bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐I‐37: A Renewed Look at CT Numbers as Encountered in the Diagnostic and Therapeutic Range
Authors: Fredriksson J , Goodenough D , Healy A , Kristbjornsson A , Kristinsson S , Levy J , Olafsdottir H
Published: June 2012

Purpose: With the development of RT treatment planning systems that use the relationship between CT number and Electron Density (ED), rather than the Linear Attenuation Coefficient (LAC) of a given material, a number of authors have pointed out the divergence between the linearity of CT numbers vs. LAC in the diagnostic range compared to the relationship between the CT number and ED. This paper will review the differences and similarities, and describe a new set of phantom test objects and automated software that can be used to automatically assess both scales. In particular the relative importance of atomic number (Z) and the relative impact of Coherent vs. Incoherent effects at high Z levels will be evaluated. Methods: A newly developed phantom (Catphan® 700) has an expanded set of sensitometric samples of known density, electron density, and chemical composition (Z effective). CT scans of the phantom were obtained at various energies (kVp) and the measured CT numbers were compared to the known physical characteristics mentioned above. Regressions between measured CT numbers, linear attenuation coefficients (with and without coherent scattering effects), and electron density for the materials were performed. Effects of different materials and the inclusion of coherent scattering on linearity scale and effective energy were established. Results: The linearity scale and effective energy are shown to be dependent on the selection of materials scanned and the inclusion/exclusion of coherent scattering effects in the linear attenuation coefficients. Electron density deviates significantly from a linear relationship with CT number. Conclusions: Caveats accompanying high Z materials are reinforced regarding application to the RT relationship between CT number and electron density. Interesting results were obtained for the influence of coherent vs. incoherent scattering, which appears to be important as the number of slices and scanning volume increases in CT. Funding provided by The Phantom Laboratory, Incorporated and Image Owl, Incorporated.
SU‐E‐I‐48: Comparison of CTDIw and Averaged CTDI Over X‐Y Plane
Authors: Liang Y , Emerson S , Schultz C
Published: June 2012

Purpose: The goal of this study is to investigate the accuracy of using CTDIw to estimate the averaged CTDI over x‐y plane. Methods: We used a Siemens Sensation 16 slice scanner, a nested 3 piece CTDI phantom with diameters of 10, 16, and 32 cm for each piece, a CT pencil ion chamber, and aluminum oxide OSL dosimeters. In axial mode, we measured the exposure with ion chamber at the 3, 6, 9, and 12 o'clock positions at distances of 4, 7, and 15 cm from the center of a 32 cm CTDI phantom, as well as at the center of the phantom. Measurements were performed at 80, 100, 120, and 140 kVp on the adult abdomen, adult head, and pediatric body phantom, using only the distances of 0, 4, and 7 cm from the phantom center for the smaller 16 cm diameter phantom. We did similar measurements using nanoDot dosimeters and the 32 cm phantom at 120 kVp in helical mode using a pitch of 0.8, 1.0, and 1.2. Results: The data obtained at four different outer clock positions was averaged and three models (linear, quadratic, and exponential) were used to fit exposure as a function of distance to the phantom center. We calculated the average CTDI over the x‐ y plane mathematically using the above models and compared the results with traditional CTDIw. In axial mode, the difference is within 6% for 32 cm phantom, with a slight increase in variance at low kVp, while the difference is within 1% for the 16 cm phantom. In helical mode with OSLs, the difference is within 2.5% for pitch 0.8–1.2. Conclusions: The current CTDIw provides an accurate estimate of the averaged CTDI over the x‐y plane for both axial and helical modes.
SU‐E‐I‐53: Optimal KVp for Image Quality and Noise in Iodine Contrast in Head CTA
Authors: Johnson L , Yester M , Barnes G
Published: June 2012

Purpose: For brain CT perfusion it is well established that 80 kVp is optimal. Although neuro‐CT angiography is somewhat similar, emphasis is on the detection of aneurysms and related vascular pathologies throughout the brain. Thus it is necessary to visualize small and large blood vessels with contrast material, as well as form multi‐planar views and 3D images, so image quality and noise in addition to contrast are important for thin slices. A study was initiated to determine the optimal kVp for neuro‐CTA. Methods: A customized version of a commercial head phantom (CIRS 007TE‐27 medium adult head CT dose phantom) was purchased to facilitate quantitative measurements with iodinated contrast material, contrast for white and gray matter, and to maintain the ability to perform dosimetry. The customization consisted of adding four 25 mm holes, 35 mm from the center arranged at 45 degree angles from the center, with solid rods equivalent with brain, white, and gray matter, as well as four fillable vials were included for study of contrast agents. Dosimetry measurements were carried out with standard pencil chamber and with 0.6 cc ionization chamber. For study of the optimal kVp for a head CTA, the vials were filled with four different concentrations of contrast, approximating low to medium concentrations that would be expected in such a study. The standard CTA protocol was followed, 64 × 0.625, pitch 0.53, rotation speed 0.5 second, and CTDIvol was kept constant for each kVp. Results: The best contrast was observed at 80 kVp; however, in order to achieve noise in CTA low enough to be clinically useful there may be issues with tube current capability for a clinical technique. Clinical investigation is underway. Conclusions: The best balance of contrast and noise currently possible will be achieved at 100 kVp in a clinical scan.
SU‐E‐I‐54: Evaluation of High Contrast Resolution for Model Based Iterative Reconstruction of Sinus Examinations
Authors: Paden R , Pavlicek W
Published: June 2012

