bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐J‐149: Validation of the Spatial Accuracy of the ExacTrac Adaptive Gating System
Authors: Twork G , Patrocinio H , Ruo R , Soisson E , Roberge D
Published: November 2011

Purpose: To evaluate the positioning accuracy of the BrainLAB ExacTrac Image Guidance System under gating conditions. Methods: Two types of phantoms were used in measurements: an anthropomorphic RANDO head phantom and a BrainLAB ExacTrac Gating Phantom. Our setup included a Varian Novalis Tx radiosurgery system equipped with the ExacTrac 6D IGRT. This system consists of an infrared positioning system for the initial patient positioning and patient tracking, and a stereoscopic kV X‐ray imaging system for final localization using internal markers or anatomy. Uncertainties were broken down into individual components, and the different BrainLAB fusion modalities (internal markers and bony fusion) were used to compare the effect of slice thickness on positional accuracy. Gating uncertainties were deduced with varying tumor motion amplitudes and window sizes in conjunction with a hidden target test. Results: Our results of CT slice thickness dependence for both fusion algorithms with the hidden target test gave similar deviation (<0.7mm), and were reasonably consistent up to a 5 mm slice width. Tumor motion and gating window size yielded an uncertainty of up to 1 mm for the parameters tested. Combining a non‐gating uncertainty of 0.9 mm with the gating uncertainty resulted in a geometrical accuracy of 1.6 ± 0.7 mm for 2.25 cm tumor amplitude and a 30% window size. For tumor motions up to 3 cm and gating window sizes up to 30%, the localization accuracy remained within 2 mm. Conclusions: We have tested the gating window and tumor amplitude effects on the spatial accuracy of the ExacTrac System equipped Novalis Tx linac for stereotactic body radiation therapy. While the CT slice thickness, mechanical deviation of the linac and gating window size contribute to uncertainty, the system provides an external modality that allows for localization accuracy of less than 2 mm for gated delivery.
SU‐E‐J‐150: Applicability of a Point Fiducial‐Based Model for the Prostatic Bed in Post‐Prostatectomy Radiation Therapy Patients: An Expedited Dosimetric Analysis Using New Functionality in Philipsˈ Pinnacle3 Treatment Planning Systemˈs Research Version
Authors: Bharat S , Zhu M , Gay H , Michalski J , Santanam L , Noel C , Parikh P
Published: November 2011

Purpose: The shape of the post‐prostatectomy volume, fiducial placement and isodose coverage is more complex than prostate radiation therapy; but to date, there have been no commercial tools to estimate the delivered dose in patients aligned with fiducials. A fiducial‐based algorithm to determine the dosimetric effects of fiducial localization has been implemented as ˈDelivered Dose Investigational Toolˈ (DiDIT), in Philipsˈ Pinnacle TPSˈ research version (Philips Medical Systems, Fitchburg, WI).Methods: 17 post‐prostatectomy patients were treated with SMLC IMRT, electromagnetic (Calypso Medical Technologies, Seattle, WA) tracking, 5mm CTV‐PTV expansions and 3mm tracking tolerances. The DiDIT tool implemented convolution of the static plan dose distribution with rigid motion (rotations and translations) calculated from the ∼150,000 fiducial tracking points, to generate estimates of the delivered dose. This analysis takes into account fiducial placement eccentricity, rotations, translations and the individual patient anatomy and treatment plan. Treatment adequacy was defined as the delivered minimum dose to the prostatic bed (CTV) >= 98% of planned minimum dose. Tracking data from subsets of the treatment course (first 3, 5 and 10 fractions) were utilized to determine the ability to predict treatment adequacy.Results: Each patient had fiducial motion of less than 2mm. Each DiDIT evaluation took 5 – 15 minutes. Treatment was adequate in 11 of 17 patients, with the CTV having cold spots of 4 to 32%. DiDIT analysis of the initial fractions was not predictive of inadequate dose delivery. Patients with inadequate dosimetry seemed to have more eccentrically placed transponders and/or more rotation though no cut‐off values were noted Conclusions: The DiDIT tool was used to quickly model delivered dose from fiducial localization. Results show that even with continuous fiducial tracking, there may be residual errors in the localization of the prostatic bed. Further work comparing these results with daily volumetric imaging is warranted.

Work sponsored by NIH R01 CA134541‐01.
SU‐E‐J‐151: Geometric Accuracy of the X‐Ray Image‐Based Dynamic Tracking for a Four‐Dimensional Image‐Guided Radiotherapy System with Gimbals Mechanism of MHI‐TM2000 (Vero)
Authors: Mukumoto N , Nakamura M , Sawada A , Takahashi K , Miyabe Y , Takayama K , Mizowaki T , Kokubo M , Hiraoka M
Published: November 2011

