Lung cancer has been the leading cause of cancer death in women for more than a decade. In 2000, about 68,000 American women died of lung cancer. That's compared to about 40,000 women who died from breast cancer.
While the lung cancer-tobacco connection is well established, researchers have suspected that adenocarcinoma -- which accounts for over 40 percent of lung cancer in women -- was linked to other, unknown risk factors. That's because adenocarcinoma, more so than other forms of lung cancer, strikes women who have never smoked.
The Mayo Clinic study used a statistical model to compare the incidence rates of the three types of lung cancer: adenocarcinoma, squamous cell carcinoma and small cell carcinoma.
"We found adenocarcinoma of the lung is more strongly associated with smoking than previously recognized," says Ping Yang, M.D., Ph.D., a Mayo Clinic epidemiologist and lead researcher on the study.
For example, among 10,000 women who do not smoke, each year three women will develop any lung cancer, two of them being adenocarcinoma. Among the same number of women who smoked a pack of cigarettes a day for 20 to 39 years, 30 will develop lung cancer each year: 14 adenocarcinoma, 8 squamous cell and 8 small cell lung cancer. All three types of lung cancer are very serious illnesses.
"Knowing the very strong association between smoking and adenocarinoma is important because researchers were beginning to look for other causes and ways to prevent it," says Dr. Yang. "The best advice remains: don't smoke cigarettes."
How the data was gathered
In 1986, a questionnaire was mailed to approximately 100,000 randomly selected women in Iowa between the ages of 55 and 69. The 41,836 respondents were surveyed again in 1987, 1989, 1992 and 1997. They responded to questions about their health and smoking history. Lung cancer cases were identified through the State Health Registry of Iowa, part of a National Cancer Institutes research program.
Through 1998, 598 women in the study were identified with lung cancer: 234 with adenocarcinoma, 115 with squamous carcinoma, 123 with small cell carcinoma and 126 with other subtypes.
Researchers used this data in a statistical model to calculate excess risk of lung cancer. Excess risk is the difference in risk between women who smoked and women who never smoked. It can only be calculated in a cohort study -- one that follows large numbers of people for many years -- smokers and those who have never smoked.
"You can't get an accurate picture of lung cancer risk by just studying smokers," she says. "You need to have reliable data from sizable number of those who have never smoked."
Dr. Yang said questions still remain about why adenocarcinoma strikes nonsmokers more often than other types of lung cancers. "The culprit is very likely to be exposure to second-hand smoke. We could not get a direct answer from this study," says Dr. Yang. "Other Mayo Clinic research projects are further examining the connection between second-hand smoke and adenocarcinoma.
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Journal
American Journal of Epidemiology