Purpose: To evaluate high contrast resolution of Model Based Iterative Reconstruction used with typical sinus examination acquisition parameters. Methods: MBIR has recently become available as a recon option on a clinical scanner (Discovery HD750, GE Healthcare). In this work, we evaluate high contrast resolution for scan and reconstruction options that are available for sinus examinations. For this study we used our adult sinus CT protocol reconstructed with filtered back projection, and two alternative scans reconstructed with IR. Our conventional adult sinus CT protocol utilizes a high resolution scan mode which is not compatible with the scanner's IR recon option. The two additional scans are with high resolution option off, one with the head SFOV, and one with the body SFOV. Using IR and the head SFOV, reconstructed images have a 1024 × 1024 pixel matrix. Using IR and the body SFOV, reconstructed images have a 512 × 512 pixel matrix. Three evaluations of high contrast resolution are made for these images. A wire phantom is scanned for assessment of image modulation transfer function. The bar patterns of the ACR phantom are visually assessed for quality in both axial and coronal reformats. Results: MTF curves show 50% values of 6.8, 7.5, and 7.7 lp/cm for Body IR 512 × 512, Head IR 1024 × 1024, and filtered back projection with HD Bone kernel. The 10% MFT curves for these reconstructions are 11.2, 11.9, and 12.1 lp/cm. Visual evaluation of the ACR phantom at 15 cm display field of view demonstrates resolution of the 10 lp/cm bar patterns for all reconstructions with better visualization of the axial versus coronal recons. Conclusions: MBIR reconstruction demonstrates high contrast resolution that is comparable with our conventional sinus examination.
SU‐E‐I‐78: Neuromelanin in the Subthalamic Nucleus of Patients with Parkinson's Disease: An Electron Spin Resonance Spectroscopy Study
Authors: Gomez J , Salmon C Garrido , Filho O Baffa , Santos J Peixoto , Pitella J
Published: June 2012

Purpose: Parkinson disease and related syndromes are associated directly with the concentrations of neuromelanin, iron and other heavy metals, and nowadays it is discussed the possible protective role of neuromelanin by the sequester redox active iron ions, reducing the formation of free hydroxyl radicals and therefore inactivating the iron ions that induce oxidative stress. The aim of this work is to study the concentration ratios between iron ions and neuromelanin in subthalamic nucleus of patients with Parkinson's disease (PD) using Electron Spin Resonance (ESR). Methods: Necropsy samples of subthalamic nucleus from eight human brains were studied: three non‐affected by any neurodegenerative disease and five with Parkinson's disease. The samples were stored in formaldehyde and washed with a solution of 0.01 molar of ethylenediaminetetraacetic acid. ESR experiments were development in a JEOL FA‐200 X‐Band spectrometer at different temperatures between −170° C to room temperature. The relative concentrations of each species were estimated from the double integral values of the fitted spectra. Results: For all samples, ESR spectra showed to be composed of three different signals following the Curie's law. One signal was attributed to high‐spin ferric ions (g∼ 4.3) in rhomboedric symmetry, Cu(II) ions (close to g=2.0) and neuromelanin (g∼ 2.01). The ferric ions concentration ratio between patients and controls was 3.0±0.2. The same ratio for neuromelanine was 0.24±0.06. Conclusions: Our preliminary results indicated a significant increment of iron concentration in PD samples which agrees with previous histochemical and biochemical reports. This finding and the clear reduction of neuromelanin concentration in PD samples suggest the possible role of neuromelanin as iron ions storage.
SU‐E‐I‐94: External Beam Radiation Cherenkov Emission in Tissue Used for Tissue Oxygen Sensing
Authors: Zhang R , Kanick S , Vinogradov S , Esipova T , Pogue B
Published: June 2012

Purpose: To show that Cherenkov emission is generated by external radiotherapy beam in tissue, and could serve as optical source to excite an oxygen sensitive phosphor, Oxyphor G4, within tissue. The intensity and lifetime of the phosphorescence was measured with a time‐gated system and reveals the oxygenation levels in the tissue phantom. Methods: A tissue phantom made with PBS, 1% v/v Intralipid‐20% (Sigma Aldrich), 1% v/v whole blood and Oxyphor G4 in 1 μM concentration is irradiated by 18MeV external radiotherapy electron beam at a dose rate of 4 Gy/min generated by a medical linear accelerator (Varian LINAC 2100C, Varian Medical Systems). On one side of the phantom, a fiber bundle is used to conduct optical signal to a spectrometer connected to a fast gating ICCD (PI‐MAX3, Princeton Instruments). For each oxygenation level, a series of spectrum of phosphorescence at different time points is measured by the time domain gating technique. Lifetime of phosphorescence is analyzed by exponential fitting and is validated by comparison to an independent analysis by frequency domain phosphorimetry. Monte Carlo simulations using GEANT4, of the fiber optic collection of Cerenkov light were performed to decide the sensitivity of the optical system for a range of specified geometries and beam types. Simulation results identify the effective depth within the phantom that is sampled by the optical collection of the Cerenkov signal. Results: Simulations show that we can detect the Cherenkov signals comes from an approximately 5 mm depth from within the tissue phantom. Lifetime of the phosphorescence and pO2 of the phantom could be measured and calculated correctly by the time domain gating system. Conclusions: This work indicates time domain gating techniques combined with an oxygen sensitive phosphor are capable of accurately monitoring tissue oxygenation from a reasonable sampling depth in tissue in vivo during external beam radiotherapy. NIH grant R01CA109558
SU‐E‐I‐96: Realistic Synthetic CT Imaging of Small Numerical Lesions with Simple CT Simulation Model Incorporating Equipment MTF
Authors: Yang Z , Kim J , Jin H
Published: June 2012

