bluuebunny/crossref_metadata_2025 huggingface dataset:
MO‐FF‐A3‐04: Biological Effects of Variable RBE in Different Proton Therapy Delivery Modalities
Authors: Mirkovic D , Titt U , Sawakuchi G , Mohan R
Published: August 2010

Purpose: To evaluate and compare dosimetric effects of variable Relative Biological Effectiveness (RBE) in scanning and passive scattering proton therapy treatments. Method and Materials: Three prostate treatment plans for an actual patient treated in our clinic were simulated using a Monte Carlo code MCNPX. Two Intensity Modulated Proton Therapy (IMPT) and one passive scattering (PSPT) treatment plan in a typical, two‐field, lateral, parallel‐opposed configuration were created using a computed tomography (CT) scan of the patient and a commercial treatment planning system (Varian Eclipse). The plans were simulated using the Monte Carlo code MCNPX scoring the total energy deposition and the fluence inside the patient volume using a three‐dimensional mesh tally. The energy deposition and the fluence were used to compute the total unrestricted stopping power, or LET. A space‐variable RBE as a function of LET, dose, and tissue type was calculated using a phenomenological model developed by Wilkens and Oelfke. Results: The RBE‐weighted dose was compared for three treatment plans for constant and variable RBE and significant differences were observed inside the treatment volume. While the three‐dimensional LET distribution in PSPT is relatively easy to predict with simple models, the LET distribution in IMPT can be much more complex and may require a detailed MC simulation in order to be estimated accurately. Conclusion: This example illustrates how very similar physical doses may have significantly different biological effects depending on the modality and the beam configuration of the treatment plan. This approach can be used for biologically based treatment planning and to explain some unexpected treatment toxicities in proton therapy patients.
MO‐FF‐A3‐05: Dosimetric Interplay Effects for Proton Uniform Scanning Lung Treatments
Authors: Flampouri S , Slopsema R , Su Z , Ho M , Li Z
Published: August 2010

Purpose: To investigate the dosimetric impact of respiratory motion on proton lung treatments delivered by uniform scanning. Method: Uniform scanning is an aperture and compensator based technique in which the pristine energy layers are delivered sequentially (‘energy stacking’). Because of high scanning frequency (3Hz) compared to breathing motion, the study ignores lateral effects and focuses on interplay between energy stacking and tissue motion/deformation. This exercise is based on actual lung patient plans that, however, were delivered by simultaneously irradiating the whole SOBP. To simulate the breathing motion the full SOBP dose is recalculated on all 10 4DCT phases. To simulate the energy stacking, the per phase dose distributions are deconvolved into energy layers with appropriate weighting. Based on delivery parameters such as total irradiation time (doserate), time offset, and switching times and patient specific parameters like average breath rate, a series of dose distributions belonging to various breathing phases and energy layers are identified to constitute a treatment. For total dose accumulation deformable registration methods are used to deform all dose maps to a mid‐exhale phase. The interplay effect of breathing motion and energy stacking is deduced by comparison of the total dose distributions over multiple breathing cycles for both all‐energies and by energy layer deliveries. Results: No dramatic discrepancies on beam dose distributions produced by the two methods are observed. For a doserate of 2Gy/min and a respiratory rate of 20bpm the PTV coverage, maximum dose, and lung mean dose differences are within 2, 2.5, and 1.5% of prescribed dose (per beam). Doserate is the sole parameter that significantly influences the interplay effect which becomes insignificant for values below 0.5Gy/min. Conclusions: Based on the cases that we applied our uniform scanning evaluation method we conclude that lung treatment can safely be delivered by uniform scanning for low doserates.
TU‐A‐BRA‐05: An Algorithm for Automated Determination of IMRT Objective Function Parameters
Authors: Vaitheeswaran R , Sathiya Narayanan V.K. , Bhangle Janhavi , Nirhali Amit
Published: August 2010

Purpose: A new algorithm for automated determination of Objective Function Parameters (OFPs) in inverse planning is proposed. Method and Materials: While in theory, optimization in inverse planning is a one‐step automatic process, in practice, planner intervention is often required to carry out a multiple trial‐and‐error process where several parameters are sequentially varied until an acceptable compromise is achieved. We propose an algorithm for automated determination of IMRT Objective Function Parameters (OFPs). The algorithm is based on a new approach called “Adapted Dose Prescription (ADP)” wherein the dose prescriptions are automatically tailored to the sensitivity of target and OARs, which immediately results in a treatment plan meeting the clinical goals. The sensitivity of a structure is estimated by calculating the difference between the expected and obtained dose values after the end of an optimization trial. We incorporated the proposed algorithm with Fast Simulated Annealing (FSA) scheme using MATLAB software package to generate Aperture‐based IMRT plans for various complex patient cases. The beam placements, aperture shaping and dose calculations were done using CMS XiO planning system in our clinic. Results: So far, three patients planned using the proposed algorithm has been treated in our clinic. Our observation is that the algorithm automatically fetches a set of OFPs that immediately results in a clinically acceptable dose distribution. This approach significantly reduces the time taken for optimization by reducing the no. of optimization trials, while providing dose distribution that is comparable to that of plans obtained using KonRad inverse planning system. Conclusion: The proposed algorithm facilitates the production of inverse solutions which, without the planner's intervention, precisely satisfy the specified constraints. Moreover, the results demonstrate that the proposed algorithm can be effectively used for clinical applications.
TU‐A‐204B‐01: The CBCT Performance of a New Treatment Platform
Authors: Oelhafen M , Munro P , Berkus T , Hassanein R , Nagy T , Graf A , Star‐Lack J , Sun M , Pavkovich J , Scheib S , Kunz P
Published: August 2010

