Abstract 1201: Breast cancer risk factors and volumetric breast density in a national breast cancer screening program
Authors: Hjerkind Kirsti V. , Ellingjord-Dale Merete , Johansson Anna L. , Aase Hildegunn S. , Hoff Solveig R. , Hofvind Solveig , Fagerheim Siri , Vos Linda , Silva Isabel dos Santos , Ursin Giske 
Published: August 2018

Abstract
Background: Studying associations between percent or absolute volumetric breast density (VBD) with age and menopausal status, and whether the associations are modified by demographic, lifestyle, reproductive, or hormonal exposures, can uncover underlying biological mechanisms and improve breast cancer risk prediction.
Material and methods: The cohort consisted of women (aged 49-71 years) who participated in the Norwegian Breast Cancer Screening Program (NBCPS) between 2007 and 2014, had information on VBD and completed questionnaires on standard breast cancer risk factors as part of the program (n=46 428). We estimated least squared means of percent and absolute VBD associated with age at mammography, menopausal status, age at menopause, reproductive and hormonal factors (ages at menarche and first birth, number of pregnancies lasting ≥6 months, duration of breastfeeding, oral contraceptives, and menopausal hormone therapy), self-reported height and body mass index (BMI), education, and lifestyle factors (physical activity, alcohol intake, and smoking).
Results: For a 5-year increase in age, the reduction in percent VBD was -0.18% in pre- and perimenopausal and -0.08% in postmenopausal women, and the reduction in absolute VBD was -0.11 cm³ in pre- and perimenopausal and -0.03 cm³ in postmenopausal women (p for interaction by menopausal status &lt;0.001). In multivariate analyses, the associations between demographic, lifestyle, reproductive and hormonal risk factors and percent and absolute VBD were highly significant, however the magnitude of the effects were modest (1-2%), and the range of percent VBD across levels of risk factors rather narrow. The strongest association was with BMI, which was inversely associated with percent VBD, with a threefold higher percent VBD in women with BMI&lt;20 kg/m² than in women with BMI&gt;33 kg/m² (12.9% versus 3.9%). Interestingly, BMI was positively associated with absolute VBD, with 1.5
times higher VBD in women with BMI≥33 kg/cm² (37.9 cm³ versus 58.4 cm³). Models were adjusted for BMI, education, and parity.
Conclusion: This large cohort analysis found percent and absolute VBD to decrease with increasing age both among pre/perimenopausal and postmenopausal women. The rate of decline was larger among pre/perimenopausal women. Percent and absolute VBD are associated with several established breast cancer risk factors, especially BMI, where the direction of the association differed for percent and absolute VBD.
Citation Format: Kirsti V. Hjerkind, Merete Ellingjord-Dale, Anna L. Johansson, Hildegunn S. Aase, Solveig R. Hoff, Solveig Hofvind, Siri Fagerheim, Linda Vos, Isabel dos Santos Silva, Giske Ursin. Breast cancer risk factors and volumetric breast density in a national breast cancer screening program [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1201.