July 16, 2009

Breast Cancer Screening May Lead to Over-diagnosis


One-third of women may be unnecessarily treated for breast cancer as a result of public screening programs that over-diagnose the disease, according to the results of a study published in the British Medical Journal.[1]

Breast cancer is the second leading cause of cancer death in women in the United States, with approximately 180,000 cases diagnosed each year. The best “treatment” for cancer is to prevent its occurrence or to detect it early when it is most treatable. As a result, screening for breast cancer has become standard for women over 40 as well as women who are at high risk of developing the disease.

In theory, the number of cases of advanced breast cancer should drop after the initiation of large-scale public screening programs; however, data indicates that this is not the case. Instead, the incidence of breast cancer, including advanced breast cancer, appears to have increased after the initiation of widespread screening. Some researchers have speculated that large screening efforts may actually lead to over-diagnosis of the disease—meaning that the screening tests detect cancers that would not cause symptoms or death. With current screening techniques, it is not possible to distinguish between a breast cancer that requires treatment and one that does not.

Researchers from Copenhagen analyzed breast cancer trends in the years before and after the initiation of government-run mammography screening programs in Australia, the United Kingdom, Canada, Sweden, and Norway. The data covered a period of at least seven years prior to screening and seven years after screening had been implemented.

The results indicated that one in three women who were identified as having breast cancer did not actually need to be treated. Some cancers grow too slowly to affect the patient. Because cancer treatment often causes harmful side effects and can be physically and emotionally stressful for patients, not all patients benefit from undergoing treatment.

The researchers concluded that “the increased incidence of breast cancer was closely related to the introduction of screening.” Although screening is widely accepted as standard protocol, it is important that women who choose to undergo screening understand the risks and benefits of the practice. In spite of the issue of over-diagnosis, mammographic screening has likely contributed to an overall reduction in breast cancer mortality.


[1] Jorgensen KJ, Gotzche PC. Overdiagnosis in publicly organized mammography screening programmes: Systematic review of incidence trends. British Medical Journal [early online publication]. July 9, 2009. DOI:10.1136/bmj.b2587.

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