Answer to Question #4720 Submitted to "Ask the Experts"

Q

My wife recently received a mammogram. I've scoured the Internet trying to find out about the risks and I haven't found any satisfactory answers. Six films were taken and I would guess that would probably equate to an effective dose of just over 1 mSv. I have read that this equates to between four and six months of background radiation, but isn't this comparing chalk and cheese? A mammogram is an x-ray radiation focused on one part of the body and background radiation comes from a number of sources, including radon.

Isn't the best comparison to be made with gamma rays, which are like x rays, rather than alpha and beta radiation, which are included in the background radiation estimates? In a year the natural exposure to gamma rays is about 0.44 mSv, so arguably my wife had over two years' worth of this deeply penetrating kind of radiation. And in terms of her life between 0 and 40, and considering that the damage caused by radiation decreases with age, her exposure from one mammogram has increased her background exposure by 6 percent, as 1.05 mSv equals 2.38 years of gamma radiation and 40 divided by 2.38 times 100 is about 6 percent.

Put another way, assuming she had no other x rays in her life, her chances of getting cancer from deeply penetrating radiation have gone up by 6 percent as a result of this procedure. And it might be higher, with reference to breast cancer, as the mammogram was specifically aimed at the breast, and the breast is more sensitive to carcinogens than other parts of the body. There is also a possibility that gamma rays and x rays are not quite comparable, that x rays actually cause more damage.

So, my question is this: Is my assumption wrong in that estimates of total background radiation are of limited use when trying to assess the potential damage caused by a mammogram?

A

We appreciate your very thorough question and the fact that you went out and did a calculation based on your wife's radiation dose. Dose calculation is a complicated issue.

Let us first consider the dose to the breast tissue. Your wife received three exposures to each breast. The U.S. Department of Energy estimates that the glandular-tissue dose per mammogram image is 138 mrem, or approximately 1.4 mSv. The total is then about 4.2 mSv for the three exposures to one breast.

The effective dose (ED) you refer to represents the relative risk associated if the entire body was being irradiated. It is useful in comparing different types of exams involving partial-body irradiation by applying a weighting factor to the organ or tissue irradiated. The weighting factor for the breast, as noted by the National Council on Radiation Protection and Measurements, is 0.05. Therefore, the ED is 4.2 mSv x 0.05 = 0.2 mSv. This is for one breast, so the risk is really double this value for the entire study performed on your spouse. This value of 0.4 mSv for six films is comparable to the standard four-film value of 0.13 mSv (NCRP Report 116) if one makes allowance for variations in equipment development and techniques.

Separately, the National Council of Radiation Protection and Measurements estimates that the risk of a fatal cancer and additional nonfatal cancer is 0.06/Sv. Some calculations result in a fatal cancer risk of 0.6 per 10,000 person-mSv. That is, if 10,000 persons receive 1 mSv each, there will be 0.6 additional cancers, or 0.24 additional cancer deaths if 10,000 people each receive 0.4 mSv.

The American Cancer Society reports that the average risk of developing cancer is about one in three for females during their lifetimes, or 38.18 percent. Adding the additional 0.24 cancers due to 0.4 mSv to the natural risk of 3,818 cancers in 10,000 would correspond to an increase of 0.006 percent in the natural cancer incidence rate, which is really an inconsequential increase.

Steven H. King, CHP, CMHP
John P. Jacobus, MS, CHP