Less overdiagnosis in breast cancer screening than previously claimed
A new study from the Cancer Registry of Norway, based on individual level data, shows that fewer women are overdiagnosed with breast cancer than previously reported. The problem of overdiagnosis in the Norwegian Breast Cancer Screening Programme may be smaller than earlier studies have reported, PhD student Ragnhild Sørum Falk says.
The new study is published online in International Journal of Cancer January 25 2013. The study follows women invited to the Norwegian Breast Cancer Screening Programme from 1996 and onwards, until the end of 2009. The number of excess cancers were estimated based on the number of cancers detected among female participants and non-participants of the program aged 50-69, and among women aged 70 and older who are no longer invited due to the upper age limit.
- We found that the incidence of breast cancer is higher among women during the time period when they are participants compared to what we would have expected had they not taken part in a screening programme. This is an expected effect of screening due to lead time. However, as the women leave the programme due to the upper age limit, what we see is an incidence reduction, a reduction that lasts during the whole follow-up period, Sørum Falk says.
Overdiagnosis is defined as cancers that would not have been detected during a woman’s life time had she not been invited to or attended a screening programme. In this study, the amount of overdiagnosis is estimated, both for precursors (DCIS) and invasive breast cancer, among women invited to the programme and among those who actually attended. The percentage of overdiagnosed tumours represents the excess incidence compared to what is expected in the absence of screening. The estimates of the excess incidence are:
- 10-11% for invasive breast cancer, among women invited to the programme
- 14-17% for invasive breast cancer and DCIS, among women invited to the programme
- 11-13% for invasive breast cancer, among attending women
- 17-20% for invasive breast cancer and DCIS, among attending women
High quality data provide more precise results
This is the first Norwegian study using individual level data for invitation and attendance in the Norwegian Breast Cancer Screening Programme and with up to 10 years of follow up after the women leave the programme due to the age limit.
- We believe that our estimates are more precise than earlier Norwegian studies that have not been based on individual level data, says Ragnhild Sørum Falk, first author of the article.
Senior researcher at the Cancer Registry of Norway, Tor Haldorsen, has been the supervisor of Sørum Falk`s PhD work.
- The study shows that individual level data on women`s screening history as well as a long period of follow up are necessary to give as precise estimates as possible, Haldorsen explains.