bluuebunny/crossref_metadata_2025 huggingface dataset:
TH‐D‐ValB‐05: Evaluation of Image Quality in Megavoltage Digital Tomosynthesis
Authors: Descovich M , Morin O , Aubry J , Aubin M , Chen H , Chen J , Bani‐Hashemi A , Geffen M , Kelly K , Pang G , Pouliot J
Published: July 2006

Purpose: We report on the characteristics of Megavoltage Cone Beam Digital Tomosynthesis (MVCB DTS) and its potential clinical application for imaging of pulmonary lesions. Method and Materials: MVCB CT refers to the reconstruction of a 3D image from a set of 2D projections, acquired using a medical linear accelerator equipped with an electronic portal imaging device (EPID). A typical MVCB CT scan is acquired over a 200 degrees arc. In the case of MVCB DTS, the angular range is limited to reduce the acquisition time. This limited angular range affects the image quality of the reconstructed tomograms. To study the image quality as a function of the angular range, phantom measurements were performed and data from a head and neck patient were analyzed. The image quality was analyzed in terms of effective slice thickness, shape distortion and contrast sensitivity. MVCB DTS of the lung was performed on patients, with localized and diffuse lesions. Results: The image quality and the capability to distinguish overlaid structures decreases with decreasing angular range: a 20 degrees arc DTS results in a slice thickness of 2.7cm (vs. 1mm), a ratio of the vertical to lateral diameter of a sphere of 0.15, and a reduced contrast sensitivity. The acquisition is faster than MVCB CT. It takes 5–10 seconds for arcs of 20–40 degrees, compared to 45 seconds for a 200 degrees arc. In lung images, the faster acquisition results in a reduced blur due to the respiratory motion. Conclusion: This study indicates some potential advantages of DTS for imaging lung patients in the treatment position. Compared with EPID, DTS provides 3D information and better soft tissue contrast. Compared with CBCT, DTS allows shorter acquisition times, compatible with breath holding.

Conflict of Interest: Research sponsored by Siemens OCS.
TH‐D‐ValB‐06: Four‐Dimensional MVCT Reconstruction Using Temporal Re‐Binning
Authors: Outten S , Ramsey C
Published: July 2006

Purpose: To reconstruct 4D‐CT images using temporal re‐binning of sinogram projections from a slow‐CT scan of an object simulating respiratory motion. Methods and Materials: A slow‐CT scan of a battery‐powered motion phantom was taken using a helical tomotherapy machine. The motion phantom consisted of an elliptical disk with maximum diameter of 8.5‐cm, mounted on a spindle offset from the center of the major axis of the disk. The rotating spindle created a wobble in the disk's motion that simulated respiration. Atop the motion phantom, a CT resolution plug rested on a platform that moved vertically 1.5‐cm with a rotational period of 7 seconds. The plug was made of water equivalent material with seven holes of with diameters ranging from 2.00‐mm to 0.5‐mm. The vertical motion of the resolution plug was measured using a real‐time respiratory gating system. The CT image sinogram obtained from the machine detector was reconstructed using a program from the manufacturer of the helical tomotherapy machine. A program was written to shuffle sinogram projections, producing reconstructed slices over a range of movement (bin). The sorted slices were compared to the original CT images of the resolution plug. Not every bin contained a full gantry rotation of projections, and missing projections were replaced. Three replacement methods were used to obtain the best possible slice reconstructions. Results: The reconstructed CT slices showed improvement in resolution over the CT image of the plug in motion over the entire range of bins examined. Smaller bins with fewer missing projections had similar resolution to the static CT image of the plug. It was shown that bin enlargement showed the finest resolution of the projection replacement methods examined. Conclusions: Temporal re‐binning of slow‐CT sinogram projections can reduce motion artifacts and improve image resolution, but methods must be devised to accommodate for missing data.
TH‐D‐ValB‐08: Commissioning of a KV Image‐Guided System
Authors: Moseley D , White E , Purdie T , Bissonnette J , Sharpe M , Brock K , Siewerdsen J , Jaffray D
Published: July 2006

Purpose: Linac‐based kilo‐voltage x‐ray systems for image‐guidance have been widely adopted in radiation therapy. Over recent months, six cone‐beam CT enabled linacs have been installed and commissioned in our clinic. In the era of image‐guidance, a reference image of the patient anatomy and the treatment plan must be delivered to the unit for matching. This requires coordination of the simulation, planning and delivery systems. This work describes the commissioning and integration process for kilo‐voltage x‐ray image‐guidance systems in the modern radiotherapy clinic. Method and Materials: Accuracy and reproducibility of the kV source was measured using the RTI Barracuda diagnostic x‐ray meter. Image quality of the cone‐beam CT acquisition system was assessed using the CatPhan 500 multi‐slice CT image quality phantom. Metrics here include spatial resolution, uniformity, CT# accuracy and linearity as well as scale, orientation and slice thickness. Geometric coincidence of the imaging and treatment iso‐centers was verified using a steel BB localized to the MV radiation iso‐center using a Winston‐Lutz technique. A custom‐made anthropomorphic torso phantom was CT simulated and planned in four orientations (head first supine, head first prone, feet first supine and feet first prone) and exported to each linac for image‐guided setup correction and “treatment”. This tested the DICOM connectivity, orientation, scale, structure sets and isocenter of the imported reference images as well as magnitude and direction of couch corrections. Results: The average absolute error after correction across all orientations and all platforms was (0.8±0.7, 0.6±0.7, 1.0±0.7) [mm±1S.D.] (L/R, S/I, A/P). Conclusion: A commissioning process for linacs with kilo‐voltage imaging was described. Image‐guided radiotherapy increases precision and accuracy of the delivered treatment but it also increases the demand for integration and coordination of other systems in the modern clinic. Conflict of Interest: This work is sponsored by the Elekta Synergy Research Consortium.
TH‐D‐230A‐01: Economics Refresher Course: How Medical Physics Services Are Reimbursed
Authors: Hevezi J , White G
Published: July 2006

