bluuebunny/crossref_metadata_2025 huggingface dataset:
TH‐C‐350‐01: Planning and Delivery of IMAT: Comparisons of Single and Multiple Arc Approaches
Authors: Wong E , McCurdy B , Oliver M , Potrebko P , Bzdusek K , Chen J
Published: July 2008

Purpose: To investigate Intensity Modulated Arc Therapy plans (IMAT) with variable dose rate single arc and constant dose rate multiple arc approaches; to provide a formula for the prediction of their delivery times, and a metric to evaluate IMAT treatment plan adaptability. Method and Materials: IMAT plans were generated for localized prostate and oropharynx cancers. For each site, we generated IMAT plans for delivery with variable dose rate single arc and with constant dose rate multiple arcs. Treatment delivery times were modeled mathematically. A maximum leaf motion per unit gantry angle metric is used here to measure the complexity of leaf motions in treatment deliveries, and we proposed to use this metric to assess IMAT plan adaptability. IMAT plans were compared based on dosimetric qualities (target conformality and uniformity indices, dose‐volume indices of critical organs), delivery times, and plan adaptabilities. All IMAT plans were generated on Pinnacle 8.1v (Philips Medical Systems) for Varian linacs. Results: Overall, both IMAT approaches have comparable dosimetric plan qualities, although generating variable dose rate single arc IMAT plans required increased angular sampling to accommodate larger leaf motions, especially when there are two or more target volumes. The delivery time for the single arc is limited by both the gantry rotational speed and the maximum leaf motion between field segments, whereas, the multiple arc approach is generally limited by the gantry rotational speed alone. For the prostate plan, the delivery times are 1.2 minutes and 2 minutes respectively. In the oropharynx case, the single arc and the multiple arc deliveries were similar at approximately 2 minutes. The adaptability indices reflected the complexity of leaf motion in the single arc compared to the multiple arc approach. Conclusion: IMAT plans with variable and constant dose rate deliveries were evaluated under multiple considerations for potential clinical utilizations.
TH‐C‐350‐02: Is Dose Rate Variation Crucial for Single‐Arc Radiation Therapy Delivery?
Authors: Tang G , Earl M , Luan S , Wang C , Naqvi S , Yu C
Published: July 2008

Purpose: Recent arc therapy techniques such as arc‐modulated radiation therapy (AMRT) developed at the University of Maryland and Varian's RapidArc™ allow variable segment‐weightings in order to expand the optimization domain. As a result, these plans may require a varying dose rate (DR) for delivery. To evaluate the necessity of DR variation in arc therapy delivery, the variable‐DR plans were translated in such a way that they can be delivered with a constant DR. Method and Materials: Four cases were selected for this study: 1 HN, 1 lung, 1 prostate and 1 brain. A single‐arc AMRT plan was generated for each case. Planning of AMRT started with optimization of ideal intensity maps with 36 equi‐spaced beams in Pinnacle followed by segmentation of the intensity maps into a deliverable AMRT MLC sequence. During leaf‐sequencing, the segment weightings are allowed to vary. In translating variable‐DR AMRT plans into constant‐DR plans, the angular spacing of the original beams were changed from equi‐spacing to spacing according to their weightings. Hence, apertures with more MUs occupy a greater angular range. To account for any field shifting in the process, a field shape correction was applied ensuring proper target coverage. Results and Conclusion: DVH comparisons show that constant‐DR plans were comparable to the corresponding variable‐DR plans in 3 of the 4 cases. Significant degradation occurred in the constant‐DR plan of the prostate case due to the large MU variations in the original variable‐DR plan, causing the beams to deviate significantly from their original positions. The estimated delivery times of the constant‐DR plans are 3 to 30 times longer than the variable‐DR plans due to large MLC shape variation within a small beam interval. It is hereby shown that DR variation is crucial to AMRT delivery in order to maintain excellent plan quality and efficient delivery time.
TH‐C‐350‐05: Performance of a Beam Tracking System for Treatment of Moving Targets with Scanned Ion Beams
Authors: Bert C , Saito N , Schmidt A , Chaudhri N , Schardt D , Rietzel E
Published: July 2008

Purpose: Scanned ion beam therapy of intra‐fractionally moving targets requires motion mitigation to prevent deteriorations of the delivered doses due to interplay. A beam tracking system was developed and experiments with various detectors were performed to assess system performance. Method and Materials: Tracking of moving targets with scanned ion beams requires simultaneous lateral and longitudinal range adaptation of pencil beam positions. Lateral beam tracking is achieved directly with the beam scanning system. Longitudinal tracking is performed with a dedicated energy modulation system consisting of fast, motorized absorber wedges. In experiments with radiographic films, a range telescope, and an array of pin‐point ionization chambers in a water phantom, the precision of lateral, longitudinal, and 3D motion compensation was measured. Motion was induced laterally by a sliding table and longitudinally by ramp‐shaped absorbers. Motion detection was performed with a laser‐triangulation sensor. Experimental tracking data were compared to results of measurements with stationary detector systems. Films were analyzed with respect to geometrical shape as well as mean, minimum, and maximum response level within the target region. Ionization chamber data were analyzed for different volumes of the setup. Analysis of range telescope data focused on Bragg‐peak shapes and ranges. Results: Film analysis yielded conservation of geometrical shape and responses (mean±SD, minimum, maximum) of (0.276±0.010, 0.241, 0.288) and (0.275±0.007, 0.252, 0.284) for stationary irradiation and tracking respectively. Within the target volume, ionization chamber data with beam tracking deviated from stationary irradiation by 0.3 ± 1.5% with −2.7% minimum and 3.7% maximum deviation. The Bragg peak range for tracking deviated by less than 0.25 mm water‐equivalent from the stationary reference. Conclusion: The tracking system for scanned ion beams successfully restored dose distributions for moving targets by adapting individual Bragg peaks during irradiations in quasi real time.

