The disadvantages of mammography screening included false-positives and overtreatment, that is, the treatment of benign tumors that would never have caused symptoms or death during a patient’s lifetime. This may be because the tumor grows so gradually that the patient dies of other causes before it produces symptoms, or the cancer remains dormant or regresses.
This demonstrates that the health risks of screening essentially offset the benefits up to 10 years, after which the benefits accumulate, but by much less than predicted when screening was first initiated. The study’s findings were published online December 2011 in the British Medical Journal (BMJ).
The Forrest report in 1986, which led to the introduction of breast cancer screening in the United Kingdom, estimated the number of screened and unscreened women surviving each year over a 15-year period. Costs and benefits were measured in quality adjusted life years (QALYs).
It suggested that screening would reduce the death rate from breast cancer by nearly one third with few hazards and at low cost. Since the Forrest report, the health risks of breast cancer screening have been recognized. Therefore, researchers from the University of Southampton (UK) set out to update the report’s survival estimates by combining the benefits and harms of screening in one single measure.
The findings are based on 100,000 women aged 50 and over surviving by year up to 20 years after entry to the screening program. Inclusion of false-positives and unnecessary surgery reduced the benefits of screening by approximately half. The best estimates generated negative net QALYs for up to eight years after screening and minimal gains after 10 years. After 20 years, net QALYs accumulate, but by much less than predicted by the Forrest report.
The investigators noted that more research is needed on the extent of unnecessary treatment and its impact on quality of life. They also call for improved ways of identifying those most likely to benefit from surgery and for measuring the levels and duration of the harms from surgery. From a public standpoint, the meaning and implications of overdiagnosis and overtreatment need to be much better explained and communicated to any woman considering screening, they added.
However, the continuing uncertainty about the extent of overtreatment is apparent in a study of French women published on online in BMJ in November 2011, which put overdiagnosis of invasive breast cancer due to screening at approximately 1%.
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