MV Treatment Beam Research Articles

Experimental validation of Fastcat kV and MV cone beam CT (CBCT) simulator.

To experimentally validate the Fastcat cone beam computed tomography (CBCT) simulator against kV and MV CBCT images acquired with a Varian Truebeamlinac. kV and MV CBCT images of a Catphan 504 phantom were acquired using a 100 kVp beam with the on-board imager (OBI) and a 6 MV treatment beam with the electronic portal imaging device (EPID), respectively. The kV Fastcat simulation was performed using detailed models of the x-ray source, bowtie filter, a high resolution voxelized virtual Catphan phantom, anti-scatter grid, and the CsI scintillating detector. Likewise, an MV Fastcat CBCT was simulated with detailed models for the beam energy spectrum, flattening filter, a high-resolution voxelized virtual Catphan phantom, and the gadolinium oxysulfide (GOS) scintillating detector. Experimental and simulated CBCT images of the phantom were compared with respect to HU values, contrast to noise ratio (CNR), and dose linearity. Detector modulation transfer function (MTF) for the two detectors were also experimentally validated. Fastcat's dose calculations were compared to Monte Carlo (MC) dose calculations performed with Topas. For the kV and MV simulations, respectively: Contrast agreed within 14 and 9 HUs and detector MTF agreed within 4.2% and 2.5%. Likewise, CNR had a root mean squared error (RMSE) of 2.6% and 1.4%. Dose agreed within 2.4% and 1.6% of MC values. The kV and MV CBCT images took 71 and 72 s to simulate in Fastcat with 887 and 493 projections, respectively. We present a multienergy experimental validation of a fast and accurate CBCT simulator against a commercial linac. The simulator is open source and all models found in this work can be downloaded from https://github.com/jerichooconnell/fastcat.git.

An Assessment of Dosimetric Characteristics of Inline 2.5 Mega Voltage Unflattened Imaging X-Ray Beam.

Purpose:The aim of this work is to study the dosimetric parameters of newly introduced 2.5 MV imaging x-ray beam used as inline imaging to do setup verification of the patient undergoing radiation therapy. As this x-ray beam is in megavoltage range but comprises of a lower energy spectrum. It is essential to study the pros and cons of 2.5 MV imaging x-ray beam for clinical use.Methods:The mean energy was calculated using the NIST XCOM table through MAC. Profile analysis was done using RFA to understand the percentage depth dose, degree of unflatteness, symmetry, penumbra and out of field dose. Dose to skin for the 2.5 MV x-ray beam was analysed for field sizes 10x10 cm2, 20x20 cm2, 30x30 cm2. Leakage measurements for treatment head and at the patient plane were done using IEC 819/98 protocol. Finally, the spatial resolution and contrast were analyzed with and without patient scatter medium. Results:The MAC at 15 cm off-axis was found to be lower than that at the CAX. Similarly, there was a decrease in mean energy from 0.47 MV to 0.37 MV at 15 cm off-axis. The reduction of mean energy towards off-axis is lower than the other high energy MV x-ray beams. The tuned absolute dose of 1 cGy/MU is consistent and within < ±1 %. The relative output factors were found to be in correlation with Co-60. The beam quality of 2.5 MV x-ray beam was found to be 0.4771. The profile parameters like the degree of unflatness of the 2.5 x-ray beam were studied at 85 %, 90 %, 95 % lateral distances, and the penumbra at different depth and field sizes are higher than the 6 MV treatment beam. In addition, out of field dose also drastically increases to a maximum of up to 30 % laterally at 5cm at deeper depths. The skin dose increases from 48.51 % to 88.15 % from 6 MV to 2.5 MV x-ray beam for the field size 10x10 cm2. Also, the skin dose increases from 88.15 % to 91.78 % from the field size 10x10 cm2 to 30x30 cm2. Although the measured leakage radiation for 2.5 MV x-ray beam at the patient plane and other than patient planes are with the tolerance limit, an increase in exposure towards gantry side compared to other areas around treatment head and the patient plane may lead to more skin dose to head and chest while imaging pelvis region. The MLC transmission of 2.5 MV x-ray beam such as inter, intra and edge effect are 0.40 %, 0.37 % and 11% respectively. The spatial resolution of 2.0, 1.25 and 0.9 LP/mm was observed for KV, 2.5MV, and 6 MV x-ray beams. The spatial resolution and contrast of 2.5 MV x-ray beam are superior to 6 MV x-ray beam and inferior to KV x-rays. Conclusions:The 2.5 MV x-ray imaging beam is analysed in view of beam characteristics and radiation safety to understand the above-studied concepts while using this imaging beam in a clinical situation. In future, if 2.5MV x-ray beam is used for treatment purpose with increased dose rate, the above-studied notions can be incorporated prior to implementation.

