EFFICACY OF BREAST-CANCER SCREENING BY AGE - NEW RESULTS FROM THE SWEDISH 2-COUNTY TRIAL

Background. Several studies have found a smaller effect of breast canc er screening on breast cancer mortality in women aged younger than 50 years compared with older women. Various possible reasons have been su ggested for this, but none firmly is established. Methods. The Swedish Two-County Study is a randomized trial of breast cancer screening of women aged 40-74 years, comprising with 133,065 women with a 13-year f ollow-up of 2467 cancers. The Breast Cancer Detection Demonstration Pr oject (BCDDP) is a nonrandomized screening program in the United State s, with a 14-year follow-up of 3778 cancers in women aged 40-74 years. The Swedish results by age were updated. The lesser effect of screeni ng at ages 40-49 years was investigated in terms of sojourn time (the duration of the preclinical but detectable phase) size, lymph node sta tus, and histologic type of the tumors diagnosed in the Swedish Study and their subsequent effect on survival using survival data from both studies. Results. In the Swedish Trial, a 30% reduction in mortality a ssociated with the invitation to screening of women aged 40-74 years w as maintained after 13-years of follow-up. The reduction was 34% for w omen aged 50-74 years and 13% for women aged 40-49 years. Results indi cated that the reduced effect on mortality far women aged 40-49 years was due to a differential effect of screening on the prognostic factor s of tumor size, lymph node status, and histologic type. The mean sojo urn times in the age groups 40-49 years, 50-59 years, 60-69 years, and 70-74 years were 1.7, 3.3, 3.8, and 2.6 years, respectively. Conclusi ons, These results suggest that much, although not all, of the smaller effect of screening on mortality in women aged 40-49 years is due to faster progression of a substantial proportion of tumors in this age g roup and the rapid increase in incidence during this decade of life. I t is concluded that the interval between screenings should be shortene d to achieve a greater benefit in this age group. It is estimated that a 19% reduction in mortality would result from an annual screening re gime.