May 3, 2012 (Miami, Florida) — The added lifetime cancer risk from multiple computed tomography (CT) scans appears to be quite minimal, David Okonkwo, MD, PhD, associate professor of neurological surgery and clinical director of the of the Brain Trauma Research Center at the University of Pittsburgh in Pennsylvania, reported here at the American Association of Neurological Surgeons 80th Annual Meeting.
Although the usefulness of CT studies in acute trauma is "indisputable," he said, public and professional concern about the potential cancer risk from frequent exposure to ionizing radiation has led to efforts to reduce the use of CT.
Younger patients are most susceptible to the additional cancer risk from ionizing radiation, but they are also the ones with the highest rates of traumatic injury. However, the overall life expectancy for those who survive 6 months after injury is similar to that of their age- and sex-matched peers. "So we have patients who are young, getting tons of CT scans, getting exposed to lots of ionizing radiation, and who, if they live past 6 months, are now facing, potentially, the downstream risk of cancer," Dr. Okonkwo said.
The Brain Trauma Research Center has a large prospective database of patients with traumatic brain injury. Therefore, Dr. Okonkwo and colleagues were able to calculate the estimated cumulative lifetime risk for cancer attributable to CT studies in a cohort of survivors of severe traumatic brain injury.
They recorded the number of CT scans each patient received and the amount of radiation, and then paired that with a national database on the lifetime attributable risk of cancer in relation to CT exposure (Biological Effects of Ionizing Radiation VII, tables 12D-1 and 12D-2, p. 311). Lifetime attributable risk is the additional risk above the baseline cancer risk.
Dr. Okonkwo questioned some of the calculated excess cancer risk in the report, saying that "if this were really true, the meningioma practice in the United States would be out of control, but it's not. I always take it with a grain of salt, but this is the best available evidence." He agreed with the some of the relative risks related to excess CT scans, such as women being more susceptible than men and younger people being more at risk than older ones.
The 2007 to 2010 study involved subjects (81% men) with an average age of 34 years (range, 16 to 80 years. They had a Glasgow Coma Scale (GCS) score of 8 or less (median, 7), could not follow commands, and had neurologic deterioration within 24 hours of admission. Exclusion criteria were a GCS score of 3, bilateral fixed pupils, imminent brain death, and loss to follow-up or death within 1 year of injury.
Patients received a median of 20 CT scans in the first year after injury (range, 8 to 71). The average cumulative effective dose was 87 ± 75 mSv (range, 34 to 234 mSv).
A CT to the head was the most common imaging performed (51% of scans), "but it only contributed 29% of the radiation," Dr. Okonkwo reported. Although there were fewer CT scans of the chest, abdomen, and pelvis, they delivered much more radiation (24% of scans but 41% of the radiation).
Very Low Excess Risk for Cancers From Multiple CT Scans
"The mean lifetime attributable risk for all types of cancer with respect to exposure to ionizing radiation was 0.81% ± 0.54%," Dr. Okonkwo said, "and the mean lifetime attributable risk of cancer-related mortalities...was 0.44% ± 0.27%." He explained that considering that a 34-year-old man has a 44% lifetime risk of developing cancer, the additional 0.81% risk is quite small.
In light of the fact that mortality from traumatic brain injury is 25% to 50%, "to me, this says just take care of the patients," he said. "Do what you've got to do to save the patient's life, and worry about this stuff 35, 40, 45 years from now because it may not actually even exist."
Odette Harris, MD, MPH, associate professor of neurosurgery at Stanford University in Palo Alto, California, and director of the Defense and Veterans Brain Injury Center at the Palo Alto Veterans Affairs Hospital, discussed the presentation and said the issue is really a matter of risk vs benefit.
She said that of the 195 patients initially enrolled, 67 were ultimately available for complete analysis. The others were lost to follow-up or had died. As study limitations, Dr. Harris identified the small number of subjects (leading to wide standard deviations), the use of models for risk estimation, and the use of doses in the literature and mathematical phantom calculations for the tissue radiation doses received (as opposed to actual doses).
"Although I believe very strongly that the conclusions are consistent with sound clinical judgment, and I have absolutely no concerns about the conclusions that were drawn...it must be noted that the conclusions do not come directly from the data presented," Dr. Harris said, adding that they are strong grounds for future research.
Dr. Okonkwo noted that when his group publishes the study, they will use actual patient data. Still, "the list of unknowns is huge," he said, such as the amount of radiation scatter and how much is actually absorbed by the head. He noted that one of his coauthors, the institutional radiation safety officer, was the person who was most shocked at the small incremental lifetime risk for cancer attributable to the large number of CT scans.
Dr. Okonkwo added that radiation exposure from CT "pales in comparison to digital subtraction angiography. We really do have a whole cohort of glowing colleagues who are doing endovascular procedures; the radiation exposure in digital subtraction angiography is insane."
Dr. Okonkwo reports financial relationships with Medtronic and Lanx, and receiving grant support from Neuren Pharmaceuticals and BHR Pharma.
American Association of Neurological Surgeons (AANS) 80th Annual Meeting: Abstract 652. Presented April 16, 2012.
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