Increased incidence of small and well‐differentiated breast tumours in post‐menopausal women following hormone‐replacement therapy

Exposure to hormone‐replacement therapy (HRT) has consistently been associated with an increased incidence of breast cancer, particularly of small tumours. Other tumour characteristics in relation to HRT have received less scientific attention. Our aim in this population‐based prospective cohort study was to assess whether HRT is associated with an increased incidence of breast‐cancer subgroups defined in terms of stage, type (according to the WHO system), Nottingham grade and the Nottingham Prognostic Index (NPI). Evaluation was based on a cohort of 5,865 post‐menopausal women followed for an average of 9.8 years. Twenty percent of women reported current use of HRT at the time of the baseline interview. Record linkage with the Swedish Cancer Registry and local clinical registries identified 141 incident invasive breast‐cancer cases. All tumours were reclassified by 1 pathologist. The incidence of breast cancer in HRT users was 377/105 and in non‐users 221/105 person‐years [relative risk (RR) = 1.72, 95% confidence interval (CI) 1.17–2.52]. This risk remained statistically significant after adjustment for established risk factors in a Cox proportional hazards analysis (RR = 1.66, 95% CI 1.12–2.45). Among HRT users, there was over‐representation of cases with stage I tumours (adjusted RR = 2.33, 95% CI 1.44–3.76), of lobular carcinomas (RR = 4.38, 95% CI 1.60–12.0) and of tubular tumours (RR = 4.81, 95% CI 1.37–16.8). Nottingham grade I/II carcinomas (RR = 2.02, 95% CI 1.29–3.16) and cases with NPI ≤ 3.4 (RR = 2.29, 95% CI 1.41–3.72) were similarly over‐represented among HRT users. Incidence of breast cancer was increased in post‐menopausal women who used HRT at baseline. Among HRT users, there was over‐representation of tumours that, with regard to stage, type and grade, are associated with a favourable prognosis. © 2001 Wiley‐Liss, Inc.