Dose management (including dose-reduction strategies) is a dominant topic of conversation throughout the imaging world. Cross-disciplinary efforts to resolve the issue are moving to the forefront of both vendor and provider dockets, spurred on not least by quality metrics that tie reimbursement rates to patient outcomes.
At UC Davis Medical Center in Sacramento, California, radiology professor J. Anthony Seibert, PhD, and his colleagues are working to lower the radiation dose that their patients receive by some 20%. It’s an ambitious goal, but one that they believe is reasonable, with the implementation of statistical iterative reconstruction techniques, dose modulation, and geometry-based modeling.
The process goes well beyond CT, Seibert says, and is needed much more in interventional radiology, where per-patient radiation doses can exceed those of CT and even PET exams, in many cases. He cites the National Council on Radiation Protection & Measurements as having raised awareness of the overall impact of increased use of CT and nuclear-medicine procedures.
To gauge the scope of that impact, Seibert points out that in the United States, the natural background radiation, at sea level, is about 3 mSv per year (including exposure to cosmic rays and radon gas), but that tally rises to about 6 mSv per year—double the normal background radiation—when it includes the impact of medical-imaging procedures.
Clouding the issue is a lack of understanding, at the provider level, of just how much radiation patients receive as a result of different imaging procedures. “We also have dose indices that may not be indicative of what the true dose to the patient is,” Seibert says.
Parameters such as the volume CT dose index and dose–length product (DLP) do not measure the dose as delivered to the patient, Seibert says; rather, they indicate the dose delivered to a calibration phantom. To the extent that these phantoms fail to match the patient’s body habitus, the inferred dose “might be significantly underestimated or overestimated,” he says.
“It gets very complicated,” Seibert says. “It’s something that’s being actively investigated, but currently, we don’t have a good handle on things as much as we would like. We can be off by a large amount—I’d speculate perhaps two to five times over or under, depending on the situation.”
On most modern equipment, CT scanner manufacturers calculate the volume CT dose index based on acquisition-technique factors and measurements of a calibration phantom that is 16 or 32 centimeters in diameter. In situations that require an accurate measurement of the dose to a specific patient, medical physicists must evaluate the size differences between the calibration phantom and the patient.
A recently published task-group report from the American Association of Physicists in Medicine1 describes a methodology for improving the estimate of the reported volume CT dose index. A large discrepancy often occurs in the imaging of pediatric patients, who can span a wide range of body sizes, complicated by the fact that some manufacturers use the 32-centimeter phantom to estimate the volume CT dose index, and some use the 16-centimeter phantom.
If medical physicists can be confused by discrepancies in the technological information that’s available to them, imagine what one concerned parent could do with some data on radiation dose from Internet sources. Seibert recalls that the mother of one pediatric patient was extremely concerned that her child was overirradiated by a postsurgical CT exam by a factor of four (according to the volume CT dose index), relative to a presurgical planning CT exam.
On further review, it was discovered that the presurgical CT scanner used the 32-centimeter phantom for a pediatric body exam (regardless of the patient’s size), and that the postsurgical CT scanner used the 16-centimeter phantom.
“We went back and performed a size-specific dose estimate to correct the volume CT dose index that’s posted on the scanner, with the patient’ imaging records,” Seibert says. “When we actually did the size-specific dose estimate, it ended up that the patient was not over-irradiated at all.”
He adds, “That just gives you an inkling” of the amount of uncorrected dose information circulating, even where best practices are in use and within the databases of dose registries. There is no doubt that estimates will be improved with time, he says; the first step is to recognize