Purpose: Quantitative analysis in CT imaging often requires image data set of lesions with ground truth, which is practically impossible in general. This study presents a simple CT simulation model which enables to create realistic synthetic CT images of numerical voxel phantoms for small lesionswith various shape and sizes. Methods: Basic CT parameters were acquired from DICOM header of target CT data, and were used in the CT simulator. Frequency response of forward and backward projectors were determined by measuring NPS of reconstructed image from white Gaussian noise input data, which were then used to iteratively calibrate those projectors so as to produce white Gaussian NPS output. Then, the MTF of target equipment were measured and incorporated into the sinogram filter before back projection in order to reflect focal spot blurring and vendor specific recon kernel. For validation, physical and numerical phantoms with rod and tube shape of 2mm ∼ 10mm were created and imaged with a real CT (Sensation 16, Siemens) and the proposed simulated CT, respectively. Images were reconstructed with 8 different kernels (B10 ∼ B80) in both CT imaging technique, and their line profiles were compared. Results: Synthesized and physically scanned CT images of phantoms with different shape, sizes, and recon kernels agreed well in visual assessment, size measurement, and line profile comparison. Conclusion: Proposed method could produce realistic synthetic CT images of numerical phantoms highly mimicking in visual appearance and quantitative comparison, which has a potential for use as ground truth data set in quantitative image analysis.
SU‐E‐I‐97: Characterizing the Modulation Transfer Function (MTF) of Proton Radiography
Authors: Seco J , Oumano M , Depauw N , Dias M , Teixeira R
Published: June 2012

Purpose: To characterize the modulation transfer function (MTF) of proton radiography using GEANT4 Monte Carlo simulations. Methods: A phantom was specifically modeled using five main materials: bone (1.92 g/cm3), muscle (1.2 g/cm3), water (1.0 g/cm3), adipose tissue (0.9 g/cm3), and lung (0.3 g/cm3). The basic geometry of the phantom consists of cube‐shaped inserts of biological materials placed in water. The thickness of the water, the size of the cube, the depth of the cube in the water, and the proton beam energy have all been varied and studied. The contrast‐to‐noise ratio (CNR) between the two materials was evaluated at multiple points along a line‐of‐interest (LOI) in order to ultimately characterize the spatial resolution by the 10% point of the modulation‐transfer‐function (MTF10% or MTF 10). Results: The MTF was generated for interfaces of water‐lung, water‐bone, water‐muscle, water‐adipose. This study indicates that proton radiography can distinguish one material from another with a resolution better than 1 mm for water‐adipose and water‐muscle or sub‐millimeter in cases of water‐bone and water‐lung interfaces. Conclusions: The sub‐millimeter resolution of proton radiography offers clinicians a potentially tool in specific tumor diagnostics (such as in lung cancer), patient‐setup for daily proton therapy, and the reduction of absorbed dose delivered when compared to photon imaging.
SU‐E‐I‐104: Detector Lag Testing for Digital Radiography
Authors: Walz‐Flannigan A
Published: June 2012

Purpose: Signal lag is an inherent property of the digital detectors used for radiography. The extent and impact of signal lag can vary. In this exhibit we aim to:'¢ provide context for detector lag testing as a matter of acceptance testing for detectors used in digital radiography (DR)'¢ provide example lag data from various DR systems. Lag measurement and analysis techniques have been published but without the expected results that are useful for benchmarking.'¢bring greater awareness and understanding to the impact of detector lag for the performance of digital detectors and the appearance of image ghosting in clinical images. Methods: A standardized method was used for the quantification of lag similar to that described in the IPEM report 32 part vii [1]. Six different vended detector options were measured and compared. Results: Lag images were visually compared and lag measurement values were normalized using the measured signal transfer properties of each system. The results are shown as normalized with exposure. The results show a wide variety of lag responses, even for the same physical detector hardware but different vendor implementations. Conclusions: The ultimate test for detector lag performance is whether residual ‘ghost’ images appear in clinical images. Some vendor systems are more prone to the appearance of lag than others. We attempt to quantify system properties at acceptance testing that can enable discriminationbetween systems which have poor or superior lag properties. A suggested easurement analysis technique which normalizes data to be linear with exposure does not appear to adequately assess the likelihood for the clinical appearance of lag. [1] Mackenzie A et al. Measurement of the Performance Characteristics of Diagnostic X‐Ray Systems: Digital Imaging Systems. Institute of Physics in Medicine 2009 (IPEM Report 32, part vii). 2010
SU‐E‐I‐106: Description of Energy Dose Deposition Kernel for the Diagnostic Beam
Authors: Kareem R , Liebmann M , Chofor N , Luellau T , Poppe B , von Ossietzky Carl
Published: June 2012

