Early smoking boosts women's breast cancer risk

NEW YORK (Reuters Health) - The breast cancer-promoting effects of cigarette smoking may be strongest in young women who have not yet had children, an international research team reports.

"Our results are consistent with the biologic data indicating that the female breast is sensitive to tobacco carcinogens before first childbirth," and may be especially sensitive before menstruation begins, Dr. Mina Ha of Dankook University College of Medicine in Cheonan, Korea and her colleagues conclude.

The risk may be lower after having a child, they add, because by then cells in the breast have finished developing and are thus less vulnerable to carcinogens.

While breast cancer is not considered to be smoking-related, recent research has linked smoking in youth to a greater risk of the disease, Ha's team reports.

To better understand the relationship between smoking at different ages and breast cancer risk, the researchers looked at 56,042 women participating in a long-term study of radiologic technologists. All were free of breast cancer when they were surveyed - between 1983 to 1993, but 906 developed the disease before the second survey, in 1994-1998.

Among women who had children, the amount they smoked before giving birth to their first child was associated with their risk of developing breast cancer, with the risk rising with the number of pack-years (the number of packs per day smoked multiplied by the number of years smoked). For example, women who had smoked for 10 pack-years before having their first child were 78 percent more likely to develop breast cancer than those who never smoked.

However, there was no association between pack years smoked after a first child was born and risk of the disease, which could explain why studies haven't found a relationship between smoking over a woman's lifetime and her breast cancer risk, the researchers say.

The lack of a relationship between later smoking and breast cancer may not mean that smoking has no effect, they add, but could represent a "competition" between smoking's carcinogenic effects and its ability to lower estrogen levels, as high estrogen levels can promote breast cancer.

SOURCE: American Journal of Epidemiology, July 1, 2007.