NEW YORK (Reuters Health) – New cardiac k-factors more accurately estimate effective doses of radiation for cardiac CT scans. Older conversion factors significantly underestimated the doses.
“We were surprised by the magnitude of the underestimation of radiation dose to patients from CT scans of the heart when currently employed methodology is used,” Dr. Andrew J. Einstein, of Columbia University Medical Center and New York Presbyterian Hospital, told Reuters Health by email. “We found that the current methodology underestimates radiation dose by an average of 46% or, in other words, that the actual effective dose of radiation averages 86% higher than estimates with the current methodology.”
The simplest and most commonly used method to estimate radiation effective dose for CT scans is to multiply the dose-length product by a conversion factor, called the k-factor. The currently used chest k-factor (0.014 mSv/mGy/cm), however, was designed for thoracic CT and not for cardiac studies.
Dr. Einstein and colleagues systematically determined k-factors for 12 contemporary coronary CT angiography (CTA) scanner designs and more than 120 contemporary CTA protocols to provide a single data source for more accurately estimating radiation effective doses. Specifically, they used solid-state metal-oxide-semiconductor field-effect transistor dosimeters placed in an anthropomorphic phantom – and determined k-factors relating these effective doses to scanner-reported dose-length products.
Their report is published in JACC: Cardiovascular Imaging, online August 16.
The mean (and median) k-factor was 0.026 mSv/mGy/cm, ranging from 0.020 to 0.035 mSv/mGy/cm, depending on which CT scanner was used. By comparison, the current chest k-factor of 0.014 mSv/mGy/cm underestimates effective dose by an average of 46% (range, 30% to 60%).
“It’s critical to use up-to-date methodology, such as that developed in this paper, when estimating radiation doses of cardiac CT scans,” Dr. Einstein said, noting the importance of this point for all medical imaging procedures.
“It is important not to take away an alarmist message from this paper,” he said. “Numerous centers already employ a combination of dose-reduction techniques to achieve low radiation doses from CT scans of the heart, even when using the updated methodology for determining radiation dose. Thus, it behooves clinicians to refer their patients to such centers, and for doctors performing these procedures to emulate these centers by implementing dose-reduction techniques and tailoring scans to each patient.”
Dr. Mahadevappa Mahesh from The Johns Hopkins University School of Medicine, Baltimore, who wrote an accompanying editorial, told Reuters Health by email, “The main clinical implications of the findings are that it may increase concern among physicians if they do not understand the limitations of effective dose estimations. If they simply focus on the proposed conversion factors (k-factors) and assume that the cardiac CT doses are now higher than before, then they may hesitate to order cardiac CT scans even when it is clinically appropriate to order for their patients. I am concerned about this, and hence in the editorial I focused specifically on providing a scenario and how to handle questions in such scenario.”
“As long as the clinical indications are strong to order a cardiac CT scan, physicians should not place too much emphasize on the effective dose estimations,” he said, “because effective dose estimations are not applicable to assessing individual risks, but rather to estimate population risk, as was the original intent when effective dose metric was proposed.”
“Another message I would hope physicians take away is to recognize medical physicists who can play a key role in radiation-dose assessment – and provide guidance regarding interpreting effective dose values and discussing benefits and risks associated with medical imaging,” Dr. Mahesh said. “Qualified medical physicists are knowledgeable with providing a balanced approach and also help physicians to review and optimize cardiac CT protocols.”
He added, “It is heartening to see the radiation dose values for cardiac CT are trending lower in the past few years due to a number of technological advances and also user awareness. This in a way should help physicians in justifying the clinically appropriate cardiac CT scans and provide a proper perspective on radiation risks.”
SOURCES: http://bit.ly/2wAZssl and http://bit.ly/2vWavM5
JACC Cardiovasc Imaging 2017.
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