Mammography utilizes ionizing radiation to image breast tissue. The examination is performed by compressing the breast firmly between two plates. Such compression spreads out overlapping tissues and reduces the amount of radiation needed to image the breast. For routine screening in the United States, examinations are taken in both mediolateral oblique and craniocaudal projections. Both views should include breast tissue from the nipple to the pectoral muscle. Radiation exposure is 4 to 24 mSv per standard two-view screening examination. Two-view examinations are associated with a lower recall rate than are single-view examinations because they eliminate concern about abnormalities due to superimposition of normal breast structures.
Purpose of This Summary
This PDQ cancer information summary for health professionals provides comprehensive, peer-reviewed, evidence-based information about breast cancer screening. It is intended as a resource to inform and assist clinicians who care for cancer patients. It does not provide formal guidelines or recommendations for making health care decisions.
Reviewers and Updates
This summary is reviewed regularly and updated as necessary by the PDQ Screening and Prevention Editorial...
Under the Mammography Quality Standards Act (MQSA) enacted by Congress in 1992, all U.S. facilities that perform mammography must be certified by the U.S. Food and Drug Administration (FDA) to ensure the use of standardized training for personnel and a standardized mammography technique utilizing a low radiation dose. (Refer to the FDA's Web page on Mammography Facility Surveys, Mammography Equipment Evaluations, and Medical Physicist Qualification Requirement under MQSA.) The 1998 MQSA Reauthorization Act requires that patients receive a written lay-language summary of mammography results.
On screening, the following Breast Imaging, Reporting and Data System (BI-RADS) assessments are used: 1, negative; 2, benign; or 0, incomplete with additional evaluation needed.
About 10% of women screened will be recalled for additional evaluation; more than 80% of these will be considered normal or benign after a full diagnostic workup, which may include additional mammographic views, ultrasound, or both. About 15% of women recalled will be recommended for biopsy, with 30% of cases assessed as BI-RADS 4, suspicious, yielding cancer; and 95% of cases assessed as BI-RADS 5, highly suggestive of malignancy, yielding cancer. About 2% of women screened will be recommended for short-interval follow-up, assessed as BI-RADS 3, probably benign, with fewer than 2% of such women ultimately found to have cancer.
Benefit of Mammography
Randomized controlled trials
Randomized controlled trials (RCTs), with participation by nearly half a million women from four countries, examined the breast cancer mortality rates of women who were offered regular screening. One trial, the Canadian National Breast Screening Study (NBSS)-2, compared mammogram plus clinical breast examination (CBE) with CBE alone; the other eight trials compared screening mammogram with or without CBE to a control consisting of usual care.
The trials differed in design, recruitment of participants, interventions (both screening and treatment), management of the control group, compliance with assignment to screening and control groups, and analysis of outcomes. Some trials used individual randomization, while others used cluster randomization in which cohorts were identified and then offered screening; one trial used nonrandomized allocation by day of birth in any given month. Cluster randomization sometimes led to imbalances between the intervention and control groups. Age differences have been identified in several trials, although the differences were probably too small to have a major effect on the trial outcome. In the Edinburgh Trial, socioeconomic status, which correlates with the risk of breast cancer mortality, differed markedly between the intervention and control groups, so it is difficult, if not impossible, to interpret the results.