dental x-rays were twice as likely to develop thyroid cancer.  More than 75% of the thyroid cancer patients were diagnosed before the age of 44. The more dental x-rays that a patient received, the more likely he or she was to develop thyroid cancer: the patients who received more than 10 x-rays had more than 5 times the risk of developing cancer than someone who had not had any dental x-rays.[1]

Mammograms: a Risk for Thyroid Cancer or Not?

The radiation that scatters from the breast to the thyroid gland is so negligible that the risk of developing thyroid cancer in a 40-year-old woman getting a mammogram is 6 in a billion.[3] This is similar to the amount of radiation your thyroid would get by standing outside for 30 minutes. Even if you got mammograms every year from the ages of 40 to 80, your risk of developing thyroid cancer would still only be 1 in 17.8 million, so there’s really no need to use a thyroid shield. In fact, using a thyroid shield during a mammogram makes the image blurry and more difficult for the radiologist to read. And, the thyroid shield can slip out of place and get in the way of the x-ray image, making a repeat exam (potentially exposing you to more radiation!) necessary.[4]

So, if mammograms don’t increase the risk of thyroid cancer, why are women three times as likely as men to be diagnosed with this cancer? Unfortunately, researchers have not yet found the answer to this question. Some believe that it could be related to better detection, and others think it could be a combination of diet, genetics, and the environment.[5]

Other Forms of Imaging

While CT scans of the head and neck are not as common as mammograms or dental x-rays, these can produce a lot of scattered radiation that can be absorbed by the thyroid. Studies have shown that wearing a thyroid shield during CT scans of the head and neck significantly limits radiation exposure to the thyroid gland.[6,7]

What You Need to Know to Keep You and Your Family Safe

  • Wear a thyroid guard during dental x-rays and CT scans.
  • You do not need to wear a thyroid guard during your mammogram. Continue regular mammography as recommended based on your breast cancer risk and age. Click here to learn about the latest mammography guidelines.
  • Try to keep x-rays of all kinds to a minimum, especially in children. Make sure that a scan (x-ray, CT, etc.) is being done only when needed, and that repeat exams are not done more frequently than absolutely necessary. Request that medical records and images be sent to all of the doctors treating you so that they don’t ask you to undergo scans that have already been done.

References:

  1. Schonfeld SJ. Lee C. Berrington de Gonzalez A. “Medical Exposure to Radiation and Thyroid Cancer.” Clinical Oncology 2011; 23:244-250.
  2. “Patient Safety: Radiation Exposure in X-ray and CT Examinations.” RadiologyInfo.org. Accessed April 23, 2012. Available at: http://www.radiologyinfo.org/en/pdf/sfty_xray.pdf.
  3. Sechopoulos I, Hendrick RE. “Mammography and the Risk of Thyroid Cancer.” AJR 2012; 198:705-707.
  4. Kopans DB. “Mammograms and thyroid cancer: The facts about breast-cancer screening” Accessed April 20, 2012. Found at: http://www.massgeneral.org/imaging/about/newsarticle.aspx?id=2720.
  5. Chen AY, Jemal A, Ward EM. “Increasing Incidence of Differentiated Thyroid Cancer in the United States, 1988-2005.” Cancer 2009; 115(16): 3801-3807.
  6. Williams L, Adams C. “Computed tomography of the head: An experimental study to investigate the effectiveness of lead shielding during three scanning protocols.” Radiography 2006; 12: 143-152.
  7. Lee YH, Park E, Cho PK, et.al. “Comparative Analysis of Radiation Dose and Image Quality Between Thyroid Shielding and Unshielding During CT Examination of the Neck.” AJR 2011; 196:611-615.