Researchers said it should be possible to set more consistent dose standards to ensure that patients are not exposed to unnecessary radiation risks.
CT scanning helps diagnose a range of conditions, but CT radiation is associated with an increased risk of cancer, so it is important to minimise exposure and reduce unnecessary variation.
Existing evidence suggests that CT radiation doses are highly variable across patients, institutions, and countries and, in many cases, doses can be reduced by 50% or more without reducing image quality and diagnostic accuracy.
Therefore an international research team set out to understand the factors contributing to this variation by analysing dose data for just over two million CT scans from 151 institutions, across seven countries – UK, Germany, Israel, Japan, Netherlands, Switzerland, and the US.
They included scans of the abdomen, chest, combined chest and abdomen, and head from 1.7 million adults between November 2015 and August 2017.
The average effective dose and proportion of high dose examinations varied substantially across institutions.
The researchers adjusted the data for a range of variables related to the patient, such as sex and size, institution (trauma centre, academic or private), and machine (manufacturer and model).
They found that most of these factors had only a small effect on dose variation across countries.
For example, after adjusting for patient characteristics, there was still a fourfold range in the average effective dose for abdominal scans and a 17-fold range in proportion of high-dose scans (4-69%).
Similar variation persisted for chest scans, and combined chest and abdomen scans.
Adjusting for institution and machine factors also had little effect on dose variation.
However, when the researchers adjusted for technical factors – how scanners were used by medical staff – this substantially reduced or eliminated nearly all the dose variation across countries.
Therefore, the researchers concluded that the variation in doses used for CT scanning of patients was primarily due to how CT scanners were used, rather than to underlying differences in the patients scanned or the machines used.
This was an observational study, so it did not establish cause and the researchers pointed to some limitations that may have influenced their results.
Nevertheless, they said: “These findings suggest that optimising doses to a consistent standard is possible, which will probably require more education of individuals who create protocols for CT, recalibration of image quality expectations targeted to answering the clinical question at hand, and greater sharing of protocols across institutions.”
*R Smith-Bindman, et al. International variation in radiation dose for computed tomography examinations: prospective cohort study. BMJ 2019;364:k4931. DOI:10.1136/bmj.k4931