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The study, which is reported in the July 26, 1997 issue of The Lancet, was conducted by Nathaniel Rothman, M.D., and colleagues at the National Cancer Institute (NCI), Bethesda, Md., in collaboration with scientists from the Johns Hopkins University School of Hygiene and Public Health, Baltimore; the National Center for Environmental Health of the Centers for Disease Control and Prevention (CDC), Atlanta; and Georgetown University, Washington, D.C.*
PCBs were once widely used in the manufacture of electrical transmission equipment, in certain cutting oils and lubricants, and in other commercial applications. Banned in the United States since the late 1970s, PCBs persist in the environment and have been detected in fish, meat, and dairy products, although levels have generally decreased over time. Past studies of workers exposed to PCBs have shown only weak and inconsistent evidence of a possible link with
non-Hodgkin's lymphoma (NHL) or other cancers.
NHL is a cancer of the lymphatic system, part of the body's immune defense system. In 1997 an estimated 53,600 new NHL cases will be diagnosed in the United States, and 23,800 people will die of the disease. Treatment most commonly involves chemotherapy with or without radiation therapy. The five-year survival rate for NHL patients is just over 50 percent. Research on the causes of NHL is needed because incidence rates have been increasing worldwide for the last few decades, and the reasons are largely unknown.
"Although studies of workers exposed to PCBs provide little evidence of an association with cancer, we believe it is important to inform the research community of this new, unexpected finding of an association between PCB levels and risk for non-Hodgkin's lymphoma," Rothman said.
The study population was selected from a group of 25,802 adults who lived in Washington County, Md., in 1974 and who participated in a study of cancer and stroke conducted by Johns Hopkins University researchers. As part of that study, the participants provided blood samples, which were stored for future studies. PCB exposure levels in the general population of Washington County were found to be similar to average U.S. levels in the 1970s.
Of the original study group, 74 persons were diagnosed with NHL between Jan. 1, 1975, and May 31, 1994. Blood levels of PCBs and eight other organochlorine chemicals from the NHL patients (cases) were compared with the levels in a control group of 147 persons selected from the study group who had not developed cancer. Cases and controls were matched by race, age, sex, year of birth, and date of blood collection.
The researchers found that persons who had blood levels of PCBs in the highest
25 percent were about four-and-a-half times more likely to have NHL compared with those who had PCB levels in the lowest 25 percent.
The primary aim of the study was to investigate whether increased risk for NHL was associated with exposure to DDT, an organochlorine pesticide that has been linked to risk for the cancer in some studies. The researchers did not find that exposure to DDT, as measured by blood levels of the compound and its metabolites in the study participants, was associated with increased risk for NHL. In addition, none of the other organochlorines, with the exception of PCBs, were associated with increased risk for the cancer.
NCI scientists are now collaborating with investigators from the CDC and other research institutions to conduct analyses in other populations to see if the findings can be confirmed.
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