Medical physics services in medicine are reimbursed through the Current Procedural Terminology (CPT) Codes published by the American Medical Association in an annual publication. Although CPT codes for medical physics services appear explicitly in the radiation oncology series of CPT codes (77XXX), diagnostic radiological physicists may use several of these codes to cover patient specific work they may be called on to perform. This refresher course will trace the advent of the Centers for Medicare and Medicaid Services (CMS) relative value system for reimbursement for medical services and explain in detail how the complex system operates for current and new technology codes. Payments differ in various settings such as hospital, free‐standing center and ambulatory surgical center and these differences will be traced for each site of service.

Additionally, private health insurance carrier are not required to follow the precepts of the CPT formalism and these reimbursement entities will be described in their methodology of reimbursement for these services. Finally, ancillary publications that impinge on these reimbursement rules (eg. Correct Coding Initiative, CCI) will be referenced. Coding compliance guides such as those published by ASTRO and ACR will form the basis for this economics refresher course.
TH‐D‐224A‐01: QA for Tomotherapy
Authors: Tome W , Jaradat H , Westerly D , Fenwick J , Orton N , Mackie T , Paliwal B
Published: July 2006

Purpose: To describe the commissioning and quality assurance for helical tomotherapy machines. Method and Materials: Helical tomotherapy is a volumetric image guided, fully dynamic, IMRT delivery system. It was been developed at the University of Wisconsin and is now commercially manufactured as the ‘Hi‐Art’. At the core of this fully dynamic IMRT Delivery system lies a short gantry‐mounted linac that is used both for treatment and per‐treatment MVCT imaging. Aside from the primary collimator and the jaws, which set the width of the beam, it is also collimated by a binary multileaf collimator generating a fan beam of intensity‐modulated radiation. Modulation varies with gantry angle. Hence, the Hi‐Art allows for the acquisition of a helical pre‐treatment MVCT scan that is used for online image guidance and allows for precise interfraction positioning of the patient while in the actual treatment position just prior to the start of treatment. Due to its unique design the Hi‐Art system, allows highly conformal dose‐distributions to be delivered to patients in a helical fashion. Patients are treated lying on a couch that is translated through the bore of the machine as the gantry rotates, therefore the Hi‐Art is the therapy equivalent of helical CT. Since this approach to therapy is fully dynamic it requires synchrony of gantry rotation, couch translation, linac pulsing and the opening and closing of the leaves of the binary multileaf collimator used for beam modulation. Results: Over the course of the clinical implementation of the HiArt we have developed a quality assurance (QA) system that covers machine specific QA. The machine specific QA system is similar to that recommended for conventional linear accelerator QA by AAPM Task Group 40; however since the Hi‐Art design and operation differs from that of conventional medical linear accelerators, the tomotherapy QA system contains also novel components, such as QA checks for synchrony of gantry rotation, couch translation, linac pulsing and the opening and closing of the leaves of the binary multileaf collimator. Conclusion: In the first part of the presentation the design and dosimetric characteristics of Hi‐Art machines are summarized and the QA system is described along with experimental details of its implementation, while in the second part the pre‐treatment patient‐specific delivery QA for helical tomotherapy is discussed.

Learning Objectives:

1. The audience will understand the rationale for the proposed machine specific QA for Tomotherapy machines and how to carry out the various tests in a systematic and controlled fashion.

2. The rationale for the proposed patient specific pre‐treatment QA and how to carry it out in a systematic and controlled fashion will be understood.
TH‐D‐224A‐02: Physics Aspects of Clinical Trials: From the Basics to IMRT and IGRT
Authors: Moran J , Olch A
Published: July 2006

Medical physicists play an important role in the success of clinical trials involving radiation therapy. An understanding of how these clinical trials work is necessary and expected of the physicist involved with these treatments. AAPM Report #86, Quality Assurance for Clinical Trials: A Primer for Physicists, provides background information to physicists participating in clinical trials and will be reviewed.

The second part of the presentation introduces a new initiative within the AAPM that will affect how clinical trials are conducted. The goal of AAPM Task Group 113 is to provide guidance to physicists and others involved in clinical trials on methods to improve the consistency and quality of data generated for trials involving external beam radiotherapy. This presentation will discuss factors that impact data quality for the entire treatment planning and delivery process with an emphasis on advanced technologies such as IMRT and IGRT. The scope of Task Group 113 includes image acquisition for volume definition, treatment planning systems, patient localization, treatment guidance and delivery, and credentialing for clinical trials.

