Relationship between the type of hormone replacement therapy and incidence of breast cancer in Korea

This study aimed to investigate the relationship between hormone replacement therapy (HRT) types and breast cancer (BC) incidence in postmenopausal women in Korea.The nested case-control study used data from the National Health Insurance Service database. Among the women aged ≥50 years who menopaused between 2004 and 2007, BC incidence up to 2017 was analyzed in 36,446 women using or having used HRT for >1 year and in 36,446 women who did not use any HRT for more than 1 year. HRT types and duration were classified into three categories.BC risk (BCR) decreased with tibolone use for all ages. With HRT initiation in women aged ≥50 years, BCR was lower with tibolone and estrogen-progestogen therapy. HRT for <3 years showed lower BCR with tibolone, while higher BCR was observed with estrogen-only therapy. BCR was lower in women of all ages on HRT for >5 years than in the control group.For women in their 50s, tibolone use lowers BCR; for all ages, the use of any HRT for >5 years showed lower BCR in Korea. These divergent results from western countries could be associated with the specific characteristics of BC in Korea.