A study published Tuesday in Lancet warns about the increased cancer risk in children who have two or three CT scans, and scary news reports about it may have left some parents in a panic. After all, a lot of kids have had more than one CT scan, after an appendicitis attack, car accident, or severe head injury.

Yet parents should understand that even though the study found that repeat CT scans were associated with a tripling in risk for both leukemia and brain cancer, the absolute risks of these two rare cancers still remain very small. Brain tumors occur in 3 out of every 100,000 children each year, while leukemia occurs in about 4 out of every 100,000 children, according to the National Cancer Institute. Thus, a tripling in risk would increase a child’s yearly chances of developing a brain tumor to 9 in 100,000 and of developing leukemia to 12 in 100,000.

That doesn’t mean, though, that doctors should order CT scans willy nilly without regard for the radiation risks -- even one CT scan delivers a hefty dose of radiation, many times what you’d get from an X-ray, that lingers in the body, perhaps for a lifetime.

“We’ve had a long-standing effort to decrease radiation dose from CT scans,” said Dr. Rick Robertson, chief of radiology at Boston Children’s Hospital. The hospital’s policy is to try to use magnetic resonance imaging or ultrasound, neither of which involve radiation, whenever possible instead of CT. And newer CT machines and adjustments to imaging methods, Robertson added, have enabled radiologists at Children’s to reduce doses by 70 to 80 percent for some types of scans, and by 20 to 30 percent for others.

Local community hospitals, however, haven’t all adopted these practices and may still use CT scans too often, especially because CT scanners are expensive machines that garner hospitals large insurance reimbursements from each image. “We see a wide spectrum of practices,” said Roberston, “and see a fair number of kids who come to us with imaging CT studies” from other institutions that weren’t warranted.

For example, most kids who have concussions on the soccer or football field usually don’t need a brain CT scan or any sort of brain imaging, though an MRI may be warranted if children have persistent post-concussive headaches, Roberston said.

A physician-led campaign to curtail excess radiation from medical imaging, called Image Gently, put together a letter for parents in response to the Lancet study to help provide commonsense guidelines about when to expect a CT scan in the emergency room and when not.

For example, a child who has a minor fall, with no bruise to the head, and is acting fine, there’s probably little chance of a brain injury and little reason to warrant a CT scan. On the other hand, a child who has been in a car accident and was injured in spite of wearing a seatbelt probably does need a CT scan to check for internal injuries.

In general, CT scans offer an advantage over MRIs in providing a quick answer to a medical emergency that might help determine, say, whether a child needs an appendectomy or other medical procedure to deal with a life-threatening situation. It also may be warranted for young children who can’t sit still for the 20 or 30 minutes it often takes to get an MRI image.

While sedation may be used to keep kids calm in MRI machines, physicians have taken the tack that the risks of sedatives are greater than radiation risks from CT scans. “I think this is probably still the case,” said Roberston, despite the new Lancet finding, “but I think it reopens the discussion.”