Breast cancer: To screen or not to screen?
Women are often told that mammography saves lives. But rarely is the question asked, 'how often?' Researchers writing in the open access journal BMC Medical Informatics and Decision Making, set out to examine how often this life-saving event occurs.
Unrealistic expectations may influence a woman's decision whether or not to participate in screening mammography. Over 90% of women think that 'early detection saves lives'. John D Keen and James E Keen, of the John H. Stroger Jr. Hospital of Cook County and the University of Nebraska, respectively, aimed to promote informed decision-making by calculating the age-dependent absolute benefit of screening in three traditional ways: the absolute risk reduction from repeated screening, the number of women needed to screen repeatedly to save one life, and the survival percentages with and without mammography. They also estimated the average benefit of a single mammogram. Their novel concept of life-saving proportion is also relevant to economic analyses of screening.
They found that the life-saving benefit of mammography gradually increases with age along with the screen-free absolute death risk, which is about 1% over 15 years starting at age 55. The corresponding risk of developing breast cancer is about 6%. Repeated screening starting at age 50 saves about 1.8 lives over 15 years for every 1000 women screened. The average benefit of a single screening mammogram is 0.034%; in other words, 2970 women must be screened once to save one life. Alternatively, twenty-three cancers must be detected. Assuming a base case 20% relative risk reduction, the survival percentage in younger women at age 40 is 99.52% without and 99.62% with screening, meaning that there is a 0.1% increased chance of survival with screening than without it.
According to the authors, "We have assumed that a 'life saved' means screening helps cure one woman with breast cancer who would otherwise have died from the disease without screening ... However, all women with breast cancer may theoretically benefit from screening mammography through slowing the disease and therefore slightly prolonging their lives".
"For a woman in the screening subset of mammography-detectable cancers, there is a less than 5% chance that a mammogram will save her life. By comparing mammography's life-saving absolute benefit with its expected harms, a well-informed woman along with her physician can make a reasonable decision to screen or not to screen for breast cancer."