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, Finla nd, 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 inter vals, depending on their year of birth. Screen-detected breast cancers numbered 385 and, during the same time period, 154 women were diagnos ed with breast cancer outside screening in the same age group in the s ame city, and 100 interval cancers were detected. Two hundred and fift y (67%) of the screen-detected cancers were of post-surgical stage I c ompared with 45 (45%) of the interval cancers and 52 (34%) of the canc ers found outside screening (P<0.0001). However, among women aged 40-4 9 the frequency of stage I cancers did not differ significantly among screen-detected cancers, interval cancers and cancers found outside sc reening (50%, 42% and 44% respectively; P=0.73). Invasive interval can cers were more frequent among women aged 40-49 ii screening was done a t either 1-year (27%) or 3-year intervals (39%) than in older women sc reened at 2-year intervals (18%; P=0.08 and P=0.0009 respectively), Ev en if adjusted for the primary tumour size, screen-detected cancers ha d smaller S-phase fractions than interval cancers or control cancers ( P=0.01), but no difference in the S-phase fraction size was found betw een cancers of women younger than 50 and those older than this (P=0.13 ). We conclude that more interval cancers were found among women young er than 50 than among those older than 50 and that this could not be e xplained by the rate of cancer cell proliferation.