Feasibility of real-time motion management with helical tomotherapy.

This study investigates the potential application of image-based motion tracking and real-time motion correction to a helical tomotherapy system. A kV x-ray imaging system was added to a helical tomotherapy system, mounted 90 degrees offset from the MV treatment beam, and an optical camera system was mounted above the foot of the couch. This experimental system tracks target motion by acquiring an x-ray image every few seconds during gantry rotation. For respiratory (periodic) motion, software correlates internal target positions visible in the x-ray images with marker positions detected continuously by the camera, and generates an internal-external correlation model to continuously determine the target position in three-dimensions (3D). Motion correction is performed by continuously updating jaw positions and MLC leaf patterns to reshape (effectively re-pointing) the treatment beam to follow the 3D target motion. For motion due to processes other than respiration (e.g., digestion), no correlation model is used - instead, target tracking is achieved with the periodically acquired x-ray images, without correlating with a continuous camera signal. The system's ability to correct for respiratory motion was demonstrated using a helical treatment plan delivered to a small (10 mm diameter) target. The phantom was moved following a breathing trace with an amplitude of 15 mm. Film measurements of delivered dose without motion, with motion, and with motion correction were acquired. Without motion correction, dose differences within the target of up to 30% were observed. With motion correction enabled, dose differences in the moving target were less than 2%. Nonrespiratory system performance was demonstrated using a helical treatment plan for a 55 mm diameter target following a prostate motion trace with up to 14 mm of motion. Without motion correction, dose differences up to 16% and shifts of greater than 5 mm were observed. Motion correction reduced these to less than a 6% dose difference and shifts of less than 2 mm. Real-time motion tracking and correction is technically feasible on a helical tomotherapy system. In one experiment, dose differences due to respiratory motion were greatly reduced. Dose differences due to nonrespiratory motion were also reduced, although not as much as in the respiratory case due to less frequent tracking updates. In both cases, beam-on time was not increased by motion correction, since the system tracks and corrects for motion simultaneously with treatment delivery.

Systematic intrafraction shifts of mediastinal lymph node targets between setup imaging and radiation treatment delivery in lung cancer patients

Background and purposeInternal target motion results in geometrical uncertainties in lung cancer radiotherapy. In this study, we determined the intrafraction motion and baseline shifts of mediastinal lymph node (LN) targets between setup imaging and treatment delivery. Material and methodsTen lung cancer patients with 2–4 fiducial markers implanted in LN targets received intensity-modulated radiotherapy with a daily setup cone-beam CT (CBCT) scan used for online soft-tissue match on the primary tumor. At a total of 122 fractions, 5 Hz fluoroscopic kV images were acquired orthogonal to the MV treatment beam during treatment delivery. Offline, the 3D trajectory of the markers was determined from their projected trajectory in the CBCT projections and in the intra-treatment kV images. Baseline shifts and changes in the respiratory motion amplitude between CBCT and treatment delivery were determined from the 3D trajectories. ResultsSystematic mean LN baseline shifts of 2.2 mm in the cranial direction (standard deviation (SD): 1.8 mm) and 1.0 mm in the posterior direction (SD: 1.2 mm) occurred between CBCT imaging and treatment delivery. The mean motion amplitudes during CBCT and treatment delivery agreed within 0.2 mm in all directions. ConclusionsSystematic cranial and posterior intrafraction baseline shifts between CBCT and treatment delivery were observed for mediastinal LN targets. Intrafraction motion amplitudes were stable.

Comparison of Bolus Materials to Highly Absorbent Polypropylene and Rayon Cloth

This study assesses the utility of a highly absorbent polypropylene and rayon cloth (HAPRC) as a bolus material in radiation therapy. A comparison of various bolus materials used clinically at our center to the test bolus material, water-saturated HAPRC, was conducted. The reproducibility of the physical density was assessed by computerized tomography (CT) scanning the water-saturated HAPRC on a uniform density block phantom on multiple occasions, allowing the cloth to dry fully between subsequent CT scans. The cloth was found to have a consistent physical density between 0.8 and 1.0 g/cm3. To evaluate the dosimetric impact of the various bolus materials, metal oxide semiconductor field effect transistor dosimetry measurements were performed for a 6 MV treatment beam on the anterior surface of an anthropomorphic thorax phantom. The ability of the bolus material to conform to an irregular surface was assessed by CT scanning the various bolus materials overlaying the nose of an anthropomorphic head phantom and another irregularly shaped test object. The HAPRC conforms well to irregular contours and is relatively inexpensive compared to Superflab and thermoplastic sheets. Before implementing the material into the clinical setting, an appropriate process for laundering the material should be determined, for hygienic reasons.

