Hormone replacement therapy and breast cancer in former users of oral contraceptives—The Norwegian Women and Cancer study

Abstract

Combined estrogen-progestin menopausal therapy (HRT) and combined estrogen-progestin contraceptives (OC) both increase breast cancer risk during current use and a few years after. We investigated risk of breast cancer in women who were users of HRT dependant on former history of OC use in a large, national population-based cohort study, the Norwegian Women and Cancer study (NOWAC). Exposure information was collected through postal questionnaires. Based on follow-up of 30,118 postmenopausal women by linkage to national registers of cancer, deaths, and emigration we revealed 540 incident breast cancer cases between 1996 and 2004. Compared to never users of either drugs current use of HRT gave a significant (p 5 0.002) higher risk of breast cancer in former OC users, RR 5 2.45 (95% CI 1.92-3.12), than among never users of OCs, RR 5 1.67 (1.32-2.12). Relative risk of current use of HRT was similar for estrogen only and combinations with progestin added in ever users of OCs. The increased risk of breast cancer in current HRT users with a history of former OC use could have potential great impact on postmenopausal breast cancer risk as the proportion of postmenopausal women with former OC use will continue to increase. ' 2007 Wiley-Liss, Inc.Key words: breast cancer; OCs; HRT; consecutive use; NOWAC Combined estrogen-progestin menopausal therapy and combined estrogen-progestin contraceptives have both been classified as carcinogenic to humans (Group 1). 1 The increased risk of 30-40% for breast cancer related to OC use is mainly connected to present and recent use, while for HRT use the risk also increases with duration of use. [2][3][4][5] For HRT use the increased risk of 1.26 was somewhat lower in the clinical trial of Women's Health Initiative 6 than in most epidemiological studies with relative risks around 2. This could partly be due to the use of different combinations of hormones. During the last decade an increasing number of former OC users have reached the menopausal age, estimated to around 300 million worldwide and exceeding 80% in some moredeveloped countries. 1 Use of treatment with HRT peaked around 2000 with about 20 million women in more-developed countries using this therapy, including as much as half of the women in US aged 50-64 years. 1 Since then use has been substantially reduced. With 1 million cases diagnosed in the world annually, breast cancer is by far the most common female cancer, comprising about 20% of all new cancers in women. 7 Among the few studies that have looked at risk related to combined use of both OCs and HRT, most studies have limited the analyses to ever use of HRT. [8][9][10][11] One French study looked at current HRT use in relation to previous OC use. 12 They found an additive effect of OC and HRT use on breast cancer risk with no significant interaction. The primary objective of this analysis was to investigate the risk of breast cancer in HRT users taking into account information on former OC use, based on the Norwegian Women and Cancer study (NOWAC). Material and meth...

Ovarian hormones and reproductive risk factors for breast cancer in premenopausal women: the Norwegian EBBA-I study

“…Greater BMI and obesity are associated with decreased breast cancer risk in studies of premenopausal women (Friedenreich, 2001; Berstad et al ., 2010) while both current and former OC users have an increased risk of breast cancer in recent studies (Lund et al ., 2007; Hunter et al ., 2010). Low parity is a well-established risk factor for breast cancer (MacMahon et al ., 1970; Braaten et al ., 2004; World Cancer Research Fund/American Institute for Cancer Research, 2007).…”

Oral contraceptives and menopausal hormone therapy as breast cancer risk categories

“…Представленные данные свидетельствуют об отсутствии повышения риска развития РМЖ у женщин, получающих МГТ [25][26][27].…”

Two hundred years of cancer epidemiology in Norway – before and during the NOFE era

“…Studies from Norway have contributed to this knowledge (47)(48)(49)(50). Norwegian scientists continue to raise new questions in this area: How is breast cancer risk among users of both hormonal medications (51)? Is there an antagonistic effect for OC-use and alcohol consumption on breast cancer risk (52)?…”