Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session Z26: Radiation Therapy and Medical Imaging Techniques |
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Sponsoring Units: DBP Chair: Paul Gueye, Hampton University Room: Baltimore Convention Center 323 |
Friday, March 17, 2006 11:15AM - 11:27AM |
Z26.00001: Monte Carlo study of interseed attenuation and tissue composition effect for clinical cases of prostate permanent implants. Jean-Francois Carrier, Luc Beaulieu Monte Carlo simulations were used to study the interseed attenuation and the effect of tissue composition on prostate implant dosimetry. Using computed tomography images of postimplant analysis, the precise anatomy of the patient was considered voxel by voxel. The physical density of each voxel was set according to the Hounsfield Unit and the specific elemental composition of each voxel was set depending on the radiation-oncologist organ contours and the local density. Mixes of different tissues were available: muscle, prostate tissue, rectum tissue, adipose tissue, bone and prostate calcification. Typically, more than 300 combinations of elemental composition and density were used for each patient. The Monte Carlo dosimetry results were compared to the clinically approved TG43-based calculations for 30 patients. The results show an interseed attenuation of about 4.5{\%} for the D$_{90 }$parameter (minimal dose received by 90{\%} of the target volume). The effect of the tissue composition varies from one patient to the other. Globally, the difference between the TG43-based calculations and the Monte Carlo results can reach more than 10 Gy for the D$_{90}$ values. From a clinical perspective, the difference level can be non-negligible for the target volume and for the surrounding organs at risk. [Preview Abstract] |
Friday, March 17, 2006 11:27AM - 11:39AM |
Z26.00002: Effects Of The Inhomogeneity of Brachytherapy Sources In Cancer Treatments Nnenna Onumah Uniformity of radioactive sources is vital in delivering accurate doses in Brachytherapy. The International Atomic Energy Agency (IAEA) defines source uniformity as no more than a 20 {\%} deviation from the average value of the dose along a transverse region. Brachytherapy induced cell damages occur at the microdosimetric levels, and as such, small deviations in dose delivered from different geometrical positions on the source can lead to huge deviations in proper treatment. A Geant4 simulation of a uniform source and a non-uniform source was simulated to check the validity of IAEA's proposed definition. A realistic source of non-uniformity, air bubbles of differing diameters (from 20 to 80 microns) were simulated and their uniformity checked against the model suggested by IAEA in two ways: (1) using the average obtained from the non-uniform source (2) using that obtained from the uniform source. Significant deviations of up to 50{\%} were observed. These results validate the need for the scintillating fiber based detector currently in development within our research group. [Preview Abstract] |
Friday, March 17, 2006 11:39AM - 11:51AM |
Z26.00003: Absolute Dose Distribution Measurements Of Beta Sources Using A Scintillating Fiber Based Detector Lawrence Tynes Brachytherapy using catheter based high dose rate $\beta $ sources has taken nowadays an important role in out-patient cancer treatments. At the present time, there is no detector system which can record accurate quantitative doses and spatial information for Brachytherapy sources. This is partly because of the short-range of the low energy beta emission of these sources (having typically an average energy of a few hundreds of keV), and the resolution limitation of existing extrapolation chambers and radiochromatic-dye films (currently the preferred method used). For the latter, transfer of data from one method to the other can lead to a loss of accuracy as high as 7{\%}, well beyond the required 2{\%} limits of current protocols. We have developed a scintillating fiber based beta detector prototype which will provide solution to this problem. It is composed of an array of 0.5 mm$^{2}$ and 1 mm$^{2}$ fibers optically coupled to photo-multiplier tubes for photon-to-current conversion. Preliminary results from high dose rate $^{90}$Sr/$^{90}$Y and $^{32}$Ru sources comparing film dosimetry, well chambers, and our scintillating fiber based detector will be presented. [Preview Abstract] |
Friday, March 17, 2006 11:51AM - 12:03PM |
