That's the conclusion of new research published online Sunday night in the British Medical Journal.
The researchers stressed that left-handed women should not be unduly worried based on these findings, nor should they be screened more closely for the disease.
"Our aim was to try to shed some more light on a possible cause for breast cancer, and we were able to show that left-handedness may play a role (among many other causes for breast cancer)," said Dr. Cuno Uiterwaal, senior author of the study and assistant professor of clinical epidemiology at the Julius Center for Health Sciences and Primary Care, University Medical Center in Utrecht, the Netherlands.
The Dutch researcher added that the finding "may be interesting from an etiological point of view, because we do not know all causes for breast cancer, and it may lead to more clues. However, it is far too early (if at all) to think about practical applications such as screening."
Other experts felt strong evidence of a real link was simply not there.
"In our patient population, the relationship of breast cancer to left- and right-handedness is really uniform," said Dr. Susan Drossman, a radiologist with a specialty in interventional breast imaging at Mount Sinai Medical Center in New York City. "I don't see any relationship in everyday practice. There is probably a huge environmental component we haven't defined."
According to new data from the American Cancer Society, death rates from breast cancer have dropped by 2.3 percent a year since 1990, with the decline most pronounced among younger women.
Except for skin cancer, breast cancer is the most frequently diagnosed cancer among U.S. women, with 211,240 new cases of invasive breast cancer expected to be found in 2005. More than 40,000 women will die from the disease this year, the society predicts.
The overall five-year relative survival rate for women diagnosed with breast cancer is now 88 percent, and the 10-year relative survival rate is 80 percent, according to the report.
While some case-control studies have shown a relationship between left-handedness and increased breast cancer risk, there has so far not been any prospective data.
For this report, the first prospective study on the subject, the researchers looked at the association between handedness and breast cancer incidence in over 12,000 initially healthy women born between 1932 and 1941.
At the beginning of the study, participants answered questions on their reproductive history, socioeconomic status, lifestyle, smoking habits and "innate hand preference." Body measurements were also taken.
Because following 12,000 women would have been prohibitively expensive, the researchers focused on a random sample of about 1,500 women.
This smaller sample of women was followed via the Dutch regional cancer registry, which provides data on all new cases of invasive breast cancer, until January 1, 2000.
Overall, left-handed women were at a 39 percent higher risk for developing breast cancer, the researchers found.
The risk for premenopausal cancer among left-handed women was more than twice as high, they added. There appeared to be no excess risk for postmenopausal breast malignancies, however.
In addition, left-handed women with a body mass index of 25 or lower were also at an increased risk for breast cancer, but not women with a body mass index over 25 (for reference, overweight begins at a BMI of 25 or above).
Any excess risk linked to left-handedness was also only seen in women who had borne children, the investigators found.
Being left-handed probably does not cause any increased risk in itself, the researchers noted. Rather, they speculated that the association may be due to fetal exposure to high levels of sex hormones in the uterus. This can induce left-handedness as well as changes in breast tissue. There is data showing a higher prevalence of left-handedness among women who were exposed to certain sex hormones before birth.
However, others remained unconvinced.
"To make the assumption that it's intrauterine exposure to hormones -- I think that's a real extrapolation," Drossman said. "They haven't proven anything."
In any case, the study authors believe their data does warrant further study. "It would make sense to have some further confirmations of this observation in other studies," Uiterwaal said. "Obviously, it would be interesting to know about other cancers as well."
More information
To learn more about breast cancer and its prevention, visit the American Cancer Society.
