bluuebunny/crossref_metadata_2025 huggingface dataset:
A Kriging Approach for CFD/Wind-Tunnel Data Comparison
Authors: Jouhaud J.-C. , Sagaut P. , Labeyrie B.
Published: July 2006

A Kriging-based method for the parametrization of the response surface spanned by uncertain parameters in computational fluid dynamics is proposed. A multiresolution approach in the sampling space is used to improve the accuracy of the method. It is illustrated considering the problem of the computation of the corrections needed to recover equivalent free-flight conditions from wind-tunnel experiments. Using the surface response approach, optimal corrected values of the freestream Mach number and the angle of attack for the compressible turbulent flow around the RAE 2822 wing are computed. The use of the response surface to gain an insight into the sensitivity of the results with respect to other parameter is also assessed.
Hot-Wire Measurements Around a Controlled Diffusion Airfoil in an
Open-Jet Anechoic Wind Tunnel
Authors: Moreau Stéphane , Neal Douglas , Foss John
Published: July 2006

The aeroacoustic measurements in the large anechoic wind tunnel of Ecole Centrale de Lyon, which previously focused on the wall pressure distribution and the far-field acoustic pressure, are extended to velocity measurements by hot-wire probes in the nozzle jet and in the vicinity of a Controlled Diffusion airfoil. The present work focuses on the flow conditions corresponding to a Reynolds number based on the airfoil chord length Rec=1.6×105 and a geometric angle of attack αg of 8°. Midspan measurements were achieved at the exit plane of the wind tunnel nozzle upstream of the test airfoil and in a large eddy simulation domain that was embedded in the potential core around the airfoil mockup. The inlet measurements by a single hot-wire probe provided insight into the free-stream turbulence intensity upstream of the profile. The X-probe measurements on the upper and lower computational boundaries show the overall deflection of the jet potential core by the cambered airfoil. These are compared to previous Reynolds averaged Navier-Stokes predictions. The X-probe measurements in the airfoil wake provide information on the development of the airfoil boundary layer and the resulting wake after separation. The measured wake velocity defect has been compared with both numerical predictions.
Experimental Investigation on the Onset of Gas Entrainment from a Stratified Two-Phase Region Through Multiple Branches Mounted on a Curved Surface
Authors: Ahmad Tariq , Hassan Ibrahim
Published: July 2006

An experimental investigation has been carried out to simulate the onset of gas entrainment phenomenon from a stratified region through branches located on a semicircular wall configuration, in close dimensional resemblance with a Canada Deuterium and Uranium (CANDU) header-feeder system. New experimental data for the onset of gas entrainment was developed during single and multiple discharge from an air/water stratified region over a wide range of Froude numbers (0 to 100), in order to thoroughly understand the onset of gas entrainment phenomenon. It was found that the critical height at the onset of gas entrainment (single or simultaneous) was a function of the corresponding Froude number of each branch, the vertical distance between the centerlines of the branches (for multiple discharge), the hydraulic resistance of the discharging lines, as well as the orientation of the branches and their diameter with respect to the main header. Concerning multiple discharge comparisons, at intermediate Fr values (1<Fr<10) the data deviates, however at higher Fr values (>10) there is convergence. The present data are necessary in validating future analytical and numerical models of the onset of gas entrainment for a curved geometry, particularly at low Froude numbers.
Experimental Validation of the Addition Principle for Pulsating Flow in Close-Coupled Catalyst Manifolds
Authors: Persoons Tim , Hoefnagels Ad , Van den Bulck Eric
Published: July 2006

Designing an exhaust manifold with close-coupled catalyst (CCC) relies heavily on time-consuming transient computional fluid dynamics. The current paper provides experimental validation of the addition principle for pulsating flow in CCC manifolds. The addition principle states that the time-averaged catalyst velocity distribution in pulsating flow equals a linear combination of velocity distributions obtained for steady flow through each of the exhaust runners. A charged motored engine flow rig provides cold pulsating flow in the exhaust manifold featuring blow down and displacement phases, typical of fired engine conditions. Oscillating hot-wire anemometry is used to measure the bidirectional velocity, with a maximum measurable negative velocity of −1m∕s. In part load and zero load conditions, instantaneous reverse flow occurs following the blow-down phase. The two-stage nature of the exhaust stroke combined with strong Helmholtz resonances results in strong fluctuations of the time-resolved mean catalyst velocity. The validity of the addition principle is quantified based on the shape and magnitude similarity between steady and pulsating flow distributions. Appropriate nondimensional groups are used to characterize the flow and quantify the similarity. Statistical significances are provided for the addition principle’s validity. The addition principle is valid when the nondimensional scavenging number S exceeds a critical value Scrit, corresponding to cases of low engine speed and/or high flow rate. This study suggests that the CCC manifold efficiency with respect to catalyst flow uniformity could be quantified using a single scalar parameter, i.e., Scrit. The results from the current study are discussed with respect to previously reported results. The combined results are in good agreement and provide a thorough statistically founded experimental validation of the addition principle, based on a broad applicability range.
Flow Geometry Effects on the Turbulent Mixing Efficiency
Authors: Aguirre Roberto C. , Nathman Jennifer C. , Catrakis Haris C.
Published: July 2006

