Individual Calculation of Effective Dose and Risk of Malignancy Based on Monte Carlo Simulations after Whole Body Computed Tomography

Abstract

Detailed knowledge about radiation exposure is crucial for radiology professionals. The conventional calculation of effective dose (ED) for computed tomography (CT) is based on dose length product (DLP) and population-based conversion factors (k). This is often imprecise and unable to consider individual patient characteristics. We sought to provide more precise and individual radiation exposure calculation using image based Monte Carlo simulations (MC) in a heterogeneous patient collective and to compare it to phantom based MC provided from the National Cancer Institute (NCI) as academic reference. Dose distributions were simulated for 22 patients after whole-body CT during Positron Emission Tomography-CT. Based on MC we calculated individual Lifetime Attributable Risk (LAR) and Excess Relative Risk (ERR) of cancer mortality. ED MC was compared to ED DLp and eD nci. ED DLp (13.2 ± 4.5 mSv) was higher compared to ED nci (9.8 ± 2.1 mSv) and ED MC (11.6 ± 1.5 mSv). Relative individual differences were up to −48% for ED MC and −44% for ED nci compared to ED DLp. Matching pair analysis illustrates that young age and gender are affecting LAR and ERR significantly. Because of these uncertainties in radiation dose assessment automated individual dose and risk estimation would be desirable for dose monitoring in the future. The Euratom council directive (2013/59/Euratom) emphasizes the need for patient radiation dose monitoring in clinical routine 1. Computed tomography (CT) is indispensable for contemporary patient care, but many studies suggest a relation between low dose protracted radiation exposure and an increased incidence of malignancy based on the linear Non-threshold Dose-Response Model 2. Therefore, detailed knowledge of radiation exposure from CT examinations is crucial for radiology healthcare professionals in clinical routine to detect and address increased risks of malignancy. Widely used parameters for radiation exposure assessment like the volumetric CT dose index (CTDI vol) and the Dose Length Product (DLP) characterize scanner radiation output, but are unable to take individual patient characteristics into account 3. Conversion to effective dose (ED DLP) is feasible using population-based conversion factors (k) that take the averaged radiosensitivity in defined anatomic regions into account 4. A variety of different k-factors are recommended in the literature for different volumes (e.g. head, thorax, abdomen, pelvis) and different CT-scanners 5,6. These k-factors are mostly derived from phantom models that try to represent average patient anatomy in western populations, but are unable to respect individual anatomy like missing organs due to aplasia or resection, organ hypo-or hypertrophy, skeletal deformations and metal implants. Nevertheless, these programs, like e.g. the National Cancer Institute (NCI) dosimetry system for CT, can be considered as current academic reference (ED NCI) 7. However, the routinely performed calculation of ED DLP is imprecise and the degree of difference to the real ED in e...

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