TH-AB-202-01: Daily Lung Tumor Motion Characterization On EPIDs Using a Markerless Tiling Model

Purpose: Tracking lung tumor motion in real time allows for target dose escalation while simultaneously reducing dose to sensitive structures, thus increasing local control without increasing toxicity. We present a novel intra-fractional markerless lung tumor tracking algorithm using MV treatment beam images acquired during treatment delivery. Strong signals superimposed on the tumor significantly reduced the soft tissue resolution; while different imaging modalities involved introduce global imaging discrepancies. This reduced the comparison accuracies. A simple yet elegant Tiling algorithm is reported to overcome the aforementioned issues. Methods: MV treatment beam images were acquired continuously in beam's eye view (BEV) by an electronic portal imaging device (EPID) during treatment and analyzed to obtain tumor positions on every frame. Every frame of the MV image was simulated by a composite of two components with separate digitally reconstructed radiographs (DRRs): all non-moving structures and the tumor. This Titling algorithm divides the global composite DRR and the corresponding MV projection into sub-images called tiles. Rigid registration is performed independently on tile-pairs in order to improve local soft tissue resolution. This enables the composite DRR to be transformed accurately to match the MV projection and attain a high correlation value through a pixel-based linear transformation. The highest cumulative correlation for all tile-pairs achieved over a user-defined search range indicates the 2-D coordinates of the tumor location on the MV projection. Results: This algorithm was successfully applied to cine-mode BEV images acquired during two SBRT plans delivered five times with different motion patterns to each of two phantoms. Approximately 15000 beam's eye view images were analyzed and tumor locations were successfully identified on every projection with a maximum/average error of 1.8 mm / 1.0 mm. Conclusion: Despite the presence of strong anatomical signal overlapping with tumor images, this markerless detection algorithm accurately tracks intrafractional lung tumor motions. This project is partially supported by an Elekta research grant

SU‐G‐BRA‐09: Estimation of Motion Tracking Uncertainty for Real‐Time Adaptive Imaging

Purpose:kV fluoroscopic imaging combined with MV treatment beam imaging has been investigated for intrafractional motion monitoring and correction. It is, however, subject to additional kV imaging dose to normal tissue. To balance tracking accuracy and imaging dose, we previously proposed an adaptive imaging strategy to dynamically decide future imaging type and moments based on motion tracking uncertainty. kV imaging may be used continuously for maximal accuracy or only when the position uncertainty (probability of out of threshold) is high if a preset imaging dose limit is considered. In this work, we propose more accurate methods to estimate tracking uncertainty through analyzing acquired data in real‐time.Methods:We simulated motion tracking process based on a previously developed imaging framework (MV + initial seconds of kV imaging) using real‐time breathing data from 42 patients. Motion tracking errors for each time point were collected together with the time point's corresponding features, such as tumor motion speed and 2D tracking error of previous time points, etc. We tested three methods for error uncertainty estimation based on the features: conditional probability distribution, logistic regression modeling, and support vector machine (SVM) classification to detect errors exceeding a threshold.Results:For conditional probability distribution, polynomial regressions on three features (previous tracking error, prediction quality, and cosine of the angle between the trajectory and the treatment beam) showed strong correlation with the variation (uncertainty) of the mean 3D tracking error and its standard deviation: R‐square = 0.94 and 0.90, respectively. The logistic regression and SVM classification successfully identified about 95% of tracking errors exceeding 2.5mm threshold.Conclusion:The proposed methods can reliably estimate the motion tracking uncertainty in real‐time, which can be used to guide adaptive additional imaging to confirm the tumor is within the margin or initialize motion compensation if it is out of the margin.

