bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐J‐120: Characterization of 4D Lung Ventilation Based on 4DCT Lung Density
Authors: Li G , Wei J , Li D , Ballangrud A
Published: June 2012

Purpose: To characterize patient's 4D lung ventilation and motion using 4DCT images aiming to extract pulmonary features for developing a biomechanical model to predict lung tumor motion. Methods: 4DCT images of sixteen patients were used for initial 4D‐ventilation evaluation. A program was developed for visualization and quantification of 4D lung ventilation. Lung density gradient and pleural pressure variation were evaluated based on the gravity. Freeform deformable image registration (DIR) was applied to map the lung density from one phase to another, with linearly interpolated voxel density. The warped lung density from a secondary CT was compared with the primary CT (full‐exhalation) for quantification. To minimize the uncertainty from motion artifacts for quantitative ventilation/motion analysis, we developed two screening algorithms (based on Fourier Transformation and Generalized Linear Regression) to quantify the regularity (score: 0 to 1) of the RPM waveform, assuming the higher the regularity in RPM the less motion artifacts in 4DCT. Ninety‐seven RPM curves (54 free‐breathing and 43 audio‐coaching patients) were used to qualitatively test these screening methods. Results: Lung density gradient in anterior‐posterior direction for supine patients is observed in 15/16 patients at scale of >100–200HU. The gravity pressure due to lung tissue weight is estimated to be ∼4mmHg, which could neutralize the pleural pressure at the posterior chest wall. The different pleural pressures at anterior/posterior chest walls may explain the difference in local diaphragm muscle engagement and motion range: 8±5mm (anterior) and 23±12mm (posterior). Motion artifacts and DIR uncertainty affected quantitative ventilation mapping. In the RPM regularity assessment, audiocoaching patients score higher than free‐breathing patients: 0.76±0.13 vs. 0.50±0.19 (Fourier method) and 0.74±0.19 vs. 0.66±0.17 (Regression method). High‐quality 4DCTs can be readily selected for further quantitative ventilation/motion analysis. Conclusions: We have characterized several basic features of lung ventilation and motion and developed two effective screening methods for high‐quality 4DCT for on‐going quantitative studies.
SU‐E‐J‐189: The Kullback‐Leiber Divergence for Quantifying Changes in Radiotherapy Treatment Response
Authors: Schreibmann E , Crocker I , Shu H , Curran W , Fox T
Published: June 2012

Purpose: Repeated imaging is an extremely powerful tool in current radiotherapy practice since it allows advanced tumor detection and personalized treatment assessment by quantify tumor response. Change detection algorithms have been developed for remote sensing images to mathematically quantify relevant modifications occurring between datasets of the same subject acquired at different times. We propose usage of change detectors in radiotherapy for an automated quantification of clinical changes occurring in repeated imaging. Methods: We explore usage of the Kullback‐Leiber divergence as indicator of tumor change and quantification of treatment response. The Kullbach‐Leiber divergence uses the likelihood theory to measures the distance between two statistical distributions and thus does not assume consistency in imaging. By it's general nature, it can accommodate the presence of noise and variations in imaging acquisition parameters that usually hinder automated identification of clinically‐relevant features. Results: In a comparison of simple difference maps and the Kullbach‐Leiber divergence operator, the difference maps were affected by noise and did not consistently detect changes of low intensity. In contrast, the proposed operator discerned noise by considering regional statistics around each voxel, and marked both regions with low and high contrast changes. Conclusions: Statistical comparison through Kullback‐Leiber divergence provides a reliable means to automatically quantify changes in repeated radiotherapy imaging
SU‐E‐J‐191: A Multivariate Framework for N‐Tissue Classification in Treatment Assessment of Glioblastomas
Authors: Schreibmann E , Crocker I , Shu H , Curran W , Fox T
Published: June 2012

Purpose: Glioblastoma is the most common primary brain tumor in adults and is rapidly fatal. Treatment monitoring of these patients has increased awareness that many patients have new areas of contrast enhancement without progressive clinical signs and symptoms. Although the enhancing areas mimic tumor progression, the lesions result from treatment effects and subsequently stabilize or improve without further treatment and are not correlated with poorer outcomes. This phenomenon has been termed pseudoprogression and is hypothesized to occur secondarily to edema and vessel permeability in the tumor area as a result of the combined effects of radiation and chemotherapy. Since the new enhancing lesions of pseudoprogression are indistinguishable from true disease progression, there is a need for a predictive model to distinguish the two phenomena. Method: We developed a classification algorithm that combines perfusion and diffusion MRI imaging to effectively partition the cases as one exhibiting true or pseudo progression based on a vector of features containing T1, rCBV and ADC imaging. The multi‐sequence classification algorithm uses an expectation maximization (EM) algorithm that learns from training cases with known clinical outcome to assigns each voxel to a type of tissue. Results: A training set of 20 where the clinical outcome is known from biopsy or from long‐term follow‐up was used by EM algorithm to model typical imaging values within tissue of pseudo, tumor, edema, necrosis, vessels or brain anatomy to construct a database of expected values for each tissue type. When presented with a new case, the algorithm automatically classifies voxels by their geographical proximities and Mahalanobis distance to the pre‐sampled values. Conclusion: Usage of advanced classification techniques allows automated labeling of voxels into normal, pseudoprogression or tumoral tissue types. The technique allows for early detection of pseudo progression to spare patients from unnecessary surgery or toxic chemotherapy.
SU‐E‐J‐201: What is the Importance of Dose Recalculation for Adaptive Radiotherapy Dose Assessment?
Authors: Pukala J , Staton R , Langen K
Published: June 2012