Flow geometry effects are examined on the turbulent mixing efficiency quantified as the mixture fraction. Two different flow geometries are compared at similar Reynolds numbers, Schmidt numbers, and growth rates, with fully developed turbulence conditions. The two geometries are the round jet and the single-stream planar shear layer. At the flow conditions examined, the jet exhibits an ensemble-averaged mixing efficiency which is approximately double the value for the shear layer. This substantial difference is explained fluid mechanically in terms of the distinct large-scale entrainment and mixing-initiation environments and is therefore directly due to flow geometry effects.
Voting strategy for artifact reduction in digital breast tomosynthesis
Authors: Wu Tao , Moore Richard H. , Kopans Daniel B.
Published: July 2006

Artifacts are observed in digital breast tomosynthesis (DBT) reconstructions due to the small number of projections and the narrow angular range that are typically employed in tomosynthesis imaging. In this work, we investigate the reconstruction artifacts that are caused by high‐attenuation features in breast and develop several artifact reduction methods based on a “voting strategy.” The voting strategy identifies the projection(s) that would introduce artifacts to a voxel and rejects the projection(s) when reconstructing the voxel. Four approaches to the voting strategy were compared, including projection segmentation, maximum contribution deduction, one‐step classification, and iterative classification. The projection segmentation method, based on segmentation of high‐attenuation features from the projections, effectively reduces artifacts caused by metal and large calcifications that can be reliably detected and segmented from projections. The other three methods are based on the observation that contributions from artifact‐inducing projections have higher value than those from normal projections. These methods attempt to identify the projection(s) that would cause artifacts by comparing contributions from different projections. Among the three methods, the iterative classification method provides the best artifact reduction; however, it can generate many false positive classifications that degrade the image quality. The maximum contribution deduction method and one‐step classification method both reduce artifacts well from small calcifications, although the performance of artifact reduction is slightly better with the one‐step classification. The combination of one‐step classification and projection segmentation removes artifacts from both large and small calcifications.
Automated classification of hyperlucency, fibrosis, ground glass, solid, and focal lesions in high‐resolution CT of the lung
Authors: Sluimer Ingrid C. , Prokop Mathias , Hartmann Ieneke , van Ginneken Bram
Published: July 2006

An automatic method for textural analysis of complete HRCT lung slices is presented. The system performs classification of regions of interest (ROIs) into one of six classes: normal, hyperlucency, fibrosis, ground glass, solid, and focal. We propose a novel method of automatically generating ROIs that contain homogeneous texture. The use of such regions rather than square regions is shown to improve performance of the automated system. Furthermore, the use of two different, previously published, feature sets is investigated. Both feature sets are shown to yield similar results. Classification performance of the complete system is characterized by ROC curves for each of the classes of abnormality and compared to a total of three expert readings by two experienced radiologists. The different types of abnormality can be automatically distinguished with areas under the ROC curve that range from 0.74 (focal) to 0.95 (solid). The kappa statistics for intraobserver agreement, interobserver agreement, and computer versus observer agreement were , , and , respectively. The question whether or not a class of abnormality was present in a slice could be answered by the computer system with an accuracy comparable to that of radiologists.
Dose uncertainty due to aperture effects in dynamic fields
Authors: Higgins P. D. , Alaei P.
Published: July 2006

Dosimetry of intensity modulated radiation therapy requires accurate modeling of the beamlets that comprise each treatment segment. Planning systems such as Varian Eclipse and Philips Pinnacle recommend measuring dose distributions and output factors for fields as small as possible, generally down to at least . Conventionally, we perform these measurements for regular fields, defined by the secondary collimators. In practice, it is the multileaf collimation system (MLC) that defines the intensity map and provides dynamic dose modulation in either a moving window or segmented step‐and‐shoot mode. For this review we have only considered the latter delivery mode. Using this method, we have studied aperture motion effects on the dynamic collimator scatter , total scatter , and phantom scatter factors for various combinations of collimator settings and dynamically stepped leaf gaps in comparison with those for static field factors. For two different Varian linear accelerators, we found similar results in a systematic dependence of collimator scatter on gap width and collimator setting. As the gap increases from the dynamic collimator scatter factors converge from a maximum difference of about 30% toward the static field values. At the same time, there is no measurable difference between dynamic field phantom scatter factors and those conventionally obtained for static fields. Second, we evaluated the two planning systems as to how well they account for collimator scatter by attempting to mimic the dynamic apertures used above by planning and measuring dose distributions to several small, cylindrical targets for a similar range of fixed collimator settings. We found that the ratio of measured‐to‐planned doses as a function of target size were similar to the measured, dynamic data for the Varian Eclipse planning system, indicating underestimation of dose for targets smaller than diameter, but were close to unity for the Philips Pinnacle system, suggestive of the underlying differences in the dose calculation algorithms. We discuss the measurements and results and potential impact on the dosimetry of small clinical targets.
X‐ray imaging in advanced studies of ophthalmic diseases
Authors: Antunes Andrea , Safatle Angélica M. V. , Barros Paulo S. M. , Morelhão Sérgio L.
Published: July 2006