SU-E-J-56: Static Gantry Digital Tomosynthesis From the Beam's-Eye-View

Purpose We have designed a novel TumoTrak™ x-ray system that delivers 19 distinct kV views with the linac gantry stationary. It images MV treatment beam above and below the patient with a kV tomosysthesis slice image from the therapy beam’s-eye-view. Results will be high quality images without MLC shadowing for notable improvements relative to conventional fluoroscopic MV imaging and fluoroscopic kV imaging. Methods A complete design has a kV electron beam multisource X-ray tube that fits around the MV treatment beam path, with little interference with normal radiotherapy and unblocked by the multi-leaf-collimator. To simulate digital tomosynthesis, we used cone-beam CT projection data from a lung SBRT patient. These data were acquired at 125 kVp and 11 fps (0.4 mAs per projection). We chose 19 projections evenly spaced over 27° around one of the treatment angles (240°). Digital tomosynthesis reconstruction of a slice through the tumor was performed using iterative reconstruction. The visibility of the lesion was assessed for the reconstructed digital tomosynthesis (DTS), using fluoroscopy MV images acquired during radiation therapy, and a kV single projection image acquired at the same angle as the treatment field (240°). Results The fluoroscopic DTS images provide the best tumor contrast, surpassing themore » conventional radiographic and the in-treatment MV portal images. The electron beam multisource X-ray tube design has been completed and the tube is being fabricated. The estimated time to cycle through all 19 projections is 700 ms, enabling high frame-rate imaging. While the initial proposed use case is for image guided and gated treatment delivery, the enhanced imaging will also deliver superior radiographic images for patient setup. Conclusion The proposed device will deliver high quality planar images from the beam’s-eye-view without MLC obstruction. The prototype has been designed and is being assembled with first imaging scheduled for May 2015. L. Partain, J. Kwon, D. Boyd: NIH/SBIR R43CA192489-01. J. Rottmann, G. Zentai, R. Berbeco: NIH/NCI 1R01CA188446-01. R. Berbeco: E. Research Grant, Varian Medical Systems.« less

SU‐E‐J‐36: Combining CBCT Dose Into IMRT Treatment Planning

Purpose:Cone beam CT (CBCT) is increasingly used in patient setup for IMRT. Daily CBCT may provide effective localization, however, it introduces concern over excessive imaging dose. Previous studies investigated the calculation of excess CBCT dose, however, no study has yet treated this dose as a source of therapeutic radiation, optimized in consideration of PTV and OARs constrains. Here we present a novel combined MV+kV inverse optimization engine to weave the CBCT and MV dose together such that CBCT dose is used for both imaging and therapeutic purposes. This may mitigate some of the excess imaging dose effects of daily CBCT and allow complete evaluation of the CBCT dose prior to treatment.Methods:The EGSnrc Monte Carlo system was used to model a Varian Trilogy CBCT system and 6 MV treatment beam. Using the model, the dose to patient from treatment beam and imaging beam was calculated for ten patients. The standard IMRT objective function was modified to include CBCT dose. Treatment plan optimization using the MOSEK optimization tool was performed retrospectively with and without assuming kV radiation dose from CBCT, assuming one CBCT per fraction.Results:Across ten patients, the CBCT delivered peaks of between .4% and 3.0% of the prescription dose to the PTV, with average CBCT dose to the PTV between .3% and .8%. By including CBCT dose to skin as a constraint during optimization, peak skin dose is reduced by between 1.9% and 7.4%, and average skin dose is reduced by .2% to 3.3%.Conclusions:Pre‐treatment CBCT may deliver a substantial amount of radiation dose to the target volume. By considering CBCT dose to skin at the point of treatment planning, it is possible to reduce patient skin dose from current clinical levels, and to provide patient treatment with the improved accuracy that daily CBCT provides.

TH‐C‐12A‐10: Surface Dose Enhancement Using Novel Hybrid Electron and Photon Low‐Z Therapy Beams: Monte Carlo Simulation

Purpose:The introduction of the TrueBeam linac platform provides access to an in‐air target assembly making it possible to apply novel treatments using multiple target designs. One such novel treatment uses multiple low‐Z targets to enhance surface dose replacing the use of synthetic tissue equivalent material (bolus). This treatment technique will decrease the common dosimetric and set up errors prevalent in using physical treatment accessories like bolus. The groundwork for a novel treatment beam used to enhance surface dose to within 80‐100% of the dose at dmax by utilizing low‐Z (Carbon) targets of various percent CSDA range thickness operated at 2.5–4 MeV used in conjunction with a clinical 6 MV beam is presented herein.Methods:A standard Monte Carlo model of a Varian Clinac accelerator was developed to manufacturers specifications. Simulations were performed using Be, C, AL, and C, as potential low‐Z targets, placed in the secondary target position. The results determined C to be the target material of choice. Simulations of 15, 30 and 60% CSDA range C beams were propagated through slab phantoms. The resulting PDDs were weighted and combined with a standard 6 MV treatment beam. Versions of the experimental targets were installed into a 2100C Clinac and the models were validated.Results:Carbon was shown to be the low‐Z material of choice for this project. Using combinations of 15, 30, 60% CSDA beams operated at 2.5 and 4 MeV in combination with a standard 6 MV treatment beam the surface dose was shown to be enhanced to within 80–100% the dose at dmax.Conclusion:The modeled low‐Z beams were successfully validated using machined versions of the targets. Water phantom measurements and slab phantom simulations show excellent correlation. Patient simulations are now underway to compare the use of bolus with the proposed novel beams.NSERC