Educational Objectives:

1. To review the primer on QA for Clinical Trials.

2. To describe the goals of TG#113 Physics Practice Standards for Clinical Trials.

3. To highlight factors that directly impact clinical trials that involve IMRT and IGRT.
TH‐D‐224C‐03: Integral Test Phantom For Dosimetric and Geometric Assurance of IG‐IMRT
Authors: Letourneau D , Moseley D , Sharpe M , Jaffray D
Published: July 2006

Purpose: To develop a combined imaging and dosimetric phantom for the quality assurance (QA) of linear accelerators capable of cone‐beam CT image‐guided and intensity modulated radiotherapy (IG‐IMRT). This integrated approach verifies image quality, registration, and delivery performance. Method and Materials: The prototype consisted of a cylindrical imaging phantom (CatPhan) combined with an array of 11 radiation diodes arranged in a plane, oriented perpendicular to the phantom axis. Single diode performance was assessed at 6 and 18 MV (profiles, depth‐dose curves and angular dependence) with comparison to ion chamber. The detection of geometric and dosimetric errors in delivery was assessed using an IG‐IMRT treatment (6 MV, 7 beams, 180 cGy, CBCT‐guided) in which known displacements relative to isocenter were applied. The minimum detectable shift was determined by comparing the discrepancy between planned and measured doses to the dose measurement uncertainty under non‐shifted conditions. Results: Diode profiles and depth dose curves agreed generally within ±1% with the chamber results. Angular dependence for the diode was low for axial beams (±1%) but increased to a maximum of 11% for out‐of‐plane irradiation. The normalized dose measurements obtained with the multi‐diode phantom agreed well with the planning results. Displacements as small as 1 mm resulted in detectable deviation dose (8.2 cGy SD, n=11) relative to the uncertainty in dose measurements for non‐shifted conditions (1.6 cGy SD, n=11). Conclusion: A phantom prototype was designed and constructed for comprehensive QA of image‐guided radiotherapy in terms of image quality and dose delivery. The results allow us to set specifications for further development. We anticipate the system will permit the localization/detection of sub‐millimeters errors in dose gradient placement. Future phantom designs will facilitate absolute dosimetry and investigate the use of additional diodes in different patterns. Conflict of Interest: Supported in part by Sun Nuclear Corporation and Elekta Inc.
TH‐D‐224C‐09: Implementation of the Varian Eclipse System for Proton Therapy Treatment Planning
Authors: Newhauser W , Zheng Y , Fontenot J , Koch N , Polf J , Schaffner B , Titt U , Zhang X , Zhu X , Mohan R
Published: July 2006

Purpose: To configure and test a treatment planning system (TPS) (Varian, Palo Alto) for use with a passively‐scattered proton therapy delivery system. Methods and Materials: Since the mechanical designs of the commercial beam delivery apparatus (Probeat, Hitachi Limited, Japan) had not yet been completed, measurements of the required data were not possible. Instead, we designed our own therapy system components using analytical methods. The MCNPX (Los Alamos National Laboratory) Monte Carlo (MC) code was used to simulate beam profiles. Separate codes were developed to create the initial proton sources, the absorbed dose and fluence tallies, and simple homogeneous water phantoms (see Zheng et al., separate contribution). Up to 108 proton histories were tracked per simulation to achieve < 2% statistical uncertainties. Additional codes were developed to generate ancillary TPS configuration files, e.g., for the range modulator wheels, flattening filters, and variable range shifter. The MC beam characteristics such as penetration range were compared with values from independent one‐dimensional analytical calculations. Results: A complete set of configuration parameters and beam profiles were generated, including 832 dose and fluence profiles, in approximately four weeks (simulation time using 60 CPUs). With the simulated configuration data, the TPS has undergone extensive development and testing during the past year. Preliminary results indicate that the MC, TPS, and independent analytical calculations are in good agreement (< 4 mm differences in penetration range). Conclusion: The results of this study demonstrate the practicality of MC models to calculate beam data for configuring a proton treatment planning system. Additional simulations with the manufacturer's preliminary equipment designs are now in progress. Validation tests of Eclipse in heterogeneous media are in progress (see Titt et al., separate contribution). Confirmatory measurements are planned. Conflict of Interest: This work was funded in part by a research grant from Varian.
TH‐D‐ValA‐02: Prepulse Effect and Maximum Energy of Protons Accelerated by High‐Power Lasers
Authors: Fourkal E , Veltchev I , Lin T , Ma C
Published: July 2006

Purpose: In recent years there has been an explosion of research work concerning the topic of charged ion acceleration using high‐power lasers. The maximum particle energy and the shape of the distribution function are the two main parameters influencing the potential utilization of the new technology in radiation therapy. The energy spectrum depends on the quality of the laser beam as well as the geometric shape of the interaction target. Recent experiments have shown that it is possible to generate quasi‐monoenergetic protons using a double target system. However, influence of the laser prepulse has not yet been fully quantified. The purpose of this study is to find how the laser prepulse changes the property of the target and under which conditions can one expect the highest proton energy. Method and Materials: A radiation‐magnetodynamics code with inline atomic kinetic modeling was used to simulate interaction of the laser prepulse with initially cold double‐layer target. The simulated target system consists of an aluminum substrate on which a thin layer of hydrogen is located. Different initial substrate thicknesses and laser contrast ratios were simulated to find an optimal interaction conditions. Results of hydrodynamic simulations were used as initial conditions for the simulation of the main pulse interaction with the altered target. Results: It is shown that interaction of the prepulse with the target leads to its partial ionization with subsequent formation of a shock wave. Propagating shock wave destroys initial configuration of matter in a substrate, greatly expanding it in laser's propagation direction. Results of simulations suggest that maximal proton energy is achieved using lasers with smallest contrasts, incident on targets of thicknesses 1–2 μm. Conclusions: Presence of laser prepulse impedes acceleration efficiency of protons. Minimizing the contrast ratio or shortening the duration of the prepulse should result in higher maximal proton energies.
TH‐D‐ValA‐04: Quantitative Evaluation of Cone Beam CT Data Used for Treatment Planning
Authors: Houser C , Nawaz A O , Galvin J , Xiao Y
Published: July 2006

