Cancer risks from CT scan use in the US

The number of CT scans performed in the US has increased three-fold in just over a decade, from 18 million scans in 1993 to 62 million scans in 2006. CT scans typically involve doses in the range where increased cancer risks have been directly observed (>10mGy). Although the risk to any individual is still probably small, the large number of people exposed annually means that even small individual risks could translate into a considerable number of cancer cases in the US population. To fully evaluate the long term radiation risks from CT scans directly would require very large scale studies with lifelong follow-up, which are considered impractical. It is possible to obtain a more timely assessment of the potential radiation risks with risk projection models. We are currently developing a research tool for calculating lifetime risk, with uncertainty, for site-specific cancers given a detailed, user-specified history of exposure to ionizing radiation. The risk estimation models will be based on the models that were developed by the National Academy of Sciences BEIR VII Committee. The software will also enable users to explore various sources of uncertainty such as the method used to transfer risk estimates between populations with different site-specific baseline cancer rates. We are using this risk calculator for a number of projects including the estimation of cancer risks from the current levels of CT scan use in the US and the risk-benefit profile of using CT for screening for lung and colorectal cancer and coronary heart disease.