Purpose: Over the recent years, the employment of kV x‐rays as a diagnostic tool into clinical routine has resulted in a significant increase in the patient's exposure to ionizing radiation. The accurate determination of the absorbed dose to patients during diagnosis is therefore necessary to avoid unnecessary exposure to the patient. This study presents an analytical model of the energy deposition kernel for the monoenergetic and polyenergetic kV beams for the fast calculation of dose in radiography. Methods: The analytical model is based on the pencil beam kernels derived from Monte Carlo simulations. DOSXYZnrc code from the EGSnrc family was employed to simulate the pencil beam of 0.1 cm width for 80, 100 and 120 keV mono‐energetic and polyenergetic beams. The lateral dose profiles were calculated at different depths within a homogenous water phantom of size 50×50×50 cm3. The evaluated dose profiles showed a high amplitude primary component at the central axis and a long range low amplitude scatter component spanning a considerable distance from the central axis. The profiles were fitted analytically with a triple exponential decay function with an offset. All coefficients of the exponential function were further fitted with appropriate analytical functions to represent their behavior relative to depth and photon energy. The accuracy of the obtained kernel was checked by the convolution of a rectangular fluence profile and comparing the calculated dose distribution with the Monte Carlo simulated dose profiles for 2×2 cm2 field size. Results and Conclusion: In a homogeneous phantom, the comparisons of the convolution method and Monte Carlo simulations showed sufficient agreement except for largest depths (deviation approx. 15%). Future developments will focus on an implementation of the method for dose calculation in the patient.
SU‐E‐I‐111: Evaluation of the Analytical Scattering Models of 1) Lynch‐Dahl 2) Highland and 3) Rossi for Proton Beams and Comparison with GEANT4 Monte Carlo Simulations as a Prerequisite for Proton Radiography Applications for Patients
Authors: Raytchev M , Safai S , Seco J
Published: June 2012

Purpose: To evaluate the approximate 1) Lynch‐Dahl 2) Highland and 3) Rossi scattering models for proton beams with GEANT4 Monte Carlo. This is a prerequisite for proton radiography applications for patients. Methods: A Matlab program developed in‐house at MGH was used to obtain a semianalytical generalized Fermi‐Eyges theory estimation of the spatial and angular spreads of a 230 MeV zero‐spread incident proton beam as a function of depth. The constants of 1) Lynch‐Dahl 2) Highland and 3) Rossi were used respectively for each model. MC simulations will determine which approximation provides the best prediction for different media configurations. Further, the calculated spreads were used to inform proton radiography imaging by calculating two limiting angles, a positional Acut and a directional Ccut. Acut is defined as the viewing angle of a point of incidence observer at which they see a point displaced by one positional standard deviation. Ccut is defined as the direction cosine of one angular standard deviation momenta. Results: Both the angular and spatial spreads as well as their respective model differences rose monotonically with depth in water. At 30 cm depth the angular spread reached values around 3 degrees with about 0.32 degrees model difference, translating to Ccut differences in the first or second significant digit. At the same depth the spatial spread reached values around 1.2 cm with about 0.7 mm model difference, translating to Acut differences in the first or second significant digit. Preliminary MC data (not shown) indicate that the signals obtained due to the influence of inhomogeneities are small and the model differences may be relevant. Conclusions: We observed non‐negligible differences between the models using MC. Further analysis is required to understand, which model provides most accurate scattering predictions for protons penetrating different media configurations.
SU‐E‐J‐01: Analysis of Acquisition Parameters That Caused Artifacts in Four‐Dimensional (4D) CT Images of Targets Undergoing Regular Motion
Authors: Sheen H , Han Y , Shin E , Min B , Kim J
Published: June 2012

Purpose: The aim of this study was to clarify the impacts of acquisition parameters on artifacts in four‐dimensional computed tomography (4D CT) images, such as the partial volume effect (PVE), partial projection effect (PPE), and mis‐matching of initial motion phases between adjacent beds (MMimph) in cine mode scanning. Methods: A thoracic phantom and two cylindrical phantoms (2 cm diameter and heights of 0.5 cm for No. 1 and 10 cm for No.2) were scanned using 4D CT. For the thoracic phantom, acquisition was started automatically in the first scan with 5 sec and 8 sec of gantry rotation, thereby allowing a different phase at the initial projection of each bed. In the second scan, the initial projection at each bed was manually synchronized with the inhalation phase to minimize the MMimph. The third scan was intentionally un‐synchronized with the inhalation phase. In the cylindrical phantom scan, one bed (2 cm) and three beds (6 cm) were used for 2 and 6 sec motion periods. Measured target volume to true volume ratios (MsTrueV) were computed. The relationships among MMimph, MsTrueV, and velocity were investigated. Results: In the thoracic phantom, shorter gantry rotation provided more precise volume and was highly correlated with velocity when MMimph was minimal. MMimph reduced the correlation. For moving cylinder No. 1, MsTrueV was correlated with velocity, but the larger MMimph for 2 sec of motion removed the correlation. The volume of No. 2 was similar to the static volume due to the small PVE, PPE, and MMimph. Conclusions: Smaller target velocity and faster gantry rotation resulted in a more accurate volume description. The MMimph was the main parameter weakening the correlation between MsTrueV and velocity. Without reducing the MMimph, controlling target velocity and gantry rotation will not guarantee accurate image presentation given current 4D CT technology.
SU‐E‐J‐02: Accuracy of Fiducial Marker Localization Using Axial and Helical CT, Cone‐Beam CT and KV Imaging
Authors: Algan O , Oyewale S , Ahmad S , Ali I
Published: June 2012