Purpose: To quantitatively evaluate the consistency of cone beam CT (CBCT) data and the deviation from helical CT if used for calculating dose to heterogeneous material. Methods and Materials: A Gammex RMI 467 tissue characterization phantom was used to generate CT numbers for both the Elekta XVI CBCT and the Picker 5000CT. For both devices, the RMI phantom was positioned using the manufacturer's recommendations. Hounsfield numbers (HU) were obtained using a default window and level and a ROI of 1cm diameter over the center of the rod (insert). The numbers were recorded and a HU vs. electron density correction curve was generated. For the cone beam portion, we used the Elekta XVI CBCT with M10 collimator and no filter. CBCT data, conventional CT data and correction curves were transferred to our CMS XIO treatment planning system. To verify the consistency of CBCT numbers, we added a 1cm of bolus to the phantom and repeated the procedure. A single field treatment plan was generated with heterogeneity and non heterogeneity corrections for CBCT and CT datasets, and for nonbolused and bolused geometries. The Gammex phantom tissue equivalent rods were modified to accept TLD for measurement verification. Results: Calculated doses for treatment plans generated without heterogeneity correction for data sets from CT and CBCT agreed within ∼1%. CBCT numbers for the same insert differed with a small change in setup (ie. adding 1cm bolus). When heterogeneity correction was turned on, a difference of 14% was found between CT and CBCT nonbolused plans. TLD measured doses agreed with those from the calculated CT set. Conclusion: Without heterogeneity correction, the dose calculation with CT and CBCT has excellent agreement suggesting a consistency in contour acquisition. One has to be extremely cautious in using CBCT data for heterogeneity corrections since significant error can occur.
TH‐D‐ValA‐05: Effect of Measured Cone‐Beam CT‐Density Calibration On Dose Calculations
Authors: Lindsay P , Zhu X , Zhang L , Wu R , Kudchadker R , Salehpour M , Liu R , Mohan R , Gillin M , Dong L
Published: July 2006

Purpose: To characterize the CT‐density conversion for a cone‐beam CT (CBCT) system, and to evaluate the accuracy of dose calculation using CBCT images in head & neck (H&N) and abdominal sites. Method and Materials: A small (16 cm) and a large (32 cm) acrylic phantom with inserts of known electron densities were scanned on conventional CT and CBCT scanners. CBCT images were acquired using full and half fan (small phantom only), with and without bow‐tie filter. CT‐density conversion tables were measured for each combination. Head and pelvis phantoms, and H&N and prostate patients were imaged. IMRT treatment plans were designed based on conventional CT and transferred to CBCT after image registration (deformable registration for H&N patient). The CBCT dose distributions were then recalculated using the measured CT‐density curves. Results: For the small phantom, minimal differences were found between CT and CBCT numbers. Different conditions (half/full fan, with/without bow‐tie) resulted in maximum differences of approximately 100 HU. For the large phantom, differences between the CT numbers and CBCT numbers were greater than 350 HU at both low and high densities. For the H&N patient and the head phantom, dose distributions calculated on CT and CBCT differed by less than 2%. For the pelvis phantom and prostate patient, there were substantial differences in the dose distributions. In the worst case, using CBCT images and the measured CBCT‐density conversion curve, the mean prostate dose was 14% higher than that calculated using the conventional CT. Due to artifacts in the CBCT, the differences in dose were larger using the CBCT‐density curves than using the default CT‐density curves. Using unit density for CBCT is a good approximation for pelvis dose calculation. Conclusion: Artifacts in CBCT images and CT‐density conversion may lead to considerable errors in dose calculation using the large pelvis CBCT images.
TH‐D‐ValA‐09: An Objective Approach to Establishing Tolerances On Photon Beam Modeling Using the Equivalent Uniform Dose
Authors: Rangel‐Baltazar A , Ploquin N , Kay I , Doll C , Dunscombe P , Baker Tom
Published: July 2006

