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According to a study of identical twin pairs, the age a woman begins menstruating may influence her risk of developing breast cancer, if her disease is caused by genes. However, researchers know that not all breast cancer cases are caused exclusively by genes; some may be caused by other factors too, such as hormones, diet, etc. In the study, the age that a woman began menstruating was significant, if her cancer was hereditary. On the other hand, the age of first menstruation was not important in women with non-hereditary breast cancer (i.e., breast cancer thought not to be caused by genes). Instead, in these cases, other factors, such as the number of children the women had, or the age at which they reached menopause, were more important in determining their risk of breast cancer. According to researchers, this study helps show that different types of breast cancer may be influenced by different risk factors. Researchers know that hormones can be related to breast cancer. In fact, approximately 80% of breast cancers are dependent on the hormone estrogen in order to grow and survive. Based on this knowledge, treatments such as the drug tamoxifen have been developed to prevent estrogen from reaching breast cancer cells, thereby "starving" and destroying these cells. In the study, Ann S. Hamilton, PhD and Thomas M. Mack, MD, MPH of the University of Southern California gathered information from 1,811 pairs of female twins, one or both of whom had breast cancer. Because identical twins share the same genes, if both twins developed breast cancer, the cancer was said to be hereditary (i.e., caused by genes); if only one twin developed breast cancer, the cancer was said to be non-hereditary (or caused by factors other than genes). The researchers found that among the twins with hereditary breast cancers, the age that they began menstruating was important in determining their risk of the disease. Women with hereditary cancers were more likely to begin menstruating at an early age. However, in the twin pairs in which only one twin developed breast cancer, the age of first menstruation was not an important factor in disease risk. In other words, it did not matter what age women with non-hereditary breast cancer began menstruating. Instead, among these women, other factors, such as age at first childbirth, the number of children the women had, and the age at which they reached menopause were more important in determining their risk of breast cancer. Researchers have long known that hormones play a key role in breast cancer, although the nature of their role is still being investigated in studies such as this one. Menstrual cycles and pregnancy both influence breast cancer risk. It appears as though the more menstrual cycles a woman has over her lifetime, the higher her risk of breast cancer. Thus, women who experience an early onset of menstruation (before age 12) or late menopause (after age 50) may be at a higher-than-average risk of breast cancer. Pregnancy also seems to decrease the risk of breast cancer if a woman becomes pregnant with her first child before age 30. This is because there is an interruption of menstrual cycles during pregnancy. Women who never become pregnant are at a higher risk of breast cancer than those who have a first child before age 30. However, pregnancy after age 30 appears to also increase the risk of breast cancer. The older a woman is, the more likely her breast tissue has already been exposed to some cancer-causing substances called carcinogens. Therefore, exposure to elevated hormone levels during pregnancy at a later age may stimulate the growth of abnormal breast tissue. In fact, some experts believe that the number of menstrual cycles that occur between a woman’s first period and her first pregnancy is a greater predictor of breast cancer risk than her age at her first menstruation or menopause. While it is too soon to conclusively say that hereditary breast cancers are influenced by early menstruation while non-hereditary breast cancer are not, this study adds to a body of research investigating the association between hormones and breast cancer.
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