Purpose: To quantify the effect of using the planned dose distributions in lieu of performing dose recalculations on daily in‐room images for adaptive radiotherapy (ART) dose assessment of head and neck cancer patients. Methods: 16 patients with cancers of the head and neck were treated using the TomoTherapy Hi‐Art II (Accuray Inc., Sunnyvale, CA). Images of all patients were acquired prior to each treatment using the megavoltage CT (MVCT) capability of the TomoTherapy unit. Overall, images from 528 fractions were evaluated. For every image set, the delivered dose was estimated by both recalculating the dose distribution using the acquired MVCT and also by simply overlaying the planned distribution on the new images. ART dose assessment was performed using deformable image registration (DIR) to deform contours from the treatment plan to the images acquired during each fraction and to accumulate the estimated dose delivered during each fraction back to the reference treatment plan. The same DIR maps were applied to both dose estimation methods. Dosimetric endpoints were then compared between the DVHs computed using the recalculated or planned dose distributions. Results: The mean PTV D95% and D05% endpoints were 0.6±0.5% and 1.4±0.8% lower using the planned dose distributions compared to the recalculated distributions, respectively, across all patients. The mean parotid D50% was 2.4±1.5% greater using the planned distributions compared to the recalculated distributions. The parotid D50% from the planned distributions was also highly correlated with the parotid D50% from the recalculated distributions for each patient (mean r2=0.97±0.09). Conclusions: For this treatment modality and site, dosimetric differences observed between overlaying the planned dose distributions and recalculating distributions on daily images were typically well within ±5%. This indicates that the dose distribution itself is robust against anatomic variations. This study was funded, in part, by a grant from Accuray Inc.
SU‐E‐J‐205: Modeling Fiducial Marker Movement in Prostate
Authors: Yao W , Corsten M
Published: June 2012

Purpose: To understand fiducial marker movement in prostate and to improve prostate patient positioning with fiducial markers. Methods: Patient planning CT and positioning CBCT were retrospectively analyzed. CBCT images were automatically registered to the corresponding CT ones based on their bony structures in Eclipse. The coordinates of the markers in the CT and CBCT were recorded. The status of bladder and rectum fillings, and gas in rectum were indexed into four levels and recorded. An algorithm was developed to optimally register the (three) fiducial markers in CBCT with those in the CT so that only the information on prostate deformation and marker migration remained. A mathematical model was built to estimate the marker migration and the relative contribution to the deformation from the status change of bladder and rectum. Results: The registration error from our optimal registration algorithm indicates the minimal patient setup error by using fiducial markers. The results from our analyzed 3 patients were average(standard deviation) = 1.10(0.40), 1.27(0.67) and 0.62(0.38) mm, but there were some fractions when the registration errors were over 2 mm. From our mathematical model, it was found that the marker migration could be as large as 2.9 mm. If 2 mm tolerance is required in such as SBRT, then soft tissue based registration may be more appreciated when the optimal registration error is greater than 2 mm or from the date when the migration is greater than 2 mm because the migrated marker may not change the location thereafter. In addition, for one investigated patient, the relative contribution to the deformation from the bladder, rectum filling and gas in the rectum is 0.43:0.31:0.26. More patient data are in analysis, and other interesting results will be reported. Conclusions: Fiducial marker migration can be over 2 mm. The migration will Result in a conclusion that small registration error does not mean good patient positioning.
SU‐E‐J‐214: Target Intrafraction Motion Dosimetric Impact on 5‐Fraction Proton Prostate SBRT
Authors: Su Z , Slopsema R , Flampouri S , Li Z
Published: June 2012

Purpose: To investigate dosimetric impact of prostate intra‐fraction motion to five fraction hypofractionated proton treatment with uniform scanning (US) and double scattering (DS) techniques using real‐time prostate tracking data from electromagnetic transponder system. Methods: Prostate intra‐fraction motion can have spatiotemporal interplay with proton treatment delivery. Five fraction (7.25Gy/fraction) prostate proton stereotactic body radiotherapy (SBRT) treatments were simulated for total 14 patients using in‐house proton treatment simulation program. The US treatment was simulated by rigidly moving CTV through a series of temporal‐spatial dose matrices indexed by energy layers, according to prostate motion traces. The CTV temporal doses of the whole treatment fraction were obtained and summed as final prostate CTV dose. The DS treatment was simulated by moving CTV through the energy layer summed dose matrix. For all patients, the fraction doses and the total dose to the CTV were presented for both DS and US treatments. Results: The CTV dose of different fractions indicated that its dose degradation depends on magnitude and direction of prostate intra‐fraction motion and is patient specific. For one of the prostate motion traces investigated, only 70% of CTV received 100% prescribed dose for a simulated US treatment and 79% CTV had 100% dose for a DS treatment. Furthermore, DVH and isodose graphs of both treatments revealed that intra‐fraction motion caused significant CTV cold and hot spots in US treatment whereas only cause CTV underdose in DS treatment. Conclusions: Intra‐fraction prostate motion causes dose uncertainty to CTV. In the 5 fraction prostate SBRT, prostate intra‐fraction motion causes significant target dose degradation. In US treatment, spatiotemporal interplay between energy layers delivery and prostate motion leads to hot and cold spots in CTV for some patients with severe prostate intra‐fraction motion. Further investigation of intra‐fraction motion management and its impact on CTV dose is necessary.
SU‐E‐T‐04: Repair Kinetics of Sublethal Damage in Rat Cervical Spinal Cord ‐ Application of the GLQ Model Incorporating Reciprocal Time Pattern
Authors: Huang Z , Mayr NA , Lo SS , McLawhorn R , Gao M , Liu T , Yuh WTC
Published: June 2012