Purpose: To verify the geometric accuracy of the X‐ray image‐based dynamic tracking for a four‐dimensional (4D) image‐guided radiotherapy system with gimbals mechanism of MHI‐TM2000 (VERO). Methods: This study was performed employing a 3D moving phantom with a steel‐ball target of 9.5‐mm‐diameter; a laser displacement gauge; orthogonal kV x‐ray imaging subsystem, a gimbaled X‐ray head, and a system controller of MHI‐TM2000. The moving phantom was driven based on the programmed motion patterns (seven periodic [Motion range: 20–40 mm, Breathing cycle: 2‐5 s] and 15 of 3D patientˈs respiratory patterns). Subsequently, the displacements of the target were measured in real time by the laser‐displacement gauge and the orthogonal kV x‐ray imaging subsystem. Meanwhile, the system predicted the future target position, and then the orthogonal gimbals were transferred to the corresponding orientation. The transferred and the current orientation of the gimbals were recorded. Predicted target positional errors (EP), mechanical errors (EM) of the gimbals, and total tracking system errors (ET) derived from EP and EM were computed. Results: Root mean square of ET (RMSET) was up to 0.68 mm for the periodic patterns. As for the entire patientˈs respiratory patterns, average RMS of EP, EM, and ET were 1.28, 0.12, and 1.24 mm, respectively. The principal component score which was computed from the patientˈs respiratory motion under the conditions of free breathing decreased with large amplitude, long breathing cycle and high breathing stability. A strong positive correlation was observed between the ratio of the RMSET to the amplitude and the principal component score of the respiratory motion (R2 = 0.76). Conclusions: This study showed that MHI‐TM2000 has the capability for high tracking accuracy. The prediction error mainly caused the total tracking system error while the mechanical error was negligible. In addition, tracking error can be predicted from the principal component analyses.

This research was in part sponsored by Mitsubishi Heavy Industries, Ltd., Japan.
SU‐E‐J‐153: Tracking Respiratory Motion in CBCT Projection Images Using an Integrated Optical Flow (IOF) Method
Authors: Godfrey D
Published: November 2011

Purpose: To introduce and evaluate a new method for automatically extracting respiratory phase information directly from CBCT or tomosynthesis projection data. Methods: In the proposed method, the Black and Anandan dense optical flow is computed in the longitudinal direction between a fixed region of interest (ROI) extracted from each projection image in a free‐breathing CBCT scan and its nearest chronological neighbor. The optical flow values are integrated over time to yield a respiratory signal curve. Free‐breathing CBCT projection data from several thoracic and abdominal subjects were used to the compare the integrated optical flow (IOF) method with the “ground truth” signals measured by manually tracking the motion of internal anatomy landmarks (e.g., the diaphragm). Finally, phase information from the proposed technique was used to reconstruct exhalation and inhalation tomosynthesis image sets, for comparison with free‐breathing tomosynthesis reconstructions of the same subjects.Results: The proposed integrated optical flow (IOF) method yielded respiratory motion curves whose phase information agreed well with measured data. However, the IOF amplitudes showed strong chronological trends, due to changing projection geometry. Furthermore, the IOF method sometimes required an adjustment of the ROI location part‐way through the scan due to the rotational motion. Phase‐based tomosynthesis reconstructions using only projections near the full‐inhale or full‐exhale phases (as determined via the automated IOF method) exhibited the expected trends, with internal anatomy and target tumor locations clearly distinguished by phase, and often more clearly visible than in the full free‐ breathing reconstructions, which contain far more motion blurring Conclusions: The proposed IOF method is an effective way to automatically sort free‐breathing CBCT projection data into respiratory phase bins for phase‐based 4D tomosynthesis or CBCT reconstruction. Future work will focus on automatic relocation of the ROI and de‐trending of the IOF measures using estimations from the free‐breathing CBCT or planning CT.
SU‐E‐J‐154: Free Breathing Motion Tracking in Emission Guided Radiation Therapy
Authors: Mazin S , Yang J , Yamamoto T , Nanduri A
Published: November 2011

Purpose: To evaluate an emission guided radiation therapy (EGRT) systemˈs ability to deliver dose to a moving PET‐avid target. Methods: We are developing a treatment system that will simultaneously deliver radiation during PET acquisition. Due to PETˈs slow imaging time, a method to compensate for motion will be to deliver radiation beam‐lets along individual PET lines‐of‐response (LORˈs) as they are detected. The EGRT system involves rotating a linac and PET detectors on a closed ring gantry while dynamically controlling a binary multi‐leaf collimator for helical delivery. A cylindrical phantom and six spherical inserts were filled with FDG to achieve an 8:1 target‐to‐background ratio. The phantom was mounted onto a motion stage programmed with a free breathing trajectory acquired from a human subject using an external marker. This trajectory was translated into a purely superior/inferior (z) motion path for the stage and used as ground truth. A planning target volume (PTV) was defined around the largest sphereˈs motion path. LOR data were acquired in list‐mode from a GE Discovery system and used as input to a voxel monte carlo simulation of radiation dose delivered by the EGRT system with an average lag time of 250 ms. The EGRT method was compared to uniform irradiation of the PTV. Results: The EGRT approach exhibited a non‐uniform dose distribution to the tumor. However, for this specific case there was a 30% relative increase in dose to 50% of the tumor volume for the EGRT approach with both methods normalized to have the same integral dose to the phantom. Conclusions: We have shown that a target exhibiting free breathing motion can be tracked and treated using individual PET emissions to guide delivery. There is dose peaking at the center of a uniform PET‐avid volume. This is the first EGRT feasibility demonstration with experimental data.

