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Reference: International variation in radiation dose for computed tomography examinations: prospective cohort study. Rebecca Smith-Bindman et al BMJ 2019;364:k4931

This week’s paper of the week is brought to you by Professor Sir Muir Gray, 3V Executive Director.

Bottom line

The authors conclusion was that:

Variation was chiefly driven by how machines were used, rather than by patient or machine manufacturer or model.




Implications for value improvement 

This is an example of what has been called a ‘one tailed curve’ namely a distribution curve where we are clear about what is good and what is not good. In this case lower doses are better than higher doses because there is no evidence that the higher dose improves outcome.

However, this is only one type of unwarranted variation. The other is a ‘two tailed curve’, namely a distribution curve for a service where we do not know whether

  • A high rate is good , or represents overuse, or
  • A low rate is good, or represents underuse

Both the NHS Atlases of Variation in Diagnostic Services have shown significant variation in the population based rate of CT scanning

Furthermore, there has been a significant growth in the number of imaging tests in the UK, although the rate is still much lower in the USA where people have now woken up the the problems that occur. The authors of a paper in JAMA emphasise that:

With aggressive testing, the yield of useful information increases only slightly. Further, some diagnostic tests generate the detection of mostly incidental findings (“incidentalomas”) with the frequency proportional to the excess of testing performed.

So, in the UK it is essential that we ask for every population:

“Is the rate if CT imaging in our population too high, too low, or just right?”


Curbing Unnecessary and Wasted Diagnostic Imaging. Ohad Oren, MD1Electron Kebebew, MD2John P. A. Ioannidis, MD, DSc3,4 doi:10.1001/jama.2018.20295

Atlas of Variation – Public Health England