Purpose: To determine localization errors of fiducial markers using axial (ACT) and helical CT (HCT), cone beam CT (CBCT) and kV imaging. Methods and Materials: A thorax phantom containing markers of various sizes (2.5, 5, 10, 20mm) was imaged using ACT, HCT, CBCT and kV imaging. The phantom was imaged with and without motion (15mm amplitude, 15 cycles/min). CT images were reconstructed at 0.625, 1.25, 2.5 and 5mm thickness. Marker location and length were measured using axial and coronal imaging. Results: The measured marker size increased almost linearly with increased slice thickness used in CT reconstruction with ACT for the stationary phantom. In HCT, the marker size varied non‐linearly with increase in slice thickness. Motion of the phantom induced further blurring and shifts of the center of the marker with different sizes. The 10 mm marker was elongated up to 13.5 mm and 16.2 mm in the ACT and HCT, respectively. The markers were elongated up to nearly 26 mm using axial and helical scanning modes by phantom motion. The displacement of maker center was more prominent in the HCT with a shift up to 2 mm due to slice thickness for stationary phantom. The marker center was displaced by up to 18 and 22 mm using ACT and HCT, respectively, which was even higher than 15 mm motion amplitude. KV imaging produced the sharpest marker image with the least difference between actual and measured marker sizes. Conclusion: Increase in slice thickness enlarged the apparent marker size and displaced the maker center in ACT and HCT. Motion led to further enlargement in the maker size and displacement of maker center that depended on the motion amplitude. These effects should be considered in CT‐based image‐guided radiation therapy to ensure accurate tumor localization and patient positioning with implanted markers.
SU‐E‐J‐03: Positioning Errors of Metal Localization Devices with Motion Artifacts on KV and MV Cone Beam CT
Authors: Lin T , Ma C
Published: June 2012

Purpose: To investigate motion artifacts of kV CBCT and MV CBCT images on metal localization devices for image guided radiation therapy. Methods: 8 MU pelvis CBCT template for Siemens Artiste MVision and Pelvis template for Varian IX on‐board Exact Arms kV were used to acquire CBCT images in this study. Images from both CBCT modalities were compared in CNRs, metal landmark absolute positions, and image volume distortion on three different planes of view. The images were taken on a breathing‐simulated thoracic phantom in which several typical metal localization devices were implanted, including clips and wires for breast patients, gold seeds for prostate patients, and BBs as skin marks. To magnify the artifacts, a 4cm diameter metal ball was also implanted in the thoracic phantom to mimic the metal artifacts. The amplitude of the sinusoidal breathing was 1cm, and the period varies from 2sec, 4sec to 8sec. Results: For MV CBCT, the CNR at 4sec breathing cycle with 1cm breathing amplitude was 5.0, 3.4 and 4.6 for clips, gold seeds and BBs, respectively while it was 1.5, 2.0 and 1.6 for kV CBCT. On the images, kV CBCT showed symmetric streaking artifacts both in the transverse and longitudinal directions relative to the motion direction. kV CBCT images predicted 89% of the expected volume, while MV CBCT images predicted 95% of the expected volume. Simulated soft tissue observed in MVCT cannot be detected in kVCT. Conclusions: MV CBCT images showed better volume prediction, less streaking effects and better CNRs of a moving metal target, i.e. clips, BBs, gold seeds and metal balls than kV CBCT images. MV CBCT was more advantageous compared to kV CBCT with less motion artifacts for metal localization devices.
SU‐E‐J‐06: A Time Dependence Analysis of CBCT Image Quality and Mechanical Stability
Authors: Oves S , Stenbeck J , Gebreamlak W , Alkhatib H
Published: June 2012

Purpose: To quantify the change, if any, in flexmap correction factors and image quality with the XVI system over a course of several years and from these results, assess their clinical impact. Methods: Flexmap, a calibration procedure which corrects for imperfect gantry rotation for cone‐beam CT reconstruction, and image quality tests were performed on three Elekta Synergy linacs equipped with XVI. Data was collected per month over three years. U and V values, corresponding to lateral and longitudinal shifts respectively, were acquired through the XVI software. Image quality parameters were obtained through CT imaging of the Catphan 500®. For each reconstruction, pixel values for low density polyethylene (LDPE) and polystyrene materials were recorded. Results: For all three linacs, analysis of the flexmap showed a significant change in the U factor for both month‐to‐month comparisons and comparisons between machines. The V correction factor exhibited a small variation month to month, and showed a slight, gradual increase over time (0.2 +/−0.08 mm). Image quality analysis showed a near consistent decrease (5–10%) in LDPE and polystyrene. Despite this decrease in pixel values, the ratio of the two pixel values remained constant, thus a similar decreasing trend in contrast was not observed. Conclusions: Analysis of monthly flexmap calibration showed the general monthly change in correction shifts and their general trend over several years. For image quality, our research exhibited roughly 0.5% per month decrease in pixel values of the Catphan®. Our results imply that CBCT images obtained from XVI are not appropriate for treatment planning and despite the decrease in panel response over time, image quality with respect to contrast will remain within acceptable clinical standards. Future studies may be carried out to assess any correlation between image quality and XVI source strength.
SU‐E‐J‐14: Evaluation of Mechanical Accuracy of Electronic Portal Imaging Devise on Its Use in Patient Specific IMRT QA
Authors: Li J , Burman C , Chan M
Published: June 2012