Microscopic characterization of pathological tissues has one major intrinsic limitation, the small sampling areas with respect to the extension of the tissues. Mapping possible changes on vast tissues and correlating them with large ensembles of clinical cases is not a feasible procedure for studying most diseases, as for instance vision loss related diseases and, in particular, the cataract. Although intraocular lens implants are successful treatments, cataract still is a leading public‐health issue that grows in importance as the population increases and life expectancy is extended worldwide. In this work we have exploited the radiation‐tissue interaction properties of hard x‐rays—very low absorption and scattering—to map distinct lesions on entire eye lenses. At the used synchrotron x‐ray photon energy of (wavelength ), scattering and refraction are angular resolved effects. It allows the employed x‐ray image technique to efficiently characterize two types of lesions in eye lenses under cataractogenesis: distributions of tiny scattering centers and extended areas of fiber cell compaction. The data collection procedure is relatively fast; allowing dozens of samples to be totally imaged (scattering, refraction, and mass absorption images) in a single day of synchrotron beam time. More than 60 cases of canine cataract, not correlated to specific causes, were investigated in this first application of x‐rays to image entire lenses. Cortical opacity cases, or partial opacity, could be related to the presence of calcificated tissues at the cortical areas, clearly visible in the images, whose elemental contents were verified by micro x‐ray fluorescence as very rich in calcium. Calcificated tissues were also observed at nuclear areas in some cases of hypermature cataract. Total opacity cases without distinguishable amount of scattering centers consist in 70% of the analyzed cases, where remarkable fissure marks owing to extended areas of fiber cell compaction are diagnosed.
Integrating respiratory gating into a megavoltage cone‐beam CT system
Authors: Chang Jenghwa , Sillanpaa Jussi , Ling Clifton C. , Seppi Edward , Yorke Ellen , Mageras Gikas , Amols Howard
Published: July 2006

We have previously described a low‐dose megavoltage cone beam computed tomography (MV CBCT) system capable of producing projection image using one beam pulse. In this study, we report on its integration with respiratory gating for gated radiotherapy. The respiratory gating system tracks a reflective marker on the patient's abdomen midway between the xiphoid and umbilicus, and disables radiation delivery when the marker position is outside predefined thresholds. We investigate two strategies for acquiring gated scans. In the continuous rotation‐gated acquisition, the linear accelerator (LINAC) is set to the fixed x‐ray mode and the gantry makes a , continuous rotation, during which the gating system turns the radiation beam on and off, resulting in projection images with an uneven distribution of projection angles (e.g., in 70 arcs each covering ). In the gated rotation‐continuous acquisition, the LINAC is set to the dynamic arc mode, which suspends the gantry rotation when the gating system inhibits the beam, leading to a slightly longer scan time, but yielding projection images with more evenly distributed projection angles (e.g., between two consecutive projection angles). We have tested both data acquisition schemes on stationary (a contrast detail and a thoracic) phantoms and protocol lung patients. For stationary phantoms, a separate motion phantom not visible in the images is used to trigger the RPM system. Frame rate is adjusted so that approximately 450 images (13 MU) are acquired for each scan and three‐dimensional tomographic images reconstructed using a Feldkamp filtered backprojection algorithm. The gated rotation‐continuous acquisition yield reconstructions free of breathing artifacts. The tumor in parenchymal lung and normal tissues are easily discernible and the boundary between the diaphragm and the lung sharply defined. Contrast‐to‐noise ratio (CNR) is not degraded relative to nongated scans of stationary phantoms. The continuous rotation‐gated acquisition scan also yields tomographic images with discernible anatomic features; however, streak artifacts are observed and CNR is reduced by approximately a factor of 4. In conclusion, we have successfully developed a gated MV CBCT system to verify the patient positioning for gated radiotherapy.
The effect of z overscanning on radiation burden of pediatric patients undergoing head CT with multidetector scanners: A Monte Carlo study
Authors: Tzedakis Antonis , Perisinakis Kostas , Raissaki Maria , Damilakis John
Published: July 2006

The purpose of this study was to investigate the effect of z overscanning on eye lens dose and effective dose received by pediatric patients undergoing head CT examinations. A pediatric patient study was carried out to obtain the exposure parameters and data regarding the eye lens position with respect to imaged volume boundaries. This information was used to simulate CT exposures by Monte Carlo code. The Monte Carlo N‐Particle (MCNP, version 4C2) radiation transport code and five mathematical anthropomorphic phantoms representing newborn, 1‐, 5‐, 10‐, and 15‐year‐old patient, were employed in the current study. To estimate effective dose, the weighted computed tomography dose index was calculated by cylindrical polymethyl‐methacrylate phantoms of 9.7, 13.1, 15.4, 16.1, and in diameter representing the pediatric head of newborn, 1‐, 5‐, 10‐, and 15‐year‐old individuals, respectively. The validity of the Monte Carlo calculated approach was verified by comparison with dose data obtained using physical pediatric anthropomorphic phantoms and thermoluminescence dosimetry. For all patients studied, the eye lenses were located in the region to from the first slice of the imaged volume. Doses from axial scans were always lower than those from corresponding helical examinations. The percentage differences in normalized eye lens absorbed dose between contiguous axial and helical examinations with were found to be up to 10.9%, when the eye lenses were located inside the region to be imaged. When the eye lenses were positioned far from the first slice of region to be imaged, the normalized dose to the lens from contiguous axial examinations was up to 11 times lower than the corresponding values from helical mode with pitch=1. The effective dose from axial examinations was up to 24% lower than corresponding values from helical examinations with . In conclusion, it is more dose efficient to use axial mode acquisition rather than helical scan for pediatric head examinations, if there are no overriding clinical considerations.
On measuring depth‐dose distribution of range‐modulated proton therapy fields
Authors: Lu Hsiao‐Ming
Published: July 2006

