(C-103) Detailed Skeletal Models Based on Tetrahedral-mesh Geometry for ICRP Reference Adults

Introduction:: The complex structures of skeletal tissues pose a significant challenge for accurate skeletal dosimetry, and faithful modeling of radiosensitive skeletal tissues such as red (active) bone marrow and endosteum is an essential task. Accordingly, previous research has tackled this problem by developing detailed skeletal models based on voxel geometry and combining them with various other techniques. Voxel models have been most commonly and widely utilized for computational dosimetry, but recent studies have emphasized the superiority of mesh geometry over voxel geometry. In the present study, therefore, we developed the first detailed skeletal models based on mesh geometry to improve the accuracy of computational bone dosimetry and will be used in many biomedical fields such as radiopharmaceutical therapy of bone cancer.

Materials and Methods:: In the present study, we developed detailed polygon mesh-based skeletal models consisting of four skeletal tissues: trabecular bone, endosteum, red bone marrow, and yellow bone marrow. First, the trabecular bone was developed directly by converting grayscale value information (first converted into binary format using a simple thresholding function) from the micro-CT images into mesh format via image-processing (i.e., FIJI) and 3D modeling software (i.e., Blender and Rhino 7). The endosteum layer was then defined to have a uniform thickness of 50 μm as measured normal to the trabecular bone surface. For this, again using the 3D modeling software, the trabecular bone was expanded in its normal direction, and any resulting mesh defects were manually removed. The yellow bone marrow, represented by marrow adipocytes, was generated in the marrow cavity using an in-house C++ program based on the Computational Geometry Algorithms Library (CGAL). Finally, the remaining region of the marrow cavity was assumed to be red bone marrow.

Results, Conclusions, and Discussions:: In the present study, a total of 32 adult male and 36 adult female detailed skeletal models were developed in high-quality mesh format, each of which accurately represents four skeletal tissues. After development, the modelling accuracy of the trabecular bone and endosteum was validated qualitatively via tests such as Dice index comparison. In addition, it was confirmed that the red bone marrow and yellow bone marrow were well defined in accordance with ICRP-70 cellularity factors. These detailed skeletal models are expected to significantly improve the accuracy and reliability of skeletal dosimetry when compared to previous models based on the voxel geometry; thus, they have potential to contribute to diverse fields such as radiopharmaceutical therapy. Future work will involve using these models to produce a new set of absorbed fractions and fluence-to-dose response functions for use in skeletal dosimetry of whole-body phantom radiation transport simulations.

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