ESTROGEN REPLACEMENT THERAPY AND BREAST-CANCER RISK - A CASE-CONTROL STUDY

The relationship between oestrogen replacement therapy and breast canc er risk was examined based on data obtained from a population-based ca se-control study of breast cancer on Long Island, New York, USA. Cases were defined as female residents of two Long Island counties, aged 20 -79, who were diagnosed with breast cancer between 1 January 1984 and 31 December, 1986. Age- and county-matched controls were selected from driver's licence files. Among all postmenopausal women, there was no significant association between ever-use of hormones to treat menopaus al symptoms and breast cancer risk. There was also no significant posi tive association in any subgroup defined by type of menopause (natural , hysterectomy with at least one ovary intact, bilateral oophorectomy) or age at menopause. Additionally, there was no increasing trend in r isk with duration of use either overall or in any subgroup, nor was th ere an effect at any interval since last use. A significant elevation in risk was observed in women with 10-19 years since first exposure, w hich was concentrated in women with a natural menopause or hysterectom y with at least one ovary remaining, and women aged > 45 at menopause. Results of logistic regression analysis revealed no important confoun ding by any of several established breast cancer risk factors. However , a significant interaction was observed between body mass index (BMI) and oestrogen use, with an effect of oestrogen use being seen only in the thinnest tercile. Although biologically plausible explanations fo r this finding exist, the effect of chance cannot be ruled out.