Purpose: Electronic portal imaging devices (EPID) have been used for both in vivo dosimetry and in vitro dose verification in intensity modulated radiotherapy (IMRT). This study is to investigate the effect of EPID mechanical precision on the accuracy of measured dose distribution. Methods: EPID energy fluences (dicom images) of H&N IMRT fields were collected daily on two Varian LINACs (Clinac‐iX & Trilogy) over 4‐week period. The energy fluences were converted to doses using EPIDoseTM (Sun Nuclear Corp). Mechanical deviations of EPIDs could be divided into two components: one with inherent detector center misalignment from the beam central axis, another caused by the ‘sagging effect’ from gantry rotation. The first component was detected by ‘best matching’ of the measured and calculated dose at zero gantry angle (G=0). The second component was computed by ‘best matching’ the 10×10cm field defined by MLC at G=0, 90,180, and 270, separately. A ‘shift’ was generated by the combination of these two components and then applied to correct the measured dose at the corresponding gantry angle for the IMRT field. Results: Inherent misalignment of the detector's center and the ‘sagging’ deviation were found to be 1–2 mm and 1–5 mm, respectively for both LINACs. Each component was found very stable (change < 1mm) over the 4‐week observational period. Using a Gamma index of 2%/2mm (DD/DTA), the ‘shift’ increased the average passing rate from 59% to more than 92%. On the other hand, blindly applying ‘auto‐shift’ from commercially available software to obtain the best match would compound true QA issues with units' misalignments. Conclusions: A false ‘mismatch’ between measured and calculated dose distribution caused by mechanical inaccuracies of EPID could be avoided by measuring the two components identified in this study. One should examine the mechanical precision of equipment prior to clinical use of EPID dosimetry.
SU‐E‐J‐16: Prostate Bed Motion during Post‐Prostatectomy Radiotherapy
Authors: Xu Z , Li T , Lee W , Hood R , Godfrey D , Wu Q
Published: June 2012

Purpose: To report the inter‐fractional prostate bed motion (PBM) during post‐prostatectomy radiotherapy using cone‐beam CT (CBCT). The correlations between PBM, anterior rectal wall and posterior bladder wall shifts were also investigated. Methods: Seventy CBCT and 8 planning CT scans from 8 patients treated with prostate bed radiotherapy were retrospectively analyzed. For each CBCT‐CT pair, two rigid image registrations were performed: one based on surgical clips and the other based on pelvic bony anatomy. Each registration gives a displacement vector. The difference between the two registration displacements represents the PBM. In addition, rectum/bladder contours on CT and CBCT scans were compared to identify the organ wall motion. Shifts of the anterior rectal wall and posterior bladder wall were assessed by averaging the slice‐by‐slice distances between contours on two image sets, measured along an anterior‐posterior line passing through the symphysis pubis. Results: The prostate bed motion in the left‐right (LR), anterior‐posterior (AP) and superior‐inferior (SI) directions was (0.1±0.9)mm, (0.9±1.6)mm, and (−0.4±1.9)mm, respectively. The derived PTV‐CTV margin for LR, AP and SI motion was 3mm, 5mm, and 6mm, respectively. Pearson's correlation coefficients between PBM and anterior rectal wall (whole length) shifts, PBM and the cranial half of anterior rectal wall shifts, and PBM and posterior bladder wall shifts, were 0.43, 0.47, and 0.67, respectively. Conclusions: The magnitude of PBM relative to pelvic bony anatomy in all three directions was small. The correlation between average anterior rectal wall shifts and PBM was weak‐to‐moderate, which may be due to rectum contouring inconsistency. Possible sources for this inconsistency include: non‐uniform rectal wall motion through its length, low CBCT image contrast, and artifacts due to filling. Significant correlation between average posterior bladder wall shifts and PBM suggests bladder wall motion may also be a suitable surrogate for PBM in the AP direction.
SU‐E‐J‐24: Image‐Guidance Using Cone‐Beam CT for Stereotactic Body Radiotherapy (SBRT) of Lung Cancer Patients: Bony Alignment or Soft Tissue Alignment?
Authors: Wang L , Turaka A , Meyer J , Spoka D , Jin L , Fan J , Ma C
Published: June 2012