Research sponsored in part by Siemens Healthcare, Particle Therapy.
TH‐C‐350‐07: Impacts of Dose Distribution Variations in Proton Therapy According to Gastro‐Intestinal Tract Air Filling and Breathing
Authors: Bouchard M , Briere T , Amos R , Crane C , Beddar S
Published: July 2008

Purpose: For upper abdominal cancers, the simulation CT represents a snapshot of the possible air distribution and radiological thickness of a patient, both of which impact proton dose distribution. The purpose of this dosimetric study is to analyze the effect of gastro‐intestinal tract air filling and breathing on the dose delivered to a superior abdomen target using passive scattering proton therapy. Method and Materials: We used free‐breathing CT (FBCT) and 4D‐CT data sets showing the same 3‐cm pancreatic tumor. Air distribution was reasonable. We reproduced a situation where proton beam parameters would be calculated on a normal breathing patient with gas (no air override: O−FBCT), and we analyzed the dosimetric impacts if air was replaced by stools (air override: O+FBCT) during treatments and vice‐versa. Target coverage for 50.4 CGE in 28 fractions and dose to critical structures were evaluated for different air‐filling and breathing‐phase scenarios. Four‐field plans including 3 incidences going through some gas were compared to 2‐posterior‐field plans. Results: For the 4‐field plans, beam parameter calculations from the O+FBCT resulted in adequate target coverage when the same beam parameters were applied to the O−FBCT. The converse situation resulted in the need for an additional 1‐cm SOBP width to achieve adequate coverage for the antero‐posterior and right‐lateral beams. The 4‐field plans provided insufficient distal target coverage to account for planning uncertainties in the expiration phase, a problem avoided with the 2‐field plan. Conclusion: Our choice of planning margins on O+FBCT for upper abdominal cancer proton therapy is safe for target coverage, even if air filling variations occur during treatments. Planning on an O−FBCT, using a passive scattering technique, results in less conformal plans. The use of posterior incidences for upper abdominal cancers prevents uncertainties, especially in specific clinical situations like ileus or breathing disorders.
TH‐C‐350‐09: 4D Treatment Planning and Dosimetric Validation of Helical Tomotherapy Treatments of Moving Tumors
Authors: Goddu S , Noel C , Chaudhari S , Parikh P , Khullar D , Santanam L , Low D
Published: July 2008

Purpose: The objectives of this study are to describe the 4D treatment planning process for moving targets and to evaluate the dosimetric consequences caused by target motion during tomotherapy delivery. Method and Materials: A lung cancer patient was CT scanned using our standard 4D‐CT protocols on Phillips brilliance 64 slice CT scanner. A maximum intensity projection CT dataset was used for treatment planning. Tumor motion trajectory and amplitude was determined from the inhale and exhale phases of the 4DCT. A three‐dimensional motion pattern was created by scaling the breathing form to an average magnitude of the target motion. The three‐dimensional trajectory was programmed into the Washington University 4D Phantom to simulate target motion. Eight EDR2 films were loaded into a phantom for dosimetric verification. Measurements were performed on a static phantom and then repeated on the oscillating phantom. Experiments were repeated for their reproducibility. Films were analyzed using Hi‐Art TPS. Results: To quantify the dose errors, gamma analysis was performed. In static phantom study, >97% of the points passed with a gamma criteria of 3% and 3mm. When the same criterion was applied for the oscillating phantom, 15–25% of the points failed. When the gamma threshold was increased to 8% and the gamma analysis window was tailored to be within the CTV, >98% of the points passed indicating that the dose errors are of the order of 5% for majority of the points. In order to find the maximum dose errors, the tolerance was increased until 100% of points passing the criteria. Conclusion: 4D treatment planning process for helical tomotherapy is discussed. Gamma analysis revealed that, the dose errors are of the order of 5% for majority of the points and the maximum dose error within the CTV was 11%.

This work is supported in part by Tomotherapy Inc.
TH‐C‐AUD A‐01: A Systematic Analysis of Errors and Uncertainties in Partial Breast Irradiation Using the MammoSite Balloon Catheter
Authors: Stojadinovic S , Todor D , Cuttino L , Arthur D
Published: July 2008

