Event

Rates of cancer are increased following low-dose radiation from computed tomography (CT) scans used for medical diagnosis. However, when the lag period between a CT scan and diagnosis of cancer is very short, it is likely that the cancer was not caused by the radiation, but rather that the CT scan was prompted by early symptoms of cancer (reverse causation) or by the need to investigate a precancerous condition (confounding by indication). We modelled the incidence of cancer following diagnostic CT scans in our cohort of almost 11 million young Australians. We find that reverse causation explains most cancers occurring at lags of less than 12 months, but explains very few at longer lags. The excess relative risk estimate for cancers apparently caused by CT scans is 0.056 per mSv, with 95% credibility intervals of 0.047-0.065, at lag periods as short as 2-3 years. Estimates from the Life Span Study (LSS) of atomic bomb survivors are much lower. We suggest that the average risk of cancer, per unit of dose, is greater at the lower doses from CT scans than at the higher average doses in the LSS.