Z26.00004: An Active Mammosite For Breast Brachytherapy Thomas Cudjoe Brachytherapy is an advanced cancer treatment that uses radioactive sources inside or in close proximity to cancerous tumors, thus minimizing exposure to neighboring healthy cells. This radiation oncology treatment unlike many others is localized and precise. The latest involvement of the Brachytherapy research group of the medical physics program at Hampton University is in the development of a scintillator fiber based detector for the breast cancer specific Mammosite (balloon device) from Cytyc Inc. Radioactive sources are inserted into a small plastic catheter (shaft) and pushed at the end of the tube. At that location, a water filled balloon surrounds the source and allow uniform gamma emission into cancer tumors. There is presently no capability for this device to provide measurements of the location of the source, as well as the radiation emitted from the source. Recent data were acquired to evaluate the possibility of measuring the dose distribution during breast Brachytherapy cancer treatments with this device. A high activity $^{192}$Ir radioactive source and a 0.5 and 1 mm$^{2}$ scintillating fibers were used. Results will be presented and discussed. [Preview Abstract] |
Friday, March 17, 2006 12:03PM - 12:15PM |
Z26.00005: Calibration Of An Active Mammosite Using A Low Activity Sr-90 Radioactive Source Jacquelyn Winston The latest involvement of the Brachytherapy research group of the medical physics program at Hampton University is in the development of a scintillator fiber based detector for the breast cancer specific Mammosite (balloon device) from Cytyc Inc. Recent data were acquired at a local hospital to evaluate the possibility of measuring the dose distribution during breast Brachytherapy cancer treatments with this device. Since sub-millimeter accuracy in position is required, precision of the device relies on the accurate calibration of the scintillating fiber element. As part of a collaboration work, data were acquired for that purpose at Hampton University and subsequently analyzed at Morgan State University. An 8 mm diameter strontium-90 radioactive field source with a low activity of 25 $\mu $Ci was used along with a dedicated LabView data acquisition system. We will discuss the data collected and address some of the features of this novel system. [Preview Abstract] |
Friday, March 17, 2006 12:15PM - 12:27PM |
Z26.00006: Modelization For Electromagnetic Electron Scattering at Low Energies for Radiotherapy applications. Vahagn Nazaryan, Paul Gueye Since release of the GEANT4 particle simulation toolkit in 2003, there has been a growing interest in its applications to medical physics. The applicability of GEANT4 to radiotherapy has been a subject of several investigations in recent years, and it was found to be of great use. Its low-energy model allows for electromagnetic interaction simulations down to 250 eV. The electron physics data are obtained from the Lawrence Livermore National Laboratory's Evaluated Electron Data Library (EEDL). At very lower energies (below 10 MeV), some of the tabulated data in EEDL have big uncertainties (more than 50{\%}), and rely on various extrapolations to energy regions where there is no experimental data. We have investigated the variations of these cross-section data to radiotherapy applications. Our study suggests a strong need for better theoretical models of electron interactions with matter at these energies, and the necessity of new and more reliable experimental data. The progress towards such theoretical model will be presented. [Preview Abstract] |
Friday, March 17, 2006 12:27PM - 12:39PM |
Z26.00007: Fully Complex Magnetoencephalography Jonathan Simon, Yadong Wang Complex numbers appear naturally in biology whenever a system can be analyzed in the frequency domain, such as physiological data from magnetoencephalography (MEG). For example, the MEG steady state response to a modulated auditory stimulus generates a complex magnetic field measurement at each MEG channel, equal to the response's Fourier transform at the stimulus modulation frequency. The complex nature of these data sets, often not taken advantage of, is fully exploited here with new methods. Whole-head, complex magnetic data can be used to estimate complex neural current sources, and standard methods of source estimation naturally generalize for complex sources. We show that a general complex neural vector source, e.g. a current dipole, is described by its location, magnitude, and direction, but also by a phase and by an additional two-dimensional perpendicular component. We give natural interpretations of all the parameters for the complex equivalent-current dipole by linking them to the underlying neurophysiology. We demonstrate biologically generated complex magnetic fields, and their equivalent fully complex current sources, with both simulations and experimental data. [Preview Abstract] |
Friday, March 17, 2006 12:39PM - 12:51PM |
