Screening for breast cancer does not affect overall mortality, and the absolute benefit for breast cancer mortality appears to be small.
A way to view the potential benefit of breast cancer screening is to estimate the number of lives extended because of early breast cancer detection.[20,21] Harris  estimated the outcomes of 10,000 women aged 50 to 70 years who undergo a single screen. Mammograms will be normal (true negatives and false negatives) in 9,500 women. Of the 500 abnormal screens, between 466 and 479 will be false-positives, and 100 to 200 of these women will undergo invasive procedures. The remaining 21 to 34 abnormal screens will be true positives, indicating breast cancer. Some of these women will die of breast cancer in spite of mammographic detection and optimal therapy, and some may live long enough to die of other causes even if the cancer has not been screen detected. The number of extended lives attributable to mammographic detection is between two and six. Another expression of this analysis is that one life may be extended per 1,700 to 5,000 women screened and followed for 15 years. The same analysis for 10,000 women aged 40 to 49 years, assuming the same 500 abnormal examinations, results in an estimate that 488 of these will be false-positives, and 12 will be breast cancer. Of these 12, there will probably be only one to two lives extended. Thus, for women aged 40 to 49 years, it is estimated that one to two lives may be extended per 5,000 to 10,000 mammograms.
A meta-analysis of randomized controlled trials conducted for the U.S. Preventive Services Task Force in 2009 (including the AGE trial) found that the number needed to invite to screen for 10 years to avoid or delay one death from breast cancer was 1,904 for women in their 40s, 1,339 for women in their 50s, and 377 for women in their 60s. A 2009 combined analysis by six Cancer Intervention and Surveillance Modeling Network modeling groups found that screening every 2 years maintained an average of 81% of the benefit of annual screening with almost half the false-positive results. Screening biennially from age 50 to 69 years achieved a median 16.5% reduction in breast cancer deaths versus no screening. Initiating biennial screening at age 40 years (vs. age 50 years) reduced breast cancer mortality by an additional 3%, consumed more resources, and yielded more false-positive results.
Population-Based Screening Programs, Including Studies of Effectiveness of Screening
Although the RCTs of screening have addressed the issue of the efficacy of screening (i.e., the extent to which screening reduces breast cancer mortality under the ideal conditions of an RCT), they do not provide information about the effectiveness of screening (i.e., the extent to which screening is reducing breast cancer mortality in the U.S. population). Studies that provide information on this issue include nonrandomized controlled studies of screened versus nonscreened populations, case-control studies of screening in real communities, and modeling studies that examine the impact of screening on large populations. An important issue in all of these studies is the extent to which they can control for additional effects on breast cancer mortality such as improved treatment and heightened awareness of breast cancer in the community.