Amid Controversies, Dose Reduction Remains Priority
Understanding the controversies surrounding the relationship between CT radiation dose and potential cancer risks better equips radiologists to find ways to reduce the dose, according to a presenter of a scientific poster on Sunday at RSNA 2013.
The National Research Council's Biological Effects of Ionizing Radiation VII (BEIR VII) report, which estimated the associated cancer risk for patients undergoing CT exams, is mathematically complex and not very digestible for radiologists, said Justin E. Costello, D.O., a third-year radiology resident at the San Antonio Military Medical Center.
"Even for most general radiologists, it's difficult to have a really good understanding of BEIR VII," said Dr. Costello, who presented "CT Radiation Dose: A Review of the Current Controversies and Dose Reduction Strategies through Clinical Scenarios and Patient/Clinician Questions." "Our goal was to present the information in a neutral way so that the general radiologist could at least make some sense of it, and may be able to convey that to an ordering provider," Dr. Costello said.
Among the controversies regarding BEIR VII are the report's reliance on atomic bomb survivor data (ABSD), the difficulty in calculating exact CT dose, and use of the linear no threshold (LNT) model, which makes a linear extrapolation of cancer risk at higher radiation doses to include individuals exposed at lower doses, Dr. Costello said.
"It's an assumption that these lower levels of radiation are actually going to cause cancer," he said. "The Health Physics Society has looked at that and said there isn't concrete data to support it. We're causing fear in patients, and we're not even sure if this causes risks."
Among the issues with using ABSD is that the radiation exposure they experienced differs from that given patients undergoing medical imaging, which is a fractionated dose compared to the one-time dose survivors were exposed. The researchers took the ABSD data and extrapolated down to levels considered medical radiation, between 5-10 or 5-100 millisieverts.
"When you compared that to people who weren't exposed to radiation, there's no statistical difference between increased risk of incidence of cancer at those levels," Dr. Costello said.
Despite the controversies, radiologists should still be seeking ways to reduce radiation dose to patients. Dr. Costello reminded radiologists the "small risk you get from that CT scan is far outweighed by the benefit of having an earlier surgery or earlier diagnosis."
He suggested several strategies and techniques to reduce radiation dose, including angular current modulation and iterative reconstruction. "The dose reductions we can get from current modulation are pretty dramatic," Dr. Costello said. "If you looked at five years ago, some of the factors we're doing to current modulation have saved at least half a dose from a CT scan."
Current modulation results in the same savings in breast dose found using bismuth breast shields. However, the shields are no longer used at most institutions due to their considerable drawbacks, which include substantial increases in CT numbers, streaking and beam hardening artifacts, and increased image noise. Iterative reconstruction, meanwhile, improves post-CT processing providing for improved image quality at higher noise indices.
The poster, LL-PHE-SU8A, can be viewed through Friday in the Lakeside Learning Center. Virtual Meeting registrants may log in to see the poster from outside McCormick Place.
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