New studies try to quantify cancer risk from CT scans

Two recent studies calculated the cancer risk that could come from radiation emitted by computed tomographic scans in the United States. The studies' authors said their conclusions were not intended to stop use of CT scans but to promote a healthy respect for them.

"It's important that we start to appreciate that although CT scanning is a marvelous technology, it has potential harm, and we need to start thinking about how we are using it and how to improve it," said Rebecca Smith-Bindman, MD, professor of radiology, epidemiology and biostatistics at the University of California, San Francisco.

She was co-author of a study, appearing in the Dec. 14/28, 2009, Archives of Internal Medicine. The study showed a mean 13-fold variation between the highest and lowest dose of 1,119 scans, representing the most common 11 types of CT procedures, performed on adult patients in the San Francisco area between Jan. 1 and May 30, 2008.

Dr. Smith-Bindman computed a cancer risk based on type of scan, age and gender. For example, she estimated that one in 270 women who underwent CT coronary angiography at age 40 would develop cancer from that scan, compared with one in 600 men. A routine head CT scan at age 40 carried a one-in-8,100 cancer risk for women and a one-in-11,080 risk for men age 40. In all cases, risks were higher in younger patients and lower in older patients.

Another study in the same issue estimated that 29,000 of 57 million CT scans done in the U.S. in 2007 would result in future cancers (excluding 15 million others in the last five years of life and those with a related diagnosis code of cancer). Nearly half -- 14,000 -- would come from scans of the abdomen and pelvis, followed by chest (4,100), head (4,000) and chest CT angiography (2,700). In part, the radiation risks were determined by using data from survivors of Japanese atomic bomb explosions in World War II.

The authors, whose affiliations include the National Cancer Institute and the research department of the American College of Radiology, estimated that a third of the projected cancers were due to scans performed between age 35 and 54. Two-thirds of future cancers would be in women. Those numbers were nearly concurrent with the demographic of those who get scans -- 30% between age 35 and 54, 60% female.

The authors acknowledged that 29,000 future cases is a small number compared with the number of scans, but they said "when a large number of people are exposed, even small risks could translate into a large number of future cancers in the population."

They wrote that the risk could be reduced in many ways, most notably "decreasing the number of unnecessary procedures as well as the dose per procedure."

Regulation of radiation doses

Dr. Smith-Bindman urged the Food and Drug Administration to regulate CT scans and establish the lowest radiation doses that produce quality images. The congressionally mandated regulation of radiation doses in mammography could provide a good model for this effort, she said.

Although radiation exposure should be monitored and limited, the medical gains contributed by CT scans are enormous, some radiologists said. They also said using an atom bomb detonation as a comparison might not be valid because most CT scans are performed in controlled settings on one part of the patient's body, rather than instantaneous exposure to the whole body.

Study authors acknowledged that long-term follow-up of large populations would be the most accurate way to quantify risks.

"Overall, medical imaging examinations have increased life expectancy and resulted in declines in cancer deaths," said James Borgstede, MD, professor and chair of the radiology department at the University of Colorado in Denver. "The overall benefits outweigh the risks."

But each individual examination should be evaluated, Dr. Borgstede said. Physicians and patients should consider whether the CT scan is the proper course or if an MRI or an ultrasound exam is an option.

Also, the patient and referring physician should determine whether the administering facility is accredited and the technologist registered, Dr. Borgstede said. If a child receives a CT scan, he or she should get a "kid-sized radiation dose."

The American College of Radiology said in a statement released in response to the studies that no imaging exam should be performed unless there is a clear medical benefit that outweighs any associated risk. The college also urged physicians to use only the minimum level of radiation needed to achieve the necessary image quality.

But the ACR also said the risk of cancer is small compared with the life-saving possibilities of CT scans. "No published studies show that radiation from imaging exams causes cancer," the statement said. Cancer assumptions based on using atom bomb victims as a basis for comparison "should be considered, but not accepted as medical fact."