Purpose: The repair rate conventionally assumed to be constant during the entire radiation course. However, increasing evidences from animal studies show that the repair process may slow down with time and the experimental data does not fit an exponential pattern. To address this enigma, we presented a generalized linear‐quadratic (gLQ) model incorporating reciprocal time repair pattern of sublethal damage and fit the model to published experimental data. Methods: In the gLQ model, the parameter G in the surviving fraction represents the repair process of sublethal damage with Tr as the repair half‐time. When a reciprocal time pattern of repair process was adopted, a generalized form of G was derived analytically for arbitrary radiation schemes (I(t) = Ii, ti‐1 < t < ti, where i=1 to N, Ii is the dose rate of the ith segment to the voxel in question, and N is the total number of segments for the entire treatment course). The published animal data was used to test the reciprocal formulas. Results: The gLQ model incorporating a reciprocal time pattern to describe the repair process was used to fit the experimental data. The reciprocal model showed better fit to the animal data than the exponential model, particularly for the ED50 data, resulting in the following parameters: the reduced chi‐square of 1.52, α/β=0.9 Gy, Tr =1.0 h for the reciprocal model and reduced chi‐square of 3.39, α/β=1.6 Gy, Tr=3.5 h for the exponential model. Conclusions: The modeling results suggest that the gLQ model incorporating the reciprocal time pattern of sublethal damage repair facilitates the interpretation of complex experimental designs, especially for irregular dose delivery schedules. These formulas can be used to analyze experimental and clinical data, where a slowing‐down of the repair process occurs during the radiation therapy course.
SU‐E‐T‐05: Comparing DNA Strand Break Yields for Photons under Different Irradiation Conditions with Geant4‐DNA
Authors: Pater P , Bernal M , Naqa I El , Seuntjens J
Published: June 2012

Purpose: To validate and scrutinize published DNA strand break data with Geant4‐DNA and a probabilistic model. To study the impact of source size, electronic equilibrium and secondary electron tracking cutoff on direct relative biological effectiveness (DRBE). Methods: Geant4 (v4.9.5) was used to simulate a cylindrical region of interest (ROI) with r = 15 nm and length = 1.05 mm, in a slab of liquid water of 1.06 g/cm3 density. The ROI was irradiated with mono‐energetic photons, with a uniformly distributed volumetric isotropic source (0.28, 1.5 keV) or a plane beam (0.662, 1.25 MeV), of variable size. Electrons were tracked down to 50 or 10 eV, with G4‐DNA processes and energy transfer greater than 10.79 eV was scored. Based on volume ratios, each scored event had a 0.0388 probability of happening on either DNA helix (break). Clusters of at least one break on each DNA helix within 3.4 nm were found using a DBSCAN algorithm and categorized as double strand breaks (DSB). All other events were categorized as single strand breaks (SSB). Results: Geant4‐DNA is able to reproduce strand break yields previously published. Homogeneous irradiation conditions should be present throughout the ROI for DRBE comparisons. SSB yields seem slightly dependent on the primary photon energy. DRBEs show a significant increasing trend for lower energy incident photons. A lower electron cutoff produces higher SSB yields, but decreases the SSB/DSB yields ratio. The probabilistic and geometrical DNA models can predict equivalent results. Conclusions: Using Geant4, we were able to reproduce previously published results on the direct strand break yields of photon and study the importance of irradiation conditions. We also show an ascending trend for DRBE with lower incident photon energies. A probabilistic model coupled with track structure analysis can be used to simulate strand break yields. NSERC, CIHR
SU‐E‐T‐11: LINAC Dose Profiling Using Cherenkov Emission Imaging
Authors: Glaser A , McClatchy D , Davis S , Gladstone D , Pogue B
Published: June 2012

Purpose: To demonstrate the potential for fast 3D dose profile imaging of a LINAC beam using images of the induced Cherenkov radiation in a water tank. A specialized time‐gated imaging system was developed as a prototype to quantify and compare with Monte Carlo, to illustrate the concept. Methods: Images were acquired from a water tank during irradiation from a 6 MV Varian‐2100C linear accelerator beam using a time‐gated CCD‐based imaging system. The camera was placed normal to the tank wall to minimize parallax reflections, and resultant images were produced by evaluating the median of each pixel in a stack of 2000 images taken at a rate of 60 Hz with an exposure time of 10 ms. Experimental data was compared to images obtained from GEANT4 simulations of the optical setup. Results: Examination of the scored quantities for dose and generated Cherenkov photons indicates that there is a strong similarity, which can be explained by considering the electron energy losses per unit path length. However, due to the complex convolution of the Cherenkov emission directionality and camera lens angular field of view, this relationship is distorted. These errors can be calibrated using the GEANT4 simulations to more accurately reflect the intrinsic dose in the water volume. Conclusions: This work demonstrates dose profiling using the induced Cherenkov radiation signal for the first time. These preliminary results serve as a proof of concept of imaging at one azimuthal angle. Analogous to SPECT, the technique could easily be translated to multiple angles yielding full dose reconstructions following filtered back projection. Further refinement of this technology could be the first step in a paradigm shift towards an alternative method for fast radiation field analysis. Advantages would include increased speed, as well as the ability to profile dynamic beam shapes within transparent solid anthropomorphic phantoms. This work has been financially supported by NIH grant R01CA109558.
SU‐E‐T‐13: Comparison of Dose Rates with and without Gold Backing of USC #9 Radioactive Eye Plaque Using MCNP5
Authors: Aryal P , Molloy J
Published: June 2012

