Techniques used to diagnose breast cancer include mammography, clinical breast examination, and breast self-examination (BSE). Of these three, mammography has been shown to be the most reliable for the detection of early stage breast cancer, which is obviously the most curable. In many randomized trials, breast cancer deaths have been reduced by 30% when screening mammography is used.

While reduction of the death rate from breast cancer is the most important benefit of screening mammography, other benefits are derived from identifying cancers at a small size and earlier stage. Early detection of breast cancer may give the woman the option of breast-conserving surgery. If she is clinically node negative, the patient may avoid removal of the lymph nodes in the axilla. Sentinel node biopsy, currently under investigation, offers the opportunity to avoid full axillary dissection, which can have significant complications. Also, earlier diagnosis may allow the patient to avoid aggressive chemotherapy and the long-term morbidity from extensive treatment.

While early detection offers clear advantages, mammography also carries potential disadvantages. These risks include the possibility of false-positives (something is found on the mammogram when there is no cancer), which may result in additional workup and increased cost, "unnecessary" surgery (excisional biopsy), and anxiety for the patient. Conversely, screening mammograms have the potential of false-negatives (the mammogram is negative when a cancer is present). This may lull the patient and physician into a false sense of security. With a negative mammogram, both the patient and the physician may ignore physical findings, resulting in a delay in diagnosis and treatment. Furthermore, the mammogram itself may carry some risk. While the risk of radiation exposure from mammography is a concern to patients, evidence shows that current techniques of low-dose mammography have a negligible risk, and that risk is only theoretical. Regardless, the unquestioned benefits of early detection of breast cancer far outweigh these arguments.

The risk of false-positive mammograms has been reported in two modern screening studies. One study from multiple mammography centers across the United States (sponsored by the Centers for Disease Control and Prevention) found a false-positive rate of 5.6% to 5.8%. A community practice at the University of California at San Francisco reported a false-positive rate of 6.4% to 6.8¡.

A study published in the New England Journal of Medicine in 1998 highlighted the problem of false-positive mammograms. In that study, 24% of women screened over a 10-year period had mammograms with false-positive findings. "False positive" was defined as a report resulting in additional workup. While the overall conclusions were rather negative, some positive aspects can be pointed out from the data. During this study, only 5% of women required a biopsy of any type. Of the cancers diagnosed, 17% were in situ cancers.

Certainly, false-positives do contribute to the increased cost of screening for breast cancer because of the additional workup that an abnormal report requires. However, if the false-positive rate is too low, then real cancers will be missed. False positives show that the radiologists are meticulous in their reading of mammograms.

Falsely negative mammograms are a much more serious problem. In this situation, a mammogram is read as "normal" when there is a cancer present. Both physicians and women must be aware of mammography's limitations. In modern screening programs, the false-negative rate for women aged 40 through 49 years is 13% to 15%. In women over 50 years of age, the false negative rate is 6% to 12%. Clinical breast examination should be performed near the time of the mammogram to avoid missing a palpable lesion that mammography does not detect. Clinical breast examination should always be a companion to mammography screening.

Guidelines for screening mammography vary. The American Cancer Society recommends mammography every year beginning at age 40. Clinical breast examination by a physician is recommended every 3 years between ages 20 and 39, and annually beginning at age 40. Breast self-examination is recommended monthly beginning at age 20. The National Cancer Institute recommends screening mammography every 1 to 2 years, beginning at age 40. Clinical breast examination is recommended annually beginning at age 40. The National Comprehensive Cancer Network (NCCN) recently published its practice guidelines for screening. For women between 20 and 40 years of age with average risk and no symptoms, NCCN recommends breast examination every 3 years. For women over 40, NCCN recommends yearly physical examination and mammography. Breast self examination (BSE) can detect early cancers and is encouraged by all organizations.