Children receiving two to three CT scans of the head have three times the risk of developing brain cancer later in life, while five to 10 scans triples the risk of developing leukemia, according to a study published online June 7 in The Lancet. It represents the first study to quantify cancer risks stemming from CT radiation in childhood, according to the authors.
While the increased risk demonstrated by the study is sure to turn heads, particularly in a climate where radiation dose concerns are paramount, the researchers reiterated that the absolute risk of developing cancers after CT is small. While steps should be taken to reduce dose, these risks should be kept in context.
"The immediate benefits of CT outweigh the potential long-term risks in many settings and because of CT's diagnostic accuracy and speed of scanning, notably removing the need for anesthesia and sedation in young patients, it will, and should, remain in widespread practice for the foreseeable future," Mark Pearce, PhD, of Newcastle University in Newcastle Upon Tyne, England, said in a release.
Close to 180,000 children and young adults who underwent a CT scan between 1985 and 2002 were included in the study. Pearce and colleagues extracted the number and types of CT scan from U.K. hospital records, estimated the dose absorbed by patients’ brains and bone marrow, and then linked the data to cancer incidence and mortality reports in the U.K. National Health Service Registry.
By the end of follow-up on Dec. 31, 2008, a total of 135 out of 176,587 patients were diagnosed with brain cancer and 74 out of 178,604 patients were diagnosed with leukemia. Because the brain and bone marrow have different radiation absorption rates, there are differing numbers of scans connected to the increased risk of disease. Two to three CT scans of the head results in a cumulative dose of around 60 mGy to the brain and nearly triples the risk of primary brain tumors. Risks of leukemia triple at between five and 10 CT scans, when the cumulative dose to red bone marrow hits 50 mGy.
The authors calculated the relative risk of brain tumors increased 0.023 per extra mGy received, and for leukemia this increased risk was 0.036 per extra mGy received. Put another way, the authors said there would be one excess case of leukemia for every 10,000 people under 20 years old receiving 10 mGy from a CT scan, and there would be one excess case of brain cancer for every 30,000 such patients.
Pearce and colleagues noted the lifetime risk of developing cancer in the general population is about one in three, and the excess cancer risks from CT are likely small compared with the benefits of the scan, assuming it is clinically justified.
“With the increasing use of CT worldwide, particularly within this young population, knowledge of the risks based on empirical data will be crucial to assess safety in relation to the benefits that CT provides,” wrote the authors. “Frequent calls have been made to decrease doses, following the as low as reasonably achievable (ALARA) principle, and only scan when justified as in the current Image Gently campaign.”
In an attached comment, Andrew J. Einstein, MD, PhD, of Columbia University in New York City, said the work of Pearce et al confirms that CT scans produce a small cancer risk, and as utilization continues to rise, efforts to control dose must continue.
"This study should reduce the debates about whether risks from CT are real, but the specialty has anyway changed strikingly in the past decade, even while the risk debate continued,” wrote Einstein. “New CT scanners all now have dose-reductions options, and there is far more awareness among practitioners about the need to justify and optimize CT doses—an awareness that will surely be bolstered by Pearce and colleagues' study."
Despite the great strides in understanding radiation dose risks, results demonstrating any connection between cancer and CT utilization, however small, could cause great concern among the general population. The American College of Radiology (ACR) released a statement urging parents not to keep their children from needed medical imaging care based on the results of Pearce et al.
“Medical imaging exams are directly linked to greater life expectancy, declines in cancer mortality rates, and are generally safer and less expensive than the invasive procedures that they replace. Scans also reduce the number of invasive surgeries, unnecessary hospital admissions, and