SRM and ASN are stockholders of RefleXion Medical.
SU‐E‐J‐155: Learning Based Tumor Motion Tracking in MV Images Using Cone‐Beam CT Projections
Authors: Yue Y , Mamon H , Berbeco R
Published: November 2011

Purpose: Tumor motion tracking in liver stereotactic body radiation therapy (SBRT) is highly desirable for improvement of treatment accuracy, dose conformity and sparing of healthy tissue. On‐board megavoltage (MV) x‐ray imaging has unique advantages for assessment of tumor location during treatment. This work proposes a novel method to track tumor motion in MV images using cone‐beam CT (CBCT) projections as prior knowledge. Methods: The learning based tumor tracking method consists of main two steps. First, a prior of 3D tumor motion is estimated from CBCT projections. Relying on the detection of implanted kV fiducials, the tumor motion trajectory is constructed along the CBCT projection angles. 3D tumor locations are then reconstructed from the trajectory. Further, a prior for tumor motion in EPID images are generated by projecting 3D motion on the MV imager plane. Second, tracking is carried out in an incremental learning framework, which adapts the sequential tracking knowledge into current estimation. The prior knowledge of tumor motion obtained in the first step is therefore integrated into estimation. For robust real‐time tracking, the algorithm is implemented by a sequential incremental update technique. Results: Patient data acquired from a liver SBRT treatment is used to validate our algorithm. Half‐fan CBCT projections acquired in three fractions were used to estimate 3D tumor motion. A total of 700 EPID images were analyzed for tumor tracking. All results were evaluated by clinician‐defined tumor positions and manual tumor tracks. The average root mean square error for tumor tracking is 0.89 ± 0.21 mm. Conclusions: A novel learning based tracking algorithm has been developed for tumor tracking in MV images. With the fast incremental online updating technique, our algorithm is well suited for real‐time tumor tracking and in‐treatment applications.

Varian Medical Systems, Inc.
SU‐E‐J‐158: Diaphragm‐Based and Direct Tumor Tracking in KV Rotational Cone‐Beam X‐Ray Projections Using Templates Derived from 4DCT
Authors: Lewis J , Li R , Jiang S
Published: November 2011

Purpose: To develop an algorithm for tracking lung tumors in rotational cone‐beam KV x‐ray projections based on combined markerless diaphragm and tumor tracking. Methods: A combined tracking algorithm which uses surrogate‐based tracking for angles at which the tumor is difficult to track directly is implemented. Direct tumor and diaphragm tracking are performed using template matching based on templates from 4DCT. A volume of interest is defined around the diaphragm apex and used to develop diaphragm templates. A linear model relating diaphragm apex and tumor positions is developed based on 4DCT. The tracked position of the diaphragm apex is used to calculate tumor positions based on the linear models. The two tracking signals are combined by selecting the method expected to be more accurate at each angle. Since direct tumor tracking is often difficult when the tumor is occluded by the spine and central structures in projections, diaphragm‐based tracking is used when the tumor is expected to overlap with these structures in the projections. Direct tracking is used when the tumor is expected to be more visible. The methods are applied to anatomical digital phantom and patient cases. Results: For the digital phantom, the e95 was 1.1 mm with direct tracking, 0.9 mm with diaphragm‐based tracking, and 1.1 mm with combined tracking. The correspond erms values were (0.6, 0.4, 0.6) mm. In patient 1, the e95 was 1.9 mm with direct tracking, 1.7 mm with diaphragm‐based tracking, and 1.7 mm with combined tracking. Corresponding erms values were (1.0, 0.9, 0.8) mm. In patient 2, the e95 was 4.6 mm with direct tracking, 3.0 mm with diaphragm‐based tracking, and 2.9 mm with combined tracking. The corresponding erms values were 2.5, 1.7, 1.6) mm.Conclusion: A combined tracking algorithm was developed which incorporates benefits of both diaphragm‐based and direct tumor tracking.
SU‐E‐J‐159: Correlation of Respiration‐Induced Motion of an External Surrogate and Implanted Internal Markers
Authors: Lu J , Brackbill E , Zheng D , Weiss E , Hugo G , Keall P , Poulsen P , Fledelius W , Williamson J
Published: November 2011

Purpose: To investigate the correlation between the respiratory motion of implanted internal markers (IM) and external RPM traces for lung cancer patients in dynamic imaging. Methods: Under an IRB‐approved research protocol, 3 patients with VISICOIL marker (10 mm long) implants near/in their lung tumor were scanned with a daily single‐slow gantry rotation arc 4D‐CBCT acquisition. The Varian Real‐time Position Management system (RPM) was used as an external respiratory motion surrogate, with the marker block placed on the patient abdominal surface near the diaphragm. The IM motion was segmented in all 2D projections with an in‐house built Matlab program. The positions in the 3D spatial domain of the markers were extracted to (LR: left‐right, CC: cranial‐caudal, AP: anterior‐posterior). A Pearson‐type auto‐correlation function of the RPM AP trace and cross‐ correlation functions between RPM AP motion and the IM motions in three orthogonal directions was used to analyze the time series. Results: Our data shows that there is a strong positive correlation between the RPM AP motion and the IM motion in the CC direction. The correlation between the external motion and the IM motions in LR and AP directions varied from fraction to fraction. In addition, phase shift is observed in LR and AP directions. Different implanted markers in the same patient presented the very similar correlations, especially in the CC direction. This result suggests that multiple implanted markers can be used to provide supplementary information of the tumor motion. When the patient coughs during their respiratory cycle, internal motion and external motion may become uncorrelated. Conclusions: The RPM respiratory trace provides a good surrogate of the internal respiratory motion in the CC direction when the patient maintains a normal breathing pattern.

