© 2002 by Oxford University Press
Journal of the National Cancer Institute, Vol. 94, No. 11, 861-860,
June 5, 2002
© 2002 Oxford University Press
CORRESPONDENCE |
Re: All-Cause Mortality in Randomized Trials of Cancer Screening
Affiliations of authors: T. Church, School of Public Health, University of Minnesota, Minneapolis; F. Ederer, School of Public Health, University of Minnesota, and The EMMES Corporation, Rockville, MD; J. Mandel, Exponent, Inc., Menlo Park, CA.
Correspondence to: Timothy R. Church, Ph.D., Division of Environmental and Occupational Health, School of Public Health, MMC 807, 420 Delaware St. SE, Minneapolis, MN 55455 (e-mail: trc{at}cccs.umn.edu).
To support their argument that results of cancer screening trials based on disease-specific mortality are unreliable, Black et al. (1) compared the treatment effect measured by disease-specific mortality with the effect measured by all-cause mortality in 12 such trials. They found "major inconsistencies" between the two measures. We disagree with their interpretation and illustrate our reasons with results from the Minnesota study of fecal occult blood testing (2). After 13 years, that study found a 33% lower colorectal cancer mortality in the annually screened group than in the control group. (Note that the number of colorectal cancer deaths per 10 000 person-years in the annual group given in Table 1 of Black et al. (1) is in error. The figure should be 82/18.4160 = 4.5, not 5.4.) Because colorectal cancer deaths constituted only 3% of deaths from all causes, the expected reduction in all-cause mortality is only 1%, i.e., 3% of 33%. The reduction in all-cause mortality actually observed in the study was 0.0 per 10 000 person-years, with a 95% confidence interval (CI) of –7.6 to 7.6. The expected 1% reduction in the all-cause mortality rate, corresponding to a decrease of 1.8 per 10 000 person-years, is consistent with this interval. Black et al. considered the treatment effect of 1.2 for disease-specific mortality inconsistent with the 0.0 for all-cause mortality but, in fact, the result 1.2 falls well within the 95% CI of –7.6 to 7.6 for the difference in all-cause mortality. Similar consistencies can be shown for most of the studies cited by Black et al. that were designed for disease-specific outcomes and, as indicated by the large CIs, are underpowered for all-cause analysis.
We agree that, in some cancer screening trials, all-cause mortality may provide assurance against the biases that Black et al. identified. However, a problem with the design of studies using an all-cause mortality end point is the enormous sample size required. For example, with all-cause mortality as the outcome, the aforementioned Minnesota trial (2) with 15 000 subjects per group would have required 20 times as many subjects, or 300 000 per group. Lung cancer trials would have to be about 10 times as large. According to the Nordic Cochrane Centre, a breast cancer screening trial would need 1.2 million women in each group (3), 25–60 times the size of some previous disease-specific studies. Given that the concerns raised may involve only certain cancers in specific populations, rejecting findings across the board based on the possibility of bias is premature.
REFERENCES
1
Black WC, Haggstrom DA, Welch HG. All-cause mortality in randomized trials of cancer screening. J Natl Cancer Inst 2002;94:167–73.
2
Mandel JS, Bond JH, Church TR, Snover DC, Bradley GM, Schuman LM, et al. Reducing mortality from colorectal cancer by screening for fecal occult blood. Minnesota Colon Cancer Control Study. N Engl J Med 1993;328:1365–71.
3 Olsen O, Gotzsche PC. Systematic review of screening for breast cancer with mammography. The Nordic Cochrane Centre, Copenhagen, Oct 20, 2001. [Accessed 05/01/02.] Available from: http://image.thelancet.com/lancet/extra/fullreport.pdf.
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