Purpose: To characterize the CBCT performance of a new radiation therapy platform (Trilogy MX). Methods: An entirely new CBCT system has been developed for Trilogy MX. The new CBCT system differs from that of the On‐Board Imager® (OBI) in the use of a beam hardening filter, which reduces patient dose, and an improved reconstructor, which uses scatter correction algorithms to account for the x‐ray scatter caused by the cone‐beam geometry. Scans of Catphan® and electron density (Model 062A, CIRS) phantoms have been compared with OBI scans. Hounsfield unit (HU) accuracy was checked by changing the z scan length and the phantom diameters for pelvis (125kVp, 680mAs, 45cm dia.) acquisitions. The Catphan was imaged using doses ranging between 5–20mGy (CTDIw). Projections acquired using clinical OBI units were also reconstructed for comparison using the new reconstructor. Results: The new CBCT system has higher dose efficiency and higher HU accuracy. When the volume length is reduced from 160mm to 90mm or when the phantom diameter is reduced from 330mm to 180mm, the HU values measured for the same inserts differ by −190 to +80HU for OBI scans and by –60 to +50HU for Trilogy MX scans (for electron densities between 0.2 and 1.2). The contrast detectability of the 1% contrast objects in the Catphan phantom improves from 9mm to 4mm diameter when using the same CTDIw dose as OBI. Clinical images exhibit much better uniformity, elimination of streaks and better definition of the skin surface. Conclusions: The new reconstruction algorithm makes substantial improvements in CBCT image quality, reduces patient dose and increases HU accuracy. The new system produces CBCT images, which are much better suited to image guidance and which may be suitable for other tasks such as adaptive RT planning.
TU‐B‐BRA‐03: Pros and Cons of Flattening Filter Free IMRT: A Comparison with Conventional IMRT with Flattened Beams
Authors: Wang L , Mok E , Xing L
Published: August 2010

Purpose: A new Varian machine, Trilogy MX with Flattening Filter Free (FFF) delivery mode at 6 MV and 10 MV, was installed at our institution. This study evaluates the performance of IMRT treatment planning with FFF delivery mode. Method and Materials: The FFF delivery mode of our Trilogy MX is calibrated to deliver 1cGy/MU at the center of the 10×10cm2 field. A number of patients who were treated previously using conventional IMRT with flattened beams were re‐planed on the new machine with the new FFF beams. Both types of plans were done using Eclipse planning system. Step‐and‐shoot mode with 10 intensity levels was used for dose delivery. The isodose distribution, dose volume histogram (DVH), total MUs and total number of segments of the two IMRT plans for each patient were compared. Results: IMRT treatment plans with FFF were found to be similar compared with conventional delivery for all the three tumor sites. For head and neck cases, slight degradations in dose coverage to tumors were observed, but they may not be clinically significant. For lung and prostate cases, plans with and without flattening filter are almost identical. However, total MUs are much larger for FFF delivery for all the cases due to the non‐flatness of the dose profile. MUs for FFF mode are 10–16% higher for lung and prostate cases, and more than 25% higher for head and neck cases. As much as 10% increase in number of segments was observed in head and neck cases. Conclusion: The quality of IMRT treatment plans with FFF delivery is comparable to that of the conventional delivery mode, but treatments of large‐sized and complicate‐shaped tumors require more MUs. Treatments of small‐sized tumors benefit more from the use of FFF delivery. Conflict of Interest: Research funding support from Varian Medical System
TU‐B‐BRA‐04: Practical Considerations for the Novel Multiple Adaptive Planning Strategy for Patients Concurrently Treated with the Prostate and Pelvic Lymph Nodes
Authors: Qi P , Pouliot J , Roach M , Xia P
Published: August 2010

Purpose: Our previous work presented the feasibility of a novel adaptive strategy, multiple adaptive planning (MAP), for the concurrent treatment of the prostate and lymph nodes. The question raised for this technique is how many plans can adequately account for multi‐directional movements of the prostate. In this study, we provide a guideline for the minimum number of plans required for clinically implementing the MAP technique. Methods and Materials: Data from five patients with high‐risk prostate cancer, who have undergone IMRT, were selected for this study. Without including lateral prostate shifts, combinations of anterior/posterior and superior/inferior shifts of 0 cm and 0.5 cm were considered. Correspondingly, a total of 9 IMRT plans were prepared for the following treatment. Double Image registrations between daily MVCBCT and the planning CT were conducted to identify independent prostate shifts. Based on this information, a most suitable plan was retrospectively selected and recalculated using the daily CBCT images. For comparison, the corresponding iso‐center shifting plans were also created. Results: For 20 fractions from 5 patients, prostate shifts in the range between 0.4 cm and 0.8 cm, were observed in 13 fractions. Of these fractions, conventional iso‐tracking method would result in adequate dose coverage of the lymph nodes in only 37% days compared to 100% days in MAP strategy. Both strategies achieved 95% of the prostate receiving a daily dose > 97% of the prescription dose in 11, 12 fractions, respectively. Furthermore, the average D50 of the rectum and bladder in the iso‐tracking plans, 29.9 Gy and 31.5 Gy, were higher than those, 28.9 Gy and 28.9 Gy, in the MAP plans. Conclusion: Using of the MAP technique with 9 pre‐created plans, which accommodate for independent prostate shifts, can achieve our treatment goals for the treatment of high‐risk prostate cancer. Research partly sponsored by the USAMRMC (PC073349).
TU‐B‐204B‐04: Ventilation from Four Dimensional Computed Tomography: Density versus Jacobian Methods
Authors: Castillo R , Castillo E , Martinez J , Guerrero T
Published: August 2010

