AJR: Rads can help put radiation risk in perspective
Although radiologists have a reasonable level of understanding of radiation risk, referring physicians don’t have ready access to information about radiation exposure and seem to have less of an understanding about the risk of radiogenic cancer, offered Stephen Balter, PhD, of the departments of radiology and medicine at Columbia University Medical Center in New York City, in an interview.
Radiation physics at a glance
Balter and colleagues sought to clarify risk and re-frame the discussion by reviewing other risks of imaging and medical procedures.
The researchers shared basic principles of radiation physics. Specifically, reducing dose can degrade image quality to the point of a non-diagnostic scan, which can lead to repeat studies and increased dose. Balter also referred to the uncertain science of radiation risk, writing, “No single radiation quantity… precisely and accurately correlates with an individual’s cancer risk.” That’s because dose varies by procedure, site, patient, organ, age and sex.
Other uncertainties stem from the mathematical extrapolations that are the basis of dose-risk models, including the Linear No-Threshold (LNT) model. That is, most patients do not reach the lifetime dose of 100 mSv that the models are based on. However, radiogenic cancers cannot usually be demonstrated at lower levels, so models extrapolate data. Consequently, models are associated with substantial uncertainty that “severely limit the utility of the model for risk estimation in individuals and small groups.”
Although both the National Council on Radiation Protection and Measurements (NCRP) and the International Commission on Radiological Protection (ICRP) caution against applying the LNT model without factoring in age and health status, some researchers and the media have largely ignored the recommendations and used the model to link CT and other imaging studies to cancer deaths, adding to the state of alarm and increased concern among patients, noted the authors.
Other math is more certain. For example, Balter pointed out natural cancer probability and its increasing incidence with age, noting that approximately 55 percent of males and 40 percent of females will be diagnosed with cancer in their lifetimes. Given the latent period between initiation of radiogenic cancer and clinical disease, “the practical risk of inducing a clinically relevant radiogenic cancer decreases with decreasing time between irradiation and end of life,” he noted.
Risks in clinical practice
Radiogenic cancer risks should be weighed with the study’s potential medical benefits as well as the risks of alternative procedures including other diagnostic exams, invasive procedures or not performing the imaging study, offered Balter.
Procedure-related acute risks and natural history effects related to the disease are often fairly clear. In contrast, radiogenic cancer is more difficult to correlate.
“There is a need to balance the immediate consequences of even low frequency of acute effects against disease progression and radiation risks,” asserted Balter, who cited the example of patients with clinically suspected appendicitis, in which CT has a diagnostic accuracy of almost 100 percent.
Leveraging data from previous studies, Balter and colleagues projected that appendiceal CT saves approximately three lives annually by preventing deaths associated with unnecessary negative appendectomies. In this scenario, appendiceal CT might produce two fatal radiogenic cancers. The net savings, stated Balter, is one life. “Two people are protected from immediate surgical death at a late (20 to 30 year) cancer risk. They’ve gained life years and quality of life.” What’s more, CT improved the differential diagnosis and saved nearly $450 per patient, offered the researchers.
Balter and colleagues then turned to coronary CT angiography (CTA), which is associated with twice the radiation dose of conventional angiography (10 mSv vs. 5 mSv, respectively), producing estimated lifetime cancer risks of 0.05 percent and 0.025 percent, respectively. However, actual risks are lower owing to the older age of most CTA patients.
In contrast, the list of conventional angiography risks is lengthy and includes myocardial infarction, major bleeding, vascular complications and infection. “Coronary CTA may be a better choice when there is a clinical need to visualize the coronary arteries but a low expectation of finding treatable disease,” offered Balter and colleagues.
Ultimately, the concern, summarized Balter, is that while radiation risks should be considered and reasonable steps taken to reduce dose, too much attention to such risks may detract attention from other risks and potential benefits. “This may not be in the patient’s best interest.”
Radiologists can help address the problem, he said, by helping to educate those on the front line of the radiation risk discussion--referring physicians and technologists.