Z26.00008: Multi-Active Catheters For Real Time Dose Distribution Measurements In Prostate Brachytherapy Treatments Carlos Velasco We have performed real time 3D dose distribution measurements of high dose rate brachytherapy sources. These data demonstrate the possibility of in-vivo dosimetry monitoring of the dose rate while treating patients with particle radiation therapy. The prototype used for these experiments is made out of 16 scintillating fibers, 30 cm long and with a 1 mm$^{2}$ transverse cross section. Each fiber was coupled to a 16 channel Hamamatsu photo-multiplier tube for photon to current conversion. Millimeter accuracy in position was attained with sub-seconds timing scale. No correction was done from Cherenkov radiation background. The (over)estimated systematic uncertainty is 10{\%} and includes electronic and computer dead times, channel-to-channel efficiency, and signal attenuation. We will discuss the impact of such device to clinical prostate cancer treatments and treatment planning softwares. [Preview Abstract] |
Friday, March 17, 2006 12:51PM - 1:03PM |
Z26.00009: Speckle Patterns in Coherence Domain Biomedical Imaging Ping Yu We have shown previously that coherence domain biomedical imaging can be used for optically sectioning small tumors such as rat osteogenic sarcoma (bone tumors). Speckle patterns of such small tumors provided quantitative measures of the health, necrotic, and poisoned tissues. However, the origins of these speckle patterns are not clear. Although the nuclei, mitochondria and other organelles inside cells are responsible for the speckle under the illumination of low coherence light source, these patterns at the imaging plane are related to the photon pathways both inside and outside the tissue. We report systematic experiments and simulation of the speckle patterns from coherence domain imaging of small tumors. The image frames are acquired at different depths inside the tumor tissue and analyzed by using a turbid medium model. The results reveal that the speckle patterns are dominated by the scattering properties of the tissue, which is characterized by the mean free path of the photons, and the collection geometry of the backscattered light photons. This work was supported by a University of Missouri Research Board grant URB-04-072 and NIH grant P50-CA-103130. [Preview Abstract] |
Friday, March 17, 2006 1:03PM - 1:15PM |
Z26.00010: Clinical implementation of proton Monte Carlo dose calculation. Harald Paganetti, Hongyu Jiang, Shashidhar Kollipara, Hanne Kooy Goal was the clinical implementation of Monte Carlo dose calculation for use in parallel to a commercial planning system. Treatment heads were modeled in detail. To describe the patient anatomy, Hounsfield Units were converted into materials with explicit element composition and density. We developed a method to dynamically assign the mass density to the materials during particle transport. Memory for CT voxels is assigned dynamically. A software link was created between the commercial planning system, the treatment machine control system and the Monte Carlo program. The prescribed range and modulation are automatically translated into the corresponding settings of the treatment head. For broad beam modulation treatment, the Monte Carlo code simulates apertures and compensators based on the milling machine files. Treatment information, like prescribed dose per field, size of the air gap, couch angle and gantry angle, is read from the departmental patient database. For absolute dosimetry, the dose delivered to the patient per monitor unit is calculated based on the simulation of the reading of a segmented transmission ionization chamber. Dose calculations are done on the CT grid resolution and have been performed for various treatment sites. Monte Carlo results can be imported into the planning system. [Preview Abstract] |
Friday, March 17, 2006 1:15PM - 1:27PM |
Z26.00011: A Universal Scaling of Proton Energy Deposition in Biological Materials Dan Fry, Wilfred Sewchand, John O'Connell We have used GEANT 4.7.0 to simulate the dosimetric properties of various materials. Bragg curves for monoenergetic pencil beams with incident energies ranging from 70 MeV to 250 MeV have been characterized by the width (90$\%$-10$\%$) of distal and lateral edges, peak-to- entrance dose ratio, and FWHM. In all materials ionization by primary protons is the dominant energy loss channel. Energy loss by electrons and secondary protons is approximately an order of magnitude lower and dose from all other secondary particles is three orders of magnitude lower. We have found that a single scaling factor (material density) is most inadequate for fully defining the characteristics of the percent depth- dose distribution. However, the distal and lateral edge widths and FWHM can be universally scaled by scaling the proton beam energy by a material dependent factor. In addition, when the peak-to- entrance dose ratio is scaled by the stopping power ratio $S_{w} $ relative to water, energy deposition properties of all materials appear to behave alike. Our results suggest that knowledge of the incident proton energy and effective $Z$ of the target are sufficient to fully predict the depth-dose distribution irrespective of material type. [Preview Abstract] |