Purpose: The objective of this study was to evaluate systematic and random errors associated with HDR brachytherapy treatment delivery of partial breast irradiation with a MammoSite Balloon. The impact of these errors on the actual delivered dose was studied. Method and Materials: The effect of respiratory motion, balloon contrast concentration and intra‐observer variability on accuracy of applicator delineation were investigated. Additionally, the consequence of variability of target delineation and dose matrix voxel size was considered in relationship to target coverage. Lastly, an analysis of 15 patients treated with the MammoSite was performed to simulate the effect of allowed source position uncertainty for each fraction. Results: Respiratory motion resulted in an under‐estimation of the balloon volume of 8% and 18% for computed tomography and cone‐beam computed tomography, respectively. The coverage was assessed based on the volume of the target receiving 95% of the prescribed dose (V95). On average, balloon volume underestimation combined with the daily ±1.0mm source position uncertainty degraded the coverage from an initial 96.3% to 91.7% over the course of 10 fractions. Additionally, a 3mm difference between the true tip of the applicator and its delineated position resulted in a decrease in coverage from 96.3% to 92.6% and from 98.7% to 79.3%, for a small and a large balloon, respectively. Conclusion: Current treatment protocols require that 90% of the target volume receives 90% of the prescription dose. Our findings suggest that significantly higher dosimetric coverage goals are necessary to compensate for the identified errors inherent in balloon based brachytherapy dose delivery. This will insure that the expected degradation of the actual dose delivered from the planned dose, as a consequence of random and systematic uncertainties, does not fall below an acceptable threshold.
TH‐C‐AUD A‐04: Calibration of Image‐Guided Robotic System for Prostate Brachytherapy
Authors: Buzurovic I , Podder T , Hu Y , Yan K , Valicenti R , Dicker A , Yu Y
Published: July 2008

Purpose: To perform calibration and evaluation of an image‐guided brachytherapy robotic system. Method and Materials: Brachytherapy robot calibration consists of three steps: mechanical robotic calibration, image calibration and overall system calibration. Mechanical calibration determines 1)system resolution, as the smallest incremental movement that the robot can physically perform, 2)repeatability as a measure of the ability of the robot to move back to the same position and orientation and 3)accuracy, as the robot's ability to precisely move to a desired position in 3D space. Imaging calibration for our system is the procedure where distance from ultrasound images is transformed to the metric distance in robot absolute coordinates. Overall system calibration is to determine the exact position of image stacks, calculated in absolute robotic system coordinates. These steps were performed using high‐resolution camera, specially‐designed phantom box for imaging calibration and CMM device for mechanical calibration. The system software allows mutual calibration between mechanical and imaging robotic modules. Mechanical calibration consists of DHV table definition for robotic system, matrix transformation, definition of composite matrices, direct kinematics solution, inverse kinematics solution, definition of robot initial position, calculation of position error, and error correction method. The robot errors gathered by position measurement are minimized by numerical optimization. Results: The calibrated precision of translation movements for the stylet and cannula are in the range of 0.03–0.08mm (depending on load); lateral and vertical movements for the gantry are 0.03mm; probe translation and rotation are 0.05mm and 0.03deg, respectively. The fiducial error for imaging is less than 0.1mm in x and y image coordinates. Conclusion: Overall robotic brachytherapy calibration plays a crucial role in accurate delivery. The calibrated precisions of the image‐guided brachytherapy robotic system are considered satisfactory for the given clinical application.

Acknowledgement: Supported by NCI‐R01‐CA091763.
TH‐C‐AUD A‐05: Experimental Validation of An Iterative Forward Projection Matching Algorithm for Seeds Center Localization Using Conebeam‐CT X‐Ray Projections
Authors: Pokhrel D , Murphy M , Todor D , Williamson J
Published: July 2008

Purpose: To experimentally validate a new algorithm for reconstructing the 3D positions of implanted brachytherapy seeds from 2D projection images. Method and materials: The iterative forward projection matching (IFPM) algorithm consists of finding the 3D seed geometry that minimizes the sum‐of‐squared‐difference of the pixel‐by‐pixel intensities between computed projection images and measured auto‐segmented images of implanted seeds. IFPM starts with an approximation to the initial seeds configuration, e.g., the pre‐implant seed arrangement. It then iteratively refines the 3D seed coordinates until the computed projections match with the measured projections. Three pairs of computed and measured projection images, with known imaging geometry, are used. Two brachytherapy phantoms were fabricated with 12 and 72 seeds in known configurations. Three projections of each phantom were acquired using an Acuity digital simulator along with a full 660 projection Conebeam CT (CBCT). Image pre‐processing steps were performed to create the binary seed centroids images for use by IFPM algorithm. To quantify IFPM accuracy, the actual seed positions were extracted from the CBCT images by the Brachy Vision‐planning system. Results: For the 12 seed phantom data, the mean reconstruction error was found to be 0.83±0.34mm where as for 72 seed phantom it was 0.97±0.37mm. The each test trials converged in 4–10 iterations with computation time of 2.8–62 min on a 2 GHz processor. Discussion: The IFPM algorithm avoids establishing seed projection correspondence required by standard back‐projection methods. In phantom studies we have demonstrated 1 mm accuracy in reconstructing the 3D positions of brachytherapy seeds from 2D projection images. This supports the potential of this algorithm for accurate and robust seed reconstruction in patients.

