Breast cancer is an important cause of death among women. Early detection through mass screening offers a possibility for earlier diagnosis and treatment, and, as a consequence, reduced mortality. However, screening may also harm, for instance if the procedure picks up forms of disease that would never have evolved into life threatening cancer (overdiagnosis and overtreatment).
In Norway women between the age of 50 and 69 are offered mammography examination every other year. Recently it has been argued that also women in their forties should be offered organised breast cancer screening. The aim of this report is to summarise existing evidence of the effect of mammography screening of women between the age 40 and 49 years as well as for all women in order to support policy makers. We also hope that the results are presented in such a way that makes them understandable for women considering screening, thereby stimulating shared decision making.
Data has been gathered from three systematic reviews that have summarised existing research on the effects of inviting large groups of women to a screening program. In addition, one randomised controlled trial published in 2006 has been included.
Women between the age of 40 and 50 years probably have a slightly smaller risk of dying from breast cancer than women who do not follow such a program. We have estimated the relative risk reduction for dying, for women in their forties who are invited to screening, to 16% (confidence interval 4-27%) after 13 years of follow-up. According to our judgement this is an optimistic estimate. The studies of the highest methodological quality show a smaller effect and some of the effect may be due to screening after reaching 50 years of age. The absolute risk reduction is 0,0003 or approximately one per three thousand after 13 years.
There are other ways to describe the effect so that women might get a clearer picture of what these risk estimates may mean. Thankfully the background risk is low so approximately 97,40 % of women in these age groups will anyway be alive after 13 years followup. The chance of being alive can be increased to 97,43 % if women are invited to participate in a screening program. If 3000 women in their forties are followed for 13 years, ca. 2922 women will anyway be alive. If all of them are invited to a screening program 2923 will survive.
On the other hand, in a group of 3000, approximately ten women will have early cancer forms revealed that would not have evolved into dangerous forms of breast cancer (overdiagnosis). These women will get an unnecessary cancer diagnosis and be treated. There are also other adverse effects. For instance several hundreds of the women will have to come back for additional examinations because of false positive results.
Again, putting these figures in perspective, each birth cohort in these age groups consists of around 44 000 women. If all are invited to yearly screening when they are 40 years old, ca. 572 will be diagnosed with cancer over 13 years. Approximately 13 of them will avoid death because of breast cancer, while ca. 143 will have an unnecessary diagnosis and have to undergo treatment.
Whether or not the benefit outweighs the harm for breast cancer screening is debatable, especially for younger women. Policy makers also have to consider ethical issues and the need for resources. At the personal level, women's own perception of risk and individual preferences, informed by balanced information, should be part of the decision making process.