Purpose: To show the effect of gold backing on dose rates for the USC #9 radioactive eye plaque. Methods: An I125 source (IsoAid model IAI‐125A) and gold backing was modeled using MCNP5 Monte Carlo code. A single iodine seed was simulated with and without gold backing. Dose rates were calculated in two orthogonal planes. Dose calculation points were structured in two orthogonal planes that bisect the center of the source. A 2×2 cm matrix of spherical points of radius 0.2 mm was created in a water phantom of 10 cm radius. 0.2 billion particle histories were tracked. Dose differences with and without the gold backing were analyzed using Matlab. Results: The gold backing produced a 3% increase in the dose rate near the source surface (<1mm) relative to that without the backing. This was presumably caused by fluorescent photons from the gold. At distances between 1 and 2 cm, the gold backing reduced the dose rate by up to 12%, which we attribute to a lack of scatter resulting from the attenuation from the gold. Dose differences were most pronounced in the radial direction near the source center but off axis. The dose decreased by 25%, 65% and 81% at 1, 2, and 3 mm off axis at a distance of 1 mm from the source surface. These effects were less pronounced in the perpendicular dimension near the source tip, where maximum dose decreases of 2% were noted. Conclusions: I 125 sources embedded directly into gold troughs display dose differences of 2 – 90%, relative to doses without the gold backing. This is relevant for certain types of plaques used in treatment of ocular melanoma. Large dose reductions can be observed and may have implications for scleral dose reduction.
SU‐E‐T‐15: Small and Nonstandard Photon Field Dosimetry Characterization Using Monte Carlo Methods
Authors: Junell S , DeWerd L
Published: June 2012

Purpose: To quantify uncertainty reduction in small photon field dosimetry through characterization of ionization chambers and calibration conditions using detailed Monte Carlo methods benchmarked against NIST‐traceable measurements. Methods: Phase space profiles were obtained using detailed EGSnrc Monte Carlo models for a Varian 6 MV photon linear accelerator, and a NIST‐traceable cobalt‐60 teletherapy unit. The responses of a farmer‐type ionization chamber, two micro‐ionization chambers, and one scanning‐type ionization chamber were simulated in multiple calibration conditions. Calibration conditions included static field sizes ranging from (10×10) cm squared to (0.5×0.5) cm squared for the 6 MV and cobalt‐60 beam qualities and an additional dynamic IMRT plan for the 6 MV beam quality. Calibration conditions also consisted of ionization chambers placed in a standard water phantom and in a specially designed acrylic phantom. Tolerance limits on the calibration conditions were investigated. All models were benchmarked against measured beam quality data, including ionization chamber beam quality correction factors for the standard absorbed dose to water cobalt‐60 calibration coefficient. Results: The majority of the simulated small field response values fell within the uncertainty of the measured values. A database was created for several proposed small field calibration conditions to provide comparisons with the Co‐60 standard reference conditions. The database includes the small field calibration conditions' beam quality correction factors, tolerances, and dose calibration uncertainties. Conclusions: The characterization of multiple calibration conditions provided an improved understanding of how the cobalt‐60 ionization chamber absorbed dose to water calibration coefficient from an ADCL can optimally be applied to small and nonstandard field calibrations to reduce the associated dose uncertainty. The developed methodology will contribute to future research of other small field radiotherapy modalities. The resulting database provides support for future recommendations on the implementation of small and non‐standard field calibration protocols.
SU‐E‐T‐24: Development and Implementation of an Automated Algorithm to Determine Radiation Isocenter, Radiation vs. Light Field Coincidence, and Analyze Strip Tests
Authors: Hyer D , Mart C
Published: June 2012

Purpose: The aim of this study was to develop a phantom and analysis software that could be used to quickly and accurately determine the location of radiation isocenter using the Electronic Portal Imaging Device (EPID). The phantom could then be used as a static reference point for performing other tests including: radiation vs. light field coincidence, MLC and Jaw strip tests, and Varian Optical Guidance Platform (OGP) calibration. Methods: The solution proposed uses a collimator setting of 10×10 cm to acquire EPID images of the new phantom constructed from LEGO® blocks. Images from a number of gantry and collimator angles are analyzed by the software to determine the position of the jaws and center of the phantom in each image. The distance between a chosen jaw and the phantom center is then compared to the same distance measured after a 180 degree collimator rotation to determine if the phantom is centered in the dimension being investigated. The accuracy of the algorithm's measurements were verified by independent measurement to be approximately equal to the detector's pitch. Light versus radiation field as well as MLC and Jaw strip tests are performed using measurements based on the phantom center once located at the radiation isocenter. Results: Reproducibility tests show that the algorithm's results were objectively repeatable. Additionally, the phantom and software are completely independent of linac vendor and this study presents results from two major linac manufacturers. An OGP calibration array was also integrated into the phantom to allow calibration of the OGP while the phantom is positioned at radiation isocenter to reduce setup uncertainty contained in the calibration. Conclusions: This solution offers a quick, objective method to perform isocenter localization as well as laser alignment, OGP calibration, and other tests on a monthly basis.
SU‐E‐T‐86: Development and Implementation of the Use of Optically Stimulated Luminescent Detectors in the Radiological Physics Center Anthropomorphic Quality Assurance Phantoms
Authors: Bergene J , Kry S , Molineu A , Bellezza D , Court L , Alvarez P , Johnson V , Followill D
Published: June 2012

