In 1996-7 around 165 000 women in four counties (Akershus, Oslo, Rogaland, Hordaland—the AORH counties) in Norway, covering 40% of the population, were invited to mammographic screening for the first time. They attended for second screening in 1998-9 and third in 2000-1. Screening in the 15 other counties started gradually later. In Sweden screening was introduced from 1986 to 1996. More than 90% of the million women in the age group 50-69 years had been invited at least once by 1992. Later some women in the age group 70-74 years were also invited. In both countries the attendance rate among the invited women has been about 75%.7,9
We obtained specific rates of invasive breast cancer for the age groups 30-49, 50-69, and > 69 years from nationwide cancer registers for the period 1971-2000. Our rates include only invasive disease; ductal carcinoma in situ has not been included.
We used Poisson regression models and relative risks to estimate the percentage change in age specific incidence rates of breast cancer after the introduction of nationwide screening programmes. We present relative risks with 95% confidence intervals and calculated estimates using the computer program EGRET.
The Norwegian dataset was analysed with an age-period model over the period 1991-2000. The period effects in 1991-5 were estimated with two linear trend parameters—one for the AORH counties and one for the non-AORH counties. As the non-AORH counties have always had a lower incidence than the AORH counties,8
we assumed this to be the case for 1991-2000.
We estimated the increase in incidence in the first screening round using an interaction parameter for age 50-69 years, the AORH counties, and the period 1996-7. The incidence increases in second and third screening rounds were estimated by using interaction with periods 1998-9 and 2000, respectively. We estimated the increase in incidence in the non-AORH counties in the period 1996-2000 analogously.
According to Boer et al the incidence is supposed to decline more in the age group 70-74 years than in age group ≥ 75.1
We therefore estimated relative risks in the AORH counties in 2000 for the age group 70-74 years.
We estimated general period effects in Sweden before and after 1986 using two separate linear trend parameters to allow for an underlying increase in incidence during the period 1971-2000. The age specific increase was estimated with interaction variables between periods 1986-9, 1990-3, 1994-6, and 1997-2000 and age 50-69 years. Here we also tested for any decline in the age groups 70-74 years and 75-79 years using interaction terms between age and periods 1994-6 and 1997-2000, respectively.
In all analyses we adjusted for age using 5 year age intervals.