Purpose: To assess the reliability of soft tissue alignment by comparing pre‐ and post‐treatment cone‐beam CT (CBCT) for image guidance in stereotactic body radiotherapy (SBRT) of lung cancers. Methods: Our lung SBRT procedures require all patients undergo 4D CT scan in order to obtain patient‐specific target motion information through reconstructed 4D data using the maximum‐intensity projection (MIP) algorithm. The internal target volume (ITV) was outlined directly from the MIP images and a 3–5 mm margin expansion was then applied to the ITV to create the PTV. Conformal treatment planning was performed on the helical images, to which the MIP images were fused. Prior to each treatment, CBCT was used for image guidance by comparing with the simulation CT and for patient relocalization based on the bony anatomy. Any displacement of the patient bony structure would be considered as setup errors and would be corrected by couch shifts. Theoretically, as the PTV definition included target internal motion, no further shifts other than setup corrections should be made. However, it is our practice to have treating physicians further check target localization within the PTV. Whenever the shifts based on the soft‐tissue alignment (that is, target alignment) exceeded a certain value (e.g. 5 mm), a post‐treatment CBCT was carried out to ensure that the tissue alignment is reliable by comparing between pre‐ and post‐treatment CBCT. Results: Pre‐ and post‐CBCT has been performed for 7 patients so far who had shifts beyond 5 mm despite bony alignment. For all patients, post CBCT confirmed that the visualized target position was kept in the same position as before treatment after adjusting for soft‐tissue alignment. Conclusions: For the patient population studied, it is shown that soft‐tissue alignment is necessary and reliable in the lung SBRT for individual cases.
SU‐E‐J‐25: Dosimetric Characterization of an OBI System Using Gafchromic XRQA2 Film
Authors: Malhotra H , Steinman J
Published: June 2012

Purpose: Accurate knowledge about the radiation dose due to the imaging component of an OBI supported linear accelerator is very important. Though films provide excellent spatial resolution, there are problems regarding sensitivity and energy dependence at kV energies. XRQA2 film (0.1–20 cGy) is primarily for measurements in kV range. We demonstrate their use for absolute dosimetry for an OBI system. Methods: H&D curves were generated using a kV therapy x‐ray unit in the range of 0–15 cGy. A 25‐cm diameter water‐equivalent cylindrical phantom with 2.5‐cm thick slabs was used on Varian Trilogy linac. XRQA2 films were cut into circular diameter and sandwiched between 2 slabs of the cylindrical phantom to measure the percentage depth dose of the CBCT scans. Additional film pieces were wrapped around the cylinder for surface dose. To get sufficient signal, 5 CBCT scans were taken per imaging acquisition template e.g. head standard dose. After 24 hours post‐irradiation, films were scanned using Epson V700 scanner in reflection mode. Only red channel was used for dosimetric analysis using RIT software. Results: High‐Quality head acquisition template resulted in a dose at 0.2, 1.3, 2.7, and 2.7 cGy on AP, left‐lateral, posterior and right‐lateral positions of the phantom, respectively. Low‐quality thorax acquisition resulted in 1.4, 1.6, 1.5, and 1.4 cGy. The PDD at 12.5‐cm (isocenter) was of the order of 40%. While the PDD was fairly uniform from left to right, the PDD along the AP/PA direction were showing almost continuous fall off with PDD at isocenter of ∼40%. Analysis of low‐quality head PDDs show little fall off dose with depth though the overall magnitude was less with PDD at the isocenter only marginally lower than the maximum along the medial and anterior posterior axis. Conclusions: Gafchromic XRQA2 film provides a useful tool to evaluate and characterize an OBI system.
SU‐E‐J‐32: Evaluation of the Dosimetric Accuracy of Stereotactic Radiosurgery with Cone‐Beam Computed Tomography Localization
Authors: Tseng T , Chang Z , Yin F , Wang Z
Published: June 2012

Purpose: To evaluate the dosimetric accuracy of stereotactic radiosurgery (SRS) with cone‐beam computed tomography (CBCT) guided patient localization. Methods: A Novalis TX linear accelerator with high definition multileaf collimator (HDMLC) was used for SRS treatments. 20 patients immobilized with the noninvasive BrainLab U‐frame system and thermoplastic mask were selected to evaluate the dosimetric accuracy of CBCT image‐guided SRS by the BrainLab iPlan Phantom Mapping module. The contours of the PTV and critical organ were transferred directly from the planning CT and MR images to the CBCT images after image registrations. The delivered dose distributions could be calculated and analyzed by copying the original treatment plans to the CBCT images and assigning the treatment isocenters on the CBCT images according to the couch shifts acquired after planning CT and CBCT image registrations. The CT electron density calibration curve used for original plans was also applied for the CBCT‐based planning. Results: The average minimum dose, mean dose, and maximum dose in PTV of the original plans and the CBCT plans were 95.45%±3.80% and 92.88%±3.25%, 110.59%±1.81% and 110.11%±2.40%, 116.55%±3.11% and 115.93%±2.78%, respectively. In the original treatment plans, the average 100% prescription dose coverage of GTV and PTV were 99.99% and 99.81%, respectively. In the CBCT plans, the average 100% prescription dose coverage of GTV and PTV were 99.90% and 98.20%, respectively. The average conformity index of the original plans and the CBCT plans were 1.846 and 1.863, respectively. Conclusions: Data demonstrated that the dose distributions calculated in the CBCT images were comparable to the original treatment plans. The CBCT plans only indicated a slightly higher conformity index and lower average minimum dose, mean dose, maximum dose, GTV coverage, and PTV coverage compared to the original treatment plans. Therefore, the CBCT guided localization was considered effective to assure dosimetric accuracy in SRS.
SU‐E‐J‐33: Geometric Agreement Check for Imaging System with Radiation Beam by KV and MV‐CBCT
Authors: Saotome N , Kida S , Haga A , Masutani Y , Ohtomo K , Nakagawa K
Published: June 2012

