title: Incidence of metachronous contralateral breast cancer in Denmark 1978-2009.
abstract: BACKGROUND
  Incidence of contralateral breast cancer (CBC) is much less studied than primary breast cancer. We aimed to assess incidence rates of CBC in relation to age, calendar period and time since first breast cancer.
  
  
  METHODS
  Using the nationwide Danish Cancer Registry, we identified 85 863 women with a first primary invasive breast cancer without distant metastases in Denmark during 1978-2009. Among these, 3120 women developed metachronous CBC. Crude incidence rates for CBC were calculated by age and calendar period at first breast cancer as well as time since first breast cancer. Mutual adjustments were made by use of Poisson regression models.
  
  
  RESULTS
  The incidence of CBC decreased with increasing age at first breast cancer. Before 1998, incidence rates of CBC showed little variation. The rates decreased by period of first primary from 546 per 10(5) person-years in 1993-97 to 328 per 10(5) person-years in 2003-09. After adjustment for age and calendar period, no clear trend was observed in the overall incidence according to time since first breast cancer.
  
  
  CONCLUSIONS
  Occurrence of cancer in the contralateral breast seems to be rather independent of time passed since the first primary. The finding of a decreasing incidence of CBC after 1997 is likely to be due to more women receiving systemic adjuvant therapy such as tamoxifen and longer duration of this treatment as well as the introduction of aromatase inhibitors.
authors.affiliation: Virus, Lifestyle and Genes, Danish Cancer Society Research Center, Copenhagen, Denmark, Department of Gynecology, Department of Oncology and Danish Breast Cancer Cooperative Group, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark.
authors.forename: Christina Bording
authors.initials: CB
authors.lastname: Rasmussen
authors.affiliation: Virus, Lifestyle and Genes, Danish Cancer Society Research Center, Copenhagen, Denmark, Department of Gynecology, Department of Oncology and Danish Breast Cancer Cooperative Group, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark Virus, Lifestyle and Genes, Danish Cancer Society Research Center, Copenhagen, Denmark, Department of Gynecology, Department of Oncology and Danish Breast Cancer Cooperative Group, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark.
authors.forename: Susanne K
authors.initials: SK
authors.lastname: Kjær
authors.affiliation: Virus, Lifestyle and Genes, Danish Cancer Society Research Center, Copenhagen, Denmark, Department of Gynecology, Department of Oncology and Danish Breast Cancer Cooperative Group, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark.
authors.forename: Bent
authors.initials: B
authors.lastname: Ejlertsen
authors.affiliation: Virus, Lifestyle and Genes, Danish Cancer Society Research Center, Copenhagen, Denmark, Department of Gynecology, Department of Oncology and Danish Breast Cancer Cooperative Group, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark.
authors.forename: Michael
authors.initials: M
authors.lastname: Andersson
authors.affiliation: Virus, Lifestyle and Genes, Danish Cancer Society Research Center, Copenhagen, Denmark, Department of Gynecology, Department of Oncology and Danish Breast Cancer Cooperative Group, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark.
authors.forename: Maj-Britt
authors.initials: MB
authors.lastname: Jensen
authors.affiliation: Virus, Lifestyle and Genes, Danish Cancer Society Research Center, Copenhagen, Denmark, Department of Gynecology, Department of Oncology and Danish Breast Cancer Cooperative Group, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark.
authors.forename: Jane
authors.initials: J
authors.lastname: Christensen
authors.affiliation: Virus, Lifestyle and Genes, Danish Cancer Society Research Center, Copenhagen, Denmark, Department of Gynecology, Department of Oncology and Danish Breast Cancer Cooperative Group, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark.
authors.forename: Rikke
authors.initials: R
authors.lastname: Langballe
authors.affiliation: Virus, Lifestyle and Genes, Danish Cancer Society Research Center, Copenhagen, Denmark, Department of Gynecology, Department of Oncology and Danish Breast Cancer Cooperative Group, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark lene@cancer.dk.
authors.forename: Lene
authors.initials: L
authors.lastname: Mellemkjær
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D018931:Antineoplastic Agents, Hormonal / Q000627:therapeutic use; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000628:therapy; D002277:Carcinoma / Q000453:epidemiology* / Q000628:therapy; D017024:Chemotherapy, Adjuvant; D003718:Denmark / Q000453:epidemiology; D005260:Female; D006801:Humans; D015994:Incidence; D008408:Mastectomy; D008875:Middle Aged; D016609:Neoplasms, Second Primary / Q000453:epidemiology*; D012044:Regression Analysis; D013997:Time Factors
chemical_list: D018931:Antineoplastic Agents, Hormonal
keywords: Breast neoplasms; incidence; regression analysis; second primary neoplasms