NEW YORK (Reuters Health) - Smokers, especially those over the age of 60, are more than twice as likely as nonsmokers to develop a specific type of acute myeloid leukemia (AML), study findings suggest.

While smoking has long been thought to be a possible risk factor for AML--the most common type of leukemia diagnosed in adults--the research results have been contradictory. The results of a new study suggest that smoking may contribute to some types of the leukemia, but not others, helping to explain the murky results.

Currently, researchers have defined eight different subtypes of AML (M0-M7), according to Dr. Janice M. Pogoda of the University of Southern California in Los Angeles, and colleagues.

It seems that smoking plays a role in M2, but not in other subtypes, Pogoda told Reuters Health. Indeed, smoking may be responsible for roughly 42% of cases of this subtype of leukemia, the researchers report in the March issue of the American Journal of Epidemiology.

"Smoking is associated with increased risk of AML," she said. "Many previous studies have not found an association, but that's because AML--and sometimes even leukemia in general--is often analyzed as a single disease when in fact it consists of several distinct subtypes."

Pagoda explained, "If subtypes are not analyzed individually, subtype-specific risk factors won't be discovered."

In the current study, the investigators looked at 412 adults living in Los Angeles who were diagnosed with AML between 1987 and 1994. The patients were interviewed about their smoking habits and were compared with 412 healthy people.

While smokers in general were 2.3 times more likely to develop M2 type AML, older adults aged 60 to 75 years were at 3.3 times greater risk of developing the illness.

Those who smoked for more than 35 years and smoked unfiltered cigarettes showed the highest risk, but even older adults who smoked filtered cigarettes still had a nearly threefold higher risk compared with nonsmokers.

"We confirmed findings from another group's work.... Amazingly, we observed almost the identical odds ratios for M2 that the other study did," Pogoda told Reuters Health.

"In epidemiology, when multiple studies show the same results, its pretty compelling evidence," she added.

Although it is difficult to say exactly how many people develop M2 type ALM since there are very little incidence-by-subtype data available for AML, Pagoda noted that in her team's study about one third of all AML cases were the M2 type.

"And, in LA County, overall AML incidence is about 39 cases per million for males and 26 per million for females," she told Reuters Health.

In AML, immature blood cells known as blasts do not mature and become too numerous.

These immature blast cells are found in the blood, bone marrow, liver, spleen and lymph nodes. People with AML tend to have severe anemia (iron-poor blood) and are prone to infection, inflammation and blood clotting disorders.

SOURCE: American Journal of Epidemiology 2002;155:546-553.

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