Recent attention to the cancer risks of ionising radiation has prompted vigorous debate about how to quantify risks of diagnostic imaging. 

In the past, models of the carcinogenic risks of ionising radiation have primarily relied on long-term surveillance of the Japanese atomic bomb survivors, which showed significant increases in the incidence of cancer after effective doses greater than about 50mSv. In a recent study in the British Medical Journal,¹ the authors presented compelling data on the magnitude of the cancer risk attributable to ionising radiation. They examined a cohort of nearly 11 million young patients in the Australian national Medicare system and compared subsequent incidence of cancer in the 680,000 patients exposed to computed tomography (CT) with that in unexposed controls.

The most striking finding is that exposure to CT in childhood increased the incidence of cancer by 24%. However, it is important to recognise that the baseline incidence of cancer in a general paediatric population is extremely small, so that a 24% increase makes this risk slightly less small. To put these numbers in context, it is necessary to consider absolute (rather than relative) cancer risk, and to relate the increase to the degree of exposure. The authors found an overall excess risk of about 0.125 cancers per sievert, which equated to roughly one excess cancer per 1,800 head CTs (each with an average estimated dose of around 4.5mSv). This would equate to roughly one excess cancer per 4,000 head CTs at the more typical doses in use with current day technology (around 2mSv). This observed increase in risk associated with the low-radiation doses delivered by CT scans supports the most widely adopted linear no-threshold dose-response model in which double the radiation dose is assumed to impart double the cancer risk. The reported risks also roughly match the lifetime attributable risks predicted by the BEIR-VII (biological effects of ionising radiation) report, one of the most commonly used linear no-threshold models.

So how should this information influence practice? There are many possible interventions to control patients’ exposure to radiation – before, during and after the CT scan. Special attention should be paid to patients undergoing recurrent imaging, because if frequently repeated scans are found to provide little clinical benefit, the cumulative risk-benefit balance may support a decision not to image again for the same clinical presentation. 

During the scan, there are many available methods to reduce the radiation dose without negatively affecting the diagnostic quality of the examination.  Although CT radiation doses vary considerably, existing dose-reduction tools and ongoing technological improvements allow CT scans to be performed using substantially lower radiation doses than was possible with previous generations of scanners. Adoption of newly developed informatics methods that enable large-scale data capture of CT scanner radiation output has resulted in databases that will be vital for institutional benchmarking, optimisation of CT protocols and quality control. With further validation of radiation-risk models, in both children and adults, we will ultimately be able to perform more accurate patient-specific risk assessment to inform imaging decisions. 

Reference 

1. Mathews JD, Forsythe AV, Brady Z et al. Cancer risk in 680 000 people exposed to computed tomography scans in childhood or adolescence: data linkage study of 11 million Australians. BMJ 2013; 346: f2360