Answer to Question #5113 Submitted to "Ask the Experts"

The confusion you mention is due in part to where the dose is being measured and the units in which they are being reported. To further confuse the situation, there are currently two sets of units being used. One is the "standard" units we have been using for decades, for example, the Roentgen (R), the rad, and the rem, and newer international units, for example, the gray (Gy) and the sievert (Sv). Based on the magnitude of the values measures, the units are expressed as the milliroentgen (mR), the milligray (mGy), or millisievert (mSv), which have a value of 1/1000 of the main unit.

In radiology and mammography, physical measurements of a machine output are reported as the entrance skin exposure in mR or as the newer entrance-skin air kerma in mGy. Based on measurements in phantoms, or calculations using x-ray technical factors set at the machine console, the absorbed dose to an organ or tissue, such as the breast, can be determined. This absorbed dose is expressed in the mrad or the mGy, where 100 mrad = 1 mGy. To be clear, the measurements in mGy need to be identified as air kerma or absorbed dose in tissue.

The organ or tissue dose is then multiplied by a tissue weighting factor which results in a radiation dose metric known as the effective dose (E). This weighting factor relates the risk from a dose to the organ to an equivalent risk associated to a dose given to the whole body and all its organs. The effective dose, E, is reported either in the mSv or the millirem (mrem), where the mrem is another older unit and 1 mSv = 100 mrem. Relating individual organ doses to whole-body doses allows for the comparison of radiation risks between different types of radiation sources and different radiological or nuclear medicine examinations. From a survey of mammography exams conducted in 2005 in the United States, the mean entrance skin exposure for the cranio-caudal (CC) view was 1082 mR (standard deviation = 211). The mean absorbed dose to the breast tissue was 1.7 mGy or 177 mrad (standard deviation = 30) (Spelic). Using the 1991 ICRP tissue weighting factor of 0.05 for breast tissue (ICRP 1991), the effective dose, E, is therefore 1.77 mGy x 0.05 = 0.09 mSv, or 9 mrem. Since most mammography exams require two views, and assuming the medio-lateral view is approximately the same as the CC view, the mean breast dose for the two x-ray views would be 0.09 mSv x 2 = 0.18 mSv, or 18 mrem. It is unclear if the 13 mrem value on our Web page is for one or two exposures, but it would clearly fall within the range of patient size distributions, since breast size and tissue composition vary significantly among any normal population.
The increased risk of cancer mortality from an effective dose, E, of 0.09 mSv is approximately 4.5 in 1 million over one's lifetime. Although multiple doses from several mammography exams will increase this risk, this risk is still considered negligible relative to the medical benefits. Please see our Web page "Risk/Benefit of Medical Radiation Exposures."

Alternative technologies, such as thermography, have either been approved for very limited indications or are at various stages of development and testing in clinical trials. Imaging techniques being studied include MRI (magnetic resonance imaging), contrast-enhanced nuclear medicine imaging, dedicated breast computed tomography (CT), ultrasound (US), and digital imaging. While some of these technologies have real potential, some are marginally better than screen-film mammography or are best for certain types of patients. For example, digital mammography has been shown to be significantly better than screen-film based mammography only for younger patients under age 50, for patients presenting with radiographically dense breasts, and for premenopausal or perimenopausal women (Pisano et al 2005). Again, for the vast majority of patients these new technologies simply do not have the diagnostic accuracy of x-ray mammography. Simply stated, we are not aware of any definitive reports stating that for the asymptomatic screening of the population for breast cancer there is anything better than x-ray screen-film mammography for the early detection of breast cancer.

Orhan H. Suleiman, MS, PhD 

References
Spelic DC PhD. Physicist, Food and Drug Administration, Division of Mammography Quality and Radiation Programs. Personal Communication.

International Commission on Radiological Protection. 1990 recommendations of the International Commission on Radiological Protection, 60. Annals of the ICRP Volume 21:1-3. Oxford: Pergamon Press; ICRP Publication 60: 1991.

Pisano ED, Gatsonis C, Hendrick E, Yaffe M, Baum JK, Acharyya S, Conant EF, Fajardo LL, Bassett L, D'Orsi C, Jong R, Rebner M. Diagnostic performance of digital versus film mammography for breast-cancer screening. N Engl J Med 353:1773-1783; 27 Oct 2005.