The situation in Norway
The Mammography Programme started in four counties in 1996 and became nationwide in 2005 (12). Women in the age group 50 – 69 years are invited to mammographic screening biennially. The Cancer Registry of Norway is responsible for administration, quality control and evaluation of the programme.
Cancer statistics in Norway are currently updated through 2010. In other words we have limited follow-up time, particularly for women resident in the counties that were late in implementing the programme. To evaluate breast cancer mortality after introduction of the Norwegian Breast Cancer Screening Programme (NBCSP) , it is important to have individual-level information from a sufficient number of invited women who have been invited to/have attended the programme more than once. It is further claimed that 10 – 15 years’ follow-up time is necessary to obtain the most exact estimates (9). Until now results from two analyses have been published on breast cancer mortality since the introduction of the programme (13, 14). Neither of the analyses used individual information on when the women were invited to or attended the screening, and the follow-up period is inadequate. Both analyses showed approximately 10 % lower breast cancer mortality among the presumably invited women. Given the shortcomings of the studies, both analyses may have underestimated the effect of the screening programme (15).
Another challenge in Norway is the fact that there is no information on the use of mammography outside of the programme. New data show that 32 % of breast cancer tumours that were diagnosed in women who did not attend the programme in Møre og Romsdal were asymptomatic and therefore may have been diagnosed through screening at private clinics (16). If this finding is representative for all the counties, it is possible that the reduction in mortality is underestimated among the attendees and overestimated among the invitees.
To calculate the extent of overdiagnosis of breast cancer tumours, a follow-up time of ten years has been proposed, but recently published studies imply that 10 – 15 years’– even lifelong – follow-up might be necessary (9, 17). Several studies have been published with figures from Norway, with estimates of 10 – 67 % (17) – (20). A number of the studies have been criticised for methodological weaknesses (21, 22), and only the one with the lowest estimate (17) uses individual data for invitation and attendance. The latter study shows 10 – 20 % excess cancer incidence, depending on the inclusion criteria.
The risk of false positive screening results in the Mammography Programme has previously been calculated to be of the same order of magnitude as the meta-analysis, 17 % for additional examinations with supplementary X-rays and/or ultrasound, and 3 % for those including a biopsy (23).