This project was supported by grant from Varian Medical Systems.
TH‐C‐AUD C‐02: Automatic Extraction of Salient Interest Points in 3D Images for Contour Propagation in IGRT
Authors: Allaire S , Pekar V , Breen S , Hope A , Jaffray D
Published: July 2008

Purpose: To efficiently propagate patient contours to multi‐fraction Cone‐Beam CT volumes. Method and Materials: An auto‐segmentation scheme was developed which uses anchor points to propagate structure delineations from one image to another, accounting for the changing patient anatomy. Four patients involved in 35‐ or 40‐fraction Head‐and‐Neck treatment schemes were selected retrospectively. The 8–12 Organ‐at‐Risk contours drawn in their initial planning CT included spinal cord, brainstem, optic structures, and parotids, to be propagated to the CBCT image acquired at each fraction. Our method involves three steps: a) Generate a patient‐specific compressed model in terms of salient points from the planning CT image. The automatic extraction of these uses a developed 3‐dimensional extension of the SIFT algorithm; b) Retrieve this model in every CBCT dataset, via automatic block‐matching maximizing the local correlation between neighborhoods around the interest points; c) Propagate the original contours based on the thin‐plate‐spline warping transformation derived from salient point pairs. Results: For each patient, over 500 salient points were extracted in 5–7 minutes, and subsequently retrieved within 45 seconds in every CBCT image, into which the 8–12 contours were propagated in 10 seconds. The results show clear improvement, compared to contours obtained by both a mere copy and voxel‐based rigid registration; the typical neck shrinking and spine flexions especially are followed. Our method is more robust and computationally affordable than voxel‐based deformable image registration. Conclusion: The contours can be successfully propagated to all fractions based on significant salient anchor points extracted to summarize one patient's gray‐level information and geometry. The thus highlighted deformations of the critical structures i) can help determine a safe dose map and be reflected in Planning organ at Risk Volume (PRV) margins, ii) may be used to support time‐efficient re‐planning in adaptive radiotherapy. Conflict of Interest: This research is partially supported by Philips Healthcare.
TH‐C‐AUD C‐04: An Online Correction Method for Interfractional Variations in Head and Neck Cancer
Authors: Ahunbay E , Chen G , Peng C , Schultz C , Li X
Published: July 2008

Purpose: Daily setup for head and neck (HN) radiotherapy (RT) can vary randomly due partially to neck rotation. This variation along with anatomy change can not be totally corrected by the current rigid translation. A full‐scope re‐optimization based on the daily CT takes too long to be practical with the current planning techniques. A novel rapid correction scheme that can be used online to correct for both inter‐fractional setup variation and anatomy change for HN RT is presented. Method and Materials: The scheme consists of two major steps after transferring planning contours to the CT of the day by means of deformable registration (MIMVISTA): (1) beam/segment apertures morphing (SAM) based on differences between planning contours and new contours, and (2) segment weight optimization (SWO) for the new apertures. SAM is accomplished by applying the spatial relationship between the planning target contour and the apertures to the new target contour. Dose distribution for each new aperture was generated using a planning system with a fast dose engine and hardware (Prowess), and was input into a newly‐developed SWO package using a fast sequential quadratic programming. The entire scheme was tested based on the daily kVCT images acquired for representative HN IMRT cases treated with a linac and CT‐on‐Rails combo (CTVision, Siemens). Results: The target coverage and/or OAR sparing degradation, arising from the current standard repositioning from rigid registration with the CT of the day, can be corrected by the present SAM/SWO scheme. The target coverage and OAR sparing for the SAM/SWO plans were found to be equivalent to the original plan. The SAM/SWO process took 5–8 min for the head and neck cases studied. Conclusion: The proposed aperture morphing with weight optimization is an effective approach for correcting inter‐fractional patient setup and anatomic changes for head and neck cancer radiotherapy.
TH‐C‐AUD C‐09: Efficacy of Virtual Reality Simulation for Noncoplanar Prostate IMRT — a Peek of Future RTP System
Authors: Shang C , Williams T , Sloman B , Ward J , Phillips R , Percy J , Beavis A
Published: July 2008

Purpose: To evaluate the efficacy of virtual reality simulation for noncoplanar prostate IMRT optimization process. Method and Materials: 14 prostate IMRT cases, randomly selected from current clinic, were re‐simulated using a pioneer virtual reality simulation (VRS) system for noncoplanar beam arrangement. Using 3D stereoscopic technology, the VRS system displays the virtual treatment environment on a DTI™ real 3D LCD screen without aid of gaggles. With the virtual patient loaded to the screen, one can simulate the treatment by realistically manipulating the gantry and table. An optimal beam projection is suggested when the inclusion of vital organ was reduced in the volumetric beam eye‐view. When exploring a noncoplanar beam position, the anti‐collision function effectively alerted the mechanical limitations. In comparing with original seven evenly‐spread coplanar beams, in the resultant new setting all beams were rotated more anteriorly, while two anterior oblique beams were tilted 20–30° inferiorly. All IMRT plans were calculated with similar modulated intensity level and number of segments. To standardize the endpoint for comparison, PTV D95 was normalized to 45 Gy. Results: VRS created non‐coplanar beam arrangements were proved clinically to be deliverable beam arrangement without risk of collision. Dose homogeneity in new plans were improved, indicated by 2.2% reduction in global maximal dose and 2.0% in high dose tail (D5) of PTV. Enhancement of rectal dose sparing was suggested by 5.7% and 3.7% lower values for rDmean and rD10cc respectively. Less inclusion of bladder in BEV of the two anterior noncoplanar oblique beams translated a 14% and 5.1% reduction in bladder mean dose and bD30cc respectively. Conclusion: The VRS effectively concluded deliverable noncoplanar beam arrangements for prostate IMRT with resultant improvement of dose characteristics. Further development of VRS may present future RTPS.
TH‐C‐351‐01: Design of the Washington University Small Animal Conformal Micro Irradiator
Authors: Izaguirre EW , Diao X , Stojadinovic S , Mutic S , Khullar D , Deasy J , Parikh P , Hope A , Grigsby P , Low DA
Published: July 2008

