SAN ANTONIO—Why is there such widespread debate about the absolute benefit of mammography screening, with an almost 20-fold difference among four major reviews in estimates of the number of women who must be screened to prevent one breast cancer death?
“We wanted to understand why these estimates differ so much,” said Robert A. Smith, PhD, senior director of cancer screening at the American Cancer Society, Atlanta, GA, at the 2013 San Antonio Breast Cancer Symposium. The estimated number of women needed to be screened ranged from 111 to 2,000 across four major reviews of screening and mortality, the Nordic Cochrane review, the U.K. Independent Breast Screening Review, the U.S. Preventive Services Task Force (USPSTF) review, and the European Screening Network (EUROSCREEN) review.
“What we found was that the estimates are all based on different situations, with different age groups being screened, different screening and follow-up periods, and differences in whether they refer to the number of women invited for screening or the number of women actually screened,” he said. “When we standardized all the estimates to a common scenario—ie, the same exposure to screening, and a similar target population, period of screening, and duration of follow-up—the magnitude of the difference between studies dropped from 20-fold to about four-fold.”
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The study standardized data from the Nordic Cochrane, USPSTF, and EUROSCREEN reviews to the scenario used in the U.K. Independent Breast Screening Review, which found that over 20 years, 180 women needed to be screened from the age of 50 to 59 years to prevent one death from breast cancer at the age of 55 to 79 years.
“When all four reviews are converted to the UK review scenario, the range of absolute benefits is now only 2.5-fold rather than 20-fold,” Dr. Smith reported, ranging from 64 to 257. In the Nordic Cochrane review, the number dropped from 2000 to 257; in USPSTF, the estimate decreased from 1,339 for women aged 50 to 59 years, to 337 for those aged 60 to 69 years, and to 193 for women aged 50 to 69 years. The EUROSCREEN estimate dropped from 111 to 64 women.
“Thus, the differences between the reviews with respect to the absolute breast cancer mortality reduction are almost entirely due to expressing the same basic result relative to different denominators,” such as choice of population mortality rates. “Thus, the so-called controversy over the benefit of mammography screening as estimated from the trials is largely contrived.”
All of the reviews indicate a substantial reduction in breast cancer mortality with screening.
“While there are genuine disagreements about overdiagnosis, methods which adjust for lead time and underlying incidence trends yield estimates which are modest and are outweighed by the mortality benefit.” Dr. Smith said. Overdiagnosis of breast tumors detected by screening is acknowledged to be a major harm of screening, accounting for 25% of all breast cancers detected by mammography.
“The debate about the value of mammography screening is not likely to fade away, and there are real, reasonable differences of opinion about various aspects of screening,” Dr. Smith concluded. “However, we hope these findings reassure clinicians and the public that that there is little question about the effectiveness of mammography screening, which should continue to play a very important role in our efforts to prevent deaths from breast cancer.”
For more details about the study, see “Real and artificial controversies in breast cancer screening,” in the November 2013 issue of Breast Cancer Management.
References
- Smith RA et al. S1-10. Presented at: San Antonio Breast Cancer Symposium 2013. Dec. 10-14, 2013; San Antonio.
