It is well known that use of both hormone treatment and mammography screening increases the risk of breast cancer but it is a challenge to separate the contributions of these interventions on trends in incidence of breast cancer given that systematic mammography screening was introduced around the time when the use of hormone treatment was at its highest, and that both interventions involve roughly the same age groups of women.

In this study, researchers used data from a Norwegian cancer registry that contained county specific information on sales of hormone treatment, and a public mammography screening programme that was gradually introduced between 1995 and 2004 thus lending itself to separate the contributions of hormone treatment use and mammography screening on recent trends in incidence of invasive breast cancer.

The cohort analysed included women aged 30-90 between 1987 and 2008, including 50,102 newly diagnosed cases of invasive breast cancer. The researcher’s found that the incidence of invasive breast cancer in Norway increased steadily until 2002, levelled off, and then declined from 2006. All non-linear changes in incidence were explained by use of hormone treatment and mammography screening activities, with about similar contributions of each factor. In 2002, when the incidence among women aged 50-69 was highest, an estimated 23% of the cases in that age group could be attributed to mammography screening and 27% to use of hormone treatment.

An accompanying editorial discusses the methodological limitations of this study.