Norwegian overdiagnosis studies
Currently, five studies have been published that estimate the extent of overdiagnosis since the implementation of the NBCSP (16) – (20) . The two oldest studies (16, 17) will not be discussed here, as Zahl & Mæhlen have overlapping data and longer follow-up time (18) .
Zahl & Mæhlen have performed an ecological cross-sectional study (18) . They concluded that 50 % of the breast cancer cases among invited women in the four pilot counties were representative of overdiagnosis. They analysed the numbers in relation to age (Fig. 1, quadrate) up to the end of 2009 (Fig 2, middle panel). My own calculations, based on data from Statistics Norway (21) , indicate that 48 % of the women-years have been erroneously included in the post-screening period (see appendix ). The proportion of overdiagnosis was measured as excess breast cancer cases in relation to cases without screening in the age group 50 – 69.
Figure 2 Follow-up after end of screening. Schematic illustration of birth cohorts invited in the four pilot counties of the Norwegian Breast Cancer Screening Program (NBCSP) in the period 1996 – 2009. The pink-shaded area indicates women who received an invitation. The yellow-shaded area indicates women previously invited. The blue-shaded area indicates women who have never been invited. All three studies evaluating overdiagnosis after the implementation of the NBCSP include women aged 70 – 79 in the post-screening period (18 – 20 ). The red line marks the authors’ demarcations. Based on the population data (21 ) and the start date of the NBCSP in the different counties (information from the Cancer Registry), I have calculated the proportion of women-years within the red demarcation line who have been invited previously (see appendix ). The study by Falk et al. (20 ) applies individual data with 100 % previously invited (left-hand panel). Zahl & Mæhlen (18 ) studied the period 1998 – 2009 with 52 % previously invited (middle panel). Kalager et al. (19 ) studied the period 1996 – 2005 with 29 % previously invited (right-hand panel)
Kalager et al. reported that 15 – 25 % of the breast cancer cases among invited women constituted overdiagnosis (19) . They studied the incidence rates of breast cancer among women in the study group compared with incidence rates in three different control groups. They considered the women in relation to age (Fig. 1, quadrate) up to the end of 2005 (Fig. 2, right-hand panel). This means, based on my calculations, that 71 % of the women-years in the four pilot counties have been erroneously included in the post-screening period (see appendix ). I calculate that this percentage was even higher for the other counties: 81 – 100 % (see appendix ). The proportion of overdiagnosis was measured as excess breast cancer cases in relation to cases without screening in the age group 50 – 79.
We carried out a cohort study using anonymised individual data (20) . The women were followed from the date of first invitation and up to the end of 2009 (Fig. 2, left-hand panel). A woman’s attendance was classified in relation to her screening history. The proportion of overdiagnosis was estimated for women who follow the national recommendations for ten screenings between the ages of 50 and 69. The time perspective without screening was calculated for age 50 and older. The proportion of overdiagnosis of breast cancer among invited women was estimated as 10 – 11 %.
There are significant differences between these studies with respect to the accuracy of the data and the length of follow-up. Figure 1 illustrates the inaccuracies which arise if age is used instead of birth cohort in the analysis. In studies of the post-screening period, it is a prerequisite that all the women included should have been invited previously, in order to include the compensatory drop in incidence. This condition is not fulfilled for two of the studies (Fig. 2). The measure of overdiagnosis is defined for different women and for different age groups in the three studies mentioned.