Purpose: The verification method of the geometry agreement between a light field and/or a laser coordinate and treatment beam should be easy and quick. In this presentation, we propose a novel QA method by using both kV‐ and MV‐CBCT for kV‐IGRT system. This method confirms the temporal unchanging the agreement of geometry in the kV‐IGRT system with the treatment beam geometry. Methods: 1) MV‐flexmap: Sequential MV‐projection images were acquired during gantry rotation by iViewGT (Elekta) and MV‐CBCT was reconstructed by in‐house software with a flexmap correction. The flexmap is displacement of gantry and detector panel related with gantry sag. The geometric change affects the deranging reconstructed image. To evaluate how much displacement of EPID panel and gantry was detectable, the images of 8mm diameter ball‐bearing (BB) located at the radiation isocenter were reconstructed with improper Flexmap.2) A comparison between the kV‐CBCT and the MV‐CBCT: The kV‐CBCT was provided by X‐ray Volume image (XVI) system (Elekta). To confirm the agreement for the geometry between kV‐IGRT system and treatment beam, the kV‐CBCTs of BB are compared with that of MV‐CBCTs. Results: The flexmaps were modified to (b)1mm / (c)3mm shifted to the rotation direction and (d)3mm to the rotation axis. The MV‐CBCT were reconstructed with the correct flexmap and with incorrect flexmap (b), (c) and (d).ã€€The geometric confirmation for MV‐CBCT was done by comparison of the width and center of the BB on the MV‐CBCT. The discrepancy of center between kV‐CBCT and MV‐CBCT was less than 1mm. Conclusions: Less than 1mm of the geometrical changing to rotation direction for MV‐detector panel could be recognized by reconstructed images of BB. Using kV‐ and MV‐CBCT enable us to perform the simple comparison for geometrical non‐idealities between the kV‐IGRT system and the treatment beam. Dr. K. Nakagawa received research grant from Elekta.
SU‐E‐J‐35: A Model for the Simulation of EPID Measurements in the BeamNRC Monte Carlo Package
Authors: Sperling N , Parsai E
Published: June 2012

Purpose: The intent of this study was to create a virtual model of a simple Electronic Portal Image Device (EPID) in the BeamNRC Monte Carlo package which would simulate EPID measurements taken in vivo. Methods: A typical accelerator was created in the BeamNRC software using published 6MV spectra from Mohan, et. al. and the basic head design of a Varian accelerator head. The particle fluence at the level of the mylar window was output into a planar ‘phase space’ file. The output of this accelerator was delivered to a phantom with a 1 mm thick slab of water acting as the detector array using the DosXYZ package. Additionally, the fluence from the same geometry was computed at the level of the virtualEPID (vEPID). The simulations were run for 2×2, 5×5, 10×10, 20×20, and 25×25 field sizes, as well as a simple IMRT field. The results of the open field simulations were then used for the parameterization of a deconvolution kernel using the form described by Renner, et. al. Results: The simulations were run with sufficient histories to produce a 1% uncertainty in the resulting fluence and dose matrices in the high dose/fluence region of the output. Additional software was created to convert ‘phase space’ files to fluence in the same grid as the vEPID. A final software package was written to iteratively determine the exponential fitting parameters described in Ritter, et. al. to deconvolve the detector function from the fluence measurements. Conclusions: The system will allow one to simulate both EPID measurements, as well as remove the detector function from vEPIDsimulations for further use in fluence studies using BeamNRC. This will allow one to simulate a system for in vivo patient measurements using an EPID to study without the need for time on a medical accelerator, and without contributing additional radiation fatigue to the EPID device.
SU‐E‐J‐38: Rotational Setup Errors in Pediatric Patients Receiving Radiation Therapy for Intracranial Tumors
Authors: Altunbas C , Miften M , Hankinson T , Tello T , Liu A
Published: June 2012

Purpose: We have clinically observed that larger rotational setup errors are more prominent in pediatric patients who received radiation therapy for brain tumors. In this work, we quantitatively evaluated the daily setup corrections in pitch, roll, and yaw axes for children who received intracranial radiation therapy under x‐ray image guidance. Methods: Daily localization data of 43 patients between the ages of 10 months and 21.9 years were analyzed in this study. Patients were immobilized with thermoplastic mask during treatments, and 2D orthogonal x‐ray images wereacquired for setup corrections before each treatment. Rotational setup corrections in pitch, roll, and yaw axes were extracted from 873 treatment fractions, and were analyzed for the whole group of patients and for two age groups: < 5 and = 5 years old. Results: The mean values for the pitch corrections were 1.91° and 1.65° (p:0.02), roll corrections were 1.37° and 1° (p<0.001), and yaw corrections were 1.93° and 1.47° (p<0.001), respectively. For patients < 5 years, 21.7% of treatments had pitch corrections more than 3°, versus 15.6% of treatments required pitch corrections more than 3° for patients >= 5 years. Similarly, 10.6% of roll corrections and 20.9% of yaw corrections were more than 3° for patients < 5 years. On the other hand, 2.1% of roll and 13.8% of yaw corrections were more than 3° for patients = 5 years old. Conclusions: Data indicate that children less than 5 years old are more prone to rotational setup errors during intracranial radiation therapy. This can be attributed to reduced efficacy of immobilization devices due to smaller and rounder anatomicalfeatures of pediatric patients, and challenges in setup while the patient is under anesthesia. The role of daily image guidance and rotational setup corrections becomes important to ensure target coverage, especially for children < 5 years old.