Mammography screening interval and the frequency of interval cancers in a population-based screening

In a population-based mammography screening, 129,731 examinations were carried out among 36,000 women aged 40-74 in the city of Turku, Finland, in the period 1987-94. Women older than 50 were screened at 2-year intervals, and those younger than 50 at either 1-year or 3-year intervals, depending on their year of birth. Screen-detected breast cancers numbered 385 and, during the same time period, 154 women were diagnosed with breast cancer outside screening in the same age group in the same city, and 100 interval cancers were detected. Two hundred and fifty (67%) of the screen-detected cancers were of post-surgical stage I compared with 45 (45%) of the interval cancers and 52 (34%) of the cancers found outside screening (P<0.0001). However, among women aged 40-49 the frequency of stage I cancers did not differ significantly among screen-detected cancers, interval cancers and cancers found outside screening (50%, 42% and 44% respectively; P=0.73). Invasive interval cancers were more frequent among women aged 40-49 if screening was done at either 1-year (27%) or 3-year intervals (39%) than in older women screened at 2-year intervals (18%; P=0.08 and P=0.0009 respectively). Even if adjusted for the primary tumour size, screen-detected cancers had smaller S-phase fractions than interval cancers or control cancers (P=0.01), but no difference in the S-phase fraction size was found between cancers of women younger than 50 and those older than this (P=0.13). We conclude that more interval cancers were found among women younger than 50 than among those older than 50 and that this could not be explained by the rate of cancer cell proliferation.