Women undergoing screening mammography experience a higher incidence of invasive breast cancer, without a corresponding reduction in symptomatic breast cancer.
Screening mammography aims to detect breast cancer at an earlier more â€œcurableâ€� stage and women who have been screened should enjoy a lower incidence of breast cancer after they stop screening.
The UK NHS breast screening programme is organised such that in Scotland, some regions are attended by a screening van for 1 year every 3 years, with no screening occurring in the interim 2 years. A graph was plotted showing the population incidence of invasive breast cancer diagnosed in one region of Scotland (Fife), from 1980 to 2004. One would expect that in the first or first two rounds of screening the cancers in this very stable populatiion would be detected and treated so that in the subsequent two years, the incidence would dip below the level before screening was introduced, by at least the same level as the level of increased diagnosis during the screening year.
In reality, the graph (figure 1) clearly shows that the population incidence of invasive breast cancer peaks every 3 years and these years correspond to the year when the region is visited by the breast screening van. However, the incidence in the following two years does not appear to reduce even after several successive rounds of screening and appears to remain at the level before screening was introduced. This suggests that these peaks represent the excess diagnoses (of early cancers) attributable to screening, without a corresponding reduction in symptomatic cancers. Although a new cohort of women would join the screening cohort every 3 years, the oldest cohort would leave- and after a few rounds of screening, the background incidence of symptomatic cancers detected in the years between screens should reduce. Firstly, the data show that it doesnâ€™t reduce and secondly, the magnitude of the rise in incidence is far more than the number of cancers detected in those who are in the 50-53 age group. Thus, the incidence of breast cancer increases dramatically upon subjecting women to screening and this is not accompanied by a reduced incidence of symptomatic cancers in the following years.
Figure 1. Invasive breast cancer incidence in Fife
This figure shows the incidence of invasive and in situ breast cancer in the Fife region of Scotland. The incidence peaks every 3 years and these years correspond to the year when the region is visited by the breast screening van. However, the incidence (of symptomatic cancers) in the other two years does not appear to reduce even after several successive rounds of screening and remains similar to the incidence before screening was introduced. This suggests that these peaks represent the excess diagnoses (of early cancers) attributable to screening, without a corresponding reduction in symptomatic cancers.
In a second analysis, (figure 2)the population incidence of invasive breast cancer from Scotland was analysed for five cohorts of women separated by 10 years of age from 1984 to 2004. The 1st cohort represents women who were 25-29 years in 1984, 35-39 in 1994 and 45-49 in 2004 and the 5th cohort was 65-69 years in 1984 and 85+ in 2004; both these cohorts never received screening. The 2nd 3rd and 4th cohorts received screening for 10, 14 and 4 years respectively.
Invasive breast cancer incidence amongst those cohorts who did not receive screening remained unchanged between 1984 to 2004. Amongst those cohorts who received screening, the incidence of breast cancer rose to a level usual for those women 10-20 years older than them. Such a rise during the period of screening, however, did not reduce the incidence during the interval between screens, or after screening stopped at 65, when it was actually higher than expected.
Figure 2. Invasive breast cancer incidence in Scotland
This graph is plotted in an unusual way and represents the incidence of breast cancer in 5 cohorts of women over 20 years, who were in the age range as specified in the legend, in 1984. Thus, the following groups of points represent incidence for the same age group: CED, JAM, LK, as well as those points joined by the green lines. Women were undergoing screening at points A, K, L and M and between B and E (cohort 4), and K and D (cohort 3). The green lines demonstrate how the incidence of breast cancer has not increased significantly over the 20 years amongst those who did not receive screening. The red line demonstrates one example of the rise of incidence of breast cancer in age group 65-59, who had 15 years of screening compared with those who did not.
Women who undergo regular screening mammography raise their incidence of invasive breast cancer by 30% to 60%. This increase does not result in a reduction in incidence in later years. In the years between consecutive screens, as well as after stopping screening, rather than enjoying a reduced incidence, these women continue to experience a higher risk of invasive breast cancer.
Competing interests: No competing interests