[Estrogen replacement therapy and breast cancer].

BACKGROUND: Sales of HRT in Norway have increased considerably over the last 10 to 15 years, as has the incidence of breast cancer. The aim of this study was to investigate the relationship between use of HRT and risk of breast cancer in a Norwegian cohort of women.

MATERIAL AND METHODS: The Norwegian Women and Cancer (NOWAC) study is a representative, national, population-based cohort study involving 31,451 postmenopausal women aged 45-64 who contributed information on reproduction, lifestyle and use of HRT. The women are followed up for breast cancer.

RESULTS: Current users of HRT, 35% of those included, had an adjusted relative risk of breast cancer of 2.1 (95% CI 1.5-2.5). A continuous regimen of estrogen/progestagen implied an increased risk compared to a sequential regimen. Longer use increased the risk (test for trend p<0.0001). The population-attributable risk of breast cancer due to use of HRT was estimated at 27%, which equals approximately 300 cases of breast cancer per year among Norwegian women aged 45-64.

INTERPRETATION: Our results suggest that current use of HRT could be considered a major determinant of the increasing incidence of breast cancer in postmenopausal women in Norway. Treatment should, accordingly, be restricted to women with serious vasomotor complaints. The dosage should be individually tailored according to potential risks and benefits, and treatment given over a short period of time.