SCATTER RADIATION INTENSITIES IN HORIZONTAL AND VERTICAL PLANES ABOUT DIGITAL BREAST TOMOSYNTHESIS SYSTEM.

Abstract

Digital breast tomosynthesis (DBT) is a main imaging modality for breast imaging. However, shielding calculations for DBT are commonly based on previous technology with softer beam spectra. In addition, shielding calculations often assume some patient attenuation is provided in locations to the posterior of the patient but without quantification of this attenuation. In this work, recent research in DBT shielding is validated by measuring scatter radiation in the vertical plane. It is also extended upon by measuring the scatter radiation in the horizontal plane and the effect of patient attenuation. These measurements are weighted by our local patient compressed breast thickness distribution to provide a simple scattering factor. Air kerma distributions are provided demonstrating the distribution of scatter radiation around the DBT system in the presence of patient body attenuation. The highest air kerma measured in any direction from a 4-view per patient screening tomosynthesis exam based on the local patient workload is 55 μGy at 1 m. This value can be used to conservatively treat the scatter radiation as an isotropic distribution for shielding assessments. This work is consistent with many recent publications, with the notable exception of the forward scattered peak which is smaller than previous studies. Under our locally encountered breast thickness distribution, the increased scatter radiation and consequent increased minimum shielding requirements is small in most cases. With 400 patients per week, the air kerma at 1 m is 22 mGy and, for a fully occupied public area 2 m away, this requires a gypsum plasterboard thickness of 18.0 mm which is 3.6 mm more than previous data suggests. Although the increase is minimal, standard manufacturing thicknesses of shielding material may not be automatically assumed to be sufficient and careful consideration of the scatter radiation is warranted.