WPM-B2 - Dosimetric ModelingRoom: B
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| Chair(s): Chris Martel
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WPM-B2.1
Estimates of Total Skeletal Spongiosa Volume for Patient-Specific Scaling of Radionuclide S-Values M.C. Hough, University of Florida
; J.M. Brindle, University of Florida; W.E. Bolch, University of Florida
Abstract: This study focuses on the utilization of spongiosa volumes (volume of all trabecular bone and hematopoietically active or inactive marrow) of both male and female cadaver subjects to create bone site-specific total spongiosa scaling factors, f-factors. These values, which are the average ratios of total spongiosa volume to a reference bone site's spongiosa volume, enable one to predict total spongiosa volumes in a given individual based on manually segmented spongiosa volumes at a single reference skeletal site from a CT scan. The total spongiosa volume may then be used to rescale reference radionuclide S-values to individual patients. Methods: Following in-vivo CT imaging of nine male and ten female criterion-specific cadavers, the spongiosa volumes of 13 reference skeletal sites were segmented via IDL 5.5. Ratios of spongiosa volumes, or f-factors, were calculated for total skeletal spongiosa volume relative to all 13 skeletal sites. These f-factors were then compared by reference site for males, females, and both genders combined. Results: Values of mean f-factors range from 3.3 – 113.6, 3.4 – 125.7, and 3.3 – 102.6 with COV% values ranging from 8.6 – 37.7%, 7.9 – 53%, and 7.9 – 36.5% for all 13 reference skeletal sites in the combined gender, male, and female populations respectively. Conclusions: Using current clinical methodologies, the mean f-factor with the lowest COV% and highest degree of manual segmentation ease was determined to be gender specific with the humeral heads as the reference site.
*This research was performed under appointment to the U.S. Department of Energy Nuclear Engineering and Health Physics Fellowship Program sponsored by the U.S. Department of Energy’s Office of Nuclear Energy, Science, and Technology.
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WPM-B2.2
Modeling Energy Deposition in Trabecular Spongiosa with PENELOPE J.A. Gersh, East Carolina University
; M. Dingfelder, East Carolina University; L.H. Toburen, East Carolina University
Abstract: As highly-energetic particles traverse the complex region
of the human trabecular spongiosa, damage to hematopoietic stem cells caused by energy deposition within red marrow can result in the onset of leukemia. Since the main effects of this disorder are delayed by a latency period as short as two years, the ability to predict biological effects following the irradiation of trabecular spongiosa is of vital importance to NASA, especially as plans proceed for a two- to three-year-long mission to Mars. While experimental methods of dosimetry in this region are hindered by a complex microstructure, Monte Carlo (MC) methods of particle transport offer a viable alternative. The aim of this project is the implementation of a transport model that maintains the integrity of dosimetric data as compared to
previously published data, while keeping the model as simple, non-bone-site-specific, non-patient-specific, and computationally-efficient as possible. Our model creates a static Cartesian-based geometry of the trabecular spongiosa that lends its internal dimensions to a postfixed comparison to path-length distributions as measured by the University of Florida (UF) and the University of Leeds. This model is
designed specifically for implementation into the general-purpose MC transport code PENELOPE. Several transport models have been developed, including tomography-based and chord-length-based models created by UF and by Eckerman and Stabin. While offering insight into the complex problem of irradiation of this region, their results
offer excellent data for benchmarking our model. As a test
of the validity of our model, comparisons are made with these transport models with respect to the calculation of absorbed dose by internal monoenergetic electron-emitters. Thus far, results from simulations using our geometry yield close values as compared to results from the aforementioned
transport models. This work is supported by NASA Grant NNJ04HF39G.
