Mammography screening may be effective in reducing breast cancer mortality in certain populations. As with any medical intervention, it has limitations, which can pose potential harm to women who participate. These limitations are best described as false-negatives (related to the sensitivity of the test), false-positives (related to the specificity), overdiagnosis (true positives that will not become clinically significant), and radiation risk.
By Hallie Levine Sklar
Young Women Who Get Breast Cancer Are More Likely to
Women who are diagnosed with breast cancer before age 40 have slightly
poorer prognoses than older women: Their five-year survival rate is about 82
percent, compared with 85 percent among women ages 40 to 74, according to the
American Cancer Society (ACS). Why? "Younger women are more likely to have
more aggressive tumors," explains Lisa Carey, M.D., medical director of the
University of North Carolina...
The specificity of mammography (refer to the Mammography section of this summary for more information) affects the number of "unnecessary" interventions due to false-positive results. Even though breast cancer is the most common noncutaneous cancer in women, only a very small fraction (0.1%-0.5%, depending on age) actually have the disease when they are screened. Therefore, even though the specificity of mammography is approximately 90%, most abnormal tests are false-positives. Women with abnormal screening test results have additional procedures performed to determine whether the mammographic finding is cancer. These procedures include additional mammographic imaging (e.g., magnification of the area of concern), ultrasound, and tissue sampling (by fine-needle aspiration, core biopsy, or excisional biopsy). A study of breast cancer screening in 2,400 women enrolled in a health maintenance organization found that over a 10-year period, 88 cancers were diagnosed, 58 of which were identified on mammography. During that period, one-third of the women had an abnormal mammogram result that required additional testing, including 539 additional mammograms, 186 ultrasound examinations, and 188 biopsies. The actuarial cumulative biopsy rate (the rate of true positives) due to mammographic findings was approximately 1 in 4 (23.6%). The positive predictive value (PPV) of an abnormal screening mammogram in this population was 6.3% for women aged 40 to 49 years, 6.6% for women aged 50 to 59 years, and 7.8% for women aged 60 to 69 years. A subsequent analysis and modeling of data from the same cohort of women, all of whom were continuously enrolled in the Harvard Pilgrim Health Care plan from July 1983 through June 1995, estimated that the risk of having at least one false-positive mammogram was 7.4% (95% confidence interval [CI], 6.4%-8.5%) at the first mammogram, 26.0% (95% CI, 24.0%-28.2%) by the fifth mammogram, and 43.1% (95% CI, 36.6%-53.6%) by the ninth mammogram. Cumulative risk of at least one false-positive by the ninth mammogram varied from 5% to 100%, depending on four patient variables and three radiologic variables. Patient variables independently associated with increased chance of a false-positive result included younger age, higher number of previous breast biopsies, family history of breast cancer, and current estrogen use. Radiologic variables included longer time between screenings, failure to compare the current and previous mammograms, and the individual radiologist's tendency to interpret mammograms as abnormal, which ranged from 2.6% to 24.4% across 93 radiologists in the study. Overall, the largest risk factor for having a false-positive mammogram was the individual radiologist's tendency to read mammograms as abnormal. The authors noted that CIs for estimates of false-positives beyond five mammograms were wide because of the relatively small numbers of women in the analysis with more than five mammograms.