Depth‐dose profile measurements are frequently performed for the characterization of spread‐out Bragg peak treatment fields in proton therapy. The measured distribution often contains a significant amount of noise with a persistent pattern. As a result, substantial smoothing has to be applied which can alter a measurement. We explored the origin of the observed noise by closely examining the sampling process in the scanning algorithm. We will show that the time characteristics of the signal in the range‐modulated proton therapy beams differ significantly from those in the photon beams, and as a result the measurement error is very sensitive to the sampling duration. The observed noise results mainly from inappropriate choices for this value, rather than from the dose distribution itself. Increasing the value arbitrarily may actually increase the noise magnitude. We will demonstrate that with an optimal value for the sampling duration and its accurate control, the noise magnitude can be reduced significantly, without increasing the measurement time.
Dosimetry of a thyroid uptake detected in seed migration survey following a patient's iodine‐125 prostate implant and in vitro measurements of intentional seed leakages
Authors: Chen Qin‐Sheng , Russell John L. , Macklis Roger R. , Weinhous Martin S. , Blair Henry F.
Published: July 2006

As a quality control procedure, a post‐implant seed migration survey has been accomplished on 340 prostate cancer patients since November 2001. Pulmonary seed embolization and intracardiac seed embolization have been detected. A case of thyroid uptake due to leaking iodine‐125 (I‐125) sources was also seized. In order to determine the dose to the thyroid, a dosimetry method was developed to link in vivo measurements and the cumulated dose to the thyroid. The calculated source leakage half‐life in the case was approximately based on the measurements and the estimated cumulated dose to thyroid was . It is concluded that one seed was leaking. In order to verify the in vivo measurements, intentional in vitro seed leakage tests were performed. A seed was cut open and placed in a sealed glass container filled with a given volume of saline. The I‐125 concentration in the saline was subsequently measured over a period of six months. Consistent in vivo and in vitro results were obtained. Recent incidents of seed leaks reported from other centers have drawn practitioners’ attention to this problem. In order to make the measurements more useful, the seed leakage tests were expanded to include I‐125 seeds from six other vendors. The results show that the leakage half‐lives of those seeds varied from nine days to a half‐year. Two seed models demonstrated least leakage. Since the measurements lasted for six months, the escape of iodine resulted from oxidation of iodide in the saline was a concern for the measurement accuracy. As a reference, another set of leakage tests were performed by adding sodium thiosulfate salt to the saline. Sodium thiosulfate is a reducing agent that prevents the conversion of iodide to iodate so as to minimize I‐125 evaporation. As a result, significantly shortened leakage half‐lives were observed in this group. Seed agitation was also performed and no significant deviations of the leakage rates were observed. Considering the body fluid is more complex than saline, the in vivo leakage half‐life, in case a source leak is encountered, may vary significantly from what is presented in this paper due to chemical reactions. In vivo measurements thus may produce a more accurate estimation of leakage half‐life and thyroid uptake dose.
Reducing respiratory motion artifacts in positron emission tomography through retrospective stacking
Authors: Thorndyke Brian , Schreibmann Eduard , Koong Albert , Xing Lei
Published: July 2006

Respiratory motion artifacts in positron emission tomography (PET) imaging can alter lesion intensity profiles, and result in substantially reduced activity and contrast‐to‐noise ratios (CNRs). We propose a corrective algorithm, coined “retrospective stacking” (RS), to restore image quality without requiring additional scan time. Retrospective stacking uses b‐spline deformable image registration to combine amplitude‐binned PET data along the entire respiratory cycle into a single respiratory end point. We applied the method to a phantom model consisting of a small, hot vial oscillating within a warm background, as well as to images of a pancreatic and a liver patient. Comparisons were made using cross‐section visualizations, activity profiles, and CNRs within the region of interest. Retrospective stacking was found to properly restore the lesion location and intensity profile in all cases. In addition, RS provided CNR improvements up to three‐fold over gated images, and up to five‐fold over ungated data. These phantom and patient studies demonstrate that RS can correct for lesion motion and deformation, while substantially improving tumor visibility and background noise.
Monte Carlo based IMRT dose verification using MLC log files and R/V outputs
Authors: Luo Wei , Li Jinsheng , Price Robert A. , Chen Lili , Yang Jie , Fan Jiajin , Chen Zuoqun , McNeeley Shawn , Xu Xiu , Ma Chang‐Ming
Published: July 2006