Purpose: To perturb a photon beam model in a controlled manner and to examine the consequences for the Equivalent Uniform Doses (EUD) of the target and organs at risk in external beam radiation therapy of the prostate. Method and Materials: We have developed seven similar but different therapy beam models in the Pinnacle® Treatment Planning System. One model generates beam data close to the golden data provided by Varian® and serves as the reference model for this study. The six other models are modifications of the reference model designed to result in controlled deviations of a particular region of the dose profiles (descending depth dose, build‐up, horns, tail, penumbra and field width). We have analyzed the consequences of planning with these perturbed models on the quality of 4 prostate treatment plans in terms of the EUDs of the PTV, rectum and bladder in comparison with the reference model. Monitor units were kept constant for all plans. Results: to maintain a change in the EUD to the prostate, bladder or rectum of less than 2%, tolerances on the various regions of the dose profiles are as follows: descending depth dose 2%; horns 3%; field width ±1mm. Deviations in the build‐up region and tail of ±10% and ±5% respectively did not change the EUDs of any structure by more than 2%. Conclusion: Currently accepted tolerances on photon beam modeling are broadly internally consistent in so far as they result in similar effects on plan dosimetry, at least for 4 field conformal prostate treatments.
TH‐D‐230C‐01: Mammography QA Workshop I and II
Authors: Geiser W , Shaw C
Published: July 2006

Purpose: to explore the impact of DR mammography on the practice of medical physics. Historical and new methods for providing scheduled and unscheduled mammography services for facilities using DR mammography will be presented. Method and Materials: clinical medical physics services provided for DR mammography facilities from various manufacturers will be presented. Quality control activities approved by the FDA for each individual manufacturer will be compared. The status of a proposed “Alternative Standard” to allow for a more uniform approach to medical physics mammography services will be reviewed. Case studies will be presented demonstrating methods and cost considerations for providing medical physics services under the “minimum standard” (prescribed by MQSA regulations) and “best practices” model. Results: Medical physics services provided in the “minimum standard” and “best practices” model have implications for the quality and cost of mammography services. Conclusion: Medical physics services may be provided in a professional and valuable manner, using combinations of the minimum standard of practice and the “best practices” model. The professional medical should consider multiple parameters when determining the appropriate model under which to deliver services.
TH‐E‐330A‐02: A Local Fourier Description of Artifacts in Circular Cone Beam Computed Tomography
Authors: Bartolac S , Noo F , Clackdoyle R , Moseley D , Siewerdsen J , Jaffray D
Published: July 2006

Purpose: Circular cone‐beam computed tomography is challenged by a lack of plane sums resulting in incomplete inversion of the radon transform. These missing plane sums have been identified by theory, and may be represented as a shift‐variant cone of missing frequencies in the Fourier domain. The aim was to verify the presence of this cone in real data, and to show the dependency of resulting image artifacts on the frequency distribution of the imaged object. Method and Materials: A mini disk phantom (mylar/foam) was constructed to probe the local frequency response at various locations in the reconstruction space. Projections obtained using an experimental CBCT benchtop were reconstructed with 120 μm3 voxel size using a modified Feldkamp filtered backprojection routine. Local Fourier transforms of the mini disks were analyzed for missing frequency data and compared with theory. Large disk phantoms of acrylic and cellular polyurethane were also imaged for further demonstration of the effect of varying the frequency content of the imaged object. Results: The cone of missing frequency was successfully identified in the mini disk phantom and agreed well with theory. Image artifact was found to have dependency on the local distribution of the object's frequency power spectrum relative to the cone of missing frequency information. Decreased resolution of the disks occurred when their dominant spatial frequency components were directionally aligned to coincide with the predicted null cone, as expected. Image reconstructions of large disk phantoms showed good detail in cellular disks even at locations that showed strong artifacts in the acrylic disk. Conclusion: The predicted null cone is observable in Fourier transforms of localized objects. Resolution in reconstruction is dependent on the relative frequency distribution of the imaged object; features that are most poorly resolved will be those with strong frequency components directed in the expected null space.
TH‐E‐330A‐03: Reduction of Ring Artifacts in Cone Beam CT: Artifact Detection and Correction for Flat Panel Imagers
Authors: Altunbas M , Shaw C , Chen L
Published: July 2006

Purpose: Defective pixels in a flat panel detector may be characterized by a nonlinear signal response drastically different from those in the neighboring pixels. They could lead to ring artifacts in cone beam CT. In this presentation, we will describe and demonstrate the use of a filter based calibration technique to detect these pixels. In addition, we will employ this technique as a flat field correction method to correct nonlinear signal response. Methods and Materials: To force the signal responses of all pixels to vary smoothly without sudden changes, we acquired images at fixed mAs but with various filtrations. Each filtered image is fitted to a smooth surface whose values are close to those of normal pixels but vary smoothly in the image. The ratios of the surface fit values to the original values were then computed on a pixel‐by‐pixel basis and used to map pixel values during subsequent image acquisition. This mapping would compensate for the nonlinear signal response associated with the defective pixels thus eliminating the ring artifacts. Using the surface fits as the reference, defective pixels were detected by automatic thresholding. Results: Using filter based calibration, defective (∼0.035 %) pixels were successfully detected and corrected for. Therefore, ring artifacts were largely eliminated in cone beam CT images. A reduction in patterned noise artifacts in the projection images was also observed. The automated surface fitting procedure was found to be robust. Conclusions: Although the conventional flat field correction addresses non‐uniform response across the detector, artifacts may still form as the pixel response varies with the beam quality and signal intensity. The filter based calibration procedure was successfully used to detect and correct for these artifacts.

