CHICAGO -- In an elderly population, cancer risk from CT scans may be overestimated, researchers said here.
In risk prediction models using Medicare data, cancer incidence related to ionizing radiation was between 0.02% and 0.04%, compared with standard estimates of 1.5% to 2% in the general population, according to Pat Basu, MD, of Stanford University.
"This is surprisingly lower than we expected," Basu said during a press briefing at the Radiological Society of North America meeting.
Action Points
- Note that this study was published as an abstract and presented at a conference. These data and conclusions should be considered to be preliminary until published in a peer-reviewed journal.
- Explain that a retrospective review of Medicare patients found that secondary cancer risk following CT scan radiation exposure was less than that of the general population.
- Note that fewer than 6% of patients had 100 mSV or more from CT scans and that most underwent head CT while abdominal CT, pelvic, and chest CT represent the studies with the highest radiation exposure.
Patients were followed for no more than four years, however -- a short period during which resultant cancers might not manifest.
"Ideally, we need large population studies over a long period of time to understand the advantages and disadvantages," Basu said.
The researchers conducted a retrospective study of Medicare claims data on 13,622,840 patients from 1998 to 2005.
They split the patients into two cohorts -- one from 1998 to 2001 and the other from 2002 to 2005 -- to reflect differences in use of imaging, and then used BEIR VII models to estimate the number of cancers induced by these examinations.
Most patients in the study had CT scans of the head, although abdominal imaging -- which delivers four times the radiation of a head scan -- accounted for the greatest proportion of radiation (40%), followed by pelvic imaging and chest imaging.
Basu and colleagues found that the proportion of patients who had imaging studies increased between the two time periods, and that more patients were receiving higher doses of radiation.
For instance, from 1998 to 2001, 42% of patients had scans compared with 50% of those in the later cohort.
Basu said that an increasing number of patients received higher doses of radiation -- more than 50 mSv -- in the 2002 to 2005 time period.
Still, less than 6% of all patients received doses greater than 50 mSv, he added.
"The majority of patients had 1 to 50 mSv, and less than 6% of patients received a significant amount of radiation," Basu said, referring to an established figure in the medical literature associating 50 to 100 mSv of radiation with "some increase in cancer incidence."
For comparison, 10 round-trip continental flights expose a passenger to 1 mSv of radiation, and healthcare workers are limited to 20 mSv of radiation per year. An astronaut receives about 200 mSv per year, Basu said.
In their risk prediction models, the researchers calculated that cancer incidence related to ionizing radiation in this study was 0.02% in 1998-2001 and 0.04% in 2002-2005.
Basu said the findings could even apply to middle-age patients -- even though this group was not assessed in the study -- because their biological sensitivity to radiation is similar to that of older patients.
Yet they would not apply to patients in their 20s and younger, he said, because the biological sensitivity in this group is much greater.
Max Wintermark, MD, of the University of Virginia in Charlottesville, agreed that the findings could be applied to those in middle age.
He also said the risk estimates may be more applicable on a population level because elderly patients account for the greatest use of CT imaging, and young people aren't subject to these scans as often.
"In younger patients we are always trying to do something else that doesn't involve radiation," he told 51˶. "We try to use MRI or ultrasound instead, if we can."
Basu added that it will be important to do longer-term, prospective studies in the future in order to capture radiation risk at all time points. Ideally, that would involve being able to follow a patient's lifetime radiation exposure and subsequent cancers.
"I would love to have that dataset," he said.
Disclosures
The researchers reported no conflicts of interest.
Primary Source
Radiological Society of North America
Source Reference: Meer A, et al "Exposure to ionizing radiation and estimate of secondary cancers in the era of high speed CT scanning" RSNA 2010; Abstract SSK08-04.