Closer Analysis of Radiation Dose from CT Scanning Warranted

Amid the climate of rising concerns of overexposure to radiation from computed tomography (CT) scanning, a panel of medical experts has advocated that more research is needed on the health effects of medical imaging and ways to reduce unnecessary CT tests, as well as industry standardization of CT units.

The recommendations, published in the November 2012 issue of the journal Radiology, were developed at the Radiation Dose Summit, organized by the US National Institute of Biomedical Imaging and Bioengineering (NIBIB; Bethesda, MD, USA). The summit included more than 100 medical physicists, radiologists, cardiologists, engineers, patient advocates, and industry representatives. The proceedings, held in Bethesda (MD, USA), in early 2011, covered currently understood risks of radiation exposure from CT scans, set priorities for future research, and called for changes to industry practices.

“The number of CT exams in the US has increased by about 10% each year over the past decade,” said John Boone, from the University of California (UC) Davis (Sacramento, USA) professor of radiology and lead author of the Radiology article. “This trend underscores the importance of developing a better understanding of the health risks of radiation exposure versus the benefits of enhanced diagnosis.”

The experts acknowledged that in spite of widespread public concern about radiation risks, the biologic effects from medical imaging are not entirely determined. Most direct evidence comes from the effects of instant, high-dose, whole-body exposures caused by industrial accidents and from survivors of the atomic bombs in Nagasaki and Hiroshima, Japan. Whether these findings can be compared to individuals exposed to sporadic and much smaller dosages applied to only regions of the body is not clear.

“The standards regarding levels of radiation were designed for workplace safety and are very conservative,” said Dr. Boone. “We don’t know whether the established thresholds are really meaningful for exposure from medical testing.”

The experts stressed that because there is a high background incidence of cancer worldwide, the small incremental increase in cancer that may be attributable to low doses of radiation from medical imaging is extremely challenging to determine. They stated that national and international registries that track cancers and patient exposures to medical radiation may soon make it possible to conduct large epidemiologic research that could help make such correlations.

“In reaction to media coverage of radiation overexposure cases, some patients refuse to undergo medical imaging procedures,” said Dr. Boone. “Yet for almost all patients, the risks of foregoing a needed medical procedure far outweigh any potential radiation-associated risks.”

Even effectively recording patient exposures of radiation from medical imaging is very difficult, according to the investigators. Although it is easy to ascertain how much radiation a machine administers during an imaging study, the amount actually received by a patient depends on various factors including body size. For example, because of differences in body mass, children and small adults can receive a dose of radiation two to three times that of larger people, even when the dose administered is the same.

Other factors, such as whether the patient lies on a stationary or moving table, also affect the radiation dose received. US sponsored studies are required to develop ways to more accurately measure patient exposures from different types of CT scans, the authors suggested.

Summit participants also discussed the role of human error in CT scanning, which has resulted in widely publicized instances of radiation overexposure. They point out that CT operators frequently are responsible for several machines made by different manufacturers, each of which may utilize dissimilar nomenclature and control consoles, thereby increasing the possibility for error.

“For some scanners, you turn a dial to the right to get a larger dose, and for others, turning it the same way gives a smaller dose,” said Dr. Boone. “There are so many differences in current CT scanners, it can be like driving a car with the brake pedal on the left in the morning, then with the brake on the right in the afternoon.”

CT scans should be constructed more like cars, Dr. Boone commented, which may have different exteriors, seat coverings, and dashboards but are standardized across the industry so that they are driven the same way, making driving errors unlikely to occur when switching from one kind of car to another. The experts call for academic and professional radiology societies, as well as industry trade organizations, to exert pressure on CT manufacturers to standardize nomenclature and control consoles to help avoid mistakes.

The experts also considered “wasteful imaging”--tests that have little impact on patient diagnoses or outcomes--resulting in unnecessary radiation exposures. Wasteful imaging can occur if physicians are not cognizant of a patient’s earlier imaging scans or cannot really determined whether a scan will help a patient with specific indications and symptoms. Moreover, clinicians may order tests they know are only slightly useful to avoid allegations of negligence in a possible future lawsuit. The experts recommend use of information technology (IT) to develop US imaging and radiation exposure registries, as well as standardized protocols that guide physicians on the use of optimal imaging modalities for different clinical problems.

The University of California recently provided grants to all five of its medical schools to develop approaches for more accurate gauges of radiation exposure from CT scans and to design protocols that improve diagnostic data and reduce radiation risks. According to Dr. Boone, the approaches being developed are very effective and will move the UC system toward achieving the goals outlined during the Radiation Dose Summit.

Related Links:
National Institute of Biomedical Imaging and Bioengineering
University of California Davis