Time interval since last test in a breast cancer screening programme: a case-control study in Italy.

STUDY OBJECTIVE To evaluate a population based screening programme for breast cancer. DESIGN This was a case-control study of women dying of breast cancer between 1977 and 1987 who had been invited to take part in a screening programme. SETTING Community based study of women aged between 40 and 70 years (total population about 35,000 at 1981 census), living in 23 small towns near Florence, Italy. PARTICIPANTS 103 cases were identified from death certification, and 515 living controls (five per case) selected for year of birth and town of residence. MEASUREMENTS AND MAIN RESULTS Screening history was obtained from computer archive. Sociodemographic information was obtained from town registry offices and directly from relatives of the deceased and from the controls by postal questionnaire, and if necessary telephone or personal interview. Analysis was carried out on two age groups--40-49 years and 50+ years at diagnosis--and considered the number of screening tests and the time interval since the last test, separately and together. In the older age group, women with at least one screening test in the previous 2 1/2 years showed a 50% reduction in risk (odds ratio 0.49, 95% confidence interval [CI] 0.25-0.95). If they had also had another previous negative screen the risk was reduced to one third (odds ratio 0.35, 95% CI 0.14-0.85). There was a significant trend of decreasing risk with increasing number of screens in older women. No clear evidence of a similar protective effect was shown for women in the 40-49 year age group. CONCLUSIONS A significant protective effect of the screening programme is evident in older women but not in younger ones. The data do not allow an assessment of optimal screening interval because of the small number of previously screened cases.