This work was supported in part by a research grant EB00117 from the NIH‐NIBIB and a research grant CA104759 from the NIH‐NCI.
TH‐E‐330A‐04: Investigation Into the Cause of a New Artifact in Cone Beam CT Reconstructions On a Flat Panel Imager
Authors: Starman J , Virshup G , Bandy S , Star‐Lack J , Fahrig R
Published: July 2006

Purpose: To investigate the source of and possible corrections for a new artifact seen in cone‐beam CT (CBCT) images acquired using an a‐Si flat panel imager (FPI) (Varian 4030CB). The new artifact is a bright circular region, tangent to the phantom edge, in images of elliptical and off‐center phantoms. Methods and Materials: Temporal response of the FPI was measured using a step‐wedge phantom, as a function of dose and irradiation history (10 cycles of 80 s exposure, with 9 off‐cycles varying in time between 2 and 30 minutes, total time 1hour 20 min). A linear time invariant (LTI) model was developed by fitting multi‐exponentials to the lag response from the step‐wedge phantom. Anthropomorphic phantoms — pelvis placed centrally, and head placed off‐center — were scanned and reconstructed with and without the developed correction. Results: Detector lag and continuous and significant monotonic gain increase (up to 10% for long irradiation periods) were observed during constant irradiation. Even after long periods of no exposure, with the detector being continuously read out, the gain did not return to the original, start‐of‐day value. Unlike Overdick et al., we did not see a saturation effect in the gain change. In CBCT reconstructions, differences up to 35 HU existed close to the edges of the artifact. After applying our correction model, differences were reduced to less than 10 HU. Our anthropomorphic phantoms did not generate streaks or comet tails, which other investigators have shown to be due to lag. Conclusion: We have determined that the source of the circular artifact observed is a non‐ideal temporal response. This artifact can be mostly eliminated by applying a correction based on a LTI model. Future work will focus on more accurate modeling to completely eliminate the artifact. Conflict of Interest: Funding was provided by Varian Medical Systems.
TH‐E‐330A‐05: Reducing Metal Artifacts in Cone‐Beam CT by Tracking and Eliminating Metal Shadows in Raw Projection Data
Authors: Zhang Y , Zhang L , Zhu RX , Chambers M , Dong L
Published: July 2006

Purpose: CT streaky artifacts, caused by metallic implants such as fiducial markers or dental fillings, remain a challenge for automatic processing of image data. The effect of these metal artifacts is magnified in cone‐beam CT (CBCT) images due to the fact that the soft tissue contrast is usually lower in these images and therefore is more sensitive to the artifacts. The goal of this study is to develop an effective offline processing technique to minimize the effect. Method and Materials: The geometry calibration cue of the CBCT system was used for tracking the position of the metal implant in the raw projection data. The 3D representation of the metallic object can be established from only two user‐selected viewing angles. The position of the shadowed region in any view can be accurately tracked by re‐projecting the 3D coordinates of the metal object. Then automatic image segmentation was performed to obtain a binary mask of the shadow at each projection angle. Finally, a Laplacian diffusion filter was used to replace the pixels in the masked region with the boundary pixels. The modified projection data were then sent back to the CBCT reconstruction engine to create a new CBCT image. Varian's Trilogy system was used in this study. The procedure was tested phantoms and patient cases. Results: It was demonstrated that this procedure can significantly minimize the metal artifacts and at the same time restore soft tissue contrast near the metallic object, even for the more difficult head and neck case with irregularly shaped dental fillings. Soft tissue visibility was improved drastically. Although not designed for on‐line applications, the processing time is approximately 1–2 second per projection on an Intel Pentium processor at 2.6GHz. Conclusion: We have implemented an effective metal artifact suppressing algorithm to improve the quality of CBCT images.
TH‐E‐ValB‐01: Cell Proliferation and Tumor Hypoxia During Radiation Therapy
Authors: Jeraj R , Simoncic U , Barbee D , Nickles J , Avila‐Rodriguez M , Forrest L
Published: July 2006

Purpose: Tumor hypoxia is an important resistant component that significantly affects response to treatment. Our aim was to concurrently monitor cell proliferation and tumor hypoxia distribution during radiation therapy. Materials and Methods: Several canine subjects with soft tissue sarcomas were repeatedly imaged with PET/CT before, during and after radiation treatment. The tumors were treated with 60Co in four 8 Gy weekly fractions. 3′‐Deoxy‐3′‐fluorothymidine ([F‐18]‐FLT) and Cu‐diacetyl‐bis(N4‐methylthiosemicarbazone) ([Cu‐61]‐ATSM) as surrogates of cell proliferation and tissue hypoxia, respectively, were used to follow the response. Approximately 200 MBq of FLT or Cu‐ATSM activity was administered per scan. The CT data between the imaging sessions was co‐registered and the corresponding PET data compared and analyzed. Results: Tumor response to therapy varied significantly between the subjects and even for different tumors in the same patient in case of multicentric disease. High heterogeneity of both cell proliferation and hypoxia (up to 50% in SUV) of the tumor was observed in several cases. Early proliferative response seems to be indicative of the overall tumor response. Both cell proliferation and hypoxia distributions changed during treatment, but their differential response remained rather constant. The distributions of cell proliferation and tissue hypoxia were often found to be complementary, but not exclusively. Conclusions: Concurrent monitoring of cell proliferation and tissue hypoxia represents a new dimension in tumor monitoring and provides basis for more efficient, biologically based treatment optimization. High heterogeneity of tumor kinetics and microenvironment together with spatially variable response calls for individualized approach to cancer management.
TH‐E‐ValB‐03: Tumor Volume Regression During Radiation Therapy to Predict Treatment Outcome for Cervical Cancer
Authors: Wang J , Mayr N , Yuh W , Zhang H , Montebello J , Grecula J , Wu D , Edwards S
Published: July 2006

