According to study results published in BMJ, the largest driver of radiation dose variation in computed tomography (CT) examination can be attributed to differences in technical parameters of machine use, while patient requirements or clinical circumstances had a more modest effect on dose variation.

The investigators who conducted this prospective cohort study sought to inform dose optimization approaches and to understand factors, including patient, institution, and machine characteristics, that influence variation in reported radiation dose used for CT examination.

Rebecca Smith-Bindman, MD, of the School of Medicine at the University of California, San Francisco, and colleagues analyzed pooled radiation and imaging data from more than 2 million adults who underwent CT examination for the abdomen, chest, and head between November 2015 and August 2017. Data gathered from 151 institutions across 7 countries was assessed for mean effective doses and proportion of high dose examinations (defined as doses above the 75th percentile) used for complete abdomen, chest, combined chest and abdomen, and head CT studies. A series of hierarchical linear and logistic regression models were used to understand the influence of patient characteristics (age, gender, and size), institution type (trauma center, 24/7 imaging provider, academic, or private), machine characteristics, and technical parameters on the variation of effective CT dose.

After adjusting for patient characteristics, the mean effective dose by institution and machine manufacturer showed only modest variation (typically 10% to 20%, sometimes up to 40%). Compared with other factors, wide variations in mean radiation doses were observed across countries; the mean effective dose for abdomen CT examination ranged fourfold from 7 to 25.7 millisieverts (mSv), and the proportion of high dose examinations ranged from 4% to 69%. For chest CT, the mean effective dose ranged from 1.7 to 6.4 mSv, and the proportion of high dose scans ranged from 1% to 26%. For combined chest and abdomen CT, the mean effective dose ranged from 10 to 37.9 mSv, and the proportion of high dose scans ranged from 2% to 78%. The variation in radiation dose for head CT was less, with the mean effective dose ranging from 1.4 to 1.9 mSv, and high dose scans ranging from 8% to 27%. The investigators suggest dose variation across countries may be attributed to institutional decisions regarding technical parameters, or how the scanners were used.

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Limitations to the study included fewer participating institutions from countries outside the United States, which restricts the ability to generalize the findings to any country as a whole, and lack of data measuring imaging quality. Technical factors affecting dose, such as iterative reconstruction software, and information on other imaging modalities were not included in the analysis.

“CT scanning doses varied widely across included countries,” Dr Smith-Bindman and colleagues concluded. “Variation was chiefly driven by how machines were used, rather than by patient or machine manufacturer or model. Optimizing doses to a more consistent standard should be possible both within and between countries by modifying the decisions made by radiology teams in developing CT protocols for patients.”

Future studies are needed to understand how institutions in different countries set up their CT protocols.

Disclosures: Multiple authors declare associations with the pharmaceutical industry. Please see original reference for a full list of authors’ disclosures.


Smith-Bindman R, Wang Y, Chu P, et al. International variation in radiation dose for computed tomography examinations: prospective cohort studyBMJ. 2019;364:k4931.