Hormone therapy use and breast cancer incidence by histological subtypes in Sweden and Norway.

Mammography screening and postmenopausal hormone therapy (HT) both influence breast cancer incidence. While breast cancer incidence increased by around 50% during the introduction of screening, a smaller decline in incidence has been reported in several countries after 2002 when the sales of HT started to decline. Data suggest that HT increases the risk of the second most common type of breast cancer, invasive lobular carcinoma (ILC) but not the most common, invasive ductal carcinoma (IDC). Breast cancer incidences stratified on histological subtypes were obtained from the national cancer registries. HT sales data from drug consumption statistics and information on the county-level introduction of mammography screening were combined, and breast cancer incidence trends were estimated using Poisson regression models, focusing on the period after 2002. From 2002 to 2007 the annual decrease in breast cancer incidence rates for women aged 50-69 was 1.5% (95% CI -2.3% to -0.7%) in Sweden and 0.8% (95% CI -2.8% to 1.2%) in the part of Norway not confounded by prevalence screening. Most of the decline was in the rates of ILC which dropped by 4.7% (95% CI -6.6% to -2.7%) and 7.0% (95% CI -12.8% to -0.9%) per year, respectively. The rates of IDC were stable in this period. Breast cancer incidence has declined in Sweden and Norway since 2002, but the reduction is moderate compared with the large increase that occurred during the introduction of mammography screening. Declining rates of ILC, but not of IDC, support the hypothesis that the drop in breast cancer incidence is associated with reduced HT use.