Results Summary

What was the research about?

Doctors often use computerized tomography, or CT, scans to take detailed pictures of the inside of a patient’s body. A CT scan can help find a wide range of health problems, including infections and cancers. The CT scan exposes the patient to a small amount of radiation, called a CT dose. That radiation slightly increases a patient’s risk of cancer.

In this study, the research team wanted to learn if CT doses vary across hospitals and countries and, if so, why.

What were the results?

CT doses varied slightly by scanner and by hospital. But CT doses varied a lot across countries. Hospital rules, such as who could change CT scanning procedures, were more important than other factors, such as type of scanner, in determining why doses varied.

What did the research team do?

The study looked at data from about two million CT scans of adult patients. Of these scans, 36 percent were of the abdomen, 24 percent were of the chest, 4 percent were of the chest and abdomen, and 26 percent were of the head. The patients were from 151 hospitals in seven countries.

The research team compared average CT dose for different scanners, hospitals, and countries. The team also compared the percentage of scans with high doses. If a dose was higher than 75 percent of the doses during the first six months of the study, the team defined it as a high dose.

To learn why CT doses differed, the team used statistical methods. The team looked at factors such as features of CT scanners and hospital policies about CT scans. The team also considered factors such as patient age and weight.

Patients, doctors who specialize in CT scans, and hospital administrators helped design the study.

What were the limits of the study?

All hospitals used the same software, Radimetrics™, to monitor CT doses. Results may differ if the study had collected data from hospitals that use other software or no software. Only a small number of hospitals outside the United States took part in the study. Data may not represent all hospitals in those countries.

Future research could collect data from hospitals that use other monitoring software. Researchers could also try to collect data from more hospitals outside the United States.

How can people use the results?

Doctors and hospital administrators can use these results when considering how to improve hospital policies to reduce CT doses.

Final Research Report

View this project's final research report.

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Related PCORI Dissemination and Implementation Project

Journal Citations

Article Highlight: Clinicians use computerized tomography (CT) scans to pinpoint a range of health problems, including infections and cancers. The scans expose patients to radiation called a CT dose, which slightly increases the patient’s risk of cancer. This study analyzed CT doses from more than 2 million CT scans across seven countries to learn if the doses varied across hospitals and countries. Doses varied modestly by hospital, but across countries, there was wide variation in radiation levels. As reported in The BMJ, these findings suggest that there is a need to implement strategies that can be used to modify and lower hospital radiation dose levels.

Peer-Review Summary

Peer review of PCORI-funded research helps make sure the report presents complete, balanced, and useful information about the research. It also assesses how the project addressed PCORI’s Methodology Standards. During peer review, experts read a draft report of the research and provide comments about the report. These experts may include a scientist focused on the research topic, a specialist in research methods, a patient or caregiver, and a healthcare professional. These reviewers cannot have conflicts of interest with the study.

The peer reviewers point out where the draft report may need revision. For example, they may suggest ways to improve descriptions of the conduct of the study or to clarify the connection between results and conclusions. Sometimes, awardees revise their draft reports twice or more to address all of the reviewers’ comments. 

Peer reviewers commented and the researchers made changes or provided responses. Those comments and responses included the following:

  • The reviewers had extensive criticisms of the background section, saying that it used out-of-date references and seemed overly biased against the risk of radiation from computed tomography (CT) imaging. They asked that the section be trimmed. The researchers shortened and simplified the background section. They felt that it was important to point out, however, that there is no meaningful controversy in the scientific literature about the carcinogenic risk from the use of ionizing radiation in medical imaging.
  • The reviewers suggested that the study could have more productive if focused on different topics, such as addressing the overuse of CT rather than the radiation exposure per scan. The researchers stated that they could not address these criticisms as the study was complete.
  • Other topics, they said, are outside the scope of their report and their response.
  • The reviewers suggested that radiation doses might have been overestimated or underestimated for patients of different sizes. The researchers replied that they reported effective dose, not organ dose, and that they accounted for patient size to reflect dose needs. They also said that authors of the study included medical physicists, including a past president of the American Association of Physicists in Medicine, who helped define how all measurements were calculated.

Conflict of Interest Disclosures

Project Information

Rebecca Smith-Bindman, MD, BS
University of California San Francisco
UCSF CT Radiation Dose Registry to Ensure a Patient-Centered Approach for Imaging

Key Dates

September 2013
July 2019

Study Registration Information


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Last updated: November 30, 2022