Abstract 2287: Number of risky lifestyle behaviors and breast cancer subtypes in a large nested case-control study from Norway
Authors: Ellingjord-Dale Merete , Vos Linda , Tretli Steinar , Hofvind Solveig , Hjartåker Anette , Russnes Hege , dos-Santos-Silva Isabel , Ursin Giske 
Published: July 2017

Abstract
Background: Alcohol, smoking, physical inactivity, high body mass index (BMI) and menopausal hormone therapy have been shown to be associated with increased risk of breast cancer. However, it is unclear how these risk factors combined influence overall breast cancer risk, and whether they are associated with only certain subtypes.
Methods: We conducted a case-control study nested within a cohort of 457,036 women who participated in the Norwegian Breast Cancer Screening Program (NBCSP) in 2006-2014, and who completed a questionnaire at baseline screening. In all, 4 686 breast cancer cases with information on risk factors and hormone receptor status (i.e. estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor 2 (HER2) occurred during the follow-up. The following surrogate definitions of breast cancer subtypes were used: ER+PR+HER2- ("luminal A-like"), ER+PR-HER2- ("luminal B-like, HER2 negative"), ER+ PR+/PR-HER2+ ("luminal B-like, HER2 positive"), ER-PR-HER2+ ("HER2 positive") and ER-PR-HER2- ("triple negative"). We defined risky lifestyle behaviors as ever smoking, weekly consumption of more than 2 glasses of alcoholic beverage, less than 3 hours leisure time physical activity per week, ever use of menopausal hormone (estrogen or estrogen and progesterone) therapy and BMI (kg/cm²) &gt;25. We used conditional logistic regression to estimate odds ratios (ORs), with 95% confidence intervals (CIs), adjusted for age, education, age at menarche, number of pregnancies and menopausal status.
Results: Number of risky lifestyle behaviors was associated with an increased risk of breast cancer overall (p-trend&lt;0.0001). Compared to women with no risky lifestyle behaviors, women with 1 (OR=1.28, 95% CI 0.87-1.88), 2 (OR=1.63, 95% CI 1.13-2.36), 3 (OR=1.85, 95% CI 1.28-2.68), 4 (OR=2.27, 95% CI 1.56-3.29) and 5 (OR=2.38, 95% CI 1.58-3.59) risky lifestyle behaviors had increased risk of the luminal A-like subtype (p-trend&lt;0.0001). However, number of risky lifestyle behaviors was not associated with an increased risk of the triple negative subtype (p-trend=0.27).
Conclusions: This study showed that number of risky lifestyle behaviors was positively associated with a marked increased risk for luminal A-like breast cancer. These findings suggest that healthy lifestyle choices may play an important role in the prevention of the commonest form of this cancer.
Citation Format: Merete Ellingjord-Dale, Linda Vos, Steinar Tretli, Solveig Hofvind, Anette Hjartåker, Hege Russnes, Isabel dos-Santos-Silva, Giske Ursin. Number of risky lifestyle behaviors and breast cancer subtypes in a large nested case-control study from Norway [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2287. doi:10.1158/1538-7445.AM2017-2287