Alarm about computed tomography scans is unjustified

To the Editor: Blecher correctly asserts that alarm is not the appropriate reaction to the potential dangers of computed tomography (CT) scans.1 Such alarm risks discouraging patients from having the CT scans they need. Appropriate CT scans are good; inappropriate ones are bad.

Blecher is also correct in stating that the linear, no-threshold (no dose is entirely safe however small) model of radiation risk is theoretical.2 However, the advice of all international regulatory radiation protection authorities is to assume that the model is correct. To do so is prudent, even if overly conservative. And while these authorities hold this position, it would be professionally irresponsible to ignore it. Notwithstanding the Health Physics Society statement that the risk of ionising radiation below 100 mSv is negligible,3 the risk is thought to be cumulative. Furthermore, a single CT scan of the chest, abdomen and pelvis may expose a patient to 30 mSv or more, and the risk in children and young adults is considered to be 2–3 times greater than the average.4

The atomic bomb data are based on radiation dose levels as low as 5 mSv,5 with the cohort exposed to doses between 5 and 20 mSv showing an 85% chance that the risk is worse than we think and a 15% chance that it is better than we think. Although not statistically significant at the 95% confidence level, these figures indicate that great care is needed.

The dimensions of the risks of low-dose radiation are contentious, but are we willing to ignore the danger of accepting Blecher’s argument?

It is a Pascal’s wager. Providing we do not spook patients from having necessary scans (and this requires better communication with patients), our possibly over-cautious approach should result in more appropriate use of diagnostic imaging. The benefits go beyond those of limiting unnecessary radiation.

Blecher finds it ironic that concerns about the dangers of CT are rising as radiation doses are falling. Doses may be falling, although that depends on where the baseline is drawn. When multidetector CT scanning was introduced, doses rose,6 but if, since then, doses have been falling, there is nothing ironic about this. Doses are falling because of the concern.

Whether a patient undergoes an imaging procedure should be subject to the process of justification mandated in the Australian Radiation Protection and Nuclear Safety Agency code of practice.7 If the potential benefit outweighs the risk, the procedure is justified and the examination should proceed, but optimised to ensure that the lowest possible dose of radiation is used to provide diagnostic images — the ALARA principle (As Low As Reasonably Achievable).

In summary:

• If a CT scan is clinically justified, then it should be performed and we should certainly avoid alarming our patients.

• If a scan is not justified, it should not be performed.

• We should adhere to the ALARA principle.

• Whenever appropriate, a non-ionising alternative to CT scans should be considered, particularly for children and young patients.