Purpose: Two calculation methods to produce ventilation images from four‐dimensional computed tomography (4D CT) acquired without added contrast have been reported in the literature. In previous work, we demonstrated a method to obtain ventilation images using deformable image registration and the underlying CT density information. A second method performs the ventilation image calculation from the DIR result alone, using the Jacobian determinant of the deformation field to estimate local volume changes resulting from ventilation. Two implementations of the Jacobian‐based methodology are evaluated, as well as a density change‐based model for calculating the physiologic specific ventilation from 4D CT. Materials and Methods: 99mTc‐labeled aerosol single photon emission computed tomography (SPECT) is the standard clinical method used to obtain ventilation images in patients. The distributions of ventilation obtained from the CT‐based ventilation image calculations are compared with those obtained from the clinical SPECT ventilation. The standard method is prone to airway deposition artifacts, which are further characterized. Seven patients with 4D CT imaging and standard 99mTc‐labeled aerosol SPECT/CT ventilation imaging obtained as part of a prospective validation study were selected. Results: Results demonstrate the equivalence of the Jacobian‐based implementations for quantifying the specific ventilation on a voxel scale. Additionally, both Jacobian‐ and density change‐based methods correlated well with global measurements of the resting tidal volume. Finally, correlation with the clinical SPECT was assessed using the Dice Similarity Coefficient, which showed significantly higher (p‐value < 10−4) correlation with density change‐based specific ventilation than with either Jacobian‐based implementation. Conclusion: Both 4D CT ventilation methods performed well globally, showing good correlation with image segmentation‐based measures of the resting tidal volume. However, CT density change‐based calculations achieved better spatial correlation with clinically acquired SPECT images. Future work will directly address the impact of spatial registration errors on degradation of both density change‐and Jacobian‐based ventilation methods.
TU‐C‐BRA‐02: Radiotherapy Plan Integrity Verification with Dynamic Pinnacle Scripts
Authors: Yang D , Moore K
Published: August 2010

Purposes: The purpose of this study was to automate elements of the quality control (QC) necessary for IMRT plan generation process. Physicists often review all dosimetrist‐generated IMRT plans prior to physician approval with the aim of detecting sub‐optimal or technically undeliverable plans. We report here an automated suite of QC tools designed to carry out a majority of these laborious physics verification tasks efficiently and accurately within the Pinnacle treatment planning system (TPS). Method: The IMRT plan integrity verification tool is implemented as a combination of Pinnacle scripting routines and Perl programs which collect/derive the plan data, compare the data against pre‐defined physical and dosimetric rules, and report the results of the logical comparisons to the user. Important technical details include (a) a unique Perl program library which has been created to allow easier processing of Pinnacle data and (b) a dynamic script generation method which was developed to allow features and flexibilities of Perl programming for plan data processing inside Pinnacle. This QC tool is able to check many rules in the categories of patient setup, contouring, beams, dose calculation, IMRT optimization and dose distribution. Results and conclusion: This tool has been in routine clinical use for several months, and initial results have shown that it improves both the efficiency and accuracy of physicists and dosimetrists, virtually eliminating some errors commonly encountered prior to the tools introduction. While this technique cannot be extended to all elements of the IMRT QC process, it has been adapted to include elements such as IMRT objective conflict analysis and IMRT cost function ranking which would be time‐consuming, if not impossible, for a human to manually verify. Further, the dynamic Pinnacle script method which facilitated the IMRT QC could also be expanded to other radiotherapy QC including 2D, 3D conformal, and brachytherapy planning.
TU‐C‐BRA‐11: Dose Calculation Accelerating: A Comparison Study of GPU and FPGA Based on Collapsed Cone Algorithm
Authors: Zhou B , Yu C , Chen D , Hu X
Published: August 2010

