Should I request a thyroid guard during a mammogram?
Answer provided by Christina Gougoutas-Fox, M.D., director, Ruth J. Spear Breast Center at Providence St. Vincent Medical Center, and radiologist, Providence Diagnostic Imaging
A chain email, forwarded by a friend, has me concerned. The email focuses on a recent episode of "The Dr. Oz Show," in which Dr. Mehmet Oz suggested that radiation from mammograms and dental X-rays could be partly responsible for increasing rates of thyroid cancer. According to the email, thyroid guards are available to protect patients during dental and breast imaging, but are not routinely used. My questions: How serious are these thyroid risks? Should I insist on using a thyroid guard when I have dental X-rays and mammograms?
We've been getting a lot of questions about this email and the show that sparked it. The topic raised so many fears about radiation exposure that women canceled their mammograms because of it. That is definitely something we don't want to happen - the risks of breast cancer going undetected and untreated due to skipped mammograms are infinitely higher than the nearly nonexistent risk that a mammogram could cause thyroid cancer. So thank you for giving me an opportunity to set the record straight.
In a printed rebuttal to this television show, the American College of Radiology and the Society of Breast Imaging called Dr. Oz's statements "erroneous" and asserted that concerns about mammograms significantly increasing the likelihood of developing thyroid cancer are "simply not supported in scientific literature."
The amount of radiation that reaches the thyroid during a mammogram, according to the American College of Radiology and the Society of Breast Imaging, is equal to the amount that you would receive from natural background sources simply from standing outside, anywhere on the planet, for 30 minutes. For women who receive annual mammograms from age 40 to 80, the total lifetime risk of developing cancer from the tiny amount of radiation scattered to the thyroid is less than one in 17.1 million. That's pretty minute.
This miniscule risk needs to be balanced against the greater risk of a thyroid guard slipping during imaging, interfering with mammogram results and, possibly, with diagnosis. Thyroid guards are made of lead, which is very heavy, and attach with Velcro, so it's very easy for them to slip or fall off after we position a patient for an image - one slight movement is all it takes. When that happens, the patient has to have her mammogram repeated, resulting in more, rather than less, radiation exposure. For these reasons, say both the American College of Radiology and the Society of Breast Imaging, "use of a thyroid shield during mammography is not recommended."
The American Dental Association does recommend using thyroid guards during dental X-rays. It clearly states, however, that the radiation dose from dental X-rays is very low, and the use of the guard is only to further minimize an already low risk of exposure to the thyroid.
I can understand the added caution with dental X-rays: the thyroid is located right behind the jaw, much closer to your teeth than to your breast. In addition, a full dental panel involves many more images, and presumably more radiation exposure, than a mammogram - and to capture some of the necessary dental angles, the X-ray beam may be pointed directly toward the thyroid.
These issues don't apply to mammograms. The breast is much farther from the thyroid, and the beam is never pointed anywhere near the thyroid during a mammogram. The only radiation that might conceivably reach the thyroid during a mammogram is "scatter" radiation - a very small percentage of the total radiation, involving tiny particles that scatter from the main beam. Even so, this scatter radiation dilutes tremendously with each millimeter that you move away from the X-ray beam, and the distance between the breast and the thyroid gland is a long way to go. Furthermore, most scatter occurs inside the body, because that's where the radiation beam is doing its work. Wearing a shield on the outside of your body would have no effect on internal scatter. (Your body's tissues prevent internal scatter from traveling far, anyway.)
My own position on the use of thyroid guards during mammograms echoes that of the American College of Radiology and the Society of Breast Imaging: I don't believe that they are necessary or helpful, and I don't recommend them. That said, Providence does make thyroid guards available to anyone who requests them, but we make sure that patients understand the risks of using the shield, as well as the low risk of radiation exposure to the thyroid with or without the shield. Our top priority is to respect our patients' preferences and to make sure that they feel completely safe when getting their mammograms.
Christina Gougoutas-Fox, M.D., is the director of the Ruth J. Spear Breast Center at Providence St. Vincent Medical Center. The Spear Center offers supportive services for women with breast cancer.