Purpose: To design a conformal small animal micro radiation therapy (microRT) instrument, consisting of a cone beam microCT subsystem for submillimeter low dose structural imaging and image guided radiotherapy and an orthovoltage conformal micro irradiator with high radiation dose rate for high throughput conformal irradiation. Method and Materials: The microCT subsystem is based on an 80kVp micro‐focus x‐ray source with 75×75 um2 focal spot and a flat panel amorphous silicon detector with 1024×1024 pixels. The microRT subsystem design utilizes a 320kVp orthovoltage source with dual focus spots (0.4×0.4 mm2 at 800W and 1×1 mm2 at 1800 W). The orthovoltage beam is collimated using two orthogonal jaws and exchangeable apertures. The resolution of the cone‐beam micro CT and the therapy beam spatial precision was determined using a numerical model. The microCT radiation dose, the orthovoltage source spectral output, and dose rate were evaluated using a mouse digital phantom (MOBY) and a pencil beam algorithm. Results: microCT reconstructed tomographic data with a resolution of 125 μm is achievable using 128 projections and a maximum radiation dose of 2cGy. Automatic animal positioning and handling is performed within a precision of 100 μm. The treatment beam can be aimed at different latitude and longitude angles in steps of 2 arc min. and translated at 50μm steps (x,y,z). The beam cross section can be modulated with submillimeter precision using steps of 50μm. A radiation dose rate of 40 Gy/min is delivered when the system is operated at an average half‐value layer of 4.6 mm Cu and a maximum beam penumbra of 0.35 mm. Conclusion: We designed and numerically evaluated the performance of a microRT system integrated with a microCT for conformal radiotherapy of murine animal models.

This work supported in part by NIH grant CA108677.
TH‐C‐351‐02: Registration Based Automatic Segmentation and Wall Motion Analysis for 4D Cardiac Micro‐CT in Mice
Authors: Schreibmann E , Badea C , Fox T
Published: July 2008

Purpose: We present an automated method for segmentation and tracking of hearth anatomy using 4D cardiac micro‐CT datasets in mice. Cardiac motion is determined and analyzed using the deformation field obtained after non‐rigidly deforming a template to all the time frames of the 4D dataset. Method and Materials: 4D micro‐CT image sets were acquired with a temporal resolution of 10 ms and 100 microns spatial resolution in four mice using the Duke in‐house developed micro‐CT scanner and cardio‐respiratory gating. Ten cardiac phases were used in the data acquisition. The images of the heart at different phases were registered using a BSpline deformable model. The contour points on the diastolic phase were automatically mapped to the corresponding points on the images of other phases following the mapping relation established by the deformable model. The method's stability to noise and artifacts in the input images was assessed using the 4D virtual mouse Moby phantom. Results: The deformable model was capable of accommodating significant variability of cardiac motion over time and across different individuals. The template was warped to the first phase of the 4D dataset with an accuracy of 0.95, 0.96 and for the left ventricle, myocardium and right ventricle as compared by the Hausdorff measure to manual segmentation. In between phases, worst segmentation accuracy was 0.93, 0.95 and 0.94 for left ventricle, myocardium and right ventricle. Additionally, the method automatically measures regional motion and deformation by probing the deformation field on the segmented contours. Conclusion: Heart wall contour evolution in 4D micro‐CT images can be easily defined and tracked using a BSpline deformable model, with no user interaction required. The method provides pre‐clinical accuracy while eliminating the labor‐intensive segmentation procedure.

Image acquisition was performed at the Duke Center for In Vivo Microscopy an NCRR/NCI National Resource (R21‐CA124584‐01, 2U24‐CA092656, P41‐RR005959).
TH‐C‐332‐02: First Implementation of High‐Resolution Dual‐Detector Region‐Of‐Interest Cone‐Beam Computed Tomography (ROI‐CBCT) for a Rotating C‐Arm Gantry System
Authors: Patel V , Ionita C , Keleshis C , Sherman J , Hoffmann K , Bednarek D , Rudin S
Published: July 2008

Purpose: Region‐of‐interest cone‐beam computed tomography (ROI‐CBCT) has the potential to reduce integral dose while providing higher‐resolution data in the ROI. Previous work used rotating object geometries. We have implemented an ROI‐CBCT system on a clinical rotating C‐arm gantry system and have developed methods for combining high‐resolution projection data within the ROI and low‐resolution, low‐dose data outside the ROI. Method and Materials: High‐resolution ROI projection data were acquired of a coronary stent placed in a rabbit using a custom‐made, high‐sensitivity, microangiographic‐fluoroscope (HSMAF) detector (35 micron pixels), attached to the C‐arm of a clinical fluoroscopic gantry. Full field‐of‐view (FFOV), low‐resolution data were acquired at the same dose as the HSMAF (standard‐dose) and at a lower dose using a commercial flat‐panel detector (FPD) (194 micron pixels). HSMAF data were spatially registered with FPD data using cross‐correlation techniques. The lower‐dose FPD pixel values were matched to the HSMAF values using linear regression. During reconstruction, HSMAF data were used within the ROI, while corrected lower‐dose FPD data were used outside the ROI. Reconstructions were performed with HSMAF/lower‐dose‐FPD and standard‐dose‐FPD data using geometric calibration data, Parker weights, a Shepp‐Logan filter, and a Feldkamp algorithm, generating 512 × 512 × 512 volumes (25 micron voxels). Results: The dual‐detector ROI‐CBCT reconstruction exhibits greater detail than that of the FPD alone. The full‐width‐at‐half‐maxima of line profiles of stent struts (100 micron diameter) are approximately 150 and 290 microns for the dual‐detector and FPD data, respectively. No truncation artifacts are visible. The integral dose of the dual‐detector system is 54% that of the standard‐dose FPD for this experiment. Conclusion: High‐resolution 3D images of stents implanted in a rabbit are successfully obtained using a new rotating‐gantry dual‐detector ROI‐CBCT system. Resolution is significantly higher and integral dose is lower with ROI‐CBCT compared to the standard FPD approach.