Purpose To provide a fast and practical solution to the dose calculation problem, two implementations of the collapsed cone convolution/superposition (CCCS) algorithm based on different technologies: Graphics Processing Unit (GPU) and Field Programming Gate Array (FPGA), are developed and evaluated. Method and Materials GPU‐based approach and FPGA‐based approach have been two most favorable design choices for application acceleration. However, the answer to the question of which one is better is strongly dependent on the target application domain and the problem's computation characteristics. To address this issue, we have implemented both approaches and evaluated them under the same metrics. Both solutions have been thoughoutly optimized to ensure the results reach the limit of hardware. Results Both implementations are compared with a commercial multi‐threaded implementation running on an Intel quad‐core computer with 2.4GHz frequency and 4GB memory. The performance data are collected for different phantom sizes and field sizes. Although the memory technology of the FPGA board is several generations behind the GPU, it still provides same level of speedup. For all test cases, the FPGA board showed a speedup in the range of 21.37–24.26X and the GPU solution (based on GTX260) demonstrates 12.90–16.33X speedup. Some other important design issues, such as cost, system specification, and design efforts are also compared. The anticipated speedups for both platforms are also provided based on the current technology. Conclusion Our results have shown that both implementations achieved significant speedup over a multi‐threaded software implementation on a Quad‐core system. Although FPGA still outperforms GPU in terms of performance, the merits of GPU such as low‐cost and off‐the‐shelf availability make it a preferred solution for many scenarios. Ultra‐high‐performance scenarios would prefer the FPGA solution as it provides a compact and powerful computation engine within a reasonable budget.
TU‐C‐204B‐01: Reduce Patient Dose and Improve Image Quality in CBCT for Image Guided Radiotherapy
Authors: Jin J , Ren L , Kim J , Movsas B , Chetty I
Published: August 2010

Introduction: We have demonstrated that cone beam CT (CBCT) image quality was substantially improved in terms of eliminating scatter related artifacts and enhancing contrast‐to‐noise ratio, using a 1D grid placed between the x‐ray source and the imaging object. The grid serves to physically reduce scatter while also enabling measurements of scatter simultaneously for post‐scan scatter correction. A full set of CBCT images can be generated from the partially‐blocked imaging data using either one‐ or dual‐rotation modes. Using one‐rotation has particularly the advantage of reducing imaging dose by a factor of two in comparison to conventional CBCT. The tradeoff is that longitudinal resolution is degraded. This study evaluates the feasibility of using CBCT images from the single‐rotation mode for image‐guided radiotherapy (IGRT). Methods and material: A lead grid of approximately 1‐mm septa width and interspace was placed at a source‐to‐grid distance of approximately 30‐cm in a Varian Trilogy CBCT system for the single‐rotation mode. An anthropomorphic pelvis phantom was used for the test. Various steps in imaging processing were performed for each projection, including measuring the scatter distribution, performing scatter correction, deriving imaging information in the grid penumbral regions by dividing the projection with a blank scan, and interpolating the remaining blocked information. Rigid body image fusion was performed for the reconstructed CBCT images with the simulation CT, and compared with CBCT images from the two‐rotation and conventional CBCT. Results: The single‐rotation CBCT images showed substantially reduced scatter‐related artifacts. Despite the degradation in longitudinal resolution, the visibility of anatomic structures (an important requirement of IGRT) was not significantly affected. The auto‐registration error was 0.8±0.3 mm in longitudinal direction, and was negligible in AP and lateral directions and three rotational directions. Conclusion: CBCT images from the single‐rotation mode can achieve reasonable longitudinal resolution for anatomic structure visualization and accurate auto‐registration in IGRT.
TU‐C‐204B‐02: Real‐Time Laser Source Tracking for Pleural PDT Treatment Planning System
Authors: Chang C , Zhu TC
Published: August 2010

Purpose: One of the most important factors determining PDT efficacy is the spatial homogeneity of light fluence rate distribution. This light uniformity requirement is especially important for treatments involving cavities, where light source moves in real‐time. For non‐small cell lung cancer, PDT has proven to be effective but its efficacy depends on the uniform light delivery to the pleural surface, which relies solely on the surgeon's balanced movement of the laser source. This study therefore aims to develop a real‐time dosimetry system capable of ensuring homogeneous light fluence delivery for pleural photo dynamic therapy (PDT). Method and Materials: An infrared camera system is used to track the laser source real‐time during PDT. Camera control and data acquisition software is developed using IGSTK. An inner surface contour is established as the boundary of the treatment area, to which the PDT dose is determined. Registration of the inner surface contour to pre‐surgical CT is used to monitor PDT dose to surrounding critical organs. Results: Inner surface contour of a phantom cavity is delineated and the accuracy of the system is determined to be <2mm. Real‐time tracking during PDT procedure is also demonstrated. No interference between the treatment laser and the IR tracking system is observed. The total time required to establish the inner surface is less than one minute. Conclusion: We showed that the movement of the surgical laser source can be tracked real‐time and recorded throughout the PDT treatment session. Developments are currently underway to utilize this 3D position information for real‐time dose calculation. Light fluence rate deposited into the pleural cavity will be quantitatively determined and displayed real‐time for the operating surgeon. With this real‐time visual guidance, the surgeon will be able to move the laser wand in such a way that ensures dose homogeneity and thus better PDT efficacy.
TU‐C‐204B‐03: Reducing Imaging Dose without Sacrificing Target Localization Accuracy: A Feasibility Study
Authors: Ruan D , Keall P
Published: August 2010

