A hyperboliod representation of the bone-marrow interface within 3D NMR images of trabecular bone: applications to skeletal dosimetry

Evaluations of the Renal Cell Carcinoma Model Caki-1 Using a Silicon Based Microvascular Casting Technique

The vascular development of orthotopically grown human renal cell carcinoma (Caki-1) was examined using a silicon based casting technique in nude mice. Images taken of these vascular casts showed Caki-1 tumors to be highly vascularized and invasive. The spread of the tumor within the cortex of the kidney revealed that Caki-1 recruits the kidney's own vasculature, destroying the functional glomeruli in the process. The loss of these glomeruli was further highlighted by the presence of enlarged glomeruli resulting from the development of super nephrons. Vessel size and density measurements were then made in this model. This was done using both computer-based and manual measurement methods. In the vessel size studies the computer-based method tended to overestimate the number of larger diameter vessels whereas the vessels density assessment showed good agreement between the two techniques. Nevertheless, both methods showed that Caki-1 tumors possessed a higher proportion of larger diameter vessels and a lower vessel density than normal kidney cortex.

In summary, silicon based vascular casting proved to be a simple and effective tool for the study of tumor vasculature. In particular this technique could readily be used to examine the invasion of tumor into normal tissue. The computer-based technique for evaluating vessel number and vascular density was found to have merit in both normal and tumor tissues, particularly in the vascular density studies. However, in both settings this technique did tend to overestimate the number of larger diameter vessels.

Investigation of effect of variations in bone fraction and red marrow cellularity on bone marrow dosimetry in radio-immunotherapy

A method is described for computing patient-specific absorbed dose rates to active marrow which accounts for spatial variation in bone volume fraction and marrow cellularity. A module has been added to the 3D Monte Carlo dosimetry program DPM to treat energy deposition in the components of bone spongiosa distinctly. Homogeneous voxels in regions containing bone spongiosa (as defined on CT images) are assumed to be comprised only of bone, active (red) marrow and inactive (yellow) marrow. Cellularities are determined from biopsy, and bone volume fractions are computed from cellularities and CT-derived voxel densities. Electrons are assumed to deposit energy locally in the three constituent components in proportions determined by electron energy absorption fractions which depend on energy, cellularity, and bone volume fraction, and which are either taken from the literature or are derived from Monte Carlo simulations using EGS5. Separate algorithms are used to model primary β particles and secondary electrons generated after photon interactions. Treating energy deposition distinctly in bone spongiosa constituents leads to marrow dosimetry results which differ from homogeneous spongiosa dosimetry by up to 20%. Dose rates in active marrow regions with cellularities of 20, 50, and 80% can vary by up to 20%, and can differ by up to 10% as a function of bone volume fraction. Dose to bone marrow exhibits a strong dependence on marrow cellularity and a potentially significant dependence on bone volume fraction.

An image-based skeletal dosimetry model for the ICRP reference adult male--internal electron sources

In this study, a comprehensive electron dosimetry model of the adult male skeletal tissues is presented. The model is constructed using the University of Florida adult male hybrid phantom of Lee et al (2010 Phys. Med. Biol. 55 339-63) and the EGSnrc-based Paired Image Radiation Transport code of Shah et al (2005 J. Nucl. Med. 46 344-53). Target tissues include the active bone marrow, associated with radiogenic leukemia, and total shallow marrow, associated with radiogenic bone cancer. Monoenergetic electron emissions are considered over the energy range 1 keV to 10 MeV for the following sources: bone marrow (active and inactive), trabecular bone (surfaces and volumes), and cortical bone (surfaces and volumes). Specific absorbed fractions are computed according to the MIRD schema, and are given as skeletal-averaged values in the paper with site-specific values reported in both tabular and graphical format in an electronic annex available from http://stacks.iop.org/0031-9155/56/2309/mmedia. The distribution of cortical bone and spongiosa at the macroscopic dimensions of the phantom, as well as the distribution of trabecular bone and marrow tissues at the microscopic dimensions of the phantom, is imposed through detailed analyses of whole-body ex vivo CT images (1 mm resolution) and spongiosa-specific ex vivo microCT images (30 µm resolution), respectively, taken from a 40 year male cadaver. The method utilized in this work includes: (1) explicit accounting for changes in marrow self-dose with variations in marrow cellularity, (2) explicit accounting for electron escape from spongiosa, (3) explicit consideration of spongiosa cross-fire from cortical bone, and (4) explicit consideration of the ICRP's change in the surrogate tissue region defining the location of the osteoprogenitor cells (from a 10 µm endosteal layer covering the trabecular and cortical surfaces to a 50 µm shallow marrow layer covering trabecular and medullary cavity surfaces). Skeletal-averaged values of absorbed fraction in the present model are noted to be very compatible with those weighted by the skeletal tissue distributions found in the ICRP Publication 110 adult male and female voxel phantoms, but are in many cases incompatible with values used in current and widely implemented internal dosimetry software.

