A case-control study on hormone therapy as a risk factor for breast cancer in Finland: intrauterine system carries a risk as well

The purpose of this study was to evaluate the association between postmenopausal hormone therapy (HT) and the risk for breast cancer in recently postmenopausal Finnish women. All Finnish women with first invasive breast cancer diagnosed between the ages of 50 and 62 years during 1995-2007 (n = 9,956) were identified from the Finnish Cancer Registry. For each case, 3 controls of the same age were retrieved from the Finnish Population Register. The cases and controls were linked to the national medical reimbursement register to assess the use of HT. The odds ratios (ORs) and 95% confidence intervals (CIs) for breast cancer were calculated with conditional logistic regression analysis, adjusting for parity, age at the first birth and health care district. Estradiol-only therapy (991 users with breast cancer, n) or oral progestagen (n = 138) was not accompanied by an increased risk. Estradiol-progestagen therapy (EPT) (n = 1,731) was associated with an elevated risk in the whole series (OR 1.36; 95% CI 1.27-1.46). The risk became detectable in less than 3 years of use. Continuous EPT use tended to be associated with a higher risk for breast cancer than the sequential EPT use. The use of tibolone (n = 80) (1.36; 1.15-1.96), a levonorgestrel-releasing intrauterine system (LNG-IUS) alone (n = 154) (1.45; 1.97-1.77) or as a complement to estradiol (n = 137) (2.15; 1.72-2.68) was also associated with an increased risk. The association between HT use and the risk for breast cancer shows a large variation between various forms of HT, and also the use of LNG-IUS may carry a risk.