Alarming Number of CTs Ordered in Some Patients
By Kerri Wachter
Elsevier Global Medical News
SAN ANTONIO -- Patients admitted and treated for subarachnoid hemorrhage are exposed to alarmingly high cumulative doses of radiation, according to findings from a retrospective, single-center study.
"Stated very simply, 35% of these patients received more radiation during one hospitalization than a worker in a nuclear power plant would be allowed to get over an entire year," Dr. Venkatesh Aiyagari said at the International Stroke Conference.
CT scans have become a routine part of neurologic evaluation, particularly in the emergent setting. "As physicians, we order CT scans all the time but most of us are unaware of the amount of radiation our patient receives from individual CT scans, and we are not in a good position to advise our patients of the risks," Dr. Aiyagari said.
Dr. Aiyagari and his coinvestigators reviewed the radiation exposure of 107 patients with subarachnoid hemorrhage (SAH) who were admitted to a neurological ICU during a 1-year period. Clinical data were obtained from the electronic medical record, and radiation dose was obtained from the CT dose reports.
The patients underwent 1,458 CT scans, 1,351 of which were noncontrast head scans. The median number of noncontrast head CT scans per patient was 9 and the median total number of CT scans per patient was 11.
The researchers performed a univariate analysis to determine predictors of the number of CT scans. They found that an aneurysmal SAH, treatment of an aneurysm, vasospasm, external ventricular drain, number of days intubated, length of ICU stay, floor length of stay, and total length of stay were all predictive of the number of noncontrast head CT scans and total number of CT scans.
On multivariate analysis, ICU length of stay, any treatment of an aneurysm, presence of an external ventricular drain, and vasospasm were independent predictors of the number of CT scans (total and noncontrast head CT) and explained 80% of the variation of the dependent variables, according to Dr. Aiyagari, who is the co-director of the neurosciences critical care division at the University of Illinois in Chicago.
The researchers next calculated the effective dose for each patient. Effective dose is used to estimate radiation risk to the entire body by converting a local (or organ) radiation dose into an equivalent uniform dose to the entire body that carries the same risk of radiation-induced cancers and genetic defects. Effective dose allows assessment of cancer risks and genetic effects regardless of the procedure.
The median effective dose per patient in this study was 28.6 millisievert (mSv) from head CT scans and 30.8 mSv for all CT scans. To put these numbers into context, the researchers divided the effective dose received by patients into four categories based on occupational exposure standards. Low doses were 3 mSv or less; moderate doses were greater than 3 mSv up to 20 mSv; high doses were greater than 20 mSv up to 50 mSv; very high doses were those greater than 50 mSv.
Roughly three-quarters of the patients fell into the high- or very-high-dose categories--35% in the very- high-dose category and 39% in the high-dose category. Only 2% of patients fell into the low-dose category and 24% fell into the moderate-dose category.
Ordinarily, Americans receive less than 3 mSv/year from normally occurring background radiation. The maximum allowable occupational radiation dose (averaged over 5 years) is 20 mSv, according to the National Council on Radiation Protection and Measurement. The maximum yearly occupational exposure) is 50 mSv/year, according to the International Commission on Radiological Protection.
The researchers next calculated the cancer risk from these doses. To do so, they used data from the Biological Effects of Ionizing Radiation VII (BEIR VII) phase 2 report. With these data, it is possible to calculate the cancer risk based on the age at exposure, sex, and dose.
They found that patients in this study have a median lifetime attributable risk of two extra malignancies per 1,000 patients over their lifetimes. The average lifetime attributable risk of malignancy for just the top quartile of patients (in terms of radiation exposure) was one malignancy per 216 patients. The average mean lifetime attributable risk of malignancy for the entire cohort was one malignancy per 488 patients.
"The elephant in the room, of course, is exposure from angiograms. We weren't able to calculate the exact radiation dose because at the time that we did the study, only fluoroscopy time was being monitored. That is an inaccurate way of estimating the total radiation dose," Dr. Aiyagari said.
Given the findings of this study, efforts should be made to reduce medical radiation exposure to a level "as low as reasonably achievable" (ALARA) through improved devices and training, alternative tests (i.e., MRI), and by questioning the necessity for every procedure, Dr. Aiyagari said.
In future studies, the researchers hope to be able to calculate radiation dose from angiograms, track postdischarge radiation exposure, examine the impact of portable CT scanners, and conduct a multicenter comparative study of test-ordering practices and radiation exposure and their correlation to clinical outcomes.
The authors reported that they have no relevant financial relationships.