Overview of dosimetry for systemic targeted radionuclide therapy (STaRT)

The purposes of systemic targeted radionuclide therapy dosimetry include compiling a database of normal organ radiation-absorbed doses that are carrier- and radionuclide-specific, and assuring that the normal organ radiation doses are within a safe range before therapy. Also of importance is quantitation of radiation delivery to tumors vs. normal tissues to correlate absorbed dose with tumor control. For agents with significant and variable excretion, estimates of individual patient distribution/clearance may be needed to optimize the dose-response relationship.

Image segmentation of trabecular spongiosa by visual inspection of the gradient magnitude

Recent advances in physical models of skeletal dosimetry utilize high-resolution 3-dimensional microscopic computed tomography images of trabecular spongiosa. These images are coupled to radiation transport codes to assess energy deposition within active bone marrow and trabecular endosteum. These transport codes rely primarily on the segmentation of the spongiosa images into bone and marrow voxels. Image thresholding has been the segmentation of choice for bone sample images because of its extreme simplicity. However, the ability of the segmentation to reproduce the physical boundary between bone and marrow depends on the selection of the threshold value. Statistical models, as well as visual inspection of the image, have been employed extensively to determine the correct threshold. Both techniques are affected by partial volume effect and can provide unexpected results if performed without care. In this study, we propose a new technique to threshold trabecular spongiosa images based on visual inspection of the image gradient magnitude. We first show that the gradient magnitude of the image reaches a maximum along a surface that remains almost independent of partial volume effect and that is a good representation of the physical boundary between bone and marrow. A computer program was then developed to allow a user to compare the position of the iso-surface produced by a threshold with the gradient magnitude. The threshold that produces the iso-surface that best coincides with the maximum gradient is chosen. The technique was finally tested with a set of images of a true bone sample with different resolutions, as well as with three images of a cube of Duocell aluminium foam of known mass and density. Both tests demonstrate the ability of the gradient magnitude technique to retrieve sample volumes or media volume fractions with 1% accuracy at 30 microm voxel size.

A comparison of skeletal chord-length distributions in the adult male

In radiation protection, skeletal dose estimates are required for the tissues of the hematopoietically active bone marrow and the osteogenic cells of the trabecular and cortical endosteum. Similarly, skeletal radiation dose estimates are required in therapy nuclear medicine in order to develop dose-response functions for myelotoxicity where active bone marrow is generally the dose-limiting organ in cancer radioimmunotherapy. At the present time, skeletal dose models in both radiation protection and medical dosimetry are fundamentally reliant on a single set of chord-length distribution measurements performed at the University of Leeds in the late 1970's for a 44-y-old male subject. These distributions describe the relative frequency at which linear pathlengths are seen across both the marrow cavities and bone trabeculae in seven individual bone sites: vertebrae (cervical and lumbar), proximal femur (head and neck), ribs, cranium (parietal bone), and pelvis (iliac crest). In the present study, we present an alternative set of chord-length distribution data acquired within a total of 14 skeletal sites of a 66-y-old male subject. The University of Florida (UF) distributions are assembled via 3D image processing of microCT scans of physical sections of trabecular spongiosa at each skeletal site. In addition, a tri-linear interpolation Marching Cube algorithm is employed to smooth the digital surfaces of the bone trabeculae while chord-length measurements are performed. A review of mean chord lengths indicate that larger marrow cavities are noted on average in the UF individual for the cervical vertebrae (1,038 vs. 910 microm), lumbar vertebrae (1,479 vs. 1,233 microm), ilium (1,508 vs. 904 microm), and parietal bone (812 vs. 389 microm), while smaller marrow cavities are noted in the UF individual for the femoral head (1,043 microm vs. 1,157 microm), the femoral neck (1,454 microm vs. 1,655 microm), and the ribs (1,630 microm vs. 1,703 microm). The mean chord-lengths for the bone trabeculae show close agreement for both individuals in the ilium (approximately 240 microm) and cervical vertebrae (approximately 280 microm). Thicker trabeculae were seen on average in the UF individual for the femoral head (ratio of 1.50), femoral neck (ratio of 1.10), lumbar vertebrae (ratio of 1.29), and ribs (ratio of 1.14), while thinner trabeculae were seen on average in the UF individual for the parietal bone of the cranium (ratio of 0.92). In two bone sites, prominent discrepancies in chord distribution shape were noted between the Leeds 44-y-old male and the UF 66-y-old male: (1) the bone trabeculae in the ribs, and (2) the marrow cavities and bone trabeculae within the cranium.