Anatomic models needed for internal dose assessment have traditionally been developed using mathematical surface equations to define organ boundaries, shapes, and their positions within the body. Many researchers, however, are now advocating the use of tomographic models created from segmented patient computed tomography(CT) or magnetic resonance (MR) scans. In the skeleton, however, the tissue structures of the bone trabeculae, marrow cavities, and endosteal layer are exceedingly small and of complex shape, and thus do not lend themselves easily to either stylistic representations or in-vivo CTimaging. Historically, the problem of modeling the skeletal tissues has been addressed through the development of chord-based methods of radiation particle transport, as given by studies at the University of Leeds(Leeds, UK) using a male subject. We have proposed an alternative approach to skeletal dosimetry in which excised sections of marrow-intact cadaver spongiosa are imaged directly via microCT scanning. The cadaver selected for initial investigation of this technique was a male subject of nominal body mass index . The objectives of the present study were to compare chord-based versus voxel-based methods of skeletal dosimetry using data from the UF male subject. Good agreement between chord-based and voxel-based transport was noted for marrow irradiation by either bone surface or bone volume sources up to (depending upon the skeletal site). In contrast, chord-based models of electron transport yielded consistently lower values of the self-absorbed fraction to marrow tissues than seen under voxel-based transport at energies above , a feature directly attributed to the inability of chord-based models to account for nonlinear electron trajectories. Significant differences were also noted in the dosimetry of the endosteal layer (for all source tissues), with chord-based transport predicting a higher fraction of energy deposition than given by voxel-based transport (average factor of about 1.6). The study supports future use of voxel-based skeletal models which (1) permit nonlinear electron trajectories across the skeletal tissues, (2) do not rely on mathematical algorithms for treating the endosteal tissue layer, and (3) do not implicitly assume independence of marrow and bone trajectories as is the case for chord-based skeletal models.
This work was supported in part by Grant No. CA96441 from the National Cancer Institute and Grant No. DE-FG07-02ID14327 from the U.S. Department of Energy with the University of Florida. We would also like to thank Scanco Medical AG for their assistance with the use of their commercial scanning service.
II. MATERIALS AND METHODS
II.A. UF trabecular microstructure acquisition
II.B. Chord length distributions for the UF old male
II.C. Chord-based infinite spongiosa transport (CBIST) model
II.D. Voxel-based infinite spongiosa transport (VBIST) model
IV.A. Comparison of CBIST and VBIST for marrow space targets
IV.B. Comparison of CBIST and VBIST for bone endosteum targets
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