Review: Breast cancer screening with mammography reduces mortality especially in women 50 to 69 years of age
ACP J Club. 1993 Sept-Oct;119:38. doi:10.7326/ACPJC-1993-119-2-038
Nyström L, Rutqvist LE, Wall S, et al. Breast cancer screening with mammography: overview of Swedish randomised trials. Lancet. 1993 Apr 17;341:973-8.
To evaluate the efficacy of breast cancer screening with mammography in reducing breast cancer mortality using meta-analysis.
Breast cancer screening trials done in Sweden.
5 randomized trials of breast cancer screening with mammography had been done in Sweden. The designs were almost identical except for the type of randomization (3 trials used the cluster method).
The magnetic tapes of information on the cohorts from each of the 5 trials were obtained. The cohorts were merged and linked to the 6 regional cancer registers to identify patients with breast cancer diagnosed between 1958 and 1989 and to the Swedish Cause of Death Register to identify women who died between 1951 and 1989. Women with breast cancer diagnosed before the date of randomization were excluded. All breast cancer deaths were reviewed by a blinded, independent, end-point committee.
The final combined cohort consisted of 282 777 women (age range 40 to 74 y) followed for 5 to 13 years. The relative risk (RR) for breast cancer as the underlying cause of death was 0.77 (95% CI 0.67 to 0.88), which translates into a 24% (CI 13% to 34%) weighted relative reduction in breast cancer mortality among those women invited to mammography screening compared with those not invited. There was a consistent risk reduction associated with screening by mammography in all 5 trials with the RR ranging from 0.68 to 0.86. The cumulative mortality per 1000 person-years by time since randomization was about 1.3 breast cancer deaths within 6 years in the screening group compared with 1.6 in the control group. The corresponding figures after 9 years were 2.6 and 3.3 and, after 12 years, 3.9 and 5.1. The largest relative reduction in breast cancer mortality (29%) was among women aged 50 to 69 years at randomization. Among women aged 40 to 49 years, a nonsignificant 13% relative reduction in breast cancer mortality among those in the screening group after 8 years of follow-up was noted. During the first 8 years the cumulative breast cancer mortality among women aged 40 to 49 years was similar in the screening and control groups. There was no evidence of any adverse effects of screening in terms of breast cancer mortality in any age group. The RR for breast cancer mortality among women aged 70 to 74 years was 0.98 (CI 0.63 to 1.53).
Among women aged 40 to 74 years invited to breast cancer screening with mammography, the relative reduction in breast cancer mortality was 24%. The greatest benefit was among women aged 50 to 69 years, who showed a relative reduction of 29% in breast cancer mortality.
Source of funding: Swedish Cancer Society.
For article reprint: Dr. L. Nyström, Department of Epidemiology and Public Health, Umeå University, S-901-85 Umeå, Sweden. FAX 46-901-38-977.
This combined analysis of the Swedish trials reports the largest aggregate experience of breast cancer screening with mammography to date. It greatly improves the precision of the estimations of the efficacy of screening (narrower confidence limits around the estimations of relative-risk reductions) and enables us to be more secure about interpreting differences in its effectiveness for various age groups. The latter is particularly important because great controversy continues about the effectiveness of screening, especially in the youngest (< 50 y) and oldest (> 70 y) women.
The practice and public health implications of this new analysis and its findings are important. The results provide a stronger scientific basis for ongoing efforts to promote and encourage breast cancer screening in the age groups for whom the effect is greatest—women between 50 and 70 years. The benefit in younger women, if present at all, appears to be much smaller and delayed by at least 8 years. No benefit is apparent in older women, but these conclusions are based on a relatively small number of women in this age group included in these trials.
The effect of screening on mortality is, however, only one part of the puzzle posed by screening for breast cancer. Clinical and public policy recommendations depend also on whether the other potentially beneficial effects of screening efforts, such as its effect on breast conserving surgery, the reduced need for systemic and adjuvant therapy, and the reassurance value to women with negative mammograms are sizable. These must be contrasted with the potentially detrimental effects of screening, including invasive procedures in women with false-positive results or in those whose tumors would never have progressed, the anxiety caused, and the added costs incurred.
The studies included in this overview have been reanalyzed in 1997 by adding 4 more years of available follow-up data and focusing specifically on the women who are 40 to 49 years of age at the time of screening (1). The methods used in the reanalysis also attempt to remove the effect attributable to cancers detected after women turn 50 because the question of the benefit of screening younger women when they are younger remains most contentious. The results continue to suggest a benefit from such screening (it is almost, but not quite statistically significant), and the benefit (if there is one) is still considerably lower than from screening women over the age of 50.
Additional studies including the relatively recent Canadian trial (2), which showed no improved survival from screening younger women, continue to make the answers to this question uncertain.
What is important from a clinical and public policy perspective, however, seems to be becoming clearer. The weight of the evidence appears to be pointing toward a relatively small (compared with older women) effect on mortality by screening younger women. Whether these benefits are added to, or subtracted from, by the “other” effects of screening previously mentioned and whether the cost effectiveness of such screening is attractive or prohibitive are now the major issues.
Alvin I. Mushlin, MD, ScM
University of RochesterRochester, New York, USA
1. Larsson LG, Andersson I, Bjurstam N, et al. Updated overview of the Swedish randomized trials on breast cancer screening with mammography: age group 40-49 at randomization. Monogr Natl Cancer Inst. 1997;22:57-61.