Computed Tomography and Radiation Exposure

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Abstract
Computed tomographic (CT) scans deliver a radiation dose of about 20 mSv. Brenner and Hall (Nov. 29 issue)1 assess the risk associated with CT radiation exposure by using the linear no-threshold extrapolation model, which assumes that cancer induction is proportional to dose even for the smallest doses. An excess of cancers has never been detected in laboratory animals or in humans for doses below 100 mSv. This model is used for analyzing data from cohorts including persons who have received doses higher than 100 mSv. This method is exposed to strong bias.2 Defense mechanisms against radiocarcinogenesis are much more effective at low doses, and the use of the linear no-threshold model in this dose range is highly debatable3,4; it greatly overestimates the risk. After repeated x-ray examinations, induction of cancer has been observed only when the cumulative dose was above 500 mSv. In patients treated with radiotherapy, a threshold was reported for irradiation doses of 0.6 Sv delivered in 30 sessions.5,6 Overestimation of the risk may deprive patients of beneficial examinations.