Friday, March 17, 2006 1:27PM - 1:39PM |
Z26.00012: Feasibility Study Of Kaon Therapy Solomon Sahle Proton therapy is a new emerging radiation modality that supersedes electron machines for cancer treatments. The strength of these beams lies in their Bragg peak distribution that allows sparing healthy tissues while depositing the majority of the dose at the (targeted) tumor location. Although promising, there are still unresolved issues with this technique due to the impossibility of viewing the beam within a patient and the production of low energy neutrons at the end of the distribution. Simulation studies on the use of kaon beams were made using the Geant4 toolkit. The strangeness content of these particles permits to extract the spatial beam information in-vivo, as well as providing a similar Bragg distribution as protons. We will discuss the impact of such beams for hadron therapy in cancer treatments. [Preview Abstract] |
Friday, March 17, 2006 1:39PM - 1:51PM |
Z26.00013: Low Energy Experimental Elastic Cross Sections for Medical Physics Application Michael Epps Elastic cross sections for electron energies below 10 MeV are fundamental quantities needed in treatment planning systems used at hospitals and health facilities. To date, there is very little if not no data within that energy regime. In collaboration with the high current, high energy resolution continuous electron beam of the Department of Energy's Jefferson Lab accelerator, we have performed a first stage of dedicated experiments with energies of 100-150 keV to collect data for this type of reactions. The targets used were gold, copper and silver. A Mott scattering chamber was used to detect the outgoing electrons. We will present the first results of this program that aims at performing a wide range of measurements including the use of polarization data for spin studies. [Preview Abstract] |
Friday, March 17, 2006 1:51PM - 2:03PM |
Z26.00014: Wavelet assessment of cerebrospinal compensatory reserve and cerebrovascular pressure reactivity M. Latka, M. Turalska, W. Kolodziej, D. Latka, B. West We employ complex continuous wavelet transforms to develop a consistent mathematical framework capable of quantifying both cerebrospinal compensatory reserve and cerebrovascular pressure--reactivity. The wavelet gain, defined as the frequency dependent ratio of time averaged wavelet coefficients of intracranial (ICP) and arterial blood pressure (ABP) fluctuations, characterizes the dampening of spontaneous arterial blood oscillations. This gain is introduced as a novel measure of cerebrospinal compensatory reserve. For a group of 10 patients who died as a result of head trauma (Glasgow Outcome Scale GOS =1) the average gain is 0.45 calculated at 0.05 Hz significantly exceeds that of 16 patients with favorable outcome (GOS=2): with gain of 0.24 with $p=4\times 10^{-5}$. We also study the dynamics of instantaneous phase difference between the fluctuations of the ABP and ICP time series. The time-averaged synchronization index, which depends upon frequency, yields the information about the stability of the phase difference and is used as a cerebrovascular pressure--reactivity index. The average phase difference for GOS=1 is close to zero in sharp contrast to the mean value of 30$^{o}$ for patients with GOS=2. We hypothesize that in patients who died the impairment of cerebral autoregulation is followed by the break down of residual pressure reactivity. [Preview Abstract] |
Friday, March 17, 2006 2:03PM - 2:15PM |
Z26.00015: Convolution Product And Lyapunov Stability In Medical Imaging Leslie Upton Resolution is one of the most important aspects of any apparatus used in medical imaging. Several mathematical algorithms have been developed to best extract anatomical features and tumors within patients suffering from cancer diseases. Distortions of images are a result of blood flow, breathing, apparatus efficiency etc. This problem is also inherent to other disciplines in physics, like nuclear physics. Here, study of the electromagnetic nuclear structure relies in the extraction of the form factors from the nuclear medium (nucleus or nucleons). Data collected depend on a number of different variables. A new approach which consists of using the mathematical convolution product combined with the (entropy equivalent) Lyapunov stability was tested in this area of physics, since easier to implement. A deconvolution technique allowed isolating the individual dependencies of the differential cross section and extracting meson form factors of interest (pion and kaon). Preliminary results will be presented, as well as a discussion on the application of this technique to medical imaging. [Preview Abstract] |
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