Use of CT scanners, like this one at UC San Francisco, on children can increase… (Justin Sullivan / Getty…)
As few as two CT scans of the head in childhood can triple the risk of developing brain tumors, while five to 10 such scans can triple the risk of leukemia, British researchers reported Wednesday. The absolute risk of developing the cancers remains small, but the study illuminates the dangers of unnecessary use of X-ray imaging in diagnosis.
The development of CT (computed tomography) scanning was one of the major developments of 20th century medicine because it allowed physicians to look inside the body more accurately than a conventional X-ray. But it also provides a much higher dose of radiation than a conventional X-ray. Based on extrapolations of data from survivors of the atomic bomb attacks in Japan at the end of World War II, some scientists have warned that such scans could increase the risk of cancer, especially in children, whose growing bodies are more sensitive to the effects of radiation. Others, however, have dismissed this as speculation because of the differences between bomb radiation and CT scans. In particular, a bomb affects the whole body, while a CT scan is focused on a small area. But there have been no previous epidemiological studies examining the risks.
A team headed by epidemiologists Mark S. Pearce and Dr. Alan W. Craft of Newcastle University studied nearly 180,000 children who underwent at least one CT scan between 1985 and 2002 in the radiology departments of 70% of the United Kingdom's hospitals. They estimated the dose of radiation absorbed in each scan, then linked the data to cancer incidence and mortality reports in the UK National Health Service Registry and calculated excess incidence of brain tumors and leukemia.
The team reported Wednesday in the British journal Lancet that 74 of 178,604 patients were diagnosed with leukemia and 135 of 176,587 were diagnosed with brain cancer. They concluded that two to three CT scans of the head among children under the age of 15 (a cumulative dose of about 60 milligray of radiation) tripled the risk of brain tumors, while for children under the age of 20, five to 10 scans (a cumulative dose to the body of about 60 milligray) tripled the risk of leukemia. For every 10,000 people younger than 20 who receive 10 milligray from a CT scan, the authors said, physicians can expect one excess case of leukemia; one excess brain tumor would be expected among every 30,000 people who received the same dose. One CT head scan before age 10 would translate into one excess case of leukemia and one excess case of brain cancer per 10,000 patients in the decade after the first exposure.
In an editorial in the same journal, Dr. Andrew J. Einstein of Columbia University Medical Center in New York wrote: "This study should reduce the debates about whether the risks from CT are real." He noted that at least 12 other groups are performing similar studies among different cohorts of patients, as well as looking at the risks of other forms of cancer. Results from those studies should begin becoming available in the next two years. In the meantime, he noted, radiologists should take extra care to minimize the doses of radiation used in CT scans and use other imaging tests whenever possible.