Abstract 887: Hormone therapy use and the risk of breast cancer
Authors: Roman Marta , Sakshaug Solveig , Vangen Siri , Graff-Iversen Sidsel , Weiderpass Elisabete , Ursin Giske , Hofvind Solveig 
Published: August 2015

Abstract
Introduction: There is substantial evidence that hormone therapy (HT) with estrogen-progestin (EPT) increases the risk of breast cancer, while it is limited for estrogen-only therapy (ET). Questions remain regarding the effect of different formulations, preparations, and routes of administration.
Aim: To assess the risk of breast cancer in women using different HT therapies.
Methods: Information on HT use was obtained from the Norwegian Prescription Database, and breast cancer incidence from the Cancer Registry of Norway. Poisson regression was used to estimate the incidence rate ratios (RR) associated with different hormone exposures.
Results: We analyzed 686,614 women born in Norway, aged 45-79 years in January 2004, followed until December 2008, of whom 178,383 (26%) were prescribed HT. During the average 4.8 years of follow-up, 7,910 invasive breast cancers were registered. Compared with non-users, tibolone users had a RR of 1.91 (95%CI:1.61-2.28), and users of EPT with estradiol-norethisterone a RR of 2.74 (95%CI:2.55-2.95). Users of ET with oral or transdermal estradiol had a RR of 1.40 (95%CI:1.16-1.68), and 1.40 (95%CI:1.00-1.95), respectively. Use of ET with oral or vaginal estriol was not associated with an increased risk. Use of continuous or sequential combined estradiol-norethisterone EPT regimens were both associated with an increased risk (RR:2.80; 95%CI:2.59-3.02, and 2.31; 95%CI:1.88-2.83, respectively). Kliogest (estradiol-norethisterone)-users had the highest risk of breast cancer (RR:3.26; 95%CI:2.84-3.73). The increased incidence rates of invasive breast cancer approximates one extra invasive breast cancer for every 475 women using tibolone and one for every 259 women using estradiol-NETA each year.
Conclusions: Use of estradiol-norethisterone and tibolone HT was associated with an increased breast cancer risk. Oral and transdermal ET preparations of estradiol seemed to confer also an elevated risk. The risks varied by route of administration and type of preparation.
Risk of invasive breast cancer with use of different types of hormone therapyHormone useNo of caseswomen-yearsRR95%CIType of componentNon-user5 6022 594 2961referenceEstradiol377155 0661.08(0.97-1.20)Estriol9639 3081.08(0.88-1.32)Tibolone13130 6991.91(1.61-2.28)Estradiol-norethisterone872144 7922.74(2.55-2.95)Mixed29292 3701.47(1.31-1.65)Type of combined regimenNon user5 6022 594 2961referenceContinuous776123 8752.80(2.59-3.02)Sequential9620 9172.31(1.88-2.83)Other896317 4441.27(1.18-1.36)Route of administrationNon-user5 6022 594 2961referenceEstradiol oral11537 4361.40(1.16-1.68)Estradiol transdermal3511 5871.40(1.00-1.95)Estradiol vaginal21899 9530.96(0.84-1.10)Estriol oral8633 2311.13(0.92-1.41)Estriol vaginal64 3680.62(0.28-1.38)Estradiol-norethisterone oral859141 5522.76(2.56-2.97)Estradiol-norethisterone transdermal82 3121.62(0.81-3.23)Other441131 7961.54(1.40-1.70)Adjusted for age (5-year), number of births, age at 1st birth and time (offset)
Citation Format: Marta Roman, Solveig Sakshaug, Siri Vangen, Sidsel Graff-Iversen, Elisabete Weiderpass, Giske Ursin, Solveig Hofvind. Hormone therapy use and the risk of breast cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 887. doi:10.1158/1538-7445.AM2015-887