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A Paired-Image Radiation Transport Model for Skeletal Dosimetry

¹ Department of Biomedical Engineering, University of Florida, Gainesville, Florida
² Department of Nuclear and Radiological Engineering, University of Florida, Gainesville, Florida
³ Deparment of Neurosurgery, University of Florida, Gainesville, Florida
⁴ Department of Health Physics, University of Nevada–Las Vegas, Las Vegas, Nevada
⁵ Department of Physics and Astronomy, Francis Marion University, Florence, South Carolina

Toxicity of the hematopoietically active bone marrow continues to be a primary limitation in radionuclide therapies of cancer. Improved techniques for patient-specific skeletal dosimetry are thus crucial to the development of dose–response relationships needed to optimize these therapies (i.e., avoid both marrow toxicity and tumor underdosing). Current clinical methods of skeletal dose 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 radiation transport (PIRT) model that can provide a more realistic 3-dimensional geometry for particle transport of the skeletal site at both microscopic and macroscopic levels of its histology. Methods: Ex vivo CT scans were acquired of the lumbar vertebra and right proximal femur excised from a 66-y male cadaver (body mass index, 22.7 kg m^–2). For both skeletal sites, regions of trabecular spongiosa and cortical bone were identified and segmented. Physical sections of interior spongiosa were then taken and subjected to nuclear magnetic resonance (NMR) microscopy. Voxels within the resulting NMR microimages were segmented and labeled into regions of bone trabeculae, endosteum, active marrow, and inactive marrow. The PIRT methodology was then implemented within the EGSnrc radiation transport code, whereby electrons of various initial energies are simultaneously tracked within both the ex vivo CT macroimage and the NMR microimage of the skeletal site. Results: At electron initial energies greater than 50–200 keV, a divergence in absorbed fractions to active marrow is noted between PIRT model simulations and those estimated under infinite spongiosa transport techniques. Calculations of radionuclide S values under both methodologies imply that current chord-based models used in clinical skeletal dosimetry can overestimate dose to active bone marrow in these 2 skeletal sites by 4%–23% for low-energy ß-emitters (³³P, ¹⁶⁹Er, and ¹⁷⁷Lu), by 4%–25% for intermediate-energy ß-emitters (¹⁵³Sm, ¹⁸⁶Re, and ⁸⁹Sr), and by 11%–30% for high-energy ß-emitters (³²P, ¹⁸⁸Re, and ⁹⁰Y). Conclusion: 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 S values for intermediate-to-high ß-emitters.

Key Words: skeletal dosimetry • marrow dose • nuclear magnetic resonance microscopy • radionuclide S value • absorbed fraction

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