Organ-based and DLP-based effective dose as representations of radiation risk in a population of 8946 patients with cumulative effective dose greater than 100 mSv
Purpose. Recent studies have shown that it is not uncommon for a patient to undergo multiple CT exams resulting in high cumulative dose above 100 mSv, the radiation risk associated with which is not negligible. The purpose of this study was to compare the estimated radiation risk in a large population of such cases based on effective dose to estimates of risk index including patient age. Materials and Methods. This IRB-approved study included 8946 patients who underwent multiple CT exams over 5 years resulting in a cumulative effective dose over 100 mSv. Organ doses were estimated using Monte Carlo methods. DLP-based effective dose (Ek) and organ-dose based effective dose (EOD) were calculated following ICRP 102 and ICRP 103 publications. The organ-dose based risk index (RI) was calculated according to BEIR VII risk coefficients. A linear regression was applied to assess each metric’s dependency to RI, assumed to be the closest patient risk surrogate. The relative sensitivity of EOD and Ek to the estimated risk was calculated in six age groups (30 to 80 years old) in terms of a Risk Sensitivity Index (RSI) computed as a normalized fit slope by the ratio of the mean value of RI for each metric. Results. Across all patients, EOD for the 100 mSv+ cohort ranged between 100.2 and 1184.7 mSv, Ek between 54.1 and 1031.9 mSv, and RI between 152.9 and 7785.1 cancers per 105 patients. Per each age group, the fit R2 was <0.004 for the linear regression of Ek vs. RI and between 0.72 and 0.97 for EOD vs. RI implying that RI and EOD are linearly related. As anticipated, the fit slope increased with patient age. The RSI was <3.15×10-4 for Ek and ranged between 0.01 and 0.26 for EOD. Conclusion. For patient exposed to high cumulative dose (>100 mSv), care should be exercised to properly assess the risk figures and to draw risk predictions from metrics unrepresentative of population risk. Compared to effective dose drawn from DLP, effective dose based on organ doses provides a closer representation of patient and population risk, provided stratification by specific age groups. Clinical Relevance statement. When patients undergo recurring CT exams, the radiation induced risks should be carefully estimated using metrics that incorporate organ dose and patient age.