Conventional IMRT dose verification using film and ion chamber measurements is useful but limited with respect to the actual dose distribution received by the patient. The Monte Carlo simulation has been introduced as an independent dose verification tool for IMRT using the patient CT data and MLC leaf sequence files, which validates the dose calculation accuracy but not the plan delivery accuracy. In this work, we propose a Monte Carlo based IMRT dose verification method that reconstructs the patient dose distribution using the patient CT, actual beam data based on the information from the record and verify system (R/V), and the MLC log files obtained during dose delivery that record the MLC leaf positions and MUs delivered. Comparing the Monte Carlo dose calculation with the original IMRT plan using these data simultaneously validates the accuracy of both the IMRT dose calculation and beam delivery. Such log file based Monte Carlo simulations are expected to be employed as a useful and efficient IMRT QA modality to validate the dose delivered to the patient. We have run Monte Carlo simulations for eight IMRT prostate plans using this method and the results for the target dose were consistent with the original CORVUS treatment plans to within 3.0% and 2.0% with and without heterogeneity corrections in the dose calculation. However, significant dose deviations in nearby critical structures have been observed. The results showed that up to 9.0% of the bladder dose and up to 38.0% of the rectum dose, to which leaf position errors were found to contribute , were underestimated by the CORVUS treatment planning system. The concept of average leaf position error has been defined to analyze MLC leaf position errors for an IMRT plan. A linear correlation between the target dose error and the average position error has been found based on log file based Monte Carlo simulations, showing that an average position error of can result in a target dose error of about 1.0%.
Three dimensional radiation dosimetry in lung‐equivalent regions by use of a radiation sensitive gel foam: Proof of principle
Authors: De Deene Yves , Vergote Koen , Claeys Carolien , De Wagter Carlos
Published: July 2006

A polymer hydrogel foam is proposed as a potential three dimensional experimental dosimeter for radiation treatment verification in low‐density tissue such as the lung. A gel foam is created by beating a radiation sensitive polymer gel mixture in an anoxic atmosphere. The mass density of the gel foam is in the order of . Both nuclear magnetic resonance (NMR) spin‐spin relaxation rate and magnetization transfer ratio (MTR) have been used to map the dose distribution from the gel dosimeter. It is found that MTR has significant advantages compared to for mapping the dose distribution in the polymer gel foam dosimeters. The magnetization transfer ratio is found to be less dependent on the density and microstructure of the gel foam dosimeter while spin‐spin relaxation dispersion has been observed making the spin‐spin relaxation rate dependent on the interecho time interval. Optical microscopy reveals a microstructure that shows great similarity with human lung tissue. It is also shown how NMR hydrogen proton density measurements can be used to map the density distributions in gel dosimeters.
SU‐CC‐ValA‐05: Advanced Integral Method for the Simulation of Diagnostic X‐Ray Spectra
Authors: Dodge C , Flynn M
Published: July 2006

Purpose: To create a fast and accurate computer algorithm for simulating the emission x‐ray spectra from diagnostic tubes as a function of tube voltages, target material, and take‐off angles. Method and Materials: The method uses an integral model to determine the radiative losses from an electron as it slows down in arbitrary media. The effect of self‐absorption and backscatter is accurately described by distribution functions for electron number, electron depth, and angular distribution that are functions of electron slowing down energy. The Monte Carlo program PENELOPE was used to determine these three distribution functions. An exact accounting of electron orientation was found necessary due to large variations in the Bremsstrahlung cross‐section as a function of emission angle. These are accounted for in the integral model by pre‐computing tables based on the Kissel Bremsstrahlung shape function. Characteristic x‐ray emissions as a function of over‐voltage are described using Monte Carlo results for both direct and indirect production. The computer algorithm is implemented as a part of a larger program for computationally simulating x‐ray production, transmission, scattering, and detection for imaging systems (XSPECT, V4.0). Results: We compared our program to the measured x‐ray spectra of Mercier, and spectral computations of PENELOPE. We found good agreement and an improvement over prior semi‐empirical estimates (Birch & Marshall, Tucker, Storm). Conclusion: We have developed a program that can simulate x‐ray spectra from tubes of arbitrary anode materials (including alloys), and target angles for tube voltages of 1 to 400 kV. After generation of target specific tables, the x‐ray spectra can be computed in a few seconds. The results are equivalent to Monte Carlo estimates that require days to compute a single spectra.
SU‐CC‐ValA‐07: Air Kerma Rate Measurements From a Miniature X‐Ray Source Using Free‐Air Ionization Chambers
Authors: Davis S , Micka J , DeWerd L , Rusch T
Published: July 2006

Purpose: To measure the air kerma rate from a miniature x‐ray source using free‐air ionization chambers (FACs), and to transfer the source air kerma rate to a well‐type ionization chamber. Method and Materials: Air kerma rates from several Xoft AXXENT™ miniature x‐ray sources were measured in air along their transverse axes using three different FACs, each with a source‐to‐aperture distance of 100 cm. The sources were operated at 50 kV and 100 μA beam current. The University of Wisconsin (UW) Attix FAC was used for the initial measurements, and follow‐up measurements were performed using the Attix and Ritz FACs at the National Institute of Standards and Technology (NIST). Two different FAC aperture sizes were used for each of the air kerma rate measurements at NIST. Additional measurements for each source were performed using a well‐type ionization chamber with a custom‐built aluminum source holder. The ratio of air kerma rate at 100 cm to well chamber current was used to determine a well chamber calibration coefficient for each source. Results: The air kerma rates for the smaller aperture were generally within 2% of the rates measured with the larger aperture, indicating that the sources were aligned properly. The well chamber calibration coefficients demonstrated some source to source variation, with an overall standard deviation of 5.3%. The results suggest that most of this variation can be attributed to azimuthal anisotropy around the long axes of the sources, and not differences in the photon spectrum emitted from each source. Conclusion: Both the Attix and Ritz FACs are appropriate for measuring air kerma rates from the miniature x‐ray sources, but further work will be necessary to develop methods suitable for traceability to national measurements standards. Conflict of Interest: Funding for this research was provided by Xoft, Inc.
SU‐CC‐ValA‐08: Air‐Kerma Strength Determination of a 169Ytterbium High Dose Rate Brachytherapy Source
Authors: VanDamme J , DeWerd L , Micka J , Culberson W , Davis S
Published: July 2006