Combined MV + kV inverse treatment planning for optimal kV dose incorporation in IGRT

Despite the existence of real-time kV intra-fractional tumor tracking strategies for many years, clinical adoption has been held back by concern over the excess kV imaging dose cost to the patient when imaging in continuous fluoroscopic mode. This work aims to solve this problem by investigating, for the first time, the use of convex optimization tools to optimally integrate this excess kV imaging dose into the MV therapeutic dose in order to make real-time kV tracking clinically feasible. Phase space files modeling both a 6 MV treatment beam and a kV on-board-imaging beam of a commercial LINAC were generated with BEAMnrc, and used to generate dose influence matrices in DOSXYZnrc for ten previously treated lung cancer patients. The dose optimization problem for IMRT, formulated as a quadratic problem, was modified to include additional constraints as required for real-time kV intra-fractional tracking. An interior point optimizer was used to solve the modified optimization problem. It was found that when using large kV imaging apertures during fluoroscopic tracking, combined MV + kV optimization lead to a 0.5%–5.17% reduction in the total number of monitor units assigned to the MV beam due to inclusion of the kV dose over the ten patients. This was accompanied by a reduction of up to 42% of the excess kV dose compared to standard MV IMRT with kV tracking. For all kV field sizes considered, combined MV + kV optimization provided prescription dose to the treatment volume coverage equal to the no-imaging case, yet superior to standard MV IMRT with non-optimized kV fluoroscopic tracking. With combined MV + kV optimization, it is possible to quantify in a patient specific way the dosimetric effect of real-time imaging on the patient. Such information is necessary when substantial kV imaging is performed.

WE‐A‐134‐02: Combined MV+kV Beam Optimization for Enabling Real‐Time KV Tumor Tracking

Purpose: Despite the existence of real‐time kV intrafractional tumor tracking strategies for many years, clinical adoption has been held back by concern over excess kV imaging dose cost to the patient when imaging in continuous fluoroscopic mode. This work aims to solve this specific problem by performing a first time investigation into the use of convex optimization tools to best integrate this excess kV imaging dose into the MV therapeutic dose as to make real‐time kV tracking clinically feasible. Methods: Phase space files for both a 125 kVp kV beam and a 6 MV treatment beam were generated with BEAMNRC, and used to make dose influence matrices in DOSXYZNRC for a lung cancer patient. The dose optimization problem for IMRT, formulated as a quadratic problem, was modified to include additional kV imaging constraints as required for real‐time kV fluorescent tracking. The MOSEK optimization toolkit was used to solve the modified optimization problem. Results: Compared to conventional IMRT optimization, combined MV+kV optimization leads to a 2.4–5.2% reduction in the total number of monitor units assigned to the MV beam due to inclusion of the kV dose. When using an open kV aperture, combined MV+kV optimization allows for a reduction of up to 50% of the excess kV dose compared to standard IMRT with kV fluoro tracking. For all kV field sizes considered, combined MV+kV optimization provided PTV coverage equal to standard IMRT without kV imaging and superior to standard IMRT plus kV fluoro tracking. Skin dose with combined MV+kV imaging was only slightly higher than the no‐imaging case. Conclusion: The use of MV+kV optimization allows for greatly reduced imaging dose to the skin, especially in cases of large kV aperture imaging. Combined MV+kV optimization demonstrates potential for real‐time tumor tracking without excessive imaging dose, paving the way for clinical implementation.

