Interval cancers following breast cancer screening in Singaporean women

Our study reports on the interval cancers arising in the subsequent 3 years in women screened as part of the Singapore Breast Cancer Screening Programme that commenced in 1994. Women, the majority of Chinese ethnicity, were either invited or not invited by random allocation to be screened. All women, other than those identified with breast cancer at screening, whether invited or not, were followed for 3 years, and information on all breast cancers arising in this period was obtained through the national registry. In particular, the cancers arising from those women who were deemed free of the disease at screening were noted. The mammographs taken at screening of those women who developed such an interval cancer were rereviewed by 3 radiologists. In the 28,099 women who were screened and deemed free of disease, 59 interval cancers were reported with annual rates of 2.1, 10.6 and 10.8 per 10,000 women-years in the succeeding 3-year periods. In 39,425 women who were invited but declined screening, the rates were 17.0, 15.5 and 11.7, while the corresponding rates from 97,294 women not invited were 12.8, 13.3 and 13.0. Whereas the annual incidence at 1 year in women who have been screened is much lower than those in the 2 groups who were not screened, it is of a similar magnitude by 2 years. This suggests that locally an optimal screening interval may be close to 1 year. The rereview of the mammograms of those 59 women with interval cancers suggested that the maximum possible number of these that might have been detected at screening was 10 (17%).