Purpose: To provide an accurate determination of the new 169Yb high dose rate (HDR) bracytherapy source in terms of air kerma strength, based on an adaptation of the current, NIST traceable, in air measurement standard in use for HDR sources. Methods and Materials: Several modifications to the seven distance technique, which is the current standard for HDR source strength measurement, were required to adapt it to the 169Yb spectrum. An Exradin A4 spherical chamber was employed, which has a relatively flat chamber response to the range of energies in the 169Yb spectrum, and has been verified to accurately measure the air kerma strength of to within the reported uncertainty of the current standard measurement technique. To convert the electrometer readings to source strength, a chamber coefficient, Nk, was determined by using the NIST calibrated chamber coefficients from several NIST H‐Beams, whose energy spectrums fall strategically within the 169Yb spectrum. Several correction factors must be applied to these electrometer readings, including corrections for temperature and pressure, air attenuation, air scatter, ion recombination, and corrections for the finite size of the chamber. Results: The decay corrected average of fourteen measurement iterations was 8.063 × 10−3 Gy‐m2/hr. Analysis of uncertainty was performed on these experimental 169Yb air kerma measurements using the standard NIST method for evaluating uncertainty. This analysis established an overall k = 2 expanded uncertainty of 2.10%. Conclusion: It is shown that, with a few modifications, the current standard for high dose rate brachytherapy source calibration could be employed to accurately calibrate the new HDR brachytherapy source in terms of air kerma strength. The uncertainties as analyzed fall within those currently used for calibration. Conflict of Interest: Research funding provided by Implant Sciences Corporation.
SU‐CC‐ValA‐09: Radiation Quality in High Contrast Imaging with Orthogonal Bremsstrahlung Beams
Authors: Sarfehnia A , Jabbari K , Seuntjens J , Podgorsak E
Published: July 2006

Purpose: To study the characteristics of orthogonal bremsstrahlung photons produced by megavoltage electron pencil beams and to evaluate the suitability of their use for improved radiation therapy imaging. Method and Materials: A 10 MeV electron beam emerging through the research port of a Varian Clinac‐18 linac was made to strike targets of carbon, aluminum and copper. The quality of resulting forward and orthogonal bremsstrahlung beams was evaluated using PDD and attenuation measurements, and the experimental findings were compared with Monte Carlo‐calculated results using the EGSnrcMP code. Images of contrast objects were acquired with Agfa 400 diagnostic films and their contrast levels were analyzed. Results: Photon yield and mean energy of the forward bremsstrahlung spectra were determined to be essentially independent of the target's atomic number Z. In comparison with forward bremsstrahlung, the yield and effective energy were lower in the orthogonal direction, and this decrease was more pronounced for targets of lower atomic number. The effective energy of a spectrum produced by carbon dropped by a factor of 10 from 1535 keV in the forward direction to 151 keV in the orthogonal direction, while for aluminum it dropped by 77% to 425 keV, and for copper by 37% to 1107 keV. The image contrast of films exposed with orthogonal beams was qualitatively determined to be superior to that obtained using the forward megavoltage beams. Conclusions: Orthogonal bremsstrahlung beams produced by megavoltage electrons have a significantly lower mean energy compared to forward beams. In the orthogonal direction, higher Z targets create higher intensity, while lower Z targets provide a more desirable low energy spectrum. Using their relatively low effective energy, orthogonal bremsstrahlung beams produced by megavoltage electrons striking low atomic number targets yield images with a higher contrast than do forward bremsstrahlung beams.
SU‐DD‐A1‐01: Advances in Co‐60 Based Tomotherapy Including Megavoltage CT
Authors: Schreiner L , Darko J , Joshi C , Rogers M , Chng N , Peters C , Salomons G , Kerr A
Published: July 2006

