Digital Breast Tomosynthesis (DBT) Dose Survey and the establishment of a DRL for a National Breast Screening Service

Digital breast tomosynthesis (DBT) has proven invaluable to BreastCheck as an adjunct to 2D imaging, and since 2014, BreastCheck has commissioned 15 Hologic Selenia Dimensions DBT systems. Risk assessment is one of the many important reasons for performing a breast dose survey. This study is a presentation of the results of the first dose survey performed on the new 3D imaging systems. Mean glandular dose (MGD) is the commonly accepted measure of mammographic radiation risk and was calculated for each 3D image according to Dance et al. The survey used the exposure data from at least 45 examinations per DBT unit and dose calculations were performed using the NCCPM breast dose survey software. Dose survey results were in good agreement with values published in the literature. As expected the average 3D breast dose exceeds the average 2D dose (Ratio = 1.28). The estimate of the 3D imaging DRL (2.6 mGy) was based on the average MGD of an oblique exposure of an average sized breast. Survey results indicated a drop in breast dose for breast thickness greater than 90 mm. The manufacturer has confirmed this occurs by design to minimize long exposure times and focal spot blur which could occur for the larger breast thicknesses. The strong comparison between calculated and displayed doses should simplify future surveys.