(Support: NIH‐Grants R01‐NS43924, R01‐EB002873, Toshiba‐Medical‐Systems‐Corporation).
TH‐C‐332‐06: Optimization and Calibration Procedures of a Contrast‐Medium Based Subtraction Technique in Digital Mammography
Authors: Rosado‐Méndez I , Palma BA , Villaseñor Y , Benítez‐Bribiesca L , Brandan ME
Published: July 2008

Purpose: To optimize radiological and clinical parameters for the application of a contrast‐medium‐based subtraction technique in digital mammography and to evaluate its clinical feasibility using a commercial mammography unit without any external or internal hardware modification. Method and Materials: This is a two‐stage project. Firstly, an extension of Lemacks' analytical formalism was implemented in order to maximize contrast‐to‐noise ratios (CNR) in simulated applications of dual‐energy and temporal subtraction modalities, and combinations of them. The formalism was validated by imaging tubular structures with an iodine‐based contrast medium embedded in a PMMA phantom. Once two optimized techniques were defined, calculations were performed to obtain exposure values limiting the total glandular dose to 2.5 mGy in a dynamical study of 1 mask + 3 post‐contrast images. Secondly, these parameters were used to calibrate the gray level in subtracted images as function of contrast medium iodine concentration contained in a multi‐well PMMA‐phantom. Optimized subtraction techniques are being applied in a clinical study which shall include 20 patients. Results: Experimental CNR results surpass Rose's criterion (CNR=5) and validate the predicted advantage of temporal techniques. Dual energy temporal subtraction, with iodine administered in the low energy image arises as the optimum subtraction technique, instead of the predicted advantage of contrast medium administration during the high‐energy acquisition. With respect to the calibration, relations between CNR and iodine concentration were found to be approximately linear beyond 8 mg/ml, although for lower concentrations, CNR is undetectable. Conclusion: The advantage of temporal over dual energy subtraction in terms of CNR has been validated, particularly of dual energy temporal subtraction. Furthermore, concentrations below 8 mg/ml do not satisfy the detection criterion.
TH‐D‐AUD A‐01: Optimizing the Laser Parameters in a Simulated Laser‐Proton Accelerator
Authors: Veltchev I , Fourkal E , Ma C
Published: July 2008

Purpose: To identify new ways of increasing the proton energy from a laser driven accelerator without increasing the laser power. To design an optimized interaction geometry regarding target positioning and laser energy delivery, that will result in more energetic protons. Method and Materials: Fully relativistic 2D3V particle‐in‐cell (PIC) simulations are used in this study. The initial conditions are chosen to correspond to a real experiment with a 40fs laser pulse (λ=800nm) and energy in the pulse in the range 4J–7J, focused to 2.8–3.4μm. The loading of the laser pulse in the simulation is adequately controlled in order to study the influence of angle of incidence and wave‐front curvature on the proton acceleration. The target is Cu with thickness of 400nm and width of 10μm. A thin proton layer attached to its back surface. Results: When the laser beam is focused on the target optimum incidence angle is found at ∼30° angle for a 21% gain in proton energy compared to normal incidence. When the laser pulse is split in two and both sub‐pulses are focused on the target at opposite angles (+30° and − 30°) the proton energy is increased and reaches maximum for equal splitting for a total energy gain of 42%. Positioning the target exactly one Rayleigh range behind the beam's waist is found to be beneficial as well. This position of the target corresponds to a wave‐front with the lowest positive radius of curvature. Conclusion: The combined optimization of angle of incidence, pulse splitting, and wave‐front curvature leads to energy gain between 65% and 140% compared to normal incidence for two realistic experimental situations. Increasing the proton energy by such a significant amount without increasing the energy in the laser pulse can prove to be the way to reach the therapeutic range of proton energies.
SU‐GG‐T‐299: An Evaluation of Solid Phantoms for the Dosimetry of Low Energy Photon Beams
Authors: Hill R , Kuncic Z , Baldock C
Published: July 2008