Purpose: To evaluate the potential for Co‐60 based tomotherapy including dose delivery and mega‐voltage CT (MVCT). Tomotherapy is a rotational implementation of IMRT that provides highly conformal doses and patient setup verification using MVCT. Current tomotherapy is limited to linear accelerators. This poster presents advances in our investigation of Cobalt‐60 based tomotherapy, including MVCT. Method and Materials: The fundamental components for the Co‐60 tomotherapy dose delivery and MVCT imaging experiments are a benchtop motion stage and a clinical Co‐ 60 MDS Nordion T‐780 unit. Film and polymer‐gel dosimetry are used to validate the tomotherapy dose delivery planned using in‐house software. Imaging is provided by a Varian Portal Vision LC250 EPID. MVCT imaging is demonstrated using a variety of phantoms, including an anthropomorphic head phantom, and various contrast phantoms. EGS Monte Carlo simulation is used to model different beam delivery approaches such as source design for increased radiation output. Results: The computer simulations, film dose measurements, and three‐dimensional polymer gel dosimetry all demonstrate that Co‐60 tomotherapy provides conformal dose delivery required of modern IMRT techniques. Film measurements show that dose delivery corresponds excellently with treatment plans, validating our in‐house planning system. Treatment planning studies show that Co‐60 tomotherapy delivery compares favourably with that from linac based 6MV tomotherapy. Dose volume histograms show identical coverage and avoidance of target critical organs. Imaging results show that Co‐60 CT provides sufficient contrast and resolution for image guidance. Results from Monte Carlo studies show that it is possible to increase beam output for a dedicated Co‐60 tomotherapy unit by modifying the source geometry. Conclusion: Co‐60 is well suited to tomotherapy and imaging applications; the development of clinical implementations of Co‐60 tomotherapy is warranted and work continues in our centre along these lines.
SU‐DD‐A1‐02: Variations of Energy Spectra and Water‐To‐Material Stopping‐Power Ratios in Three‐Dimensional Conformal and IMRT Photon Fields
Authors: Jang S , Liu H , Siebers J , Mohan R
Published: July 2006

Purpose: Complex dose distributions and dose gradients in IMRT may cause spatial variations in photon‐ and electron‐energy spectra. This study examined the change of photon‐ and electron‐energy spectra, and their effects on dosimeter response and water‐to‐material stopping power ratios (SPR) for 3D and IMRT beams. The later term is an important factor for dosimetry protocols and obtaining dose‐to‐water conversion in Monte Carlo dose calculations. Method and Materials: The Monte‐Carlo BEAM‐EGSnrc system was used to simulate external‐beam photon fields with 3D or IMRT features. Electron and photon energy fluences and spectra were calculated on a voxel‐by‐voxel basis using track‐length estimation for 3D and IMRT treatment plans. The water‐to‐material SPR were averaged over the voxel of interest with the electron spectra using the Spencer‐Attix theory. The relative response of ion chambers, films, and TLDs were modeled using the photon and electron spectra. Results: There was a strong spatial dependence of photon‐energy spectra in both the 3D and IMRT fields. The low‐energy (<100 keV) component of the photon spectra increased inversely with doses because of the contribution of the scattered photons. A similar effect was observed for electrons but to a much smaller extent. As a result, the response of film could increase by more than 10% in the low‐dose region, whiles the changes of ion chamber and TLD response were within 3%. On the other hand, the variation of the water‐to‐material SPR with energy spectra and spatial locations was not clinical significant (< 1%) for soft tissue, cortical bone, and lung, and was less than 2% for dry air. Conclusion: Photon‐ and electron‐spectra are spatial‐ and dose‐dependent in 3D and IMRT photon fields. The spectra variation should be considered for certain dosimeters whose responses are energy dependent. For patient‐like materials, the water‐to‐material SPR was relative stable in spite of the spectral variation.
SU‐DD‐A1‐04: In Vivo Prostate IMRT Dosimetry With MOSFET Detectors Using Brass Build‐Up Caps
Authors: Varadhan N , Garrity B , Miller J , Weber M
Published: July 2006

Purpose: To develop a general formalism with various correction factors to predict dmax entrance dose with the new hemispherical brass buildup caps to be used with MOSFET detectors in anterior prostate IMRT fields and thereby integrate in vivo IMRT dose measurement as part of routine QA process in IMRT radiotherapy Method and Materials: We have used the new wide energy hemispherical build‐up caps for this study. Due to its high density and high atomic number it provides the minimal amount of metal needed to achieve full build‐up at Dmax for a range of photon energies. We have developed a general formalism to predict Dmax entrance dose by applying necessary correction factors after studying the response of MOSFET with brass build up caps for energy, dose rate, dose reproducibility, SSD and patient specific IMRT correction factor. Results: In vivo Prostate IMRT dose measurements with MOSFET detectors using brass buildup caps was performed and compared against dose predicted by two different treatment planning systems. We used both 6 MV and 10 MV for this study and compared the in vivo MOSFET detector reading with dose predicted by Philips Pinnacle (6 MV) and CMS XiO (10 MV) treatment planning systems respectively. We achieved a overall accuracy of better than ± 5% on measured patient doses. Conclusion: Routine IMRT QA in most institutions today only involves verifying the optimized fluence map delivered to the patient in a test phantom at a certain preset depth. Based on our work here, we believe adding in vivo IMRT dosimetry with MOSFET detectors using the new brass build up caps along with routine fluence map verification in phantoms and MLC quality assurance offers greater accuracy and confidence in actual dose delivered to the patient.
SU‐DD‐A1‐05: A Ray Tracing Method to Generate Initial Conditions for IMAT Optimization
Authors: Oliver M , Gladwish A , Craig J , Chen J , Wong E
Published: July 2006