Purpose: We develop and investigate the feasibility of reducing concomitant imaging dose without sacrificing tumor localization accuracy in IGRT, by making “smart” imaging decisions in real‐time. This study focuses on demonstrating the utility of adaptive imaging triggers instead of uniform triggering. Method and Materials: We demonstrate the sub‐optimality of the current uniform imaging scheme, in terms of accuracy v.s. imaging dose tradeoff. We developed an oracle imaging triggering strategy, which retrospectively distributes the kV imaging events to maximize localization accuracy over the treatment duration. The tradeoff between localization accuracy and total imaging dose from the oracle solution provides a performance upper bound and its deviation from the corresponding uniform image triggering scheme alludes to the potential of imaging less with the same localization accuracy. The proposed concept is tested on 159 patient derived 3D abdominal and thoracic tumor traces. Results: The oracle tradeoff curve dominates the current practiced uniform image triggering scheme, in the sense that for the same localization accuracy, it consistently reduces the required total number of images. Quantitatively, it manage to decrease the average imaging frequency by 40% to 50% in most cases. Furthermore, significant potential dose reduction is observed throughout a large range of motion patterns and under various total imaging dose conditions. Conclusion: We have proposed a feasibility study to investigate the As Low As Reasonably Achievable (ALARA) principle in IGRT, by reducing imaging dose while maintaining tumor localization accuracy. Future work will focus on developing real‐time heuristics to approach the oracle performance and experimental verification of geometric and dosimetric accuracy. Research supported by NIH/NCI R01 93626 and AAPM Seed Funding Initiative.
TU‐C‐204B‐05: First Clinical Development of a Failure Detection‐Based Online Repositioning Strategy for Prostate IMRT — Experiments, Simulation, and Dosimetry Study
Authors: Liu W , Xing L , Qian J , Luxton G
Published: August 2010

Purpose: To implement and evaluate clinic‐ready protocols for failure detection‐based online patient repositioning using treatment MV beam imaging with minimal usage of on‐board kV imaging during prostate IMRT. Methods and Materials: We propose a two‐step decision‐making strategy: (1) to detect marker/target motion potentially beyond a pre‐defined threshold through ciné‐MV imaging and online‐updated characterization of prostate motion and (2) to assess over‐threshold displacement and as‐needed reposition the patient based on paired MV‐kV 3D target localization. Two levels of clinical implementation were evaluated: (1) field‐by‐field based motion correction for present‐day linacs, i.e., with an MV‐kV pair and possible repositioning done directly before the next field if an over‐threshold event is detected in the current field; (2) instantaneous repositioning, for new‐generation linacs with capabilities of simultaneous MV‐kV imaging and remote automatic couch control during treatment delivery. Experiments were performed on a Varian Trilogy linac in clinical mode using a 4D motion phantom programmed with typical patient prostate motion trajectories. Dosimetric impact was examined using a 2D ion chamber array. Simulations were done using 536 trajectories from 17 patients. Results: Compared to no intervention, despite deterioration of marker detection efficiency caused by MLC blockage, the field‐by‐field repositioning protocol halved (23% to 10%) the mean percentage of time that target displacement exceeded a 3mm threshold, at minimal cost in additional imaging (one MV‐kV pair per 2–3 treatment fractions) and with a very small number of repositionings (once every 4–5 fractions). The instantaneous repositioning approach reduced the over‐threshold time tenfold (to <2%), also with low kV usage (2.5/fraction). Information acquired for repositioning using combined MV‐kV data was found to have sub‐millimeter accuracy. Conclusions: This work demonstrated in current clinical setup, significant reduction of any adverse effects of intrafraction prostate motion. The method incurs minimal imaging dose to the patient as compared with other stereoscopic imaging techniques.
TU‐C‐204B‐07: A Comparative Study for Daily Localization with 3D Ultrasound, Cone Beam CT, Implanted Prostate Fiducial Markers and Calypso 4D Localization System for Patients Undergoing IGRT for Prostate Cancer
Authors: Neicu T , Ren L , Chetty I , Pradhan D , Stricker H , Movsas B , Elshaikh M
Published: August 2010

Purpose: To assess the accuracy and dosimetric implications of multi‐modality, online image‐guided localization methods, including intra‐modality 3D prostate ultrasound, cone beam CT (CBCT), implanted prostate markers and Calypso 4D localization for patients undergoing external beam radiation treatment with IMRT for prostate carcinoma. Materials and methods: Fifteen prostate cancer patients were imaged using IGRT techniques using the following daily imaging sequence: CBCT, kV orthogonal x‐rays of implanted fiducial markers using the On Board Imager (OBI) on a Varian Trilogy TX linac (Varian Medical Systems, Palo Alto CA), perform patient shifts based on ClarityTM intramodality 3D U/S‐IGRT system (Resonant Medical Inc., Montreal, Canada) per an IRB‐approved protocol, kV x‐rays (post patient‐shift), and Calypso 4D Localization System (Calypso Medical, Seattle, Washington) for intrafraction motion monitoring, finally treat patient. Residual motion was assessed from differences between kV images of marker positions performed prior to and after U/S‐based shifts. Dosimetric differences were evaluated by computing treatment plans using the average shift values generated by the image‐guided localization methods. Results: The anterior — posterior direction had the largest recorded average shifts, overall being less than 7 mm. Most (65%) of the average shifts calculated between the various IGRT modalities were within 3 mm of each other, and the rest within 3 – 7 mm. Changes in inter‐fiducial distances between simulation and treatment were found to be on average 1.5 mm (maximum of 5.8 mm). Possible causes are rotations, migration and/or prostate size/shape changes. DVHs for the rectum, and bladder show in some instances significant increases in dose received when measured setup shifts were included. Conclusion: The comparison of multi‐modality IGRT techniques is subject to many uncertainties, including prostate intrafraction motion (89% within 3 mm range), changes in the prostate position due to bladder/rectal filling during the time between image acquisitions, and migration of fiducial markers.
TU‐C‐204B‐08: Robust Fluoroscopic Tracking of Multiple Implanted Fiducial Markers: Exploiting the Spatial Constraints
Authors: Li R , Sharp G
Published: August 2010