Supported by NIH grant P01 CA 116602.
SU‐E‐J‐162: Tomotherapy Real‐Time Tumor Tracking: An Innovative Method for Magnetic Tracking in Enclosed Bores
Authors: McGary J , Chen J , Zubiao X
Published: November 2011

Purpose: Real‐time tumor tracking for Tomotherapy systems is a problem in need of a solution. Currently, magnetic tracking systems, such as Calypso, are unable to localize a magnetic source due to the conducting bore. Eddy currents generated in response to the source create additional magnetic fields, at the source frequency, that are difficult to model and filter from the signal. Therefore, we are presenting an innovative method to solve this problem. Method: We have developed a localization algorithm based on a transponder source within a conducting cylinder, and a cost effective sensor configuration. The algorithm uses magnetic field gradients, as well as the magnetic fields, to effectively filter magnetic fields generated from surrounding eddy currents induced along the gantry. Solution accuracy depends on the sensor geometry and configuration; and therefore, the method is a combination of algorithm and sensor parameters to maximize accuracy, while minimizing cost to create a practical system. We modeled the gantry and transponder system to simulate the magnetic fields throughout the environment. The sensor fields were used as algorithm input to calculate the transponder location. The location of the transponder was compared with the calculation. Results: We investigated different algorithms, sensor geometry, and sensor configuration. We compared our method to traditional localization methods of Wilson, Nara, Chao, Hashi, and McGary. Those methods were unable to attain suitable clinical treatment accuracy due to the conducting bore. The method presented here was able to locate the transponder within 1–2 mm over a large extent of possible transponder positions and orientations. In certain regions, the localization error increased to about 3–4 mm due to the nearby eddy currents. Conclusions: The method, algorithm and sensor design, is capable of locating a transponder, for real‐time tumor tracking, in a modeled Tomotherapy gantry bore within 2–4 mm.

Financial support from Tomotherapy, Inc. was provided, in part, to fund some of the research presented here. Other support came from the Golfers Against Cancer.
SU‐E‐J‐163: Tracking Interfractional Patient Positioning Consistency When a Robotic Couch Is in Use
Authors: Liu H , Yu Y , Werner‐Wasik M , Andrews D
Published: November 2011

Purpose: To track interfractional patient positioning consistency during a course of stereotactic radiotherapy using BrainLAB frameless immobilization and localization system, with ExacTRAC IGRT and Robotic couch. Methods: One relationship between the BrainLAN iPLAN coordinates and the Varian couch parameters was developed so that three translational couch positions were incorporated into Aria R&V system as part of the plan parameters. Therapists use these positions to verify patientˈs initial set‐up. After ExacTRAC calculates 3 shifts and 3 rotations of a patientˈs current position to the expected position based on the registration of a pair of x‐ray images with DRR, the robotic couch can be driven to place the target at Linac isocenter. However, since the rotational axes for pitch and roll of the robotic couch are not coincident with the Linac isocenteral axes, the couch positions will be changed to allow for pitch and roll corrections. Current R&V system only records 3 translational and 1 couch angle (no pitch and roll), and the interfractional couch position difference can be > 8 cm vertically. A second relationship was developed between the recorded translational parameters and the actual target position, taking into account the pitch and roll from the ExacTRAC system, so that the interfractional patient position can be tracked. Results: The differences between the estimated and actual translational couch positions are < 2 mm on average for 28 patients. The interfractional couch position variance is < 1.5 mm for 3 patients (10 fractions per patient). Discussions: If the EaxcTRAC IGRT system is mis‐calibrated to a wrong position other than Linac isocenter, the x‐ray image registration system cannot in itself detect the error. Therefore, this patient position tracking methodology can be a very important safety tool for preventing errors, so that the treatment team can deliver stereotactic radiation at a higher confidence level.
SU‐E‐J‐168: Proton Radiography for Pediatric, T‐Spine and Lung Malignancies; Development and Enhancement of a Proton Imaging Technique
Authors: Seco J , Dias M , Depauw N , MacDonald S
Published: November 2011

Purpose: Proton radiography (PR) was investigated for daily set up prior to proton radiation treatment, which would provide improved imaging capabilities along with reduced radiation dose to pediatric, lung and T‐spine tumors. Pediatric patients are more susceptible to radiation induced malignancies and toxicities from therapeutic and diagnostic radiation. While, in the case of T‐spine and lung tumors, motion and lung heterogeneities can significantly reduce the benefit of using proton beam therapy. Methods: A group of 5 pediatric patients with 1) neuroblastoma, 2) spinal malignancy, or 3) thoracic tumor were used to evaluate proton radiography. In addition, 7 lung/T‐spine tumors were also used to evaluate the ability of proton radiography to allow real‐time tumor tracking, while the tumor is moving within the lung with the patient respiration. Proton radiographic images were generated using a Monte Carlo imaging tool, PR‐Imaging, developed at MGH. Results: Radiographic images of the lung tumors were generated with simulated PR‐Imaging using a scanned proton pencil beam (PBS) at 230MeV, 330MeV and 490MeV, and compared to diagnostic X‐ray digitally reconstructed radiographs (DRR). In addition, whole body PR‐Images of pediatrics was compared to conventional X‐ray radiographs. PR images with high resolution comparable to that of a diagnostic x‐ray were generated. The X‐ray portal images and the X‐ray radiographs present better edge detection, while lung tissue to soft tissue boundaries of the tumor were better distinguished in PR images. The PR‐imaging allows the oncologist and radiologist to change windowing level and tumor contrast level relative to soft tissue background. Conclusions: PR‐Imaging prior to therapy allows for high quality images that can provide information on range degradation of the proton therapy beam. PR‐Imaging during therapy allows for daily quality assurance and tumor tracking during radiation to guarantee that it has been appropriately treated with radiation.
SU‐E‐J‐169: A Guidance System Using Optical Patient Alignment (OPA) during Breast Radiotherapy
Authors: Lozada J Paz , Nazareth D , Schmitt J , Malhotra H
Published: November 2011

