Multiscale Monte Carlo simulations for dosimetry in x-ray breast imaging: Part II - Microscopic scales

BackgroundAlthough the benefits of breast screening and early diagnosis are known for reducing breast cancer mortality rates, the effects and risks of low radiation doses to the cells in the breast are still ongoing topics of study.PurposeTo study specific energy distributions (f(z,Dg)$f(z,D_{g})$) in cytoplasm and nuclei of cells corresponding to glandular tissue for different x-ray breast imaging modalities.MethodsA cubic lattice (500 mu m length side) containing 4064 spherical cells was irradiated with photons loaded from phase space files with varying glandular voxel doses (Dg$D_{g}$). Specific energy distributions were scored for nucleus and cytoplasm compartments using the PENELOPE (v. 2018) + penEasy (v. 2020) Monte Carlo (MC) code. The phase space files, generated in part I of this work, were obtained from MC simulations in a voxelized anthropomorphic phantom corresponding to glandular voxels for different breast imaging modalities, including digital mammography (DM), digital breast tomosynthesis (DBT), contrast enhanced digital mammography (CEDM) and breast CT (BCT).ResultsIn general, the average specific energy in nuclei is higher than the respective glandular dose scored in the same region, by up to 10%. The specific energy distributions for nucleus and cytoplasm are directly related to the magnitude of the glandular dose in the voxel (Dg$D_{g}$), with little dependence on the spatial location. For similar Dg$D_{g}$ values, f(z,Dg)$f(z,D_{g})$ for nuclei is different between DM/DBT and CEDM/BCT, indicating that distinct x-ray spectra play significant roles in f(z,Dg)$f(z,D_{g})$. In addition, this behavior is also present when the specific energy distribution (Fg(z)$F_{g}(z)$) is considered taking into account the GDD in the breast.ConclusionsMicrodosimetry studies are complementary to the traditional macroscopic breast dosimetry based on the mean glandular dose (MGD). For the same MGD, the specific energy distribution in glandular tissue varies between breast imaging modalities, indicating that this effect could be considered for studying the risks of exposing the breast to ionizing radiation.