Despite the growing use of MDCT in seniors, the risk of developing a secondary cancer as the result of CT scans remains very low, concluded researchers from Stanford University, who examined MDCT use in the U.S. Medicare database over eight years and estimated the number of cancer cases resulting from the scans using a model-based analysis.
The researchers divided patients into low-risk and high-risk groups based on cumulative radiation exposure over a four-year period, with low-risk patients categorized as those who were exposed to 50 to 100 mSv of radiation and high-risk patients those who were exposed to more than >100 mSv, according to lead author Dr. Aabed Meer (JACR, April 2012, Vol. 9:4, pp. 245-350).
The authors analyzed eight years of Medicare claims (1998-2005) to evaluate MDCT use. Data were split into two four-year cohorts, examining 5,267,230 claims from 1998-2001, and 5,555,345 claims between 2002 and 2005. The investigators looked at the number and type of scans received over each period to determine the percentage of patients considered to have received low or high threshold levels of cumulative radiation exposure.
CT scans were by far the most common exams in both four-year periods (60% to 58% of all scans, 14% to 10% of total radiation in the first and second periods, respectively), but abdominal imaging delivered the greatest proportion of radiation -- 43% in the first four-year cohort and 40% in the second, representing, 45% to 51% of all scans, the group reported.
Chest CT was third, representing 28% and 36% of all scans, corresponding to 20% and 24% of the radiation dose, respectively, during the first and second four-year periods.
The National Research Council's Biological Effects of Ionizing Radiation (BEIR IV) models were used to estimate the number of radiation-induced cancers that would have occurred in the population. Overall, 4.2% of the patients received low doses and 2.2% received high cumulative doses. The authors estimated 1,659 cancers related to ionizing radiation in the first four-year cohort and 2,185 cases in the second cohort.
Combining both low-risk and high-risk patient groups (50-100 and >100 mSv), as well as those who received < 50 mSv, that authors estimated that only 1,659 cases of cancer (0.03%) would develop in the 1998 to 2001 cohort and 2,185 (0.04%) in the 2002 to 2005 cohort.
The authors cited several limitations to the study, including the theoretical nature of radiation risk, drawn mostly from the analysis of bomb-survivor data. In addition, the use of Medicare claims data makes it impossible to determine study indications and underlying diagnoses. Also missing is data from young adults and children, whose sensitivity to ionizing radiation is known to be greater than that of Medicare-aged adults.
"With advances in high-speed CT technology, the risk for secondary cancers is still very low in the elderly, but radiation doses have been increasing along with the increasing reliance on CT scans for diagnosis and management," Meer and colleagues wrote.