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, according to research published April 2 in the BMC Medical Informatics and Decision Making.
Unrealistic expectations may influence a woman's decision whether or not to participate in screening mammography. More than 90 percent of women think that early detection saves lives, according to study authors John D. Keen and James E. Keen, of the John H. Stroger Jr. Hospital of Cook County and the University of Nebraska in Lincoln.
The authors 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 concept of life-saving proportion is also relevant to economic analyses of screening.
The results showed that the life-saving benefit of mammography gradually increases with age along with the screen-free absolute death risk, which is about 1 percent over 15 years starting at age 55. The corresponding risk of developing breast cancer is about 6 percent. Repeated screening starting at age 50 saves about 1.8 lives over 15 years for every 1,000 women screened. The average benefit of a single screening mammogram is 0.034 percent; in other words, 2,970 women must be screened once to save one life.
Alternatively, 23 cancers must be detected. Assuming a base case 20 percent relative risk reduction, the survival percentage in younger women at age 40 is 99.52 percent without and 99.62 percent with screening, meaning that there is a 0.1 percent increased chance of survival with screening than without it.
"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," according to the authors. "For a woman in the screening subset of mammography-detectable cancers, there is a less than 5 percent chance that a mammogram will save her life."
Stephen Duffy, MD, of Cancer Research UK in London, argued that direct results from empirical data might be more trustworthy than modeled estimates derived by combining data from disparate sources. On the other hand, Michael Retsky, MD, from the Harvard Medical School in Boston, praised the study, noting that it is "a positive step in the right direction considering that too often women aged 40 to 49 are asked to sign informed consent for mammography without being properly informed of the potential risks."