Purpose: To develop and evaluate an optical patient‐alignment system to assist in patient positioning during breast radiotherapy. Since the breast is a deformable organ of variable size and shape, standard setup procedures may be inadequate to position the breast properly for treatment. Methods: This method employs a standard digital camera, and an in‐house constructed calibration phantom (CP), composed of ten identifiable reflecting spheres, mounted on a wooden platform. A CT image of the CP was acquired, to determine the 3D configuration of the spheres. For calibration, the CP is placed on the treatment couch and aligned to the collimator. A digital photograph of the CP is acquired, and a centroid‐finding technique is applied to this image to obtain the 2D coordinates of the spheres. These 2D coordinates, along with the known 3D model of the spheres, serves as input to an optimization routine based on the downhill‐simplex algorithm. The routine solves for the geometry of the camera location; i.e., the rotation and translation relating the camera to the CP, and therefore to the treatment room geometry. Using this information about the camera parameters, the 3D CT data can be projected onto the cameraˈs imaging plane, and displayed on a monitor, superimposed on a live video of the patient. This would enable the therapist to view both the patientˈs current and desired positions, and guide in proper positioning.Results: In order to evaluate our method accuracy, simulated images were created of a virtual CP. Our results demonstrate an accuracy of 0.2 cm and 0.1 degrees, respectively, in translation and rotation Conclusions: we developed a method to calibrate an optical camera system, which can be used to superimpose a perspective projection of a CT image on a patientˈs real‐time optical image. Displaying this visual information will assist in accurate setup during breast radiotherapy
SU‐E‐J‐178: Image Guidance on the TrueBeam STx: Evaluation of CBCT Imaging Dose and Quality
Authors: Fahimian B , Xing L , Maxim P
Published: November 2011

Purpose: To evaluate the cone beam computed tomography (CBCT) dose and imaging quality of the TrueBeam STx linear accelerator equipped with the X‐Ray Imaging System. Methods: In accordance to the recommendations of TG‐111, a 0.6 cm3 Farmer thimble ionization chamber was used for in‐phantom measurements at the center and periphery (4 cm from edge, 12 oˈclock) of 30 cm diameter × 30 cm length cylindrical phantom. A total of six new CBCTscan protocols were evaluated: 1) head (full trajectory, 100 kVp, 264 mAs, 2) head (half trajectory, 100 kVp, 146.40 mAs), 3) low dose thorax (full trajectory, 125 kVp, 264.00 mAs), 4) pelvis (full trajectory, 125 kVp, 686.40 mAs), 5) pelvis spotlight (half trajectory, 125 kVp, 731.27 mAs), and 6) pelvis spot light (full trajectory 125 kVp, 1318.68 mAs). Doses from MV, portal imaging and kV fluoroscopy were evaluated as well. Image quality was evaluated using the Catphan 504 placed at isocenter and scanned under the exact same parameters as the dosimetric study. Results: The measured dose for CBCT settings 1–6 (center, periphery) are: 1) 0.37cGy, 0.37cGy; 2) 0.2cGy, 0.34cGy; 3) 0.64cGy, 0.64.cGy; 4) 1.64cGy, 1.66cGy; 5) 3.5cGy, 7.27cGy and 6) 6.05cGy, 8.14cGy.With regards to image quality, we observed two general trends: First, comparing the half trajectory reconstructions to the full trajectory reconstructions, shading artifacts are visible in the half trajectory reconstructions. Second, by nature of the half reconstructions under the Thorax and Pelvis protocols, the resolution is degraded by 56%; which is due the fact that the pixel size is larger for the half fan methodology.Conclusion: In this study we evaluated the imaging dose and quality of the TrueBeam STx linear accelerator. The measured doses and image quality are consistent with previous measurements reported in the literature for older generation Varian On‐board Imaging systems (OBIs).
SU‐E‐J‐180: SPECT‐Based Target Localization: Comparison between Full‐Size and Compact Detectors
Authors: Roper J , Bowsher J , Yin F
Published: November 2011

