BOSTON: The bone marrow transplants that often cure leukemia can themselves cause cancer years after treatment, according to a study published in Thursday's New England Journal of Medicine. The reason appears to be that patients are affected by the high doses of radiation used in association with the transplants. Doctors give massive doses of chemotherapy and radiation to kill the patient's diseased bone marrow and then replace it with donor marrow. In a follow-up study of nearly 20,000 people who received bone marrow transplants between 1964 and 1992, scientists from the National Cancer Institute found that the marrow patients had developed 80 new cases of cancerous tumors, while 30 would be the statistical norm for the group. The risk rose as time passed and was eight times higher than expected 10 years after the transplants. The researchers also found the risk was greatest in children who were younger than 10 when they had the procedure. Despite such results, the study's authors believe marrow transplants are still one of the best treatments available to patients with leukemia and other cancers of the blood because it can prolong their lives significantly. As the scientists concluded, the benefits for most patients seem to far outweigh the risk of complications for a few later on.