Purpose: We report and compare two new fluoroscopic fiducial trackers that exploit the spatial constraints among the multiple implanted fiducial markers to achieve fast, accurate and robust tracking. Method and Materials: The spatial constraints are modeled as Gaussian distributions of the pairwise distances between the markers. The means and standard deviations are learned from training data. Pairwise distances that deviate too much from the learned distributions would have a low spatial matching score. The first method (M1) is a variant of the particle filters. In M1, the importance of the sampled marker locations is a product of an image matching term (s_image) and the spatial matching terms (s_spatial). Patient specific motion models need is required for M1. The second method (M2) tracks the markers via nonserial dynamic programming without using motion models. In each frame of the fluoroscopic video, template matching is used to detect candidate marker locations. Optimal marker locations are determined through optimizing a score function which is the sum of s_image and s_spatial. We compare the performance of M1 and M2 with an existing tracking method (M3). 16 fluoroscopic videos with groundtruth from different sessions of 3 liver patients are used in this retrospective study. Results: M2 gave the smallest error (mu: 1–2 pixels, sigma: ∼1 pixel) and run at ∼5 frame per second (fps). M1 performed well (mu: 1–5 pixels, sigma: 1∼2 pixels) at ∼7.5 fps when patient breathed consistently, the accuracy dropped (mu: ∼8 pixels) when the patient breathes differently across sessions. Conclusion: Exploiting spatial constraints makes M1 and M2 robust and accurate. M2 enables tracking without motion models but tracks at a lower frame rate. M2 is promising for applications like tracking marker locations using intermittent imaging to reduce the imaging dose in IGRT.
TU‐D‐204B‐01: Scatter Correction for On‐Board Cone‐Beam CT in Radiation Therapy Using Planning MDCT Images
Authors: Niu T , Gao H , Star‐Lack J , Sun M , Fan Q , Zhu L
Published: August 2010

Purpose: The applications of cone‐beam CT (CBCT) imaging in radiation therapy are greatly hampered by the poor image quality mainly due to scatter artifacts. The current use of CBCT is only limited to treatment setup, and an optimal scatter correction solution still remains unclear. Here, we propose a new scatter correction method for CBCT based on the current workflow of radiation therapy. Methods and Materials: With much smaller inherent scatter signals, diagnostic multi‐detector CT (MDCT) provides more accurate CT images and is routinely used for radiation treatment planning. Using the MDCT image as the “free” prior information, we first estimate the primary projections in the CBCT scan via forward projection on MDCT data. Since the deviation between patient geometries in the CBCT and the registered MDCT data only leads to high‐frequency primary projection differences and scatter has dominant low‐frequency components, CBCT scatter signals are accurately estimated by low‐pass filtering and effectively corrected for after subtraction. The proposed method is evaluated using two phantom studies on tabletop systems. Results: On the Catphan©600 phantom, the reconstruction error is reduced from 348 HU to 4 HU around the phantom center. In the selected regions of interest, the average image contrast is increased by a factor of 3.3. This contrast increase improves low‐contrast detectability, as seen in the pelvis phantom study. Besides a significant improvement of image quality, a 25 HU object, which is otherwise buried in the scatter artifacts, can be clearly identified after the proposed scatter correction. Compared to the kernel‐based method, our approach shows an improved performance. Conclusions: Effective scatter correction is achieved on CBCT using our MDCT‐based approach. The increased accuracy of CBCT imaging substantially facilitates CBCT‐based clinical applications, such as tumor delineation and dose calculation. As such, the proposed method will be very attractive in current radiation therapy.
TU‐D‐204B‐04: 4DCT Reconstruction from Undersampled Projections Using Edge‐Preserving Total Variation and Non Local Means
Authors: Tian Z , Xun J , Jiang S
Published: August 2010