SU-E-J-127: An Initial Application of Evaluating Lung Tumor Motion Throughout Radiotherapy

We present a novel method for evaluating lung tumor motion incorporating planning CT scan, imaging for patient localization, and during treatment delivery. Tumor motion is evaluated at three stages of the treatment process. Following the acquisition of 4D CT scans for treatment planning, tumors are contoured at one phase and registered to other phases to obtain tumor motion patterns. Tumor motion prior to each treatment is evaluated by identifying tumors directly on every raw projection of the daily localization CBCT scans. Finally, tumor motion during treatment is evaluated by analyzing MV treatment beam images. Every frame of image is evaluated as three components, each with separate DRRs: all non-moving structures, the tumor, and other moving structures. The locations of three components are determined though a registration process. In order to quantify tumor motion, the cumulated probability, the percentage of time when the tumor motion is equal to or smaller than a given range, is evaluated at every stage. This methodology has been applied to five lung cancer patients undergoing radiotherapy. More than 5400 frames from MV treatment beam images and 24 CBCT scans were acquired from 12 fractions. Both the average tumor position and the cumulated probability with the range were evaluated. The tumor motion ranges are generally larger than those obtained from 4D CT. Significant variation in motion patterns was observed as well. Information obtained from 4DCT is insufficient for motion assessment. In contrast, our novel methodology is capable of accurately characterizing lung tumor motion on a daily basis, without the need for implanted fiducial markers and without additional imaging dose. It provides daily verification that the tumor range is within prior estimates and covered by the treatment planning volume. This research is supported by CPRIT Individual Investigator Award RP110329.

Image quality improvement in megavoltage cone beam CT using an imaging beam line and a sintered pixelated array system

To quantify the improvement in megavoltage cone beam computed tomography (MVCBCT) image quality enabled by the combination of a 4.2 MV imaging beam line (IBL) with a carbon electron target and a detector system equipped with a novel sintered pixelated array (SPA) of translucent Gd(2)O(2)S ceramic scintillator. Clinical MVCBCT images are traditionally acquired with the same 6 MV treatment beam line (TBL) that is used for cancer treatment, a standard amorphous Si (a-Si) flat panel imager, and the Kodak Lanex Fast-B (LFB) scintillator. The IBL produces a greater fluence of keV-range photons than the TBL, to which the detector response is more optimal, and the SPA is a more efficient scintillator than the LFB. A prototype IBL + SPA system was installed on a Siemens Oncor linear accelerator equipped with the MVision(TM) image guided radiation therapy (IGRT) system. A SPA strip consisting of four neighboring tiles and measuring 40 cm by 10.96 cm in the crossplane and inplane directions, respectively, was installed in the flat panel imager. Head- and pelvis-sized phantom images were acquired at doses ranging from 3 to 60 cGy with three MVCBCT configurations: TBL + LFB, IBL + LFB, and IBL + SPA. Phantom image quality at each dose was quantified using the contrast-to-noise ratio (CNR) and modulation transfer function (MTF) metrics. Head and neck, thoracic, and pelvic (prostate) cancer patients were imaged with the three imaging system configurations at multiple doses ranging from 3 to 15 cGy. The systems were assessed qualitatively from the patient image data. For head and neck and pelvis-sized phantom images, imaging doses of 3 cGy or greater, and relative electron densities of 1.09 and 1.48, the CNR average improvement factors for imaging system change of TBL + LFB to IBL + LFB, IBL + LFB to IBL + SPA, and TBL + LFB to IBL + SPA were 1.63 (p < 10(- 8)), 1.64 (p < 10(- 13)), 2.66 (p < 10(- 9)), respectively. For all imaging doses, soft tissue contrast was more easily differentiated on IBL + SPA head and neck and pelvic images than TBL + LFB and IBL + LFB. IBL + SPA thoracic images were comparable to IBL + LFB images, but less noisy than TBL + LFB images at all imaging doses considered. The mean MTFs over all imaging doses were comparable, at within 3%, for all imaging system configurations for both the head- and pelvis-sized phantoms. Since CNR scales with the square root of imaging dose, changing from TBL + LFB to IBL + LFB and IBL + LFB to IBL + SPA reduces the imaging dose required to obtain a given CNR by factors of 0.38 and 0.37, respectively. MTFs were comparable between imaging system configurations. IBL + SPA patient image quality was always better than that of the TBL + LFB system and as good as or better than that of the IBL + LFB system, for a given dose.

Clinical Implementation of Gated RapidArc for SBRT

Purpose/Objective(s): The newly acquired Varian TrueBeamSTx linac is capable of delivering volumetric modulated arc therapy (RapidArc) under respiratory-gated conditions. When operating under flattening-filter free (FFF) mode, dose rate of up to 2400 MU/min. is achievable. This has made gated RapidArc a suitable choice for Stereotactic Body Radiotherapy (SBRT) for sites with respiratory motion, such as lung and pancreas. This study describes the implementation process.