Purpose: To investigate the outcome predictive power of tumor volume measured by serial MR imaging (MRI) of cervical cancer, including the sensitivity and specificity to identify patients at risk of local failure. Method and Materials: Seventy‐nine patients with cervical cancer stages IB2‐IVA, treated with radiation/chemotherapy (RT/CT), underwent serial MRI: MRI 1(pre‐RT), MRI 2(at 20–25 Gy/2 weeks), MRI 3(at 40–50 Gy/4 weeks), and MRI 4(at 1–2 months post‐RT). Mean follow up was 6.2 (0.2–9.4) years. Tumor volumes (V1,V2,V3,V4) and regression ratios (V2/V1,V3/V1,V4/V1) were measured by MRI 3D volumetry, and correlated with local tumor‐control and disease‐free survival using Mann‐Whitney rank‐sum test. Results: The volume data collected in this study were analyzed and the predictive power in terms of p‐value to discriminate local tumor‐control and disease‐free survival was computed. The absolute tumor volumes (V2,V3,V4) and the regression ratios (V2/V1,V3/V1,V4/V1) strongly correlated with local tumor‐control (p<0.001). These parameters also correlated with disease‐free survival, but only the last measurement (MRI 4) showed significant predictive value (p=0.02). Four methods had been developed to identify patients at risk for tumor recurrence (sensitivity 61%–100% and specificity 87%–100%). The most powerful method is based on the volume regression measured in MRI 3 and MRI 4 (V3/V1 >20% and V4/V1 >10%), which have a sensitivity of 89% and a specificity of 100%. Local failure can also be predicted as early as 2–3 weeks (MRI 2), the method of V1 >40 cc and V2/V1 >75% shows a sensitivity of 61% and a specificity of 93%. Conclusion: MRI‐based volumetric tumor measurement provides important predictive information about tumor response to the ongoing RT/CT. The methods developed in this study demonstrate a high specificity (87%–100%) for patients at risk of local failure based on long‐term follow‐up. These methods may classify patients who require more aggressive therapeutic intervention.
TH‐E‐ValB‐06: Quantitative Characterization of Tumor Vascular Dysfunction in High‐Grade Gliomas Prior to and During Radiotherapy
Authors: Nagesh V , Chenevert T , Junck L , Tsien C , Lawrence T , Cao Y
Published: July 2006

Introduction: Vascular properties within and adjacent to tumors may not be distinguishable by cerebral blood flow [CBF] or cerebral blood volume [CBV] alone, since the rates of CBV change may not be proportional in magnitude to CBF change. Hence, the empirical and physiological relationships between CBF and CBV were examined to estimate vasculature‐specific hemodynamic characteristics in high‐grade gliomas. Methods: Twenty patients with gliomas were studied with dynamic contrast‐enhanced T2* MRI [DCE‐MRI] before and during radiotherapy [RT]. CBF and CBV were calculated from DCE‐MRI and the relationships between the two were evaluated using two different metrics: The physiological measure of Mean Transit Time [MTT]=CBV/CBF; and, Empirical fitting of CBV and CBF using the power law, expressed as CBV=Constant*(CBF)β. Three tissue types were assessed, Gd‐enhanced tumor volume [GdTV], non‐enhancing abnormal tissue located beyond GdTV but within the abnormal hyperintense volume on FLAIR images [NEV], and normal tissue in hemisphere contralateral to tumor [CNT]. Effects of tissue types, CBV magnitudes (low[L], medium[M] and high[H] CBV), before and during RT, on MTT and β were analyzed by factorial ANOVA. Results: Both, MTT and β were significantly different (p<0.009) among the three tissue types. MTT increased from CNT(=1.60s) to NEV(=1.93s) to GdTV(=2.28s) (<0.0005). The power exponent β was significantly greater in GdTV(=1.079) and NEV(=1.070) than CNT(=1.025), but β in NEV and GdTV were not significantly different from each other. β increased with increasing CBV magnitude. There was a significant decrease in MTT and a significant increase in β in tumor (GdTV) and peritumoral (NEV) tissue during RT compared with pre‐RT values. Conclusions: β was strongly dependent on CBV magnitude and MTT on tissue type. Progressive abnormalities in functional characteristics of the vascular bed were noted, with significant disorder in tumor, but mild abnormality in peritumoral tissue. Early vascular response to radiation was first observed in functional rather than structural properties.
TH‐E‐ValB‐07: Blood‐Tumor‐Barrier Permeability Changes in High‐Grade Gliomas During Radiation Therapy Using DCE MRI
Authors: Cao Y , Tsien C , Nagesh V , Ross B , Chenevert T , Junck L , Lawrence T
Published: July 2006