Purpose: To study the angular dependence of optically stimulated luminescence dosimeters (OSLD) in the Radiological Physics Center anthropomorphic quality assurance pelvic phantom to provide accurate dosimetric measurements as a replacement for TLD. Methods: A spherical phantom was constructed to investigate the angular response of the OSLD as oriented in the RPC pelvic phantom. Three OSLD per irradiation angle, placed at the center of the spherical phantom, were irradiated with 100 cGy from six different angles. The angular response at each angle was determined relative to the OSLD response when the beam was incident normally on the OSLD surface. A pelvic phantom dosimetry insert was modified to include both TLD and OSLD. Three treatment plans were developed in Pinnacle v9.0 and one in Accuray's Multiplan, each with increasing angular beam delivery (4 field, IMRT, SmartArc, CyberKnife) for the pelvic phantom using a common dose prescription and constraints. Each plan was delivered to the phantom three times, containing two TLD and two OSLD, oriented in the transverse plane, at the center of the PTV. The dose delivered to the TLD and OSLD was calculated for each treatment and then compared. Results: The angular dependence correction factor for the spherical phantom was found to be uniformly 1.041 ± 0.003 from single beam edge‐on irradiations. The angular dependence correction in the pelvic phantom from multiple beam orientation irradiations was 1.024 ± 0.002, such that the OSLD dose agreed with the TLD dose. Applying the OSLD pelvic phantom correction factor, the RPC measured dose to planning system calculated dose ratio was 0.995 ± 0.009. The established RPC phantom TLD dose to calculated dose ratio was 0.995 ± 0.010. Conclusions: An anthropomorphic phantom OSLD angular dependence correction factor was established such that the final OSLD dose measurements agreed with RPC's TLD dose measurements to within 1%. Work supported by grant CA 10953, awarded by NCI, DHHS.
SU‐E‐T‐88: Evaluating Gantry Sag on Linear Accelerators and Introducing an MLC‐Based Compensation Strategy
Authors: Du W , Gao S , Wang X , Kudchadker R
Published: June 2012

Purpose: Gantry sag is one of the well‐known sources of mechanical imperfections that compromise the spatial accuracy of radiation dose delivery. This study aims to quantify the gantry sag on multiple linacs and to investigate a multiple leaf collimator (MLC)‐base strategy to compensate for gantry sag. Methods: We used the Winston‐Lutz method to measure the gantry sag on three Varian linacs. A ball‐bearing phantom was imaged with a square radiation field during gantry rotation. The images were analyzed to derive the radiation isocenter and subsequently the gantry sag, that is, the superior‐inferior wobble of the radiation field center from the radiation isocenter as a function of gantry angle. Compensation for gantry sag was attempted by offsetting the MLC leaves at 90‐degree collimator angle. The amount of offset was the opposite of measured gantry sag, which was gantry angle‐specific. Results: Gantry sag was reproducible within a six‐month period. On the three linacs, the maximum gantry sag was found to vary from 0.7 mm to 1.0 mm, depending on the linac and the collimator angle. The radiation field center moved inferiorly, or away from the gantry, when the gantry was rotated from 0 to 180 degrees. Comparison of gantry sag at 0‐ and 90‐degree collimator angles showed that the uncertainty in MLC leaf positions did not increase the gantry sag. Instead, gantry sag was caused primarily by nonideal gantry rotation. After the MLC compensation was applied, the maximum gantry sag was reduced to less than 0.2 mm. Conclusions: The results indicated that gantry sag on a linac can be quantitatively measured with sub‐millimeter precision, using a simple ball‐bearing phantom and the electronic portal imaging device. Reduction of gantry sag is feasible by applying a gantry angle‐specific correction to MLC leaf positions at 90 degree collimator angle.
SU‐E‐T‐89: Characterization of Dental Restoration Material for Cs‐137 Radiation Dosimetry
Authors: Ratliff S , Gustafson B , Barry K
Published: June 2012

