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Breast Cancer Onset in Susceptible Groups Differs

Risk for unaffected twin and of bilateral disease not influenced by age at diagnosis

MONDAY, July 14 (HealthDay News) -- The onset of breast cancer in the unaffected twin of a sister with breast cancer and the onset of bilateral breast cancer based on family history differs from that normally seen in unilateral disease and is largely unaffected by age and time since diagnosis, according to a report released online June 30 in advance of publication in the Journal of Clinical Oncology.

Mikael Hartman, M.D., from the Karolinska Institute in Stockholm, Sweden, and colleagues estimated the risk of breast cancer in the unaffected twin of 2,499 Scandinavian female twin pairs (855 monozygotic, 1,644 dizygotic) where the other sister had breast cancer. They also estimated the risk of bilateral breast cancer in 93,448 Swedish women with breast cancer, of whom 4,429 developed bilateral cancer without a family history and 443 developed bilateral cancer with a family history (affected first-degree relative).

The researchers report that the incidence of breast cancer in the twin was 0.64 percent per year for monozygotic twins and 0.42 percent per year for dizygotic twins, compared with 0.09 percent per year for unilateral breast cancer. For bilateral breast cancer, the incidence was 1.03 percent per year for familial cases and 0.68 percent per year for non-familial cases. Neither risk was affected by age or time since first event, the investigators found. Compared with the risk of non-familial bilateral cancer, the risk of familial bilateral cancer was significantly higher (incidence rate ratio 1.52) and the risk of cancer in the dizygotic twin sister was significantly lower (incidence rate ratio 0.75).

"We observed that the onset of breast cancer in susceptible subpopulations differs from that normally seen in unilateral disease," Hartman and colleagues conclude. "Interestingly, age at diagnosis did not influence risk of breast cancer in bilateral breast or unaffected twin sisters."

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