Purpose: To evaluate water equivalency of solid phantoms for the dosimetry of low energy photon beams. Method and Materials: The physical properties and composition of the solid phantoms studied were taken from the IAEA TRS398 dosimetry protocol. Doses were calculated using the PENELOPE Monte Carlo code V2006 for photon beams in the energy range of 50 to 280 kVp. The incident photon spectra were calculated using an analytical method (Ma and Seuntjens 1999). The following was calculated: (1) central axis depth doses through the phantoms and (2) dose to a small voxel of water located at the surface of the solid phantom. The number of incident particles was selected to provide statistical uncertainties of less than 0.5%. Results: For the depth dose calculations, the greatest variation occurred using the 50 kVp x‐ray beam. The depth doses measured in white polystyrene (RW3) and Plastic Water had the greatest variation of up to +14% and −19% respectively as compared to water. For the dose to the water surface voxel, the changes in dose varied between −4.5% to 10.6% for the 50 kVp x‐ray beams with the polystyrene and RW3 solid phantoms giving the worst results. In comparison, the doses measured in the RMI457 Solid Water were in excellent agreement with doses to water, giving differences of less than 2%. Conclusion: We have demonstrated that some solid phantoms can give significant dose differences if used for dosimetry of low energy photon beams. This occurs whether the solid phantom is used for depth dose measurements or for providing backscatter material. These differences should be quantified before clinical use of the solid phantom as a substitute for water.
Poster — Thurs Eve‐39: Full 3D dose calculation for total body irradiation: A comparison study between treatment planning systems in homogeneous and heterogeneous conditions
Authors: Lavallée M‐C , Gingras L , Aubin S , Côté C , Larochelle M , Chrétien M , Beaulieu L
Published: July 2008

Purpose: To perform full 3D heterogeneous dose calculations for total body irradiation (TBI) cases and compare different treatment planning softwares (TPS). Method: A retrospective study was performed on 7 patients. Dose distributions obtained with Pinnacle3 v.7.9u (Philips Medical Systems) were compared with the ones calculated using our actual TBI planning system Theraplan Plus (TPP) by MDS Nordion/Nucletron. Two different Pinnacle3 models were studied: standard beam commissioning (std_Pinnacle3) and TBI commissioning (TBI_Pinnacle3). For the later case, commissioning was adapted for the special TBI conditions (extended SSD of 190cm, large field, acrylic beam spoiler, and out of field dose (OFD)). Results: Significant differences are found between the TPP, std_Pinnacle3 and TBI_Pinnacle3 dose distributions. For the relative mid‐line doses, differences up to 12% were observed. Systematic overestimations of 5% were found in patients extremities between TPP and TBI_Pinnacle3. Average dose underestimation of 3% was observed between std_Pinnacle3 and TBI_Pinnacle3. Differences in patient extremities are attributed to the OFD contribution which is not correctly computed in TPP and std_Pinnacle3. Dose comparison outside the patient's mid‐line showed greater differences (up to 20%) between models. Accurate 3D heterogeneous dose calculations with TBI_Pinnacle3 model show major differences (homogeneous versus heterogeneous) in high and low density regions. Dose overestimation of 5% was observed in bony regions and dose underestimation of 5% to 10% was observed in lung regions. Conclusion: Those results are of major interest since they show a strong dependence of the dose calculation outcome on both TPS and commissioning used, potentially leading to significant dose misevaluation.
Poster — Thurs Eve‐43: Verification of dose calculation with tissue inhomogeneity using MapCHECK
Authors: Korol R , Chen J , Mosalaei H , Karnas S
Published: July 2008

MapCHECK (Sun Nuclear, Melbourne, FL) with 445 diode detectors has been used widely for routine IMRT quality assurance (QA)1. However, routine IMRT QA has not included the verification of inhomogeneity effects. The objective of this study is to use MapCHECK and a phantom to verify dose calculation and IMRT delivery with tissue inhomogeneity. A phantom with tissue inhomogeneities was placed on top of MapCHECK to measure the planar dose for an anterior beam with photon energy 6 MV or 18 MV. The phantom was composed of a 3.5 cm thick block of lung equivalent material and solid water arranged side by side with a 0.5 cm slab of solid water on the top of the phantom. The phantom setup including MapCHECK was CT scanned and imported into Pinnacle 8.0d for dose calculation. Absolute dose distributions were compared with gamma criteria 3% for dose difference and 3 mm for distance‐to‐agreement. The results are in good agreement between the measured and calculated planar dose with 88% pass rate based on the gamma analysis. The major dose difference was at the lung‐water interface. Further investigation will be performed on a custom designed inhomogeneity phantom with inserts of varying densities and effective depth to create various dose gradients at the interface for dose calculation and delivery verification. In conclusion, a phantom with tissue inhomogeneities can be used with MapCHECK for verification of dose calculation and delivery with tissue inhomogeneity.
Sci‐Fri PM: Planning‐05: Saving trees and improving workflow
Authors: Angers C , Renaud J , MacPherson M , Clark B
Published: July 2008

Patient treatment preparation is a multi‐step process requiring inputs from a range of disciplines and technologies. Our centre generates just over 4500 treatment plans annually and operates from two main campuses. A large, split program presents unique challenges during treatment preparation and has provided the impetus for a completely electronic treatment process. Throughout 2006 and 2007, we migrated our external beam treatment planning to the Computerized Medical Systems (CMS) product line. Utilizing a thin‐client architecture, CMS supports distributed (multi‐site) planning. Coincident with the treatment planning upgrade, IMPAC Multi‐Access was configured to provide a paperless and filmless treatment record and electronic patient workflow. Standardized treatment objectives were also implemented in the form of site‐group approved care plans. Details of the pretreatment process and the CMS / IMPAC implementation will be presented as well as a workflow time analysis. To date, treatment preparation times have been reduced by 25% (2.5 days) as a result of workflow improvements, representing a clear benefit to both staff and patients.
Sci‐Fri PM: Planning‐07: A low diffusion radiochromic gel dosimeter for three‐dimensional radiation dosimetry
Authors: Babic S , Battista J , Jordan K
Published: July 2008

