Journal of the National Cancer Institute Advance Access published online on May 13, 2008
JNCI Journal of the National Cancer Institute, doi:10.1093/jnci/djn146
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© The Author 2008. Published by Oxford University Press.
ARTICLES |
Timing of Familial Breast Cancer in Sisters
Affiliation of authors: Department of Medical Epidemiology and Biostatistics, Karolinska Institute, Box 281, Stockholm, Sweden
Correspondence to: Marie Reilly, PhD, Department of Medical Epidemiology and Biostatistics, Karolinska Institute, Box 281, 171 77 Stockholm, Sweden (e-mail: marie.reilly{at}ki.se).
Background: Women who have had a first-degree relative diagnosed with breast cancer (ie, a positive family history) have a rate of breast cancer that is approximately twice that of all women their age, but it is unclear how they should perceive this risk at different ages or if they should be considered at higher risk for the remainder of their lifetime.
Methods: We used Swedish population–based data to assess the risk of breast cancer in 23654 sisters of women diagnosed with breast cancer and in 1 732 775 sisters of unaffected women from 1958 through 2001. Poisson models were used to express the rate of breast cancer as a function of current age, whether a woman had an affected sister, time since the first diagnosis in the family, and family size (number of sisters). The effect of the age of the index case (the first sister diagnosed in the family) at diagnosis and whether her "at-risk" sisters had achieved this age were examined in stratified analyses. Incidence rate ratios of breast cancer in exposed compared with unexposed sisters were calculated with 95% confidence intervals. All estimates were adjusted for calendar time.
Results: Sisters of breast cancer patients had higher breast cancer incidence than unexposed sisters at all ages. The association of exposure (ie, a diagnosis of breast cancer in a sister) with the risk of breast cancer was most pronounced in young women (age 20–39; incidence rate ratio = 6.64, 95% confidence interval = 4.66 to 9.48), and the relative risk decreased to approximately 2 in women older than 50 years. The risk associated with having a sister diagnosed with breast cancer was not modified substantially by the age of the index case at diagnosis (
45 years vs >45 years). The risk was similar for women who were approaching the age at which the first sister was diagnosed in their family and those who had already attained it. The incidence rate ratio of breast cancer in exposed sisters compared with unexposed sisters was constant over time for all age categories of at-risk women.
Conclusions: Women who have a sister diagnosed with breast cancer have an increased risk of breast cancer throughout much of their lifetimes.
| CONTEXT AND CAVEATS Prior knowledge Having a sister diagnosed with breast cancer increases a woman’s risk of breast cancer, but how this increase in risk depends on the time elapsed since the sister's diagnosis, the age of the at-risk sister, and other factors was unclear. Study design Using data on birth dates and parentage that were linked to a national cancer register, Poisson models were constructed to determine the risk of breast cancer associated with having a sister diagnosed with the disease and how it varied according to the age of the at-risk woman, the time elapsed since the diagnosis of her sister, and other parameters. Contribution The increased risk of breast cancer associated with having a sister diagnosed with the disease was most pronounced in younger women, and for all women the increased risk was constant over the time elapsed since the sister's diagnosis. Implications Women with a sister diagnosed with breast cancer remain at increased risk for much of their lifetime. Limitations The study could not assess the risk associated with having a sister diagnosed with breast cancer in women older than 70 years.
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Manuscript received November 9, 2007; revised March 17, 2008; accepted April 7, 2008.
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J Natl Cancer Inst 2008 100: 687.