Trabecular Bone Dosimetry
My primary field of research has been in the area of trabecular bone dosimetry. Dosimetry is the quantitative assessment of how much energy radiation deposits in a specific tissue. Trabecular bone is the "spongy" bone where bone marrow is located. In this region, the skeleton is composed of a lattice of thin bone structures (trabeculae) which surround cavities filled with bone marrow.
Pictured at right is an image of human trabecular bone. The empty spaces are where the bone marrow would reside. Stem cells in the bone marrow are responsible for the production of blood cells.
The trabecular skeleton is of importance to health physicists because radiation damage to the bone marrow is often the limiting factor in radiation therapy procedures that involve injection of a radionuclide into the body. The small size and complex structure of the interlacing bone and marrow regions complicates accurate assessments of radiation dose. An electron may traverse both marrow and bone regions while depositing its energy.
The method for handling this complexity (and the data that is still used today) was published 25 years ago. This method involved using optical microscopy to obtain distributions of path lengths (chord distributions) from two-dimensional slices of trabecular bone.
Methods for Acquiring Chord Distributions.
My first work in trabecular dosimetry involved developing methods for obtaining chord distributions from improved three-dimensional NMR images of trabecular bone. This work was published in December 1998 in Health Physics and was titled "NMR Microscopy of Trabecular Bone and Its Role in Skeletal Dosimetry."
Chord distributions are a collection of the frequency of straight-line path lengths across marrow cavities and bone trabeculae. Obtaining these measurements from digital images is not trivial. Geometrical artifacts (such as those depicted to the right) arise due to the stair-step interfaces associated with digital images.
Computational Model Utilizing Chord Distributions. The chord distributions can then be used to calculate radiation dose to the marrow and bone regions. This entails using a Monte Carlo transport code to transport particles through alternating regions of bone and marrow. My work involved choosing an improved hemispherical geometry for the alternating regions. The model is represented at left where the shaded regions are bone and the clear regions are marrow.
This work was published in the Journal of Nuclear Medicine in November 1999 as one of a three-part special contribution that highlighted our work in skeletal dosimetry. The paper was titled "A Three-Dimensional Transport Model for Determining Absorbed Fractions of Energy for Electrons Within Trabecular Bone."
Calculating Dose Via Coupling of 3D Images. The final part of my doctoral work involved developing a method for calculating dose that does not depend on the use of chord distributions. Using chord distributions has assumptions that I wanted to test. The work consisted of acquiring a three-dimensional NMR image of a human thoracic vertebra (pictured at right) and directly coupling it to a computational computer code that simulates radiation transport. The code simulates how electrons and photons travel through a medium depositing energy. The code transports the particles in geometry provided by the NMR image.
As a result of this work, two articles were published in July of 2001 in Medical Physics. One major conclusion in "Chord Distributions Across 3D Digital Images of a Human Thoracic Vertebra" is that an assumption made in using chord distribution dose models is incorrect. Namely, the use of sampled chord distributions in dosimetry relies on statistical independence of the bone and marrow distributions. The graph ton the left shows that this is not the case. It turns out that large bone chords tend to be followed by smaller marrow chord lengths.
This suggests that alternative methods should be explored. One possible solution is to sample from a three dimensional distribution (shown at left) based on the previously sampled chord length.
Instead of dealing with chord distributions, however, modern image acquisition and computing allow us to simulate radiation transport in the exact geometry. Or rather, the geometry provided by a three-dimensional imaging modality. The second paper published in Medical Physics in July 2001 details this work. "Beta-Particle Dosimetry of the Trabecular Skeleton Using Monte Carlo Transport Within 3D Digital Images" presents the non-chord distribution based model and its consequences.
In this work, the macrostructural boundaries of the skeletal site were modeled for the first time. For the thoracic vertebra, the geometry shown at the left was used to contain the cube of trabecular bone.
Effects of the Bone/Marrow Interface. I undertook a separate problem in trabecular dosimetry in the spring and summer of 2000. It had been theorized by others in published papers that the region of marrow closest to the bone segments would have enhanced doses due to backscattering from the nearby bone surface. I developed another computational code and found that the opposite occurs. That is, the region of the marrow nearest to the bone/marrow interface actually receives less dose than the marrow in the center of the cavity. In other words, the bone shields the nearby marrow. This has very important consequences depending on the actual location of the radiosensitive blood-forming cells within the marrow cavity. The graph to the right shows the fraction of energy absorbed in soft tissue voxels as a function of distance from the bone surface.
An abstract of this work, "The Effects of the Bone-Marrow Interface in Trabecular Bone Dosimetry of Beta-Particles Utilizing Voxel-Based Transport," was published in Health Physics in June of 2000. I presented the results at the Annual Meeting of the Health Physics Society in Denver.
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