BREAST-CANCER MORTALITY IN A NONRANDOMIZED TRIAL ON MAMMOGRAPHIC SCREENING IN WOMEN OVER AGE 65

Recent case-referent studies in the Nijmegen breast-screening programm e have shown a reduction in breast-cancer mortality of approximately 5 0% due to screening of women aged 65 years and older. In this type of study, however, the results may be biased because of self-selection. T he purpose of our present study was to compare the breast-cancer morta lity rate in a population invited for screening with that of a referen ce population from an area without a screening programme. In 1977-1978 , 6773 women aged 68-83 years were enrolled in the mammographic screen ing programme in Nijmegen, The Netherlands. The women were followed up until 31 December, 1990. The reference population consisted of women from the same birth cohort from Arnhem, a neighbouring city without ma ss screening, for whom the entry date was 1 January, 1978. The ratios of the Nijmegen and Arnhem breast-cancer mortality rates with 95% conf idence intervals (CI) were calculated. In the study period, 173 patien ts were diagnosed with primary breast cancer in Nijmegen vs. 183 in Ar nhem; 40 Nijmegen patients had died of breast cancer vs. 51 Arnhem pat ients. The cumulative mortality-rate ratio was 0.80 (95% CI = 0.53-1.2 2). In the periods 1978-1981, 1982-1985 and 1986-1990, the mortality r ate ratios were 1.44 (95% CI = 0.67-3.10), 0.81 (95% CI = 0.37-1.79) a nd 0.53 (95% CI = 0.27-1.04), respectively. After adjustment for the d ifference in incidence rate that existed between the Nijmegen and Arnh em populations, mammographic screening of women older than 65 can be e xpected to yield a 40% reduction in breast-cancer mortality after 10 y ears. (C) 1997 Wiley-Liss, Inc.