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WPM-B2.3
A Skeletal Reference Dosimetry Model for the Adult Female Kayla Kielar, University of Florida
; Amish Shah, MD Anderson Cancer Center; Wesley Bolch, University of Florida
Abstract: Absorbed dose estimates to the skeletal tissues (active bone marrow and endosteum) are an essential feature of risk estimates in both occupational and medical dosimetry. At present, the vast majority of skeletal reference models (SRMs) used for these purposes are based on studies in the late 1960s and early 1970s at the University of Leeds in which a novel optical scanning method was used to obtain linear chord-length distributions across several skeletal sites of a single 44-year male subject. These data form an essential component of the ICRP’s SRM published in ICRP Publications 30, 70, and 89. Recently, researchers at the University of Florida’s Bone Imaging & Dosimetry Project have developed an image-based skeletal reference model for the adult male at an age representative of cancer patients undergoing radionuclide therapy (66-year). In the present study, initial work on an adult male cancer patient was further developed to add a companion SRM, the adult female cancer patient. A 64-year-old female cadaver was selected having a body-mass index of 22.5 kg m-3 and a cause of death presenting a low probability of skeletal deterioration. In-vivo CT images were acquired prior to bone harvesting at 13 skeletal sites, all with high percentages of active bone marrow. Next, high-resolution ex-vivo CT images were acquired from which volumes of both cortical bone and trabecular spongiosa were determined via image segmentation. Finally, physical sections of spongiosa were cut and imaged via microCT. Both sets of images (ex-vivo CT and ex-vivo microCT) were combined under Paired-Image Radiation Transport (PIRT) via methods described previously by Shah et al. (JNM 46:344-353; 2005). Once fully established, skeletal dose estimates from the UF reference female skeletal model may be scaled to individual patients via CT-based measurements of spongiosa volume (adjustments at the macroscopic level) and potentially CT-based measurements of bone mineral density (adjustments at the microscopic level).
*This research was funded by the USDOE Office of Nuclear Energy, Science, and Technology under the NE/HP Fellowship Program.
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WPM-B2.4
An Image-Based Skeletal Dosimetry Model for the Pediatric Male Deanna Hasenauer, University of Florida
; Christopher Watchman, Department of Radiation Oncology, University of Arizona; Amish Shah, MD Anderson, Orlando, FL; Wesley Bolch, University of Florida
Abstract: Accurate predictions of the radiation dose to bone marrow are important in radiation protection of both medical patients and members of the general public. Current pediatric skeletal models use chord-length distributions that do not account for energy loss to cortical bone or particle escape into soft tissue at high electron energies. Absorbed fraction results for these models are also scaled by cellularity after transport, which has been shown to be an accurate assumption only at high electron energies. Recently, Shah et al. introduced the PIRT (Paired-Image Radiation Transport) model for skeletal dosimetry which accounts for both the trabecular microstructure and the 3D shape and dimension of the adult skeletal site. In this study, we extend this approach to the pediatric skeleton of a 9 month male. Extremely limited data exist on the bone microstructure of children, and thus we have adopted the chord-length distributions for a 1.7-year male acquired at the University of Leeds in the early 1970s. These distributions were used as input to the 3DCBIST (chord-based infinite spongiosa transport) model for electrons and beta-particles, which accounts for cellularity during transport. For the bone macrostructure, we have rescaled each ex-vivo CT image of the UF male skeletal model using dimensions given by the 9 month-male from the UF series of pediatric tomographic models. This model was then used in SIRT (Single-Image Radiation Transport) simulations to account for electron energy escape from spongiosa tissues. The 3DCBIST and SIRT results were then combined to obtain an overall indication of skeletal tissue absorbed fraction as a function of 22 bone sites. The resulting 3DCBIST-SIRT values of AF clearly indicate that as the size of bone decreases, existing skeletal reference models for pediatric individuals (solely based on infinite transport methods) increasingly overestimate the absorbed dose to active bone marrow and trabecular endosteum by not accounting for particle escape to the bone cortex. Furthermore, unlike current pediatric models which are tied to stylized skeletal masses, the SIRT-3DCBIST model is more anatomically refined and applicable to tomographic phantoms. Future efforts include 3D microimaging of an 18 year-old male spongiosa, thus permitting PIRT model simulations as has been done in the adult male.
*This research was funded under the United States Department of Energy Health Physics Fellowship Program.
