One of the more complex decisions in the cancer screening decision-making algorithm centers on the risk-benefit ratio. For patients in the screening pools, CT-based screening for lung and colon cancer meets the mark.
The review published in the July issue of American Journal of Roentgenology explored the risks of radiation-induced cancer from CT exams. Data suggest as many as 0.9 to 2 percent of cancers in the U.S. may be traced to x-ray or CT exams.
Despite these risks, Jeffrey M. Albert, MD, from The University of Texas M.D. Anderson Cancer Center in Houston noted near consensus on the risk-benefit balance of diagnostic CT exams. “However, controversy exists regarding whether CT should be used for disease screening in asymptomatic patients.”
He continued, “Although the cancer risks associated with the radiation doses typically used for CT are small, they are not negligible when examined at the population level.”
The topic is fraught with uncertainty as estimated doses for CT exams vary by exam and patient. In addition, the linear no-threshold model, the established model for estimating the risk of radiation-induced cancer, relies on extrapolation of intermediate- and high-dose exposure to estimate risk of malignancy.
Lung cancer screening
The National Lung Screening Trial (NLST) estimated a 20 percent relative reduction in lung cancer-specific mortality among high-risk individuals ages 55 to 74 years who underwent annual CT screening for three years. The average effective dose of the exams was 1.5 mSv. Although research suggests a single low-dose exam carries a 0.01 to 0.06 percent lifetime cancer risk and some critics have expressed concerns about overdiagnosis of lung cancers, multiple organizations, including the American Cancer Society, recommend that patients who meet NLST eligibility criteria be offered low-dose screening in a multidisciplinary center.
Colon cancer screening
Albert referred to general poor compliance with conventional colonoscopy and the emergence of CT colonography as a more comfortable alternative that might increase participation in screening. The average CT colonography dose is 8-10 mSv, and a model estimated the risk of a radiation-induced malignancy at 0.14 percent with one exam for a 50-year-old individual. Another model estimated a 0.15 percent risk for an individual undergoing screening every five years from ages 50 to 80 years.
Nevertheless, the consensus regarding CT colonography is that the benefits exceed risks for individuals ages 50 years old and older. Furthermore, Albert indicated ongoing efforts to optimize scanning to reduce radiation exposure, which would further decrease risk.
The evidence supports recommending screening CT colonography every five years for average-risk patients, according to the American Cancer Society.
Albert concluded with an eye to the future and referred to ongoing efforts to optimize CT technology to reduce exposure while maintaining diagnostic accuracy, which would produce further gains in the risk-benefit ratio. He also suggested, “As has been done with CT-based screening for lung and colorectal cancer, future studies that explore CT screening for various malignancies should consider the risks of radiation-induced malignancy in addition to the benefits of screening.”