Purpose: Four‐dimensional computed tomography (4DCT) is widely used in radiotherapy planning for thorax and upper abdomen to take respiratory motion into consideration. However, its high radiation dose becomes a major concern and impedes its wide application. In this work, we proposed an iterative 4DCT reconstruction algorithm with both an Edge‐Preserving Total Variation (EPTV) regularization and a Non‐Local Means (NLM) regularization to accurately reconstruct 4DCT images from highly undersampled projections to reduce imaging dose. Methods and Materials: In 4DCT, images of neighboring phases usually share similar structures, though these structures are apart from each other spatially in neighboring phases due to breathing motion. To take this temporal correlation into the reconstruction process, we propose to use NLM as a temporal regularization in 4DCT reconstruction. For this purpose, each phase of the 4DCT is iteratively reconstructed by a modified total variation method, namely EPTV method. NLM regularization is imposed in each iteration step to enhance image quality. Specifically, we consider every possible small patch in all phases. Patches spatially close to each other in successive phases are weighted averaged, with high weights assigned to similar patches. We tested our reconstruction algorithm on a NCAT phantom at thorax region and a real patient 4DCT thorax data. 4DCT images obtained from our EPTV+NLM approach are compared with those obtained from the EPTV method. Relative errors are computed and are used to evaluate the reconstruction quality. Results: About 40 x‐ray projections for each breathing phase are enough to accurately reconstruct the 4DCT image in both test cases. The images reconstructed with NLM regularization are found to have lower relative error and hence better image quality than those reconstructed otherwise. Conclusions: Our proposed EPTV+NLM method is a robust approach for the 4DCT reconstruction, where NLM as a temporal regularization can considerably enhance the reconstructed image quality.
TU‐D‐204B‐06: Noise Reduction with Detail Preservation for Low‐Dose Kilovoltage CBCT Using Nonlocal Means Algorithm
Authors: Lu W , Yang D , Wang J
Published: August 2010

Purpose: kV‐CBCT has shown enormous benefits for patient setup and treatment verification in radiotherapy. With the frequent uses of CBCT, the cumulative imaging dose to normal tissues may not be insignificant. A lower mAs protocol in CBCT acquisition reduces the dose, but dramatically degrades image quality due to excessive noise. This work studies the effectiveness of a denoising algorithm, namely nonlocal means (NL‐means), in reducing noise while preserving details in low‐dose CBCT images. Method and Materials: NL‐means algorithm estimates the true value of a pixel as a weighted average of all pixels in the image, where the weights depend on the similarity between the pixels. Unlike the local smoothing or filtering methods, NL‐means can reduce noise while preserve details. The parameters of NL‐means were optimized for the low‐dose CBCT. A non‐stationary correction for avoiding blurring out nonrepeated structures was examined. A CatPhan and an anthropomorphic head phantom were used to evaluate its performance. Results: In all images tested, the noise was clearly reduced by the NL‐means algorithm. In the five contrast objects, the contrast‐to‐noise ratios were increased significantly (mean 1.1 to 5.5), and even higher than those in high‐dose CBCT (mean 3.0). Line pairs with spatial resolutions 1 to 5 lp/cm were more distinguishable. Finer lines (6 –7 lp/cm) were blurred out compared to the high‐dose CBCT. For the anthropomorphic head phantom, the suppressed noise resembled the desired white noise. The noise in all structures was reduced while the details and fine structures were well preserved. The image appears to be of similar or even higher quality than high‐dose CBCT. Conclusion: Results on a CatPhan and an anthropomorphic phantom demonstrated that NL‐means effectively reduced the noise while preserved most fine structures in low‐dose CBCT.
TU‐D‐204B‐07: Correction for the ‘Black Hole’ Artifact in Cone‐Beam Computed Tomography
Authors: Sun M , Nagy T , Oelhafen M , Star‐Lack J
Published: August 2010

Purpose: An artifact typically seen in axial cone‐beam computed tomography (CBCT) reconstructions is a dark region or ‘black hole’ situated beneath the scan isocenter. We have traced the cause of the artifact to scattered radiation coming from the patient table. The artifact is most severe for offset‐detector scans and is especially significant in image‐guided radiotherapy (IGRT) where patient tables have flat tops and are larger than those used for diagnostic radiology. To address this problem, we have developed a hybrid scatter kernel superposition (SKS) method designed to correct for scatter from both the object‐of‐interest and the patient table. Method and Materials: The proposed algorithm uses separate kernels to estimate scatter from the patient table and the object‐of‐interest. For each projection image, the location of the shadow cast by the patient table on the imager is determined. The region not shadowed by the table is processed using asymmetric kernels that best model scatter transport through body‐like objects. The patient table region is processed using a combination of the asymmetric kernels and slab‐derived symmetric kernels. The composition of these hybrid kernels is projection‐angle dependent. To test the algorithm, pelvis phantom and patient data were acquired using the Varian On‐Board Imager (OBI) and processed. Artifact intensities and HU accuracies were assessed. Results: The hybrid kernel algorithm produced substantially better scatter estimates than previous SKS methods. HU non‐uniformities in the pelvis phantom were reduced from 97HU to 29HU. In the in vivo scan, the black hole artifact was reduced by 128HU and anatomical details were made more visible. Conclusions: Radiotherapy patient tables can produce significant scatter‐related artifacts. The proposed hybrid SKS algorithm effectively addresses this problem. Scatter from both the object‐of‐interest and the patient table was accurately estimated, and image uniformity and HU accuracy were greatly improved. Employees of Varian Medical Systems.
TU‐D‐204C‐04: Machine Learning as New Tool for Predicting Radiotherapy Response
Authors: El Naqa I
Published: August 2010