Introduction: Radiation could affect vascular permeability in tumor and normal tissue. A previous study using high‐resolution MR images and a contrast uptake index demonstrated that an increase in contrast uptake in the tumor occurs after 30 Gy and persists up to one month after radiation therapy (RT). This finding suggested that an optimal time window exists to increase the efficiency of drug delivery to tumors. In this study, a quantitative method, dynamic contrast enhanced (DCE) MR imaging, was used to assess tumor vascular permeability changes during RT, in order to maximize potential therapeutic benefit. Methods: Twenty patients with high‐grade gliomas who underwent conformal RT participated in a MRI protocol. DCE T2* weighted images were acquired before RT, after ∼30 Gy, and one month after the completion of RT. A transfer constant (K) of Gd‐DTPA from blood to tissue was estimated voxel‐by‐voxel and used as a metric for assessment of vascular permeability. In the tumor volume (TV) defined on FLAIR MRI, statistically significant changes in K after ∼30 Gy and one month after RT were evaluated, compared to before treatment, using a students' t test. Results: An average fractional volume of 29.6% in tumor manifested substantial contrast leakage with K>0.005 min−1 pre RT. In the TV where there was no substantial leakage pre RT, the mean K increased significantly from K=0.0003 min−1 to 0.0073 min−1 after ∼30 Gy (p<0.0005) and to 0.0053 min−1 one month after RT (P<0.003). The fraction of TV that showed substantial contrast leakage significantly increased by 23% after ∼30 Gy (p<0.02), but not one month after RT (p>0.5). Conclusion: DCE MR imaging reveals vascular permeability increases after ∼30Gy in the portion of tumor where leakage is not substantial before RT. This finding suggests that the optimal time to administer chemotherapy is during the course of radiation therapy.
TH‐E‐224A‐02: A Systematic Analysis of Patient Specific IMRT QA Data
Authors: Su Z , Jin H , Fox C , Li H , Lynch B , Yang C , Li J , Liu C , Palta J , Kim S
Published: July 2006

Purpose: This is a retrospective analysis of patient specific IMRT QA data. The goal is to systematically evaluate IMRT plans and analyze factors that influence quantitative differences between calculations and measurements. Method and Materials: For each IMRT beam, planar dose is calculated using Pinnacle treatment planning system (TPS) at 10cm depth, 100 cm SAD in solid water with normal incidence of the gantry and Mapcheck measurement is performed accordingly. Absolute dose comparisons are performed between the planned and measured planar dose distributions with a 3% and 3mm criteria. A threshold is set at 10% of normalization point dose. The pass rates are categorized into the versions of TPS and delivery system. We also analyzed the outliers to see if we could a priori predict the differences between measured and calculated dose using a recently published Dose Uncertainty Model in Medical Physics. Results: A total of 427 plans with 2246 beams were analyzed; 57.7% of all beams have passing rates of at least 95%; 36% are between 85% and 95%; 6.3% are below 85%. The passing rate correlates strongly with the accuracy of beam modeling in TPS. The TPS version that explicitly modeled the MLC characteristics (leaf‐end and leaf‐side effects) had a better passing rate than the TPS version that had a simpler MLC model (60.7% versus 52.7% of at least 95%). The QA failure rate increases with the complexity of intensity modulation in a treatment field and with larger uncertainty of MLC beam model in the TPS. Conclusion: The accuracy of IMRT delivery is strongly correlated to the accuracy of MLC beam model in TPS. The pass rate for patient specific QA is strongly influenced by the complexity of the intensity modulation in a field. Strong relationship between position of failure and expected uncertainty was observed.
TH‐E‐224A‐03: Orientation, Position, and Temperature Dependence On Optical Density Measurements When Using Sensitive Radiochromic Film and Fluorescent CCD Scanner
Authors: Lynch B , Kozelka J , Ranade M , Li J , Simon W , Dempsey J
Published: July 2006

Purpose: We report systematic artifacts that were found to occur when scanning a commercial radiochromic film (RCF) (GAFCHROMIC EBT) with a charge‐coupled device (CCD) film scanner. Potential systematic sources of optical density (OD) measurement error were found be related to film orientation with respect to the scanner bed, film position on the scanner bed, and film temperature from repeated scanning. Method and Materials: We investigated the use of two flatbed CCD scanners that have been previously reported for use with the RCF, as well as two point densitometers that measure diffuse OD. Change in OD with film orientation was studied with small uniformly irradiated RCFs that were rotated and repeatedly scanned. A RCF from an IMRT QA measurement was scanned in portrait and landscape orientations to assess the magnitude of possible clinical errors. Film uniformity was assessed by evaluating profiles in the scan direction for large RCFs with graded uniform ODs produced by exposure to sunlight. A thermocouple was placed on the bed of a CCD scanner RCF was repeatedly scanned while recording temperature. Results: Sinusoidal variations of 15% were observed for rotating film measurements made with one CCD scanner (Epson) and 80% for the other (Microtek). The point densitometers did not demonstrate any variation with rotation. Changes in temperature in the range of 18–34°C lead to OD changes as large as 7%. The edges of the CCD scan beds demonstrated non‐uniformities as large as 15%. Conclusion: Care must be taken to avoid these systematic errors when using RCF for clinical dosimetry. We recommend: maintaining the orientation of films; scanning in the central portion of the scanner bed; and limiting the number of consecutive scans.

This work was supported by NCI grants R01‐CA‐100636 and N43‐CM‐52214, Department of Health and Human Services Contract No HHSN261200522014C, and by Sun Nuclear Corporation.