Quantifying variation in DLP- to- effective-dose conversion factors in CT using a Monte Carlo-based dose-estimation software.

BACKGROUND: A common method to estimate the effective dose in CT is to multiply the dose length product in a CT scan by a conversion factor (k). The k-factors are determined by dividing the effective dose (calculated using a weighted summation of organ doses) by the dose length product (DLP) for specific digital phantoms. The k-factors determined in this way may vary substantially depending on the effective dose calculation methods and phantom models used. PURPOSE: The purpose of this study was to estimate variation associated with different acquisition and patient characteristics in the quantification of k-factors. METHODS: K-factors were calculated using VirtualDoseCT for a diverse set of computational phantoms representing different patient models, scanner models, and seven CT exams. The tube potential varied from 80 to 120 kV. The scan length was varied by ± 5 cm relative to the default setting. The CTDIvol was set to 10 mGy when tube current modulation was not used; otherwise, the software default was used. A multivariable regression method was employed to derive an empirical formula for k-factors, along with model-uncertainty analysis and Sobol's sensitivity analysis to identify the influence of each variable. RESULTS: K-factors for all exams increased with kV. Negative dependence of k-factors on BMI was observed, with lower values in males than in females. Moderate to high variation was observed in the quantification of k-factors without TCM, which remained unchanged with TCM. Based on Sobol's indices (0.8-1), k-factors were influenced most by scan length and patient size for adults, and scan length and age for pediatric phantoms. DISCUSSION: The study showed that a large spread in k-factors was predominantly influenced by scan length, patient size and age. The heterogeneity in k-factors may lead to large heterogeneity in the effective dose estimate if the DLP×k method is used. These results indicate that age and size specific k-factors to estimate effective dose from DLP may be warranted. CONCLUSIONS: A set of k-factors should be established to account for these factors and improve effective dose estimations.