title: Background risk of breast cancer and the association between physical activity and mammographic density.
abstract: BACKGROUND
  High physical activity has been shown to decrease the risk of breast cancer, potentially by a mechanism that also reduces mammographic density. We tested the hypothesis that the risk of developing breast cancer in the next 10 years according to the Tyrer-Cuzick prediction model influences the association between physical activity and mammographic density.
  
  
  METHODS
  We conducted a population-based cross-sectional study of 38,913 Swedish women aged 40-74 years. Physical activity was assessed using the validated web-questionnaire Active-Q and mammographic density was measured by the fully automated volumetric Volpara method. The 10-year risk of breast cancer was estimated using the Tyrer-Cuzick (TC) prediction model. Linear regression analyses were performed to assess the association between physical activity and volumetric mammographic density and the potential interaction with the TC breast cancer risk.
  
  
  RESULTS
  Overall, high physical activity was associated with lower absolute dense volume. As compared to women with the lowest total activity level (<40 metabolic equivalent hours [MET-h] per day), women with the highest total activity level (≥50 MET-h/day) had an estimated 3.4 cm(3) (95% confidence interval, 2.3-4.7) lower absolute dense volume. The inverse association was seen for any type of physical activity among women with <3.0% TC 10-year risk, but only for total and vigorous activities among women with 3.0-4.9% TC risk, and only for vigorous activity among women with ≥5.0% TC risk. The association between total activity and absolute dense volume was modified by the TC breast cancer risk (P interaction = 0.05). As anticipated, high physical activity was also associated with lower non-dense volume. No consistent association was found between physical activity and percent dense volume.
  
  
  CONCLUSIONS
  Our results suggest that physical activity may decrease breast cancer risk through reducing mammographic density, and that the physical activity needed to reduce mammographic density may depend on background risk of breast cancer.
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, Stockholm, 17177, Sweden. Thang.Trinh@ki.se.
authors.forename: Thang
authors.initials: T
authors.lastname: Trinh
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, Stockholm, 17177, Sweden. Mikael.Eriksson@ki.se.
authors.forename: Mikael
authors.initials: M
authors.lastname: Eriksson
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, Stockholm, 17177, Sweden. Hatef.Darabi@ki.se.
authors.forename: Hatef
authors.initials: H
authors.lastname: Darabi
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, Stockholm, 17177, Sweden. Stephanie.Bonn@ki.se.
authors.forename: Stephanie E
authors.initials: SE
authors.lastname: Bonn
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, Stockholm, 17177, Sweden. Judith.Brand@ki.se.
authors.forename: Judith S
authors.initials: JS
authors.lastname: Brand
authors.affiliation: Centre for Cancer Prevention, Wolfson Institute of Preventive Medicine, Queen Mary University of London, Charterhouse Square, London, EC1M 6BQ, UK. j.cuzick@qmul.ac.uk.
authors.forename: Jack
authors.initials: J
authors.lastname: Cuzick
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, Stockholm, 17177, Sweden. Kamila.Czene@ki.se.
authors.forename: Kamila
authors.initials: K
authors.lastname: Czene
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, Stockholm, 17177, Sweden. Arvid.Sjolander@ki.se.
authors.forename: Arvid
authors.initials: A
authors.lastname: Sjölander
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, Stockholm, 17177, Sweden. Katarina.Balter@ki.se.
authors.forename: Katarina
authors.initials: K
authors.lastname: Bälter
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, Stockholm, 17177, Sweden. Per.Hall@ki.se.
authors.forename: Per
authors.initials: P
authors.lastname: Hall
mesh_terms: D000328:Adult; D000368:Aged; D000071060:Breast Density; D001943:Breast Neoplasms / Q000453:epidemiology / Q000209:etiology* / Q000473:pathology*; D005260:Female; D006801:Humans; D042361:Mammary Glands, Human / Q000002:abnormalities*; D008327:Mammography; D008875:Middle Aged; D009043:Motor Activity; D011159:Population Surveillance; D011446:Prospective Studies; D012306:Risk; D013548:Sweden / Q000453:epidemiology