Purpose: The purpose of this work is to characterize the radiation‐induced thermoluminescence properties of a dental restoration material and to see if the material might be feasible for use in retrospective radiation dosimetry. Methods: Retrospective, or accidental, dosimetry is the study of using nearby materials to measure radiation received by individuals. In this project we obtained samples of Ivoclar Vivadent e.max CAD material, a glass‐ceramic used for making dental restorations such as full or partial crowns. The samples were machined into square chips .32 cm × .32 cm × .089 cm and annealed in the same furnace used by the dentist. The samples were exposed to a Cs‐137 source using a PMMA source holder and then read in a Harshaw 3500 TLD reader. The samples were read without nitrogen gas flux using heating rates of 5 degrees C/s or 10 degrees C/s up to a maximum temperature of 400 degrees Celsius. The glow curves were analyzed using Systat PeakFIT peak‐fitting software and Microsoft Excel spreadsheets. The authors gratefully thank Dr. Aaron Imdieke and the staff of River City Dental, St. Cloud, MN for the dental restoration materials and the use of their dental furnace. Results: A sample subjected to a radiation exposure of .04 C/kg exhibits a glow curve with a prominent peak at approximately 140 degrees Celsius, which is well‐modeled by the first order glow curve deconvolution formula developed by Kitis, Gomez‐Ros, and Tuyn. The activation energy corresponding to this peak is approximately 1 eV. The thermoluminescent signal fades with time after exposure. Conclusions: Ivoclar Vivadent e.max CAD dental restoration material has the potential to be used as a material for retrospective Cs‐137 radiation dosimetry. Future work could look at its thermoluminescent dosimetry properties in more detail and also at other dental restoration materials. The authors would like to thank Dr. Aaron Imdieke and the staff of River City Dental, St. Cloud, MN, for the donation of scrap dental restoration materials and the use of their dental furnace.
SU‐E‐T‐94: Multileaf Collimator Performance and Validation of Quality Control Tolerances
Authors: Letourneau D , Wang K , Norrlinger B , Nurul A , Homer P , Lee P , Jaffray D
Published: June 2012

Purpose: The automated quality assurance system (AQUA) is a centralized quality control (QC) software designed to automate QC tests. Statistical analysis of AQUA results was performed to assess the geometric accuracy and long‐term reproducibility of a commercially available multileaf collimator (MLC) and examine the applicability of the American Association of Physicists in Medicine (AAPM) tolerances for MLC QC. Methods: The MLC was first calibrated with AQUA by minimizing leaf‐positioning errors on megavoltage images for 5 different leaf‐bank positions (−60 to 100 mm from radiation isocenter). Leaf‐positioning accuracy and reproducibility was assessed by repeating the AQUA test 5 times/week. The range of leaf‐positioning error over leaf‐bank positions and time was reported. Measured leaf‐positioning errors were then separated into systematic and random error components. The systematic error corresponds to the variation (standard deviation) in mean positioning errors between leaves over leaf‐bank positions and time. The random error quantifies the leaf position variations around its mean and is calculated as the root‐mean‐square of the individual leaf position standard deviations. Results: To date, 2 different MLCs have been calibrated using AQUA and 9–18 datasets have been acquired to assess performance. For the unit with the longest follow up, the range of leaf‐positioning errors was −0.62 to 0.85 mm and 98% of the measured leaf positions (n=7200) were within ±0.5 mm of the nominal position. The systematic error was the main error component (±0.15 to ±0.2 mm) and was attributed to the residual errors after calibration. The random error was ±0.07 mm for both units and demonstrated good leaf‐positioning reproducibility and limited uncertainty of the AQUA measurements. Conclusions: Preliminary results show that after MLC calibration with AQUA, leaf‐positioning errors on two different units are well within the AAPM‐recommended ±1 mm tolerances. Additional MLC performance improvement is possible if residual errors after calibration can be reduced further as the MLC demonstrated high reproducibility. Funded in part by Elekta Inc.
SU‐E‐T‐100: How to Improve the Dose Accuracy for Gantry Angle Dependent Patient Specific IMRT QA Using 2D Ion Chamber Array with Octavius Phantom
Authors: Choi D , Nookala P , Patyal B
Published: June 2012

Purpose: To determine the cross calibration factors which can predict more accurate dose distribution for fixed beam IMRT QA using Octavius phantom. Methods: The ion chamber based Octavius 2D‐array detector (PTW, Freiburg, Germany) is a step in the right direction to measure the absolute dose and dose distribution for patient specific IMRT QA. However, the directional dependency of this detector made it less than desirable for angle dependent IMRT QA. We evaluated the new Octavius system (PTW, Freiburg, Germany) for angle dependent IMRT QA which compensates the response due to directional dependency. The system is designed for full arc VMAT QA, but does not always work for the discrete angle IMRT QA due to non‐averaging of errors caused by directional dependence of detectors. The proposed method uses correction factors for each gantry angle. The dose for a 10cm × 10cm open field for each gantry angle was calculated by treatment planning system and measured using the Octavius phantom. The correction factors were determined at each gantry angle and the dose distribution was renormalized at each angle using correction factors. Results: The discrepancy between measured and planned dose per monitor unit depended on the gantry angle and were in the range of +−4% using the PTW method. Using our method, uncertainty due to the detector angle dependency was eliminated. Conclusions: The new method removes the angle dependency of ion chamber based 2D array detector for the fixed beam IMRT QA. It provides fast, accurate and more realistic results for angle dependent IMRT QA.
SU‐E‐T‐101: Dosimetry Intercomparison for a Synchrotron‐Produced Monochromatic X‐Ray Beam
Authors: Brown TAD , Hogstrom KR , Alvarez D , Ham K , Matthews KL
Published: June 2012

