Cigarette smoking appears to be a major risk factor for the development of leukemia and may be responsible for more than 1 in 5 cases of the feared blood cancer, according to a study by Loma Linda University researchers.
The researchers consider the findings particularly significant because almost all of the 34,000 Seventh-day Adventists studied who had smoked had stopped years earlier, suggesting that some of the adverse effects may not be reversible. The report increases the credence of other recent studies that have suggested a link between smoking and leukemia.
The results, being published this month in the Journal of the National Cancer Institute, add to a growing list of cancers that have been linked to cigarette smoking. The list includes cancers of the lung, bladder, pancreas, esophagus, throat and perhaps the cervix.
The study is part of the ongoing Adventist Health Study, a major epidemiological investigation of the causes of various cancers among California residents.
Although the vast majority of Adventists do not smoke by church proscription, many are adult converts to the religion who smoked cigarettes prior to their baptism into the church.
The researchers found that the Adventist ex-smokers had twice the risk of developing leukemia than similar individuals who never had smoked.
The results are “very discouraging,” said Paul K. Mills, an assistant professor of preventive medicine at Loma Linda and principal author of the study. “We saw a strong association between prior cigarette smoking and the risk of developing leukemia.”
The findings “obviously strengthen the gathering impression that cigarette smoking is related to leukemia risk and that the association is likely to be causal,” Dr. Clark W. Heath Jr., a vice president of the American Cancer Society, wrote in an editorial published in the medical journal along with the study.
The risk of developing leukemia was also increased by a greater number of cigarettes smoked daily and a longer duration of smoking, which Mills said was “important evidence of a causal relationship.”
Individuals who regularly smoked more than 25 cigarettes a day had a threefold increased risk of developing leukemia. The Adventists who had smoked for more than 15 years had a 2 1/2-fold increased leukemia risk.
Leukemia is a bone-marrow cancer in which the marrow is replaced by immature, abnormal white blood cells. Leukemia is a relatively uncommon tumor compared to breast, colon or lung cancer. Each year, there are about 27,000 newly diagnosed cases and 18,000 deaths attributed to leukemia in the United States.
According to Heath, until 1986 it was generally accepted that tobacco use was not a significant cause of leukemia. Since 1988, several studies, including a large study of U.S. military veterans and two American Cancer Society studies, had provided preliminary evidence of a link.
The causes of leukemia are largely unknown. Those that have been identified, such as the chemical benzene and ionizing radiation, appear to account for only a small percentage of cases, according to Heath.
The Loma Linda study calculated that about 20% of leukemia cases were associated with prior smoking. In all, there were 46 new cases of leukemia diagnosed during the six-year study period.
On the basis of this and earlier preliminary studies, Heath estimated that from 20% to 30% of leukemia cases may be linked to smoking.
Because cigarette smoking is very common in the general population--more than 25% of adults smoke--the proportion of total leukemia risk that may be attributed to cigarette smoking is significant, according to the researchers.
In a related finding, the Loma Linda researchers found that ex-smokers among the Adventists had a threefold increased risk of developing myeloma, another bone-marrow tumor, as compared to individuals who had never smoked. Mills cautioned that this result should be considered “preliminary” because it was based on 23 cases. Previously, this association has not been closely examined.
The Adventist Health Study began in 1974. Participants in the current study completed a questionnaire about their lifestyle in 1976 and were followed for the development of new cancers through the end of 1982.