CT radiation exposure has dominated consumer and professional headlines for several years, yet mechanisms, effects and measures to control dose are not well-understood. Research is needed to plug knowledge gaps in nearly all areas of CT radiation exposure, according to the proceedings and recommendations of the Radiation Dose Summit, published in the November issue of Radiology.
The National Institute of Biomedical Imaging and Bioengineering held the summit in Bethesda, Md., in February 2011, and gathered more than 100 medical physicists, radiologists, cardiologists, engineers, industry representatives and patient advocates.
Experts identified four primary topics and developed recommendations for research in each area.
The first topic—state of knowledge about adverse effects of low doses of ionizing radiation—is the subject of many debates. Although scientists have studied survivors of atomic bombs dropped on Hiroshima and Nagasaki for six decades, these exposures differ dramatically from CT exposure, creating a gap in knowledge.
Dose is expressed as effective dose, or radiation dose that would have to be delivered to the entire body to produce the cancer induction risk received by exposed organs. However, the concept was developed to establish standards for occupational exposure, rather than medical exposure.
Another issue is the application of the linear nonthreshold (LNT) model of radiation-induced cancer risk. “The LNT model was not intended for use in the manner used by some to predict increased cancer deaths in a population of individuals exposed to medical radiation,” wrote John M. Boone, PhD, from University of California Davis Medical Center in Sacramento, and colleagues.
After the summit, the experts recommended research to:
- Further identify subgroups sensitive to radiation, and examine underlying biologic mechanisms;
- Define and clarify repair mechanisms in irradiated cells, tissues and organs;
- Determine the population impact of anxiety induced by predictions of radiation-induced cancer;
- Determine methods to identify specific cancers caused by radiation exposure; and
- Evaluate the validity of the LNT hypothesis.
The second issue centered on the complexities of recording and tracking radiation dose. Boone and colleagues emphasized that the CT Dose Index (CTDI) measures are recognized and standard, but they do not translate into patient specific measurements as they are based on phantoms and do not account for differences in patient size. These measures also don’t account for table motion or dose-saving technologies.
Although the American College of Radiology has launched a dose registry, it faces multiple challenges in enabling direct comparison. The reasons include: use of multiple different scan names, lack of a universally accepted descriptor of patients size and an inability to determine the cause for variations in CTDI VOL.
The experts recommended research focus on development of methods to:
- Convert scanner output to reliable patient estimates; and
- Integrate improved dose estimates into the patient record and track dose over time using informatics.
They also suggested researchers consider how dose information might be used for individual patients.
Another overarching issue identified at the summit is selection of optimum protocols and implementation of fail-safe mechanisms. Attendees pointed out the variability in protocols and interfaces across vendor models, which challenges development of best practices. They also described the need for additional comparative effectiveness research for CT imaging, which could inform appropriate use criteria. Finally, they noted that CT technologists in the U.S. have fallen behind their international colleagues in terms of education.
Boone and colleagues suggested federally sponsored research to:
- Develop CT scanner-based methods to better measure patient dimensions for dose assessment;
- Engineer IT tools to automate dose estimates;
- Assess organ dose based on patient size;
- Quantify benefits of CT exams to inform benefit-risk determinations; and
- Expand educational opportunities for CT technologists.
The final topic area centered on leveraging IT to optimize CT scanning. Experts at the summit emphasized the value of decision support on CT ordering, and offered that decision support tools might address protocol selection to minimize radiation exposure. They also considered the lack of a national repository of organ-based radiation dose information in the U.S.
In this area,