Adipocyte Spatial Distributions in Bone Marrow: Implications for Skeletal Dosimetry Models

Few studies have been conducted to quantify the spatial distributions of adipocytes in the marrow cavities of trabecular bone. Nevertheless, such data are needed for the development of 3-dimensional (3D) voxel skeletal models where marrow cellularity is explicitly considered as a model parameter for dose assessment. In this investigation, bone marrow biopsies of the anterior iliac crest were examined to determine the size distribution of adipocyte cell clusters, the percentage of perimeter coverage of trabecular surfaces, and the presence or absence of adipocyte density gradients in the marrow space, all as a function of the biopsy marrow cellularity (5%-95%). Biopsy slides from 42 patients were selected as designated by the hematopathologist as either normocellular or with no evidence of disease. Still-frame video image captures were made of 1-3 regions of interest per biopsy specimen, with subsequent image analysis of adipocyte spatial characteristics performed via a user-written MATLAB routine. A predictable shift was found in cluster size with decreasing marrow cellularity from single adipocytes to clusters of >or=3 cells; the percentage of 2-cell clusters remained relatively constant with changing cellularity. Also, a nonlinear increase in trabeculae perimeter coverage was found with increasing fat tissue fraction at marrow cellularities between 50% and 80%. Finally, it was demonstrated that only in the range of 20%-50% marrow cellularity was a slight gradient in adipocyte concentration indicated with adipocytes localized preferentially toward the trabecular surfaces. Electron transport simulations were conducted in 4 different 3D voxel models of trabecular bone for sources localized in the active marrow (TAM), bone volume (TBV), bone endosteum (TBE), and bone surfaces (TBS). Voxel model simulations demonstrated that absorbed fractions to active marrow given by the ICRP 30 model (MIRDOSE2) are exceedingly conservative for both TBV and TBS sources, except in the case of high-energy particles (>500 keV) at high values of marrow cellularity (>70%). Values of both phi(TAM