The Swedish two county trial of mammographic screening for breast cancer: recent results and calculation of benefit.

Abstract

The Swedish two county trial of breast cancer screening is now in its tenth year. This paper presents detailed results on mortality from breast cancer and from all other causes, and on the population denominators at risk for each of the first 8 years of follow up, for each county separately. These data represent a two year update on the last major report. Results show an increasingly significant deficit in deaths from breast cancer among the 77 092 women invited to screening relative to the 56 000 not invited (RR = 0-68, p = 0-002), with no significant difference between the effects of screening in the two counties (p = 0-5). These results remain the same when adjusted for age. Analysis of all cause deaths shows no significant effect of screening (p = 0 5), nor was there any significant effect of screening on deaths from all causes other than breast cancer (p = 0 9). The rates of deaths from intercurrent illness in breast cancer cases were almost identical in the group invited to screening and the group not invited (p = 0 7). This result remained the same when adjusted for age. We calculate that in the age group 50-69 at entry, one breast cancer death was prevented per 4000 woman/years, per 1460 mammographic examinations, per 13-5 biopsies, and per 7-4 breast cancers detected. The purpose ofthis paper is to present in fuller form the results on mortality, stressing the relationship of screening both to breast cancer and to total mortality. These results allow one to assess both the benefit of screening and the extent ofany possible adverse effects on mortality. In addition some questions about the value of breast cancer screening are addressed.Y9These include concern about the overall mortality rate among women invited to screening (with the related question of deaths from intercurrent illness among women who developed breast cancer), the reduction in mortality from breast cancer in age subgroups and the comparison of results in the two counties.The trial, as originally analysed, contained women who had previously had breast cancer among the population at risk (although not among the breast cancer cases or deaths as they were excluded at that stage). This imperfection has now been rectified and a definitive analysis is possible.Finally, it is possible that the slight difference in the age distributions of the study and control groups might have modifying effects on the results, since randomisation took place at the level of small communities in this trial. Although the age distributions of the study and control groups were closely similar, an analysis which takes into account the potential age affects, in addition to the updated information, is timely. MethodsThe design and management of the two county trial have been described elsewhere.' The numbers of individuals in the trial are shown tabulated by age and county in table 1. It is important to note that the control group is slightly younger than the group allocated to screening. Randomisation into the trial took place in the period 1977-80 and data a...

Mammography screening in African American women

“…2,3,10 However, limited attention has been given to whether these results apply to African American women. To our knowledge, among the eight randomized clinical trials that have evaluated screening mammography, [11][12][13][14][15][16][17][18][19][20] only the Health Insurance Plan of Greater New York (HIP) Study 11 included a substantial proportion (approximately 20%) of black participants and provided race-specific results. 21,22 In a single report of race-specific survival rates, Shapiro et al showed that substantially more white than black women in the control group survived breast cancer five years from diagnosis (60.9 % versus 47.1 %).…”

Quality of life following a false positive mammogram

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The reduced breast cancer mortality found in several major studies (Shapiro et al, 1982;Collette et al, 1984;Verbeek et al, 1984;Tabar et al, 1985Tabar et al, , 1989Palli et al, 1986) is the rationale for screening with mammography. In order to justify the continued use of a screening procedure, subjects correctly classified as positive at screening should receive a benefit.

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Verification of the cause of death in the trial of early detection of breast cancer

“…However, even in the New York trial and the Two Counties trial the validity of reviewers conclusions are not immune from a bias which could favour screening. Both studies (Shapiro et al, 1988;Tabar et al, 1989) (Chamberlain, 1982). The purpose of making populationbased mortality comparisons rather than case-fatality comparisons is the avoidance of lead time bias and length-biased sampling effects but, if stage is taken into consideration in judging between competing causes of death, review of the cause of death is unhelpful.…”

“…The records of 99% of the women were successfully flagged at the NHS Central Registries and it is these women, aged 45 to 72, whose mortality has been analysed. For (Tabar et al, 1985;Tabar et al, 1989) (Table II). If, however, in those cases where the assessors were uncertain because records were scanty, the certifying doctors were given the benefit of the doubt, the number of cases of disagreement between assessor and death certificate would have been reduced from 56 (6.0%), as shown in Table II, to 12 (1.3%).…”