Effect of various forms of postmenopausal hormone therapy on the risk of ovarian cancer--a population-based case control study from Finland.

Postmenopausal hormone therapy (HT) associates with an increased risk of ovarian cancer, but its' influence on tumor histology is not as well known. Therefore, we evaluated the effect of various types of HT on the risk of epithelial ovarian cancer by histological subtype. All Finnish women diagnosed with ovarian cancer (n = 3,958) aged over 50 during 1995-2007 were identified from the Finnish Cancer Registry. For each case, three controls, matched for age and place of residence, were recruited from the Finnish National Population Register, which also provided data on parity and ages at deliveries. After exclusion of controls with oophorectomy, 11,325 controls remained. The prescription register provided HT use from age 50. Odds ratios (OR) for different HTs were estimated by conditional logistic regression: adjusted for parity, ages at deliveries and hysterectomy. Estradiol-only therapy use for 5 years or more associated with an increased risk (OR 1.45; 95% confidence interval 1.20-1.75) of a serous subtype, but with a decreased risk of mucinous subtype (0.35; 0.19-0.67). Use of sequential estradiol-progestin therapy (EPT) for 5 years or more associated with an increase in overall ovarian cancer risk (1.35; 1.20-1.63) and with an increase in the endometrioid subtype (1.88; 1.24-2.86) particularly. Continuous EPT, estradiol + levonorgestrel-releasing intrauterine system or tibolone had no effect on overall ovarian cancer risk. In conclusion, only sequential EPT use for 5 years or more associates with an increased risk of overall ovarian cancer. Furthermore, HT regimens differ significantly in their association with various histological types of ovarian cancer.