The purpose of this work was to estimate effective and organ doses from a low-dose lung cancer screening protocol using tube current modulation (TCM) and patient models of various sizes.
METHOD AND MATERIALSMonte Carlo simulation methods were used to estimate effective and organ doses from a low-dose lung cancer screening protocol for a 64-slice CT (Sensation 64, Siemens Healthcare) that used TCM. Scanning parameters were from the AAPM’s Alliance for Quality CT on-line protocols. Ten GSF voxelized patient models that had all radiosensitive organs identified were used to facilitate estimating both organ and effective doses. Predicted TCM schemes for each patient model were generated using a validated method wherein tissue attenuation and scanner limitations were used to determine the TCM output as a function of table position and source angle. The water equivalent diameter (WED) was determined by estimating the attenuation at the center of the scan volume for each patient model. Monte Carlo simulations were performed using the unique TCM scheme for each patient model. All organ doses were tallied and effective doses were estimated using ICRP 103 tissue weighting factors. All dose values were normalized by scan-specific dose-length product (DLP) from 32 cm CTDIvol values that used the average tube current across the entire length of the simulated scan. Absolute and normalized doses were reported as a function of WED for each patient model.
RESULTSFor all ten patient models, the effective dose using TCM protocols was below 1.5 mSv. Smaller sized patient models experienced lower absolute doses compared to larger sized patients. DLP-normalized effective, lung, thyroid, and breast doses possessed an exponential relationship with respect to patient size with coefficients of determination of 0.73, 0.72, 0.24, and 0.73, respectively.
CONCLUSIONEffective doses for a low-dose lung screening protocol using TCM were below 1.5 mSv for all patient models used in this study. Strong correlations existed between DLP-normalized effective, lung, and breast doses, while thyroid doses showed some dependence on patient size.
CLINICAL RELEVANCE/APPLICATIONThese results, along with the scanner-reported DLP and WED, can be used to estimate effective, lung, thyroid, and breast doses from lung screening CT exams that use TCM.