Radiotherapy outcomes are determined by complex interactions between treatment techniques, cancer pathology, and patient‐related physiological and biological factors. A common obstacle to building maximally predictive treatment outcome models for clinical practice in radiation oncology is the failure to capture this complexity of heterogeneous variable interactions and the ability to apply outcome models across different multi‐institutional data. Methods based on machine learning can identify data patterns, variable interactions, and higher order relationships among prognostic variables. In addition, they have the ability to generalize to unseen data before. In this work, we will provide an overview of the current role of machine learning methods for predicting post‐radiotherapy tumor control probability (TCP) and normal tissue toxicities (NTCP). We will discuss some of the current challenges in the field and highlight the potential opportunities of machine learning methods for future treatment outcomes research in radiation oncology.
TU‐D‐201B‐02: Current and Future Measurements of CT Dose
Authors: Cody D
Published: August 2010

The theory behind the current method for assessing radiation dose in CT (the CT Dose Index) will be reviewed. This method has many positive features, and a growing number of limitations. In many ways, technology and imaging equipment has extended beyond the scope of our current CTDI methodology. Several groups have been formed to develop a different approach to assessing CT radiation dose, and many issues must be carefully considered. Dr. Cody will describe a new CT dose measurement process as outlined by Task Group 111, and will also provide her perspective on other interesting developments in the area of CT dose assessment. The related issues of estimated CT dose and patient dose will also be addressed, and relevant Monte Carlo modeling results will be reviewed.
TU‐E‐204B‐05: Feasibility of Markerless 3D Tumor Trajectory Tracking in CBCT Projections Using Digital Subtraction Method
Authors: Chen S , Poulsen P , Weiss E , Keall P , Hugo G
Published: August 2010

Purpose: To evaluate feasibility of markerless 3D tumor trajectory tracking in CBCT projections using a digital subtraction technique for tumor enhancement in low contrast lesions. Method and Materials: 3D tumor position was estimated in CBCT measured projections (MP) by registering digital subtraction projections with templates. A fan beam 4DCT scan served as the planning CT (pRCCT), from which two sets of DRRs were generated using an in‐house GPU‐based algorithm. Tumor templates (TT) were generated by masking the GTV+0.5cm and projecting at the MP view angles for each 4D phase image. Subtraction templates (ST) were generated by first removing the GTV+0.5cm from the pRCCT, positioning the pRCCT at the pose of the patient during CBCT acquisition, and projecting at the MP angles for each 4D phase image. The ST was subtracted from the corresponding MP to create subtraction projections (SP). The 2D tumor position in each SP was calculated by matching with the corresponding TT using a robust correlation metric, and transformed to 3D positions by backprojection with a prior depth estimate. 3D position error was assessed by comparing to 3D positions of implanted markers, estimated with a separate algorithm, in a clinical scan. Results: The mean absolute error for the conventional method was 2.1mm, 2.5mm, and 2.9mm in mediolateral, anterior‐posterior, and superior‐inferior. The 90% error level was 4.1mm, 6.4mm, and 4.5mm. For the digital subtraction method, the mean errors were 0.8mm, 0.5mm, and 0.6mm and 90% errors were 1.4mm, 0.9mm, and 1mm. Conclusions: Digital subtraction is feasible for tumor enhancement to reduce error in tumor trajectory measurement. A framework for validation of markerless tracking algorithms using implanted markers was developed. Supported by NIH R01CA116249.
TU‐E‐204B‐07: Real‐Time 3D Target Position Estimation Using a Single KV Imager Combined with an External Respiratory Monitor during Arc and Static Beam Delivery
Authors: Cho B , Poulsen P , Ruan D , Sawant A , Keall P
Published: August 2010

Purpose: To experimentally investigate a real‐time 3D target position estimation method, using a single kV imager combined with an external respiratory monitor, integrated with a DMLC tracking. Method and Materials: The experimental DMLC tracking system employed a Varian Trilogy with kV imager and Respiratory Position Management (RPM). The internal/external correlation between 3D target position T(t) and RPM signal R(t) was modeled by a state‐augmented linear model: T(t)=a*R(t)+b*R(t‐ τ)+c. The model parameters were determined by solving least‐squares estimation of the error Σ∥pi ‐ P(θi)T(ti)∥2, where pi was the projected marker positions in kV images and P(θi) was the projection operator at the gantry angle θi. During arc and static 5‐field beam delivery, DMLC tracking was performed with ten patient lung tumor motion traces. A 3D motion stage with a gold marker and a separate 1D motion stage with an RPM marker‐block were used to reproduce the tumor motion and external respiratory signals, respectively. The gold‐marker positions were measured by 1‐Hz kV imaging, while the external marker positions were measure by RPM system at 30Hz. To initialize the correlation model rotational kV images over 120° of angular span were acquired. The estimated 3D marker positions were sent to the DMLC to reposition the beam. The tracking accuracies were quantified as “beam‐target” alignment mismatch in cine MV images acquired during the experiments. Results: With tracking, the average errors of the ten lung tumor traces were 0.6–1.4mm and 0.8–1.5mm during the arc and static 5‐field beam delivery, respectively. Without tracking, the average errors were 3.7–6.1mm and 3.0–7.3mm. Conclusion: The DMLC tracking system integrated with a new estimation method shows typical accuracy of 1mm for patient tumor motion traces during arc and static delivery, and thus has the potential for accurate motion management of thoracic and abdominal tumors. Conflict of Interest: Supported by NTH/NCI R0193626.