Cigarette smoking has been clearly linked to increased risks of a number of cancers, including cancers of the lungs, colon, pancreas and bladder. But studies have yielded conflicting results as to whether smoking may boost a woman's odds of developing breast cancer.

The new study, published in the American Journal of Epidemiology, looked at the relationship between smoking and the risk of ductal carcinoma in-situ, or DCIS -- abnormal cells in the milk ducts of the breast that can progress to cancer that invades the breast tissue.

If smokers have a higher risk of DCIS, the researchers reasoned, that could mean that tobacco exposure acts at the very early stages of breast cancer development.

However, the study found that among more than 63,000 postmenopausal women followed for eight years, smokers and former smokers were no more likely to be diagnosed with DCIS than women who had never smoked.

The findings provide little support for a relationship between smoking and the risk of DCIS after menopause, write the researchers, led by Dr. Geoffrey C. Kabat of the Albert Einstein College of Medicine in New York.

The results come from an analysis of more than 63,000 U.S. women ages 50 to 79 who took part in the government-sponsored Women's Health Initiative clinical trial, begun in the 1990s.

Over eight years, 486 women were newly diagnosed with DCIS. Of women who developed the condition, 4.5 percent were current smokers; among women who remained free of DCIS, 7 percent were current smokers. In both groups, about 42 percent of women were former smokers.

Overall, Kabat's team found no differences in the risk of developing DCIS among former, current and non-smokers, when other factors -- like age, weight, education levels and self-reported exercise habits -- were taken into account.

Nor could the researchers find a relationship between DCIS and the length of time a woman had smoked, how heavily she had smoked or the age at which she had started smoking.

Given the myriad cancer-promoting chemicals in cigarette smoke, the lack of an association between smoking and DCIS -- and, in many studies, invasive breast cancer -- seems counterintuitive, Kabat and his colleagues write.

However, they point out that cigarette smoke also has anti-estrogen effects, and estrogen can fuel the growth of breast tumors. That, the researchers note, makes the relationship between smoking and breast cancer more complex.

Whatever the effects of smoking on estrogen, however, the known risks of smoking -- from various cancers to chronic lung disease to heart disease -- far outweigh any potential benefit.

Since mammography screening came into widespread use in the 1980s, diagnoses of DCIS have been increasing. It's been estimated that by 2020, 1 million U.S. women will be living with the diagnosis. Because of the chance that DCIS can progress to invasive cancer, most women receive treatment -- usually with surgery to remove the abnormal cells, sometimes followed up with radiation.

In general, the risk factors for DCIS are the same as those for invasive breast cancer -- such as older age; having a mother, sister or daughter who had breast cancer; and long-term use of hormone replacement therapy after menopause.

Medicine Breast cancer Cancer