Review

 Many studies have looked at the various risk factors that increase an individual woman’s risk of breast cancer. The recognised risk factors include early menarche (age at first period) and late menopause, which reflect a woman’s cumulative lifetime exposure to estrogen. In addition, age greater than 30 years for first pregnancy, low parity (few pregnancies) and a family history of breast cancer in a first-degree relative (mother or sister) also increase a woman’s risk of breast cancer.

This particular study looked at identical and non-identical twins in whom, one or both had developed breast cancer. Cases of breast cancer occurring in both identical twins were felt to represent forms of breast cancer with a strong genetic predisposition. Although it is known that certain women carrying some genes such as BRCA1 and BRCA2 have a high risk of breast cancer, the reality is that these genetic conditions are rare in themselves and probably only account for 2 - 3% of all breast cancers. It is also known that 15 – 20% of breast cancers have a genetic basis, but the genes involved have yet to be discovered.

The finding of note in this study was that in identical twins where both had developed breast cancer, the earlier onset of menarche in one twin was more likely to lead to that twin being diagnosed with breast cancer first. In identical twins where only one of them had developed breast cancer, earlier age at menarche did not increase the risk of breast cancer in the affected twin. In cases where only one twin was affected by breast cancer the risk factors of later first pregnancy, lower parity and later menopause were associated with increased risk of breast cancer in the affected twin. However, these risk factors did not predict an earlier diagnosis when both twins were affected.

This study raises the possibility that in breast cancers where there is a genetic predisposition as found in identical twins who both have developed breast cancer, that early onset puberty rather than the usual hormonal milestones of later life influences the age at diagnosis of breast cancer. It also appears that risk factors of lower parity, late onset menopause and older age at first pregnancy play more of a role in increasing the risk of breast cancer in those women without a genetic predisposition to breast cancer. This study suggests that breast cancers with a genetic basis may be more sensitive to the affects of hormones in puberty, rather than the reproductive milestones of adulthood.

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Content updated November 12, 2003