SEATTLE—Use of computed tomography (CT) scans—and thus exposure to ionizing radiation—increased over 15 years in children at a set of nonprofit health care delivery systems in a new study. But currently available strategies could greatly reduce this cancer risk, according to the HMORN Cancer Research Network study, published in JAMA Pediatrics.
Led by Diana L. Miglioretti, PhD, and Rebecca Smith-Bindman, MD, the study also documented substantial variation in the radiation doses that were used in CT scans. This is an important area where quality can be improved by lowering and standardizing the doses to which children are exposed, the authors said. Dr. Miglioretti is a senior investigator at Group Health Research Institute and a professor of biostatistics at the UC Davis School of Medicine. Dr. Smith-Bindman is a professor of Radiology and Biomedical Imaging; Epidemiology and Biostatistics; and Obstetrics, Gynecology, and Reproductive Medicine at UC San Francisco (UCSF) and a radiologist at the UCSF Medical Center.
"We estimated that the number of cancers caused by CT scans performed on children could fall dramatically—by 62 percent—if dose-reduction strategies like that instituted by the Image Gently initiative were targeted to exams with the highest quarter of doses and if CT scans were used only when medically necessary," lead author Dr. Miglioretti said. Image Gently, an initiative of the Alliance for Radiation Safety in Pediatric Imaging, aims to change practice by increasing awareness of chances to promote radiation protection in medical imaging of children.
"Many people have focused on eliminating unnecessary CT scans, which is a worthy goal and could reduce future cancers caused by radiation from CT scans by around a third, since experts believe around a third of exams are unneeded," Dr. Miglioretti added. "But we were surprised to find we could reduce cancers caused by CT imaging even more than that—by 43 percent—simply by lowering the radiation from the highest-dose exams. We found the biggest gains would come from targeting the highest quarter of doses." Combining the two strategies would reduce 62 percent of cancers.
In Europe, setting target levels for CT radiation dose metrics (diagnostic reference levels) does just that. When the dose for an exam exceeds the 75th percentile for that exam type, the high dose must be justified. "Following this strategy in the United States, which now has no diagnostic reference levels, could greatly reduce the number of cancers that CT scans cause, especially in children." said Dr. Miglioretti's co-author Dr. Smith-Bindman.
Use of CT doubled for children younger than age 5 years, and nearly tripled for children age 5-14 years, at six U.S. health care systems—Group Health, Marshfield Clinic, and Kaiser Permanente Colorado, Georgia, Hawaii, and Northwest—from 1996 to 2005. But CT use was stable between 2006 and 2007, and it started to decline from 2008 to 2010, particularly among younger children. In 2011, Dr. Miglioretti started working with Group Health's radiation technologists and radiation safety committee to minimize radiation exposure from CT scans in children and adults.
The research team also calculated radiation doses absorbed by each organ and the effective (overall) dose from 744 CT scans in children conducted between 2001 and 2011 at four of these systems (Group Health, Marshfield Clinic, and Kaiser Permanente Hawaii and Northwest) and at the Henry Ford Health System. Those doses per scan varied widely, and many scans delivered an effective dose of 20 mSv or higher—the yearly limit set for people who work with radiation in Europe.
"Effective dose is a somewhat crude way to summarize how much radiation the patient's whole body is exposed to," Dr. Miglioretti explained. But it is useful, because it accounts for not only the settings that are used on the machine (i.e., how much radiation the CT scan produces) but also the sensitivity of the different organs to developing cancer from radiation.
"Children tend to absorb more radiation from imaging than adults do, because their bodies are smaller," Dr. Miglioretti said. "And the radiation children absorb will lead to greater harm, because they are more likely than adults to develop cancer from radiation exposure." Children's small size also means that they need less radiation exposure from CT than do adults to produce images that are sharp enough for clinicians to use for diagnosis. "That's why radiology technologists should use lower settings for children than for adults," she said. "But that doesn't necessarily always happen."
She and her team used a new, improved approach to estimating both organ and effective doses in children, which National Cancer Institute researchers recently developed. And they used models developed by the Biological Effects of Ionizing Radiation (BEIR) committee, based on sources including the Life Span Study of survivors of the atomic bombing of Hiroshima and Nagasaki, Japan, to estimate how the radiation doses they observed translated into increased risk of developing cancer during the children's remaining lifetimes.
The researchers project that the 4 million CT scans of the head, abdomen/pelvis, chest, or spine that are performed each year in American children could cause 4,870 future cancers. But these could be reduced by 62 percent if the highest doses were reduced—by using standardized protocols and guidelines like Image Gently—and by eliminating unnecessary imaging. "These are all things we could easily do right now," Dr. Miglioretti said.
"CT scans are extremely useful for diagnosing many conditions in children," Dr. Smith-Bindman said. But they deliver ionizing radiation doses that are 100 to 500 times higher than those from conventional X-rays. "We need to be judicious in using CT in children because of their greater sensitivity of developing cancer from radiation exposure, and the long life spans ahead of them," she said. "CT exams should be used only when this information will improve diagnosis and clinical care." To detect some conditions, alternatives to CT scans include magnetic resonance imaging (MRI) and ultrasound, which involve no ionizing radiation.
"The most important result of our study is that we can substantially reduce the risk of cancer from CT simply by reducing the doses used at the highest dose range," Dr. Smith-Bindman said. "There is rarely any advantage in using such high doses."
Projected risks of solid cancers are higher for younger children and girls than for older children and boys—and also for patients who get CT scans of the abdomen/pelvis or spine rather than other sites, Dr. Smith-Bindman said. In girls, one radiation-induced solid cancer is projected to result from every 300� abdomen/pelvis scans, 330� chest scans, and 270� spine scans, depending on age. The risk of leukemia is highest from head scans in children younger than 5 years: 1.9 cases per 10,000 CT scans.
Other recent related research found CT increased the risk of cancer in Australian children and teens; but those authors surmised that radiation doses from newer scanners, since 2005, might be lower than the ones they studied, suggesting that radiation exposure may be less concerning now than in the past. "But we really do not have evidence that the doses from CT scans are coming down," Dr. Smith-Bindman said. "New scanners often have greater capacity to perform sophisticated exam types that sometimes deliver higher doses. And in fact we found children were typically exposed to higher radiation doses than the Australia team estimated."