Purpose: To investigate the utility of using ray tracing to extract intrinsic information from CT, contour and primary dose data in order to determine initial conditions (number of arcs, arc weights, arc ranges and leaf positions) that can be input into an Intensity Modulated Arc Therapy (IMAT) optimization routine. Methods and Materials: Patient CT and contour data was ray‐traced to determine PTV and PTV‐OAR arcs. An additional arc was determined by the calculation of a ray importance factor (RIF) through ray tracing of the primary dose ray‐tracing of the PTV. All three sets of arcs were then input into a previously described leaf position optimization algorithm. This method was tested on two geometries by ray tracing 27 equi‐spaced beams. The optimized arc deliveries (number of arcs, arc weights, arc ranges and leaf positions) were then input into a fast dose calculation algorithm, NXEGS (NumeriX LLC) for dose calculation and comparison with primary dose as calculated by ray tracing. Results: RIF arc addition reduced the objective function by 20% for geometry 1 and 8% for geometry 2. Leaf position optimization further reduced the objective function by 27% for geometry 1 and 29% for geometry 2. Calculation of dose using NXEGS provides accurate dose distributions for IMAT. Conclusions: Ray tracing can quickly provide information about number of arcs, arc ranges, arc weights and leaf positions with very little user input. Leaf position optimization can improve leaf positions once the initial number of arcs and arc ranges are determined. Together these two steps can produce intensity modulated arcs for further optimization with a more accurate dose calculation algorithm.
SU‐DD‐A1‐06: Prostate IMRT Dose Escalation with Urethra Sparing: Dose Painting with IGRT
Authors: Zhang M , Moiseenko V , Liu M
Published: July 2006

Purpose: With IGRT, the geometric uncertainty in treatment can be reduced, which makes it feasible to implement IMRT dose painting with a reasonable resolution. In this cancer center, an on‐line realignment protocol is utilized for prostate cancer patients. This IGRT protocol is based on use of implanted gold fiducial markers and EPI. In this study, dose escalations with urethra sparing have been tested by using IMRT dose painting. Method and Materials: CT scans of three patients were chosen from the IGRT group. The original 3D‐CRT plan (74Gy/37fr, 10mm PTV margin) was used as a reference. In test IMRT plans two PTVs were generated. PTV1 was defined as 5mm extension of prostate. PTV2 was generated from PTV1 with 5mm margin subtracted for bladder, rectum, and urethra. Two raw plans were generated. Plan 1 was 74Gy/37fr to PTV1, and Plan 2 was 74Gy/37fr to PTV2. Then, the urethra sparing IMRT boost plan was generated as a weighted sum of the two raw plans, e.g. . Different combinations of weighting factors were tested: w1∈[0.6, 1], w2∈. [0.1, 0.5]. The dose to each organ was calculated with organ motion simulated based on actually recorded EPI image mismatches. The tumor control probability (TCP) and effective dose were used to evaluate the plans. Results: To achieve the same urethra D50 (minimum dose to 50% volume) as the reference plan, the highest weighting combination was w1=0.7, w2=0.5 . This yields significant dose reduction in bladder and rectum. For the considered patient the TCP increases from ∼74% to ∼95%. Conclusion: With IGRT, the urethra sparing IMRT dose painting is superior to the 3D‐CRT plan. The total prescription dose can be as high as 88Gy, with TCP of ∼95% and lower GI complication. Since urethra has been spared, the GU complication will be less.
SU‐DD‐A2‐01: On the Accuracy of a Moving Average Algorithm for Tracking Respiratory Motion During Radiation Therapy Treatment Delivery
Authors: George R , Williamson J , Murphy M , Weiss E , Keall P
Published: July 2006

Introduction: Real‐time motion tracking (RTT) treatment delivery has several advantages toward the improvement of accuracy for radiotherapy. However, currently there are certain limitations to this technique. The purpose of this study was to investigate an alternative treatment scenario using a moving average algorithm (MA) for treatment which could potentially be approaching the accuracy of RTT. Method: A comparison was performed between three different treatment scenarios.

(1)RTT: ;

(2)MA: ;

(3) Static beam delivery(SB) .

Where Xest(t) and Xact(t) are the estimated and actual position at time t, n in seconds is the averaging period (5–25 seconds range). The data used for this analysis was 331 respiration‐motion traces from 24 lung‐cancer patients acquired using three different breathing types (free breathing(FB), audio coaching(A) and audio‐visual biofeedback(AV)). The metrics used for comparison were the group systematic error(M), the standard deviation(SD) of the systematic error(Σ), and the root mean square of the random error(σ). The averaging period was varied to study the effect on the various metrics. Margins were calculated using the formula by Stroom et al.(IJROBP 1999;43(4)) Results: M and Σ are negligible for both MA[M ∈ (−0.01,0), Σ ∈ (0,0.01)] and RTT[M ∈ (0), Σ ∈ (0)] compared to SB[M ∈ (−0.15,−0.02), Σ ∈ (0.05–0.20)]. MA(0.48–0.54) has a slightly reduced σ than SB(0.53–0.57). Negligible improvements were found by varying the average periods for M and Σ . σ was found to be insensitive to the different averaging periods(0.53–0.56 for A). From the margin calculations FB is most affected by the different treatment scenarios. (All values in cm). Conclusions: MA has accuracy advantages over SB and practical advantages over RTT. MA significantly reduces M and Σ compared with SB. MA and SB require less margins for AV than that for FB and A. The margins required for RTT are independent of breathing training type. There is a group systematic error caused by intrafraction motion during FB.