Purpose: To develop a low diffusion radiochromic leuco crystal violet (LCV) hydrogel utilizing micelles, for three‐dimensional (3‐D) radiation dosimetry. Method and Materials: Concentrations of LCV dye, Triton X‐100 and trichloroacetic acid were varied to determine the optimal gel sensitivity for optical computed tomography (CT). Using a laser optical CT scanner at λ = 594 nm, diffusion rate measurements were performed on half‐irradiated (6 MV x‐rays) cuvette gel samples made with and without surfactant, respectively. A cylindrical 1 L gel volume was irradiated with a 12 MeV electron beam (Varian Clinac 2100C) to a dose of 30 Gy and scanned with cone‐ beam optical CT at λ ∼ 590 nm (Vista™, Modus Medical Devices Inc.). Results: The most radiation sensitive gel formulation was found to be: 1 mM LCV, 4 mM Triton X‐100, 30 mM trichloroacetic acid and 4% gelatin. The diffusion rates of a LCV gel without and with surfactant present were about 2 and 20 times lower than the Fricke xylenol‐orange gel system, respectively. Comparison of the central axis gel attenuation coefficients normalized at depth of maximum dose (dmax) with TG21‐corrected ion chamber data, were in agreement, thus, indicating energy and dose‐rate independence. Conclusion: Radiochromic LCV micelle gels show minimal diffusion effects and a dose response that is linear, energy and dose‐rate independent. Optical CT scanned LCV micelle gels are a promising system for 3‐D dose verification. Conflict of Interest: Two of the authors (JB, KJ) have a licensing agreement with Modus Medical Devices Inc. concerning the commercialization of Vista™.
Sci‐Fri PM: Planning‐08: Zero diffusion radiochromic genipin‐gelatin dosimeter
Authors: Jordan K
Published: July 2008

Purpose: To develop a zero diffusion radiochromic hydrogel by cross‐linking gelatin polymers with genipin, for three‐dimensional (3‐D) radiation dosimetry. Method and Materials: Gelatin and genipin were dissolved in distilled water and continuously stirred at 40C for thee hours and then sulfuric acid was acid prior to placing gel in containers. Concentrations of genipin, gelatin and sulfuric acid were varied to determine the optimal gel sensitivity for optical computed tomography (CT). Photon beams (4 MV x‐rays, 1×1, 2×2, 3×3 cm) irradiated a 10 cm diameter cylinder of gel. Using a cone beam optical CT scanner at λ = 590 nm, zero diffusion was observed by recording sequential 3D scans separated by 18 hours. Additional evidence was provided by sequential transmission photographs of a cuvette containing a sample that had been exposed to a 0.8 mm diameter, 1 mW, 594 nm, He‐Ne laser beam and photobleached. Results: Radiochromic bleaching is a fast process, complete within seconds of termination of irradiation. Dose sensitivity increases with cross‐ linking density and sulfuric acid concentration. The initial optical density limits dose sensitivity, requiring approximately 30 Gy for 10 cm diameter samples to have reasonable contrast. More sensitive but darker gels require smaller samples. A comparison of profiles through 3D reconstructed dose distribution showed identical profiles for a time interval of 18 hours. Visually the sample maintained this image for six months when stored at 4C in a dark refrigerator. Transmission photographs of the laser beam image recorded by photobleaching remained constant when stored a 23 C in the dark for ten months. Conclusion: Radiochromic genipin gelatin gels form a permanent and stable image when protected from bright visible light. This result is a new, non‐toxic material for 3D dosimetry.
Sci‐Fri PM: Planning‐09: An EGSnrc investigation of ion chamber response to Co‐60 beams
Authors: La Russa DJ , Rogers DWO
Published: July 2008

The EGSnrc Monte Carlo code was evaluated for its ability to calculate the relative response of a variety of ion chambers to Co‐60 beams as a means of justifying the use of this code in future investigations of cavity theory. EGSnrc calculations were compared to measurements with four separate ion chambers, which were each configured with several wall materials (ranging from plastic to lead) and cavity sizes (or cavity air pressures). The experimental results included measurements by Nilsson et al. in 1992, and experiments by Whyte, Attix et al. and Cormack and Johns in the mid‐to‐late 1950's designed to evaluate Spencer‐Attix cavity theory. Experiments by Whyte involved measurements of the response per unit mass as a function of cavity air pressure for a large cylindrical chamber, whereas the other experiments consisted of measurements of the response per unit mass (or ionization current) as a function of the distance between the front and back wall (cavity height) of a plane‐parallel chamber. EGSnrc calculations, which could account for the change in response associated with changes in wall material in most cases, were generally within 1–3% of experimental values, even for experimental data that required calculations of unreported wall corrections determined using experimental techniques. The ability of EGSnrc to accurately model these experiments, which showed variations up to 300%, confirms its suitability for detailed Monte Carlo studies of cavity theory.