AJR: Cumulative dose estimates should not guide CT decision-making
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The linear no-threshold model (LNT), which links any radiation exposure to cancer risk, has emerged as the dominant mechanism to estimate risks of radiation exposure. However, the model is based on studies of atomic bomb survivors who were exposed to higher radiation levels than medical patients, wrote Daniel J. Durand, MD, from Johns Hopkins University School of Medicine in Baltimore.
Meanwhile, a movement to include cumulative dose estimates for individual patients in the EMR has emerged. “It is important to consider, however, that with the inclusion of cumulative dose estimates in the EMR comes the temptation—or even the implied obligation—to use this information clinically in real time,” wrote Durand.
Durand outlined several flaws with the scenario. For starters, linearity implies that absolute risk stays constant, remaining the same for the first and the nth imaging studies. Relative risk actually decreases with each exam, which means that CT studies are not riskier for patients with high cumulative dose estimates.
“[Any] cumulative exposure threshold used to identify patients at high risk will be arbitrary and of little use in caring for individual patients,” continued Durand. Neither feasible option—100 mSv nor 50 mSv—is backed by the weight of evidence because the evidence suggests there is no threshold, he asserted.
Durand opined that patients with higher cumulative dose estimates face a lower relative risk of radiation-induced cancer and a potential greater benefit in terms of early cancer detection. In clinical practice, physicians tend to be more likely to image patients with high lifetime risks of cancer, regardless of the source of the risk. “[A] high cumulative dose estimate alone should never make us less likely to image a patient,” emphasized Durand.
Durand suggested that physicians focus on imaging history rather than cumulative dose estimates in the decision-making process. Specifically, he noted that the EMR contains imaging history, which can be accessed to avoid unnecessary duplicate studies and minimize dose. At the same time, clinically justified CT exams, even those repeated within a short time frame, should be performed.
A final group that merits consideration is high-risk patients, those with chronic conditions who are much more likely to undergo frequent scanning. Durand referred to calls for more sparing use of chest CT in cystic fibrosis patients and head CT in hydrocephalus patients. This is “logically and ethically sound policy” from a population research standpoint; however, patients are high-risk once diagnosed. Their high cumulative dose estimates follow the diagnosis. He recommended that physicians review high-risk disease status to inform CT decision-making, pointing out that disease status is already available in the EMR and does not require cumulative dose estimates.
“Almost certainly, radiologists will be contacted on a routine basis by patients and referring clinicians seeking rationale guidance on what to do with high cumulative dose estimates,” wrote Durand. He suggested that radiologists stress that inclusion of dose estimates in the EMR is not useful in the care of individual patients.
Durand emphasized the need for a thoughtful consensus regarding the relevance of such estimates in individual patient care and noted “patients should not be scanned any less based on high cumulative dose estimates alone.
"Instead, the risks and benefits of each radiologic examination should be carefully weighed for each individual patient, a process that involves reviewing information already contained the EMR,” he concluded.