CT radiation dose falls over time, but still varies widely

CT radiation dose dropped a little over the course of a large seven-year study of patients undergoing repeat scans. But just as noteworthy were large variations in dose for repeat procedures in the same patients over time, researchers concluded in a paper published March 28 in the American Journal of Roentgenology.

The study from Duke University Medical Center looked at nearly 3,000 subjects undergoing more than 12,000 repeat identical CT scans over seven years in four anatomical regions. Thanks to the implementation of new CT technologies and procedures, the team found a trend toward lower radiation doses. But analysis of dose differences in the same individuals between scans showed wide variations across the four types of exams investigated (AJR, March 28, 2017).

Dr. Achille Mileto from the University of Washington.

"If you look at the overall period of seven years, radiation doses are decreasing, and there's an overall global trend toward reduction of radiation doses," said Dr. Achille Mileto, who has since moved from Duke to the University of Washington. "But there is also substantial variability, so if you look at an individual patient, the overall trend toward dose reduction might not coincide with the individual radiation doses for that patient."

Chest doses showed the least intrapatient variability between scans, while renal stone protocols showed the most variability, he added.

"When you get new technology, you tend to be more aggressive with radiation dose reduction, so this tends to create more room for variability," Mileto said.

Dose optimization and repeat scans

The rise in CT usage has been accompanied by the growing use of repeat CT across various patient populations. At the same time, radiation dose optimization efforts have been trying to drive doses down, the authors wrote.

Aiming to create benchmarks for optimized practice, health oversight organizations and agencies have advocated the development of diagnostic reference levels (DRLs), as developed by organizations such as the Joint Task Force on Adult Radiation Protection on behalf of the American College of Radiology (ACR) and the RSNA.

The campaigns have gained momentum and successfully increased adherence to recommended guidelines among main stakeholders in healthcare. The dose-optimization initiatives have also coincided with reports of substantial variability in radiation dose levels "due to the complex interplay of differences in CT equipment and variances in scanning technical factors," they wrote.

Few studies have delved into this variability, however, and the problem is compounded by the heterogeneity of various patient populations being treated at separate institutions. It is critical to understand whether patients assigned to repeat scanning with the same protocol but perhaps different equipment are receiving similar radiation doses.

A longitudinal assessment of radiation dose can be used to clarify whether global trends in dose reduction are actually benefiting patients. This study aimed to conduct such an analysis of radiation dose data from adult patients undergoing clinically indicated, repeat identical thoracoabdominal CT exams.

In the study, Mileto, along with Dr. Rendon Nelson, Dr. Douglas Larson, and colleagues, collected electronic dose data from 2,851 subjects who underwent a total of 12,635 identical repeat CT scans in a single health system. Each patient had a mean of 4.8 scans (range, 2-33). The scans included four anatomical areas:

• Chest-abdomen-pelvis with contrast: 4,261 studies of 1,064 patients
• Abdomen-pelvis with contrast: 876 CT studies of 261 patients
• Renal stone CT: 1,053 studies of 380 patients
• Chest: 6,085 studies of 1,146 patients

Images were acquired on seven different scanners from GE Healthcare and Siemens Healthineers and configurations ranged from four to 128 detector rows, according to the authors. Accompanying the scanner protocols were some major changes in scanning parameters over the seven-year study, including a reduction in peak tube voltage from 140 kV to 120 kV and a reduction in tube current in combination with implementation of iterative reconstruction in GE scanners (adaptive statistical iterative reconstruction, ASIR) and Siemens scanners (sinogram affirmed iterative reconstruction, SAFIRE). 2D and 3D modulation of tube current were introduced for all scanner platforms, and pitch was increased from 1.75 to 3.2 for dual-source CT systems.

Radiation dose was measured in terms of size-specific dose estimates (SSDEs). The results showed a trend toward a global reduction in SSDE values across all four anatomical areas; for example, the mean radiation dose for chest-abdomen-pelvis exams fell from 24.8 mGy in 2007 to 14.4 mGy in 2013.

But at the same time, the analyses of radiation dose distribution across different patients showed substantial variability in SSDE values across the four protocols, from a high of 34.18 mGy for renal stone exams to 6.74 mGy for chest exams.

The analyses also showed that only renal stone CT protocols yielded consistent radiation dose reductions within the same patients throughout the seven-year study period, which is not surprising considering the aggressive dose-reduction methods used in these exams, which are carried out most frequently in young adults.

"Successful efforts to reduce overall radiation doses may actually direct attention away from other critical pieces of information that have so far been underappreciated, namely the widespread variability in global radiation dose values across clinical operation volumes," the authors wrote. "These data may provide a foundation for the future development of best-practice guidelines for patient-specific radiation dose monitoring."

The sheer diversity in CT hardware and software used in exams, including scanner design and geometry, the scanning approach, and proprietary tube current modulation schemes, contributes significantly to the wide range of radiation doses seen in the same type of CT exams on the same patient, they noted.

"We are kind of obsessed with radiation dose reduction, but I think we should keep in our minds the concept of radiation dose optimization, which means trying to adjust the dose to the specific clinical task," Mileto said. "With technology we are reducing the dose, but we are increasing the room for variability. This is great if you are consistently reducing the dose, but we really want to understand what's going on in terms of variability. So I think the main lesson is to try to develop best-practice guidelines for patient-specific radiation dose monitoring. I think basically the scenario in the near-term future will be to create some kind of shared library for radiation doses."

Limitations of the study included its single-center design, the use of only thoracoabdominal scans, and the use of scanners from only two vendors. Also, head CT is a common procedure, but it was not included in the study, the authors wrote.

"The natural follow-up would be a multivariate study that includes all of the factors -- for example, looking at the years of experience of the technologist, different technologists, the CT fleet, the radiation dose parameters, etc.," Mileto said. "You implement protocols as part of the standard of care at the institution, but there are human variables like the technologist experience that you are not able to fully control, so it would be nice to look at these human factors."