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Re-examination of data from four large studies of the benefits and harms of mammography screening shows that the benefits are more consistent across these studies than previously understood and that all the studies indicate a substantial reduction in breast cancer mortality with screening, according to results presented at the 2013 San Antonio Breast Cancer Symposium, held Dec. 10-14.
There is widespread debate in academic literature and the media about the absolute benefit of mammography screening, commonly defined as the number of women who must be screened to prevent one breast cancer death. Four major reviews of screening and mortality each paint a different picture, with estimates of the number of women who must be screened ranging from 111 to 2,000, an almost twentyfold difference.
"We wanted to understand why these estimates differ so much," said Robert A. Smith, Ph.D., senior director of cancer screening at the American Cancer Society in Atlanta. "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. When we standardized all the estimates to a common scenario - i.e., 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 twentyfold to about fourfold."
"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," Smith continued. "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."
The four reviews compared by Smith and colleagues were 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. The researchers chose to apply the data from each of the reviews to the scenario used in the U.K. Independent Breast Screening Review. This review investigated the effect of screening women in the United Kingdom for 20 years, from age 50-69, on breast cancer mortality from age 55-79, and estimated that 180 women needed to be screened to prevent one breast cancer death.
After standardizing the Nordic Cochrane, USPSTF, and EUROSCREEN reviews to the scenario in the U.K. Independent Breast Screening Review, the magnitude of the difference between studies in the estimated number of women needed to be screened to prevent one breast cancer death dropped dramatically. The adjusted estimates ranged from 64 to 257 instead of the original 111 to 2,000. Specifically, the Nordic Cochrane review estimate for the number of women who must be screened to prevent one breast cancer death dropped from 2,000 to 257. The USPSTF estimate dropped from 1,339 for women age 50-59 and 337 for women age 60-69, to 193 for women age 50-69. The EUROSCREEN estimate dropped from 111 to 64.
Publication Number: S1-10 Title: Disparities in the estimates of benefits and harms from mammography: Are the numbers really different?
Authors: Robert A Smith1, Stephen Duffy2, Tony Hsiu-Hsi Chen3, Amy Ming Fang Yen4 and Laszlo Tabar5. 1American Cancer Society, Atlanta, GA; 2Wolfson Institute of Preventive Medicine, Queen Mary University of London, London, United Kingdom; 3Graduate Institute of Epidemiology and Preventive Medicine, National Taiwan University, Taipai, Taiwan; 4School of Oral Hygiene, Taipei Medical University, Taipai, Taiwan and 5Falun Central Hospital, Falun, Sweden.
It is generally accepted that screening with mammography prevents deaths from breast cancer, although debate continues about the absolute size of the mortality benefit, and the concomitant risks associated with screening. A number of recent observational studies have claimed to find low rates of benefit in terms of reducing mortality rates or late stage disease, and high rates of overdiagnosis, defined as the diagnosis by screening of cancer which would not have been diagnosed in the patient's lifetime if screening had not taken place. These publications have achieved a high profile in the mass media and stimulated further debate. We review the apparent disparities between different reviews of the effects of mammography screening on mortality from breast cancer and on overdiagnosis. The four major reviews are the UK Independent Review, the Nordic Cochrane review, the US Preventive Services Task Force (USPSTF) review, and the EUROSCREEN review. The estimated number needed to screen/invite to prevent one death from breast cancer ranges from 111 to 2000, almost a 20-fold range. The estimated number required varies by age group, whether the intervention described is actual screening or invitation to screening, follow-up time and other factors. To assess whether these represent genuine disagreements or whether the differences are mainly due to such factors as follow-up time and target population, we converted all four to pertain to the same scenario as used in the UK Independent Review, that is to the effect of screening for 20 years from age 50 to 69 on breast cancer mortality in ages 55-79, in a UK population. 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. 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, choice of population mortality rates, etc. Thus, the so-called controversy over the benefit of mammography screening as estimated from the trials is largely contrived. When expressed relative to the same denominator, with the same screening and follow-up periods, and using the same absolute mortality rates, absolute benefit estimates are all of the same order of magnitude, and all 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.
Additional details about this study are available in the November 2013 issue of Breast Cancer Management. This study team was led by Stephen W. Duffy, M.Sc., professor of cancer screening at Queen Mary University of London, and was funded by the Centre for Cancer Prevention, Wolfson Institute of Preventive Medicine, Barts and The London School of Medicine and Dentistry, Queen Mary University of London, United Kingdom. Smith declares no conflicts of interest.
This research was presented at the 2013 San Antonio Breast Cancer Symposium Wednesday, Dec. 11, 7:30 a.m. CT, during a press conference hosted by C. Kent Osborne, M.D., director of the Dan L. Duncan Cancer Center and director of the Lester and Sue Smith Breast Center at Baylor College of Medicine. Press conferences will be held in Room 217D of the Henry B. Gonzalez Convention Center, San Antonio, Texas.
Article adapted by Medical News Today from original press release. Click 'references' tab above for source.
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