CT Scans Increase Patients' Risk of Cancer

Imaging specialists with the University of California in San Francisco (UCSF) say routine computed tomography (CT) scanning is associated with a 1-in-80 chance of developing cancer in certain groups. According to the investigators, radiation doses from CT scanning are higher than previously thought and vary between procedures.

“In day-to-day clinical practice, we found significant variation in the radiation doses for the same type of CT procedures within institutions and across institutions,” said lead investigator Dr Rebecca Smith-Bindman, professor of radiology at UCSF.

According to the researchers, approximately 70 million CT procedures are performed annually, up from 3 million in 1980. The process is typically much faster than it used to be, but Dr Smith-Bindman describes this as a “double-edged sword.” She explained, “Because the images can be obtained so quickly, it has been very tempting to do multiphase studies...increas[ing] the radiation dose.”

The UCSF team looked at the 11 most frequently ordered CT procedures and assessed radiation exposure associated with each. Then they calculated the potential risk of cancer related to each procedure. The review involved 1119 patients treated at four institutions in the San Francisco Bay Area in a 5-month period. All the procedures involved the head and neck, chest, or abdomen and pelvis.

They measured a mean 13-fold variation between the lowest and highest radiation doses delivered with one CT procedure. The number of CT scans required to cause cancer was greater for women than for men, which Dr Smith-Bindman said was what the investigators had expected. Age was also a factor.

The estimated cancer incidence for women was 1 in 270 for a CT coronary angiogram at age 40 years; in men, the same procedure was associated with an estimated 1 in 600 incidence. CT scans of the head were associated with far lower rates of cancer, at 1 in 8100 for 40-year-old women and 1 in 11,080 for men of the same age. Patients less than age 20 years, however, had nearly double the risk. “The risk associated with obtaining a CT is routinely quoted as around 1 in 1000 patients who undergo CT will get cancer,” Dr Smith-Bindman said. “In our study, the risk of getting cancer in certain groups of patients for certain kinds of scans was as high as 1 in 80.”

Archives of Internal Medicine

The dose of radiation delivered with a single CT scan was as high as 74 mammograms or 442 chest radiographs. To minimize patients' exposure, the investigators recommended reducing unnecessary studies or those not likely to affect clinical decision-making. They suggested standardization for CT scanners regarding low-dose and lower-dose protocols and the adoption of federal standards for radiation doses across patients and facilities, as well as FDA oversight on the performance of CT scans. The study was published in the .