Mammography utilizes ionizing radiation to image breast tissue. The examination is performed by compressing the breast firmly between a plastic plate and an x-ray cassette that contains special x-ray film. For routine screening in the United States, examination films are taken in mediolateral oblique and craniocaudal projections. Both views should include breast tissue from the nipple to the pectoral muscle. Two-view examinations decrease the recall rate compared with single-view examinations by eliminating concern about abnormalities due to superimposition of normal breast structures.
Did You Know?
Under the Affordable Care Act, many health insurance plans will provide free women’s preventive services, including mammograms, birth control and well-woman visits. Learn more.
Under the Mammography Quality Standards Act (MQSA) enacted by Congress in 1992, all facilities that perform mammography must be certified by the U.S. Food and Drug Administration (FDA). This mandate has resulted in improved mammography technique, lower radiation dose, and better training of personnel. Refer to the list of FDA Certified Mammography Facilities. Image contrast has improved with the use of lower voltage, specialized aluminum grids, and higher film optical density. The 1998 MQSA Reauthorization Act requires that patients receive a written lay-language summary of mammography results.
Mammography can identify breast cancers too small to palpate on physical examination and can also find ductal carcinomain situ (DCIS), a noninvasive condition. Because all cancers develop as a consequence of a series of mutations, it is theoretically beneficial to diagnose these noninvasive lesions. A large increase in the frequency of DCIS diagnosis occurred in the United States beginning in the early 1980s  because of the increased use of screening mammography. Appropriate management of DCIS is not well understood because its natural history is incompletely defined. (Refer to the PDQ summary on Breast Cancer Treatment for more information. Also refer to the Ductal Carcinoma In Situ section of this summary for more information.)
Numerous uncontrolled trials and retrospective series have documented the capacity of mammography to diagnose small, early-stage breast cancers, including those that have a favorable clinical course. These trials also show that cancer-related survival is better in screened women than in nonscreened women. These comparisons are susceptible, however, to a number of important biases:
Lead-time bias: Survival time for a cancer found mammographically includes the time between detection and when the cancer would have been detected because of clinical symptoms, but this time is not included in the survival time of cancers found because of symptoms.
Length bias: Mammography detects a cancer while it is preclinical, and preclinical durations vary. Cancers with longer preclinical durations are more likely to be detected by screening; these cancers tend to be slow growing and to have good prognoses, irrespective of screening.
Overdiagnosis bias: An extreme form of length bias; screening may find cancers that are very slow growing and that would never have become manifest clinically.
Healthy volunteer bias: The screened population may be healthier or more health conscious than the general population.