WE‐E‐BRB‐06: An EPID Based Method for High Accuracy Calibration of an Optical External Marker Tracking Device with the LINAC Reference Frame

Purpose: The accuracy of an optical camera system used for patient motion monitoring is typically limited to 1–2 mm due to use of the light field crosshair and in‐room lasers for initial camera calibration setup making it unsuitable for highly accurate patient monitoring procedures such as frameless SRS. To provide submillimeter accuracy in calibrating a 3D stereoscopic infrared optical camera with the treatment frame of a LINAC, a semi‐automated calibration method was developed based on simultaneous viewing of external markers with both the MV treatment beam and optical camera system. Methods: A phantom comprised of 9 steel BBs covered in highly IR reflective tape was placed on the LINAC treatment couch and imaged with both a 3D stereoscopic IR imager, and the on board MV EPID imager at a variety of gantry angles. Using the relative 3D BB positions given by the camera, simulated annealing was used to optimize 9 parameters including source‐to‐imager distance, detector tilt, and deviations from nominal isocenter. Singular value decomposition was then used to calculate the transformation between different reference frames. Additionally, a traditional method of calibration was used for comparison. Results: Using modeled data, the simulated annealing process was able to determine the locations of the BBs with .2 mm accuracy. Using projection images acquired with an MV imager, initial preliminary results show the ability to determine locations of BBs with a .2 mm planar standard deviation and a 1 mm axial standard deviation. Conclusions: The method can be used to provide a highly accurate spatial registration between an external 3D camera frame and the LINAC frame. The real MV imager results, while not as precise as the simulated results, exceed current standards and are expected to improve with continued refinement.

SU‐E‐T‐220: Towards Automatic Quality Assurance of Kv‐CBCT Based IGRT Systems: A Time Series Analysis of KV‐MV Isocentre Coincidence

Purpose: Regular measurement of the coincidence of CBCT image with the MV treatment beam isocentre is an important component of quality assurance for CBCT based IGRT. In this work the coincidence of the imaging isocentre (kV‐CBCT) with the treatment isocentre measured using the EPID (MV‐EPID) has been measured over an extended period using an algorithm developed to perform the measurement automatically. Methods: The treatment isocentre was computed by analysing EPID images of the PentaGuide phantom (MODUS Medical Devices) for gantry angles of 0°, 90°, 180° and 270° with collimator angles of 90° and 270°. The position of the centre of the PentaGuideˈs central air cavity was calculated relative to the radiation isocentre using the EPID images and relative to the CBCT isocentre in the CBCT images. The vector difference between these two measurements gave the coincidence of the MV radiation isocentre and kV‐CBCT image origin. Processing was performed on the development platform of the ARTISCAN software (AQUILAB, Lille, France) which utilised image processing to automate these calculations directly from the DICOM images. The software was first validated using simulated MV and CBCT images of the phantom with known offsets. Site validation was performed with the Synergy system (ELEKTA, Crawley, UK) and using the Hexapod system (ELEKTA) to offset the phantom. In addition, 10 measurements were performed on two Synergy systems over a 17 months period during site validation. Results: No deviations were observed using the simulated images, and phantom position measurements were confirmed with the offset phantom during on‐site validation with minor deviations of: absolute mean error MV and kV was 0.33 mm and 0.35 mm respectively. The distance between isocentres was less than 1 mm over the period of testing (max=0.53, mean=0.26, std=0.16). Conclusions: The algorithms developed show efficiency and reliability for automatic QA of the isocentre position.

MO-D-BRC-05: Reducing Megavoltage Cone Beam CT Imaging Dose without Loss of Spatial Resolution Using a Sintered Pixelated Array System

