Current methods of skeletal dose assessment in both medical physics (radionuclide therapy) and health physics (dose reconstruction and risk assessment) rely heavily on a single set of bone and marrow cavity chord-length distributions in which particle energy deposition is tracked within an infinite extent of trabecular spongiosa, with no allowance for particle escape to cortical bone. In the present study, we introduce a paired-image radiationtransport (PIRT) model which provides a more realistic three-dimensional (3D) geometry for particle transport in the skeletal site at both microscopic and macroscopic levels of its histology. Ex vivoCT scans were acquired of the pelvis, cranial cap, and individual ribs excised from a male cadaver (BMI of ). For the three skeletal sites, regions of trabecular spongiosa and cortical bone were identified and segmented. Physical sections of interior spongiosa were taken and subjected to microCT imaging. Voxels within the resulting microCT images were then segmented and labeled as regions of bone trabeculae, endosteum, active marrow, and inactive marrow through application of image processing algorithms. The PIRT methodology was then implemented within the EGSNRCradiationtransport code whereby electrons of various initial energies are simultaneously tracked within both the ex vivoCT macroimage and the CT microimage of the skeletal site. At initial electron energies greater than , a divergence in absorbed fractions to active marrow are noted between PIRT model simulations and those estimated under existing techniques of infinite spongiosa transport. Calculations of radionuclide values under both methodologies imply that current chord-based models may overestimate the absorbed dose to active bone marrow in these skeletal sites by 0% to 27% for low-energy beta emitters (, , and ), by to 49% for intermediate-energy beta emitters (, , and ), and by to 76% for high-energy beta emitters (, , and ). The PIRT methodology allows for detailed modeling of the 3D macrostructure of individual marrow-containing bones within the skeleton thus permitting improved estimates of absorbed fractions and radionuclide values for intermediate-to-high energy beta emitters.
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. Cadaver selection
II.B. In vivocomputed tomography scanning
II.C. Bone harvesting and ex vivocomputed tomography scanning
II.D. Image segmentation of spongiosa and cortical bone regions
II.E. Micro-computed tomography of trabecular spongiosa
II.F. Voxel-based infinite spongiosa transport (VBIST) model
III.G. Paired imageradiationtransport (PIRT) model
III. RESULTS AND DISCUSSION
III.A. Absorbed fractions to active marrow within the pelvis
III.B. Absorbed fractions to active marrow within the cranium
III.C. Absorbed fractions to active marrow within the rib cage
III.D. Absorbed fractions to endosteal tissues
III.E. Radionuclide values between VBIST and PIRT model simulations
Data & Media loading...
Article metrics loading...
Full text loading...