As breast cancer increases in size, there is a higher frequency of axillary metastases and a higher mortality rate. Tumor size for infiltrating breast carcinomas has been classified as T1 (<2 cm), T2 (2-5 cm), or T3 (>5 cm).  The frequency of axillary metastases is 24-31% for T1 carcinomas, 44-49% for T2 tumors, and 60-70% for T3 carcinomas.  If breast cancers are detected when they are smaller, breast cancer mortality can be decreased. This is the basis for screening mammography.

Technique of Screening Mammography
The standard screening mammogram consists of two views of each breast:  the craniocaudal view (the view from the top) and the mediolateral oblique view (the view from the side).  Performance of both views is essential in order to determine the location of a lesion in three dimensions. It is important to include the posterior tissues of the breast on the films. On the mediolateral oblique view, the pectoral muscle should be identified to approximately the level of the nipple.

On the craniocaudal view, the pectoral muscle is often not identified, but there should be approximately the same amount of tissue (as measured by dropping a perpendicular line from the nipple to the chest wall) as on the mediolateral oblique view.

The breast is compressed during mammography in order to eliminate motion that could cause blurring of the image, to create a more uniform thickness of breast tissue to allow better quality images, and to minimize radiation dose.

Radiation Dose of Screening Mammography
Although excess breast cancers have been detected in women exposed to high doses of radiation (0.25-20 Gy) related to atomic-bomb radiation, multiple chest fluoroscopies, or radiation therapy, there is no direct evidence of a carcinogenic effect of mammography. The Biological Effects of Ionizing Radiation (BEIR) V Report of the National Academy of Sciences estimates a mean glandular dose of 4mGy for a two-view bilateral mammogram.

Kopans has stated that the excess risk of death incurred by undergoing screening mammography is comparable to risks of daily life that are under-taken routinely, such as breathing Boston or New York City air for 2 days, riding a bicycle 10 miles, driving in a car 300 miles, flying 1,000 miles by jet, or eating 40 tablespoons of peanut butter.

Even among the youngest women who undergo screening mammography, the risk-benefit ratio favors the use of mammographic screening. Feig estimated that annual mammography of 100,000 women for 10 consecutive years from age 40 to 50 could theoretically result in eight breast cancer deaths during their lifetime. Assuming the demonstrated 24% mortality reduction due to biennial mammographic screening in this group, the benefit-to-risk ratio will be 48.5 lives saved per life lost. An assumed mortality reduction of 36% from annual screening would result in 36.5 lives saved per life lost.

Feig concludes that the proven benefit from screening mammography far outweighs the theoretical radiation risk.