WE-A-201B-05: Dosimetric Characterization of a Dedicated Breast Computed Tomography Clinical Prototype System

Abstract

Purpose: To determine the radiation dosimetry characteristics of a dedicated breast computed tomography (BCT) clinical prototype system. Method and Materials: The relevant parameters of the BCT system were replicated in a Monte Carlo simulation program based on the Geant4 toolkit. To model the system's x‐ray spectrum, empirical measurements of exposure under varying conditions and of the first half‐value layer were performed. To validate the Monte Carlo program, the exposures at the five insert positions of a standard head CT phantom while undergoing both a scout image and a full BCT scan were estimated, and the results compared to empirically measured values. The average glandular dose (AGD) was characterized for simulated homogeneous semi‐ellipsoidal breasts of varying sizes and glandular fractions. In all cases, the AGD was normalized to the exposure measured with a CTionization chamber in air located at the isocenter. Results: The modeled x‐ray spectrum was able to predict the empirically measured x‐ray exposures after varying thicknesses of aluminum to within 9.4%, while the exposure distribution at the inserts of the standard head CT phantom were estimated to within 11.5%. The normalized AGD to the homogeneous breasts varied from 2.51 mGy/R to 5.21 mGy/R, depending on breast size and glandular fraction. For an average breast, for which the system automatically sets the tube current at 100 mA, the AGD would be 7.55 mGy. Conclusion: The glandular dose from this BCT system was characterized for a range of breast sizes and compositions, and was found to result in similar values compared to typical multi‐view diagnostic mammographic examinations but higher values than a standard two‐view screening mammography examination. Further studies including the variation between mean glandular dose to homogeneous breasts and actual glandular dose to breasts with real tissue distribution are ongoing.