Purpose: Single photon emission computed tomography (SPECT) is being investigated for imaging inside radiation therapy (RT) treatment rooms to target tumor biology. Because of the space occupied by RT devices and the size and weight of typical gamma cameras, it may be more practical to image with a compact detector. However, data truncation due to smaller detector size can potentially degrade target localization. Here target localization is evaluated for full‐size and compact parallel‐hole‐collimated SPECT detectors. Methods: Twelve 1.5‐cm‐diameter tumors, with 6:1 activity ratios, were simulated in a XCAT phantom. Modeled scan time was 5 minutes. Ensembles of 100 scans were simulated for 3 cases: (a) 45‐cm‐wide detector, no image truncation; (b) 20‐cm‐wide detector with surface‐connected, completely‐sampled common volume (CV); and (c) 20‐cm‐wide detector with interior, incompletely‐sampled CV. On average, the 20‐cm‐wide detector could achieve closer proximity to the phantom, thus improving spatial resolution. Images were reconstructed using OSEM. Localization error was evaluated as the distance between the true tumor location and the location estimated by a numerical observer. Differences in localization were assessed for significance using the Wilcoxon rank‐sum test. Results: Localization errors from the 45‐cm‐wide detector were used as a baseline for evaluating the performance of the 20‐cm‐wide detector. Finding 1: For tumors inside the CV of the 20‐cm‐wide detector, localization performance was generally comparable to the 45‐cm‐wide detector. However, localization was significantly better (p=0.012) using the 20‐cm‐wide detector in one case. Finding 2: Localization was degraded for tumors on the CV boundary. Finding 3: Localization performance of the 20‐ cm‐wide detector was typically worse, and substantially so, for tumors outside its CV. Conclusions: These preliminary results show that, with 180‐ degree parallel‐hole‐collimated detector trajectories, localization performance of a compact detector was comparable to that of a full‐size detector within the CV of the smaller detector, even an interior CV.
SU‐E‐J‐181: Evaluation of Motion Artifacts of Metal Localization Devices on KV and MV Cone Beam CT
Authors: Lin T , Hossain M , Ma C
Published: November 2011

Purpose: To investigate the strengths and weaknesses in motion effect of kV CBCT and MV CBCT for image guided radiation therapy with metal localization devices. Method and Materials: 15 MU pelvis CBCT template for Siemens Artiste MVision and Pelvis template for Varian IX on‐board Exact Arms kV were used to acquire CBCT images in this study. Images from both CBCT modalities were compared in CNRs, image landmark coordinates, and image volume distortion on three different planes of view. The images were taken on a breathing‐simulated thoracic phantom in which several typical metal localization devices were implanted, including clips and wires for breast patients, gold seeds for prostate patients, and BBs as skin marks. To magnify the artifacts, a 4cm diameter metal ball was also implanted in the thoracic phantom to mimic the metal artifacts. The thoracic phantom was placed on a home‐made moving phantom performing a sinusoidal motion in the patient superior‐inferior direction. The amplitude of the sinusoidal waves was 1cm, and the period varies from 2sec, 4sec to 8sec. Results: For MV CBCT, the CNR at 4sec breathing cycle with 1cm breathing amplitude was 5.0, 3.4 and 4.6 for clips, gold seeds and BBs, respectively while it was 1.5, 2.0 and 1.6 for kV CBCT. On the images, kV CBCT showed symmetric streaking artifacts both in the transverse and longitudinal directions relative to the motion direction. kV CBCT images predicted 89% of the expected volume, while MV CBCT images predicted 95% of the expected volume. Conclusion: MV CBCT images showed better volume prediction, less streaking effects and better CNRs of a moving metal target, i.e. clips, BBs, gold seeds and metal balls than kV CBCT images. MV CBCT was more advantageous compared to kV CBCT with less motion artifacts for metal localization devices.
SU‐E‐T‐01: Tumor Control Probability Models Derived Using Machine Learning for Head and Neck and Lung Tumors including a New Metric, Total Clonogen Survival (TCS)
Authors: Oh J , Zuniga A , Thorstad W , Bradley J , Deasy J
Published: November 2011

Purpose: Empirically validated and highly predictive models of tumor control probability could help guide daily clinical practice, but are currently unavailable. Using a newly‐identified dosimetric predictor (Total Clonogen Survival (described elsewhere at this meeting)), and a newly acquired dataset on head and neck tumors, we explore the predictive power available in a TCP model using machine learning methods.Methods: We modeled two different datasets collected at Washington University School of Medicine: (A) lung cancer data consisting of 56 non‐small cell carcinoma patients (a local failure group (22) and a control group (34)) who received 3D conformal radiation therapy with a median prescription dose of 70 Gy (60–84 Gy) and had a median follow‐up of 32 months; (B) head and neck cancer data consisting of 80 squamous cell carcinoma patients (a local failure group (23) and a control group (57)) who received IMRT as definitive treatment with a median prescription dose of 70 Gy (66–72 Gy) and had a median follow‐up of 19 months. Using dose‐volume parameters extracted from these datasets, we found significant parameters for predicting local failure and evaluated the performance using support vector machines (SVM) with leave‐one‐out cross‐validation (LOO‐CV) in conjunction with several feature selection strategies. Results: For lung cancer data, V75 and TCS were chosen as significant parameters with a Matthewˈs correlation coefficient (rCV) of 0.462 and a Spearmanˈs rank correlation coefficient (RsCV) of 0.642. For head and neck cancer data, with Min dose, V70, and TCS, we achieved the best performance: rCV=0.367 and RsCV =0.554. Conclusions: Combining the newly identified metric TCS with other relevant metrics within a machine learning framework allowed us to produce a predictive TCP model that should be further tested for potential clinical use.
SU‐E‐T‐03: Proximity Effects and Intra‐Track DSB Interactions within the Repair‐Misrepair Fixation (RMF) Model
Authors: Park J , Carlson D , Stewart R
Published: November 2011

