Breast Cancer: The Sister Risk

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It turns out that sisters may share more than nail polish, advice, and parents; the newest research tells us that women who have a sister diagnosed with breast cancer are at an increased risk of developing the disease. While the threat remains throughout their lives, the increased risk is most apparent in younger women, no matter the age at which the first sister was diagnosed. Though research has determined that women who have a first degree relative stricken by breast cancer have an increased risk for this disease, it remains unclear how the risk varies in relation to a woman’s current age and the age at which her relative was diagnosed.

Marie Reilly, Ph.D., along with colleagues at the Karolinska Institute in Stockholm, Sweden looked at a national database of families that had been linked up with the national cancer registry. Researchers analyzed data on breast cancer patients from the years 1958 to 2001.The incidence of breast cancer was compared between 23,654 sisters of breast cancer patients to 1,732,775 patients who did not have a sister with the disease.

Researchers found the familial risk for a diagnosis of breast cancer in young women aged 20 to 39 had an astonishing and significant 6.6 increase in the development of breast cancer as compared to women of the same age bracket who did not have a sister with the disease. There was a decline in cancer diagnoses for women aged 50 and older, who had an approximate two fold familial risk of developing the disease. The risk of breast cancer diagnosis was similar for sisters of a breast cancer patient regardless of whether they were nearing or had passed the age at which a sister had been diagnosed.

Breast cancer is the most common malignancy in women and the leading cause of death among the female population aged 35-54. While the cause of breast cancer is still unknown, researchers believe that combinations of genetic and environmental factors are at play in the development of the disease. Women diagnosed with early stage, non-invasive breast cancers that carry the same mutations in two inherited breast or ovarian cancer genes as women (such as first degree relatives) diagnosed with more invasive breast cancer, may benefit from high risk treatment.

The Burden of a Lifetime

The authors of the report comment that, “Sisters of women diagnosed with breast cancer still have an increased risk of breast cancer 20 years after diagnosis of the sister, suggesting that women live with the burden of familial breast cancer for their lifetime.”

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