Adapted from the NCI Cancer Bulletin.
In one of the largest prospective studies of its kind, researchers from the University of Southern California (USC) Keck School of Medicine report significant racial and ethnic differences in smoking-related lung cancer risk.
In addition to the large size of the cohort, this new study, published in the January 26, 2006, New England Journal of Medicine (see the journal abstract), included far broader ethnic and racial representation than previous studies. It found that both African Americans and Native Hawaiians had significantly greater risks of lung cancer related to smoking compared with whites, Hispanics, and Japanese Americans.
National statistics have long documented significant differences in lung cancer incidence rates across populations, explains the study's principal investigator, Dr. Christopher Haiman, of the Keck Department of Preventive Medicine. And, he adds, it's well established that smoking behaviors also vary widely among these groups.
"In our study, among smokers we found quite striking racial and ethnic differences in the effect of smoking on lung cancer risk," he says, "and that these racial and ethnic differences were found to be modified by the number of cigarettes smoked per day."
To conduct the study, Dr. Haiman and colleagues from USC and the Cancer Research Center of Hawaii analyzed data on nearly 184,000 African American, Japanese American, Hispanic, Native Hawaiian, and white men and women in the Multiethnic Cohort Study (MCS). The National Cancer Institute-funded MCS was established in Hawaii and Los Angeles beginning in 1993 to explore the relationship of diet and other lifestyle factors to cancer.
The analysis included 1,979 new cases of lung cancer diagnosed over an eight-year period. Among those who smoked 10 cigarettes or less a day, whites had a 55 percent lower risk of lung cancer than African Americans, and among those who smoked 11 to 20 cigarettes a day, a 43 percent lower risk. For Hispanics and Japanese Americans, the percentages were lower still. However, once smoking rates reached 30 cigarettes a day - the equivalent of a pack and a half - or more, the risk difference was minimal.
According to Dr. Mukesh Verma, acting chief of the National Cancer Institute's (NCI) Analytic Epidemiology Research Branch (AERB), Epidemiology and Genetics Research Program, this latter finding could be because biologic or genetic mechanisms that modulate risk may have modest effects compared with the significant carcinogenic effect of heavy smoking.
The analysis ruled out differences in diet, occupation, and level of education as underlying factors to explain the risk disparities. Differences in education levels were related to risk but, the researchers argued, "Education is very likely a surrogate variable for other important exposures, but what these are and whether they are distributed disproportionately in the observed high-risk groups of African Americans and Native Hawaiians are not clear."
Research into the environmental factors that influence lung cancer risk across populations clearly is still necessary, says Dr. Verma. But, he adds, ongoing research as well as whole genome scans, such as those NCI has planned for breast and prostate cancer through the Cancer Genetic Markers of Susceptibility study, may eventually pinpoint smoking-related lung cancer susceptibility genes that may help explain these ethnic differences.
The study's findings don't change the public health message on smoking, stresses Dr. Virginia Hartmuller, the AERB program director for the grant that funds MCS. "The bottom line is health care professionals should still tell their patients that they are far more likely to get lung cancer if they smoke, and they can reduce their risks by quitting."