Purpose: Formulas linking Linear‐Quadratic (LQ) cell survival parameters to double strand break (DSB) induction are derived from the Repair‐ Misrepair‐Fixation (RMF) model for mixed radiation fields. Measured data for several cell lines irradiated by radiations of varying quality are used to examine the impact of proximity effects on intra‐ and inter‐track binary misrepair. Methods: With the RMF, estimates of alpha and beta for any particle type are determined by a well‐defined physical parameter (mean specific energy), two biological parameters (theta and kappa) that are independent of radiation quality, and a biological parameter (sigma) that depends on radiation quality. To minimize the number of ad hoc adjustable parameters, we used the published Monte Carlo Damage Simulation (MCDS) to estimate sigma. The effects of radiation quality on alpha and beta were examined by performing a regression analysis of survival data for 55 and 250 kVp x‐rays (V79 cells), for 55 kVp, 250 kVp and gamma‐rays from 60Co (CHO cells) and human kidney T‐1 cells irradiated by x‐rays, protons, deuterons and alpha particles with an LET up to 200 keV/um. Results: In CHO, V79 and T‐1 cells irradiated by widely varying types of radiation, the probability per unit time DSB formed by the same track interact in pairwise fashion is 100 to 300 times larger than the probability per unit time DSB formed by different tracks interact in pairwise fashion. Conclusion: The RMF is a useful conceptual and mathematical framework to quantify the effects of radiation quality on intrinsic radiation sensitivity for monoenergetic charged particles and mixtures of charged particles of varying quality. Although not an especially important cell killing mechanism for low LET radiations, intra‐track binary misrepair becomes the dominant one‐track cell killing mechanism for intermediate and higher LET radiations.
SU‐E‐T‐06: Investigating in Vitro Gamma‐H2AX Response and Chromosome Damage in Human Lymphocytes and Lymphocyte Subsets for Biomarkers of Radiation Sensitivity
Authors: Beaton LA , Ferrarotto C , Ringuette N , Malone S , Samiee S , Wilkins RC
Published: November 2011

Purpose: To examine the in vitro gamma‐H2AX response in lymphocytes and lymphocyte subsets from patients who have shown severe radiosensitive response (severe late toxicity) in order to determine whether radiation responsive subsets would provide more specific markers for radiosensitivity. To concurrently examine cytogenetic endpoints as potential markers for radiosensitivity in addition to providing additional information about the mechanisms of radiosensitivity. Methods: Ten prostate cancer patients were identified with grade 3 Late Proctitis (RTOG/EORTC Late Radiation Morbidity Scoring Schema) along with twelve matched normal responding patients (Grade 0 Toxicity). These patients had been treated with dose escalated (76Gy) 3D Conformal Radiotherapy as part of an ongoing Phase III clinical trial. Peripheral blood samples were taken from each patient, irradiated with 250kV, 12.5mA x‐rays and examined for gamma‐H2AX response in lymphocytes, CD4+ and CD8+ T‐cells, and CD19+ B‐cells. A dose course experiment was conducted with 6 dose points ranging from 0 to 10 Gy, processed 1 hour after irradiation. The time response of gamma‐H2AX was also monitored, with 2 Gy irradiated samples being incubated for 7 time points ranging from 0 to 24 hours before processing. Samples were fixed stained with gamma‐HAX‐FITC, CD4‐PE, CD8‐APC and CD19‐PC7 before being analyzed by flow cytometry. Furthermore, 0 and 6 Gy irradiated blood samples were analyzed for chromosome aberrations and excess fragments per cell.Results: In a subset of the data for the radiosensitive population, the gamma‐H2AX response tended to be higher in the time course experiment. It was also found that, at 6 Gy, the number of excess fragments per cell in the radiosensitive population (1.65 +/− 0.29) was higher than in the normal population (2.20 +/− 0.27) (p‐value < 0.001) Conclusions: These preliminary results suggest the existence of potential for markers for predicting radiosensitivity which could be useful for tailoring radiotherapy treatments.
SU‐E‐T‐07: Dose to Medium or Dose to a Water Cavity Embedded in Medium?
Authors: Enger S A. , Ahnesjo A , Verhaegen F , Beaulieu L
Published: November 2011

Purpose: To investigate, by means of Monte Carlo (MC) simulations, the relationships of the dose to a local medium, Dm,m, and the dose to a water cavity embedded in the medium, Dw,m, the dose to a cell in the medium, Dc,m and the dose to a cell nucleus in the medium Dn,m for different energies used in brachytherapy and different cavity sizes. Methods: The Geant4 9.3 MC package was used to calculate Dm,m, Dw,m, Dc,m and Dn,m for a mono‐energetic photon beam impinging on one side of a cubic phantom. Concentric spherical cavities of 2–14 mum representing cells and their nuclei with four different chemical compositions were placed inside the phantom. Absorbed dose and electron fluence inside the cell and nucleus were scored. The dose to the cytoplasm and nuclei cavities was also estimated by means of large cavity theory (LCT) and small cavity theory (SCT) calculations. Results: Low brachytherapy energies are more sensitive to chemical composition of the cytoplasm and nucleus. Three of the simulated cell lines have water‐like behavior for adipose tissue while for breast and bone tissue the variations are larger. One cell line deviates for all the simulated tissues. Estimated results calculated with SCT deviates from MC calculated results for the low energies up to 50 keV for 2, 3 and 5 mum nucleus . For the 7 mum nucleus the difference is large up to 300 keV. The largest differences between LCT and MC results are observed for energies up to 100 keV. Conclusions: There is no universally applicable set of conversion factors used to convert Dm,m into the dose received by the cell nuclei. Water equivalent behavior varies depending on the chemical composition of the cell and nucleus and the surrounding tissue.
SU‐E‐T‐08: A PO2‐Based Vascular Tumour Model for Iterative Radiobiological Modeling
Authors: Kirkby C , Ghasroddashti E
Published: November 2011