Purpose: This study performed a dosimetry intercomparison for synchrotron‐produced monochromatic x‐ray beams. Ion chamber depth‐dose measurements in a polymethylmethacrylate (PMMA) phantom were compared with the product of MCNP5 Monte Carlo calculations of dose per fluence and measured incident fluence at 25 and 35 keV. The ion chamber measurements are being used to calibrate dose output for cell irradiations designed to investigate photoactivated Auger electron therapy at the LSU Center for Advanced Microstructures and Devices (CAMD) synchrotron facility. Methods: Monochromatic beams of 25 and 35 keV were generated on the tomography beamline at CAMD. A cylindrical, air‐equivalent ion chamber was used to measure the ionization created in a 10×10×10‐cm3 PMMA phantom at depths of 0.6 – 7.7 cm. AAPM TG‐61 protocol was applied to convert measured ionization into dose. MCNP5 simulations of the irradiation geometry were performed to determine the dose deposition per photon fluence in the phantom. Photon fluence was determined using a NaI detector to make scattering measurements of the beam from a polyethylene target at angles 15 – 60 degrees. Differential Compton and Rayleigh scattering cross sections were used to derive the incident fluence. Results: At 35 keV dose measurements for equal exposures determined using the MCNP5‐fluence results underestimated those of the ion chamber by 1.8 – 4.8% for PMMA depths from 0.6 – 7.7 cm, respectively. At 25 keV there was an overestimate of 6.6 – 1.9%. Conclusions: These results show that TG‐61 ion chamber dosimetry, used to calibrate the dose output for the cell irradiations, is accurate within approximately 7% for beam energies 25–35 keV. This research was supported by contract W81XWH‐10‐1‐0005 awarded by The U.S. Army Research Acquisition Activity, 820 Chandler Street, Fort Detrick, MD 21702‐5014. This report does not necessarily reflect the position or policy of the Government, and no official endorsement should be inferred.
SU‐E‐T‐111: Single Line Multi‐Detector Scintillation Dosimetry: Demonstration of Feasibility
Authors: Jozsef G , Pagliazzi M , Saxena I , DeWyngaert J
Published: June 2012

Purpose: Obtain feasibility data on the use of multiple scintillators on a single optical line for dose measurements. Methods: A CsI (Tl doped) crystal and a plastic (Rexon Inc, Rp‐408) scintillator detectors, both transparent, were attached to the end of a fiberoptic line and connected to an Ocean Optics USB‐2000 spectrometer. After baseline spectra, spectra with the two scintillators adjacent to each other and then separated by a 7.6 cm plexiglass spacer were obtained. Irradiations were performed using 6 MV X‐ray beam from a Varian EX linear accelerator. Utilizing the baseline spectra the dose received by each scintillator were calculated from the measured spectral peaks of the linear scintillator assemblies. Linearity tests were performed by varying dose and the dose rate in a homogeneous radiation field covering both scintillators. Unequal doses were delivered to the scintillator by gradually closing the collimator from one direction, blocking one detector at a time. Doses to the scintillators were modulated by different amount of solid water placed over the two detectors, as well. Results: Measured scintillation spectra agreed with the published spectra. The spectra did not change with depth in the phantom. The multi‐scintillator system response was strictly linear between 1.67 and 40 MUs, (approx. 1.3 to 31 cGy) and dose rate independent between 100 to 600 MU/min. The profile curves obtained by closing the collimator agreed with qualitatively expected curves. Doses measured under different phantom thicknesses were in good agreement with ion chamber measurements on the same locations (+/− 3%). The linearity and dose rate independence allow absolute dose calibration for given beam energies and scintillator arrangement. Conclusions: Multi‐probe scintillation dosimetry along a single optical fiber is possible in therapeutic irradiation conditions. This is feasible by using signals from multiple select scintillators with distinct spectroscopic responses arranged along an optical fiber.
SU‐E‐T‐113: Volume Effect Correction Factor KV for Small‐Field Photon Dosimetry with Ionization Chambers
Authors: Looe HK , Stelljes TS , Harder D , Poppe B
Published: June 2012

Purpose: The volume effect of ionization chambers gives rise to a spatial averaging effect that can be expressed mathematically as the convolution of the true dose profile with the detector's response function. The latter has been shown to be best described by Gaussian distribution. Based on this knowledge, the volume effect correction factor kV is derived. Methods: To derive kV, a sixth degree polynomial is fitted to the true dose profile: D(x) = a0 + a2×2 + a4×4 + a6×6. The measured dose profile M(x) is calculated as the convolution product of D(x) with a one‐dimensional normalized Gauss function with standard deviation s. Therefore kV at the dose maximum has the value D(0)/M(0), which is a function of the coefficients a0,2,4,6 and the detector specific s. In the case where D(x) is unknown, kV can be derived analogously from M(x) so that M(x) = b0 + b2×2 + b4×4 + b6×6, where kV can now be expressed as a function of the coefficients b0,2,4,6 and s. Results: The magnitudes of kV,lat and kV,long were calculated for 1 to 5 cm dose profiles using measured s values, both in the lateral and the longitudinal directions, for a set of common ionization chambers. At field widths above 2 cm, the values of kV,lat fall below 1.01 for all the chambers evaluated, whereas it needs field widths above 4 cm to get all values of kV,long below 1.01. Since the detector's signal is integrated over the sensitive volume, the total kV can be calculated as kV,total = kV,lat . kV,long. Conclusions: In this work, a correction is developed to eliminate the volume effect of ionization chambers when they are positioned in the maxima of dose profiles, particularly for the performance of output factor measurements for the calibration of narrow photon beams.
SU‐E‐T‐116: The Water Equivalence of Organic Liquid Scintillators for Proton Dosimetry
Authors: Ingram WS , Robertson D , Beddar S
Published: June 2012