Purpose: To demonstrate that megavoltage cone beam CT (MVCBCT) image quality can be significantly improved without increasing imaging dose or reducing spatial resolution for both head‐ and pelvis‐sized volumes. The improvement is achieved with the combination of an imaging beam line (IBL) with a low atomic number electron target and a novel sintered pixelated array (SPA) detector. Methods: Three Siemens Oncor linear accelerators were equipped with an IBL+SPA system, an IBL system with a conventional Kodak Lanex Fast B scintillator (IBL+LFB), and a 6 MV treatment beam line system with an LFB (TBL+LFB). Head‐ and pelvis‐sized phantom images were acquired with all three systems at imaging doses ranging from 2‐60 cGy. Contrast to noise ratio (CNR) and modulation transfer function (MTF) were calculated from the phantom images. Head and neck, prostate, and lung cancer patients were imaged with the three imaging systems at doses ranging from 2–15 cGy. Results: For head‐ and pelvis‐sized phantom images acquired at 5 cGy or above, the CNR average percentage increases for imaging system upgrades from TBL+LFB to IBL+LFB to IBL+SPA were 52% (p < 1E‐7) and 42% (p < 1E‐6), respectively. The MTFs do not change with imaging system by statistically significant percentages. Soft tissue contrast is generally more easily differentiated on IBL+SPA images than TBL+LFB and IBL+LFB in the patient images. Conclusions: Since CNR scales with the square root of imaging dose, each step in the TBL+LFB to IBL+LFB to IBL+SPA upgrade halves the imaging dose required to obtain a given CNR. No statistically significant change in spatial resolution was observed with any upgrade, suggesting that the pixelation of the SPA prevents a loss in spatial resolution. IBL+SPA patient image quality was always better than that of the TBL+LFB system and as good as or better than that of the IBL+LFB system. Sponsored partially by Siemens Oncology Care Systems.

TU‐G‐BRC‐03: A Novel Markerless Technique for Real‐Time Lung Tumor Tracking

Purpose: A critical component missing in the radiotherapy process for lung cancer is real‐time monitoring of lung tumors. The large planning margin that is typically required to account for rumor motion compromises sparing of critical structures and normal tissues. A novel markerless method has been developed to monitor lung tumor motion during treatment.Methods: The method automatically identifies lung tumors directly on continuous MV treatment beam images. Prior information from planning CT and contours is used to generate three sets of DRRs for lung tumors, other moving organs, and static organs, respectively. Composite DRRs are produced by combining three sets of DRRs, with weighting factors assigned to compensate for different attenuation coefficients between MV and kV x‐ray energies. Individual locations of three set of DRR are optimized by comparing with acquired MV treatment beam images using a normalized cross correlation algorithm. The method was evaluated by monitoring a target moving inside a chest cavity phantom. It was also applied retrospectively to online MV images from lung cancer patients. Results: In phantom studies, the target and target motion characteristics were successfully located within seven different treatment fields; the detected target locations have a maximum deviation of 0.8 mm from expected locations. Lung tumors in lung patients were successfully tracked. Although the planning 4D CT indicated that the lung tumor motion range was within 0 mm, real‐time tracking suggested that: a) the lung tumor motion pattern is not regular; and b) the average tumor locations, the respiratory frequencies, and the tumor motion amplitude (range) vary considerably during treatment. Conclusions: This tracking method, which requires no implanted markers and can be performed with no additional imaging dose, will enable clinicians to reduce margins and safely deliver higher doses by accounting for the effects of displacement and motion during treatment.

TU‐G‐BRC‐07: Multileaf Collimator Tracking of Respiratory Motion Using a Novel X‐Ray Monitoring System

Purpose: To demonstrate for the first time MLC tracking of respiratory motion based solely on x‐ray motion monitoring in the unique in‐line geometry. Methods: A Siemens Artiste linac was equipped with two flat‐panel imagers (FPI), one directly underneath the linac head, the other underneath the patient couch. The kV x‐ray beam pointed towards the linac head and was tilted by 5.4deg from the MV treatment beam axis, such that the two beams were spatially separated on both FPIs. A programmable phantom with an embedded metal marker reproduced a 3D lung tumor motion trace. The marker position was automatically detected in the kV images, which were acquired on the upper FPI at a frame‐rate of 7.1Hz. The marker coordinates were sent to our tumor tracking system, which adapted the aperture of a Siemens 160 MLC in real‐time. System latency was compensated through a linear regression predictor, which was trained on a 40s breathing data window. The lower FPI was used to assess both the system latency and the geometric tracking accuracy by identifying MV field and marker position. System latency was determined by tracking a sinusoidal motion trace without prediction and calculating the phase difference from least squares fits to the phantom and treatment field trajectories. Radiochromic films were used to assess dosimetric tracking accuracy. A static radiation delivery to the non‐moving target served as reference for the delivery to the moving phantom with and without tracking. Results: The total system latency was 740ms. Respiratory motion tracking yielded root mean squared deviations between phantom and MV field positions of 1.71mm (1.26mm) parallel (perpendicular) to the leaf travel direction. 2%/2mm gamma failure rates were decreased from 42% to 20%. Conclusions: We achieved accurate MLC tracking of irregular breathing motion (perpendicular to the treatment beam) based solely on x‐ray imagery.Supported by Siemens Healthcare/CR.