Purpose: A 2D model of a tumour incorporating the microvasculature environment is developed and used in an iterative manner to model the changing levels of oxygen through its volume over the course of a radiation therapy treatment. Methods: Tumour vasculature is created by iteration, using published pO2 histograms. A 4 mm2 vessel map is randomly generated then convolved with an oxygen‐spreading kernel to generate pO2 map. A cost function is calculated based on the relative square differences between the input and newly created pO2 histograms. Vessel number and spatial position are optimized. Radiation therapy treatment fractions are modelled by iteration. A linear‐quadratic surviving fraction response map is generated incorporating local hypoxia reduction factors (HRFs) that are calculated from the pO2 map. A threshold, dose‐dependent response that modifies the microvasculature environment is also introduced. Vessel, pO2, and HRF maps are updated before calculating the i+1 map of tumour response. Parameters for a squamous cell carcinoma were incorporated for the initial testing. Results: Weighting the cost function allowed us to keep differences in the extremely hypoxic bin (0 – 5 mmHg) and fully oxic bin to less than 1 % of the overall cell count. Differences of as much as 12 % in intermediate bins may indicate limitations of the model to conform to the measured data. The total surviving fraction (TSF) increased by 123% in the vascular response model without any specific vascular dose enhancement compared to the stationary vascular case. Vascular dose enhancement by factors of 1.5 and 2.0 further increased the TSF by 26 % and 50 %, respectively. Conclusions: This work provides a framework for answering questions relating to tumour vasculature response during radiation therapy. These preliminary results suggest potential for caution in enhancing dose to tumour vasculature.
SU‐E‐T‐09: Investigate the Impact of Prolonged Treatment Delivery Time on Tumor Control Probability
Authors: Wang L , Uzan J , Nahum A , Fan J , Jin L , Ma C
Published: November 2011

Purpose: Stereotactic body radiotherapy (SBRT) can be delivered by a linac machine using multiple beams or a Cyberknife machine. The Cyberknife‐ based SBRT usually allows greater dose conformity to the tumor target and reduce normal tissue toxicity. However, it requires much longer treatment time. This work explores the potential impact of such prolonged delivery times on treatment outcome. Methods: The tumor‐control probability (TCP) utilizing an extended linear‐quadratic (LQ) model, which accounts for sublethal damage repair and clonogen proliferation, was used to calculate the cell‐killing efficiency of clinical SBRT plans for 30 patients. LQ parameters derived from compiled clinical data for lung cancer (α=0.31, =0.062, α/β=10 Gy, Tk = 21d, Td = 3d and a 0.025 s‐1 repair constant) were used to compute changes in the TCP due to prolonged delivery time associated with Cyberknife‐ delivery. The dose‐volume histograms were obtained from actual SBRT plans using 12 Gy × 4 or 10Gy ×5 fractions. The impact of delivery time at 10, 50, and 70 min on TCP were studied for all 30 patients. For a few typical patients with small, intermediate, and larger tumors, the TCP was calculated for a range of delivery time from 5 to 100 min. Results: Our calculations showed that an average 99.8% of TCP was reached for most of the patients, if 10 minute delivery time was assumed, except for a few patients with large or multiple targets. By increasing treatment delivery time from 10 to 50 and 70 minutes, the TCP decreased by an average of 3% for most of the patients. Significant decrease was found for those same patients with larger tumors or multiple targets, with the TCP vales in the 67% range. Conclusions: The delivery time may have a significant impact on Cyberknife‐based SBRT treatment outcomes for patients with larger or multiple tumors.
SU‐E‐T‐10: A Computational Model for the Estimation of Biological Dose in a Clinical Proton Beam
Authors: Das I , Anferov V , Besemer A , Moskvin V , Nichiporov D
Published: November 2011

Purpose: The variability of relative biological effectiveness (RBE) of a proton beam with spatial distribution is well known. It is dependent on the proton beam characteristics, type of tissue and biological indices. A computational model based on a proton spectrum and LET is used for the estimation of the biological dose. Methods: The depth dose curve of the pristine beam is divided into two regions: the proximal region from the surface to the Bragg peak and the distal region beyond the peak. Knowing the beam energy at the surface in the proximal region, and using the published energy‐range tables, we estimated the effective proton energy at any depth proximal to the Bragg peak. Based on the energy, the LET and RBE is estimated. In the distal region, we assume that the effective beam energy changes monotonically from the value calculated at the peak to the final energy of about 3MeV at the 3% dose level on the distal edge. Applying the calculated RBE factor to the measured depth dose profile we obtain the biological dose in the pristine beam. The method can easily be applied to modulated beams. Results: Our computation model shows that it is possible to predict the biological dose for any combination of beam energy, and for SOBP size based on the available RBE data. Our results showed that the biological dose for a SOBP has finite slope rising to higher values close to distal edge of the peak. Conclusions: A computational model provides an unique tool to estimate the biological dose in any combination of beam energy and SOBP size. If the RBE vs LET data for a tissue is know our computational approach could provide the biological dose accurately in a clinical beam.