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WPM-B2.5
Use of Realistic Phantoms in Medical Internal Dosimetry Michael Stabin, Vanderbilt University
; A B Brill, Vanderbilt University; W Paul Segars, Johns Hopkins University
Abstract: Traditional anthropomorphic phantoms used in radiation dosimetry calculations for the past 30 years are based on simple geometric primitives. New medical imaging technologies are now allowing the development and use of more realistic phantoms, providing the radiation dosimetry community with improved models for use in dose calculations. Both standardized phantoms (to replace the widely accepted ‘family’ of models of adults, children, and pregnant women) and models for individual patients may now be based on models based on segmentation of individual medical scans, or on models employing non-uniform rational B-splines (NURBS). Monte Carlo radiation transport methods are then applied to calculate specific absorbed fractions and dose conversion factors for electron and photon sources in whole organs or other regions of the body. This talk will describe current models and methods and provide an update on the current status of the use of realistic organ and body models for internal dosimetry in nuclear medicine.
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WPM-B2.6
Preliminary Effort to include Organ Deformation and Motions in VIP-Man Model * Juying Zhang, Nuclear Engineering and Engineering Physics Programs, Rensselaer Polytechnic Institute, Troy, New York
; Xie Xu, Nuclear Engineering and Engineering Physics Programs, Rensselaer Polytechnic Institute, Troy, New York; Chengyu Shi, Cancer Therapy and Research Center, San Antonio, TX
Abstract: Radiation treatment requires accurate dose planning and radiation delivery according to the target organ volumes. However, respiratory motions of the patient make radiation treatment very challenging. Research has been performed to develop motion-simulating human phantoms for Monte Carlo dose calculations. This paper presents the simulation of respiratory motions in the previously developed VIP-Man (VIsible Photographic Man) model. Control points for each organ are first sampled from the 3D VIP-Man model. Then, Non-Uniform Rational B-Splines (NURBS) are used to simulate the motion of each organ. NURBS uses the transformed 4D control points by clinically obtained respiratory motion data. An 8-field radiation treatment plan of lung tumor is used to study the effect of the respiratory motion using Monte Carlo simulations. Results show that the motion simulation ability allows each of the respiratory cycles to be accurately studied and the 4D VIP-Man model has the potential for improved radiation treatment planning strategies.
*Research funded by National Institutes of Health/NCI (1 R42 CA115122-01) and National Institutes of Health/NCI (1 R01CA116743)
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WPM-B2.7
Post-implant Dosimetry Analysis of Iodine-125 Permanent Seed Brachytherapy Patients by Delineation of Prostate Volumes Using MR Pre-implant and Post-implant Imaging Modality Deana Tuttle, University of Nevada Las Vegas
; Curtis Mack, Arizona Oncology Associates, PC; Michael Taylor, Northwest Permanente PC, Physicians and Surgeons; Mark Yoshino, Southern Arizona Diagnostic Imaging
Abstract: The purpose of this study was to obtain more accurate prostate volumes of iodine 125 brachytherapy patients by fusing pre-implant ultrasound images with post-implant CT and MR images. Post-implant dosimetry analysis determines the dose profile of the prostate due to the location of the implanted seeds and is highly dependent on accurate prostate volume delineation. Research has shown differences of seven percent in volume estimates between pre-implant ultrasound and MR images with a difference as much as forty percent between post-implant CT and MR images, indicating that current imaging modalities are unable to adequately delineate the prostate from surrounding tissue. Therefore, to determine the dose received by the prostate for comparison to the dose prescribed by the physician, standard brachytherapy, in addition to pre and post-implant MR scans, were performed on ten patients that elected for permanent seed implant as treatment of prostate cancer. A pre-implant Transrectal Ultrasound (TRUS) and MR scan were performed to determine prostate volumes with a prescribed dose of 145 Gray. Two weeks afterwards, the patients received iodine-125 permanent seed implant surgery. During the post-operative exam 30 days later, CT and MR scans were performed. Differences in prostate volumes were determined by contouring the prostates in the pre-implant ultrasound and post-implant MR and CT and then fusing the images using the Varian’s VariSeed™ software. Results show various differences between the calculated total dose and the prescribed dose received by the prostate, indicating an over-dosage of the prostate and surrounding tissue.
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