The finding may help explain why only about 15 per cent of smokers eventually develop lung cancer, and it suggests hormones might play an important role in the leading cancer killer of women in Canada.
Hysterectomy -- removal of the uterus -- is the second-most frequently performed surgical procedure for Canadian women, after cesarean sections. More than 36,000 women had a hysterectomy in 2007-08, according to the Canadian Institute for Health Information, and about 30 per cent had both ovaries removed as well.
About 90 per cent of hysterectomies are done for benign or non-cancerous problems such as irregular menstruation and fibroids.
"We found that women who experienced non-natural menopause are at almost twice the risk of developing lung cancer compared to women who experienced natural menopause," Anita Koushik, a researcher at the Université de Montréal's Department of Social and Preventive Medicine, said in announcing her team's findings Tuesday.
"This increased risk of lung cancer was particularly observed among women who had non-natural menopause by having had both their ovaries surgically removed."
It's the second study this year to associate bilateral oophorectomy -- removal of both ovaries -- with higher odds of developing lung cancer.
A study published in April involving more than 29,000 women participating in the U.S. Nurses' Health Study found that oophorectomy increased the risk of lung cancer, as well as fatal and non-fatal coronary heart disease -- compared to women whose ovaries were kept intact.
Researchers are at a loss to explain the findings.
"Many things, if not most things in medicine are found by serendipity," said Dr. William Parker, lead author of the American study and a faculty member at the John Wayne Cancer Institute at St. John's Health Center in Santa Monica, Calif.
"The smart people go, OK, we don't understand it, but let's try to figure it out. It may be true, it may not be true," Parker said. "But now that you've got two papers within a six-month period saying the same thing, you have to pay attention to it."
Doctors have long recommended removal of the ovaries at the time of hysterectomy to reduce the risk of ovarian cancer.
"People thought, you go through menopause, your ovaries stop working, you don't need them and you could get ovarian cancer," Parker said. But ovaries make hormones after menopause that are converted in the body to estrogens. "There's mounting evidence those hormones protect your heart to some degree," and possibly the lungs, he said.
"Ovarian cancer is a terrible disease," he said. "But heart disease and lung cancer are the major killers of women."
His team's study found that removing both ovaries decreased the risk of ovarian and breast cancer, but at no age was it associated with increased survival.
For women not at high risk for ovarian cancer, "There is no question the ovaries should stay in," Parker said.
The Canadian study, published this week in the International Journal of Cancer, involved 422 women with lung cancer and 577 without at 18 Montreal-area hospitals that together diagnose 98 per cent of all lung cancers that occur in the greater Montreal area.
The team looked at income, age, gender, occupational risk factors, medical and smoking history, menstruation and pregnancy histories.
Women were considered menopausal if their periods had stopped naturally, surgically (via hysterectomy with removal of both ovaries) or because of chemotherapy or radiation.
Having a "non-natural" menopause, usually as a result of having both ovaries removed, was associated with a 92 per cent increased risk of lung cancer, "which suggests almost a doubling of risk," Koushik said in an interview.
-- Canwest News Service