Purpose: Organic liquid scintillators are currently under investigation for use in proton dosimetry. The purpose of this work is to evaluate the water equivalence of these materials as a preliminary step to identify scintillators that are well‐suited to this purpose. Methods: Stopping powers were calculated for 0.001–1000 MeV protons in water, polystyrene, and two organic liquid scintillators: BC‐531 and OptiPhase ‘Hi‐Safe’ 3 at 0%, 25%, and 50% concentrations of water. Angular scatter was quantified by theta0, a characteristic multiple Coulomb scattering angle analogous to the standard deviation of a Gaussian distribution of proton angles relative to the incident beam axis. Theta0 was calculated as a function of depth over the range of 200 MeV protons in these materials. Results: Collisional stopping power in BC‐531 ranged from +44% to +1% ofthat in water. It remained within 6% from 2–600 MeV. OptiPhase ranged from +24% to −2%, with smaller deviations at increased water concentrations. At all concentrations, OptiPhase showed smaller deviations than polystyrene and BC‐531 and remained within 1% of water from 2–600 MeV.Theta0 was very similar for all materials, with deviations from water of 5 milliradians or less over the majority of the proton range. BC‐531 showed deviations of 10 milliradians or more in the last few millimeters of the range. OptiPhase showed smaller deviations than BC‐531 or polystyrene, and these deviations decreased with increasing water concentration. Conclusions: OptiPhase was found to be more water equivalent than BC‐531 or polystyrene in stopping power and angular scatter, and increased water concentration improved both quantities. Large deviations in stopping power were only found below 2 MeV for any material, where proton range is less than 0.1 millimeter. The deviations from water found in angular scatter were less significant, and probably too small to affect measurement.
SU‐E‐T‐117: A Point Densitometer Scanner for EBT2 Film Dosimetry
Authors: Hrinivich T , Jensen M , Wong E
Published: June 2012

Purpose: To create and characterize a device that can accurately and reproducibly digitize Gafchromic(R) EBT2 film for dosimetry. Methods: We have constructed a point densitometer scanner that allows the acquisition of 1 dimensional optical density profiles of EBT2 dosimetry film at submillimeter resolution. An LED with peak output at 636nm collimated and focused with an aspheric lens was used as a light source. A large area photodiode was used as a detector. A custom transimpedence amplifier was developed to optimize the detector signal response for optical densities corresponding to a clinically relevant dose range (0 – 3Gy). A custom motorized translational stage was developed allowing 1 dimensional scans and manual fine adjustment in the lateral direction. The entire scanner was contained in a light tight enclosure to eliminate the effects of ambient light. Dose profiles were generated from EBT2 films exposed uniformly to varying doses using a superficial unit with the films placed on top of a solid water phantom. Dose profiles were generated from the same films using a flatbed scanner (Epson 10000XL) implementing two and three optical channel corrections for film inhomogeneity and the results were compared. Results: Initial analysis of the point densitometer shows a beam FWHM of 0.7mm and position error of the translational stage at 0.02mm in the scan direction and 0.2mm in the lateral direction. Reproducibility of the scans has been shown to within 1%. Orientation dependence and measured dose homogeneity have been improved over corrected flatbed digitization techniques. Conclusions: Preliminary characterization of the point densitometer shows its potential as an improved quality assurance tool for EBT2 film dosimetry.
SU‐E‐T‐119: Evaluation of New VMAT and IMRT Planning QA Devices: Portal Dosimetry Vs Compass
Authors: Kum O , Shin H , Han Y
Published: June 2012

Purpose: Wide spread clinical use of advanced radiation treatment delivery techniques such as volumetric modulated arc therapy (VMAT) and IMRT needs an easy‐to‐use, well‐defined, and effective quality assurance (QA) system to ensure the accuracy of dose delivery. The comprehensive evaluation of two delivery QA systems (DQA) (Portal Dosimetry and Compass) for the advanced radiation delivery techniques is necessary for the clinical setting. Methods: Portal Dosimetry QA system with Varian PortalVision (PV) provides a quantitative comparison of measured electronic dose profile to a predicted dose profile with Varian's Eclipse™ treatment planning system (TPS) for the VMAT and IMRT plans. Quantitative comparisons of the two images, including dose difference, gamma analysis, points dose measurements, and line profiles are easily acquired in two dimensions. Meanwhile, Compass system with MatriXX can provide an accurate three‐dimensional quantitative analysis of dose delivery. We applied the two planning QA systems to our RapidArc patient‐ specific QAs. Validation of the Compass and Portal Dosimetry systems were performed with ionization chamber measurements for absolute dosimetry and film measurements for gamma index calculations for phantom and patient treatment plans. Results: Compass system was very sensitive to the MLC leaf gap variations in three dimensional gamma analyses. However, Compass system was not enough to fix the errors in dosimetric leaf gap by itself. DQA process with Portal Dosimetry took approximately 6 minutes for the two Arcs VMAT plan. Compass took about 40 minutes for the same plan in our first clinical trial. For the three typical IMRT and VMAT plans, the two systems showed the same passing rates in 95% pixels passing gamma criteria 3%/3mm. Conclusions: Portal Dosimetry DQA system has the advantages of effectiveness and practicality in the clinical setting. On the other hand, Compass was the next supportive system because of its accuracy and reality with patient's anatomy.