OBI System Research Articles

BRAILLE METHODS OF TEXT RECOGNITION USING "DEEP LEARNING"

The Uzbek Braille Identifier System (OBI), which will help visually impaired and blind people to independently read and understand texts with the help of artificial intelligence, is being developed in the republic. Braille is the primary means of written communication for the visually impaired, but users who are not profi cient in Braille may have difficulty understanding it. OBI aims to fill this gap by translating braille into a more readable form for nonbraille users. The OBI system specifi cally targets punctuation recognition in braille documents and is part of the broader field of optical braille recognition (OBR), which is still new and requires significant research. The goal of OBI is to improve written communication for the visually impaired in Uzbekistan.

IGRT stereotassica intracranica con Brainlab Exactrac e con sistema integrato OBI: feedback dell’equipe dell’U.O.C. Di Radioterapia del P.O. Perrino di Brindisi

The stereotactic intracranial Radiotherapy (SRS) Imaging-guided (IGRT) has increased its developments in few years because of the possibility to target lots of brain tumors also very small thanks to the new hardware tecnologies. SRS represents the main surgical alternative for patients with high risk complicances. This treatments requires a technology ensuring a great accuracy during X-ray exposure on the target as planned. In Perrino's Radiotherapy there are two linacs with two different tecnologies ensuring the patient's set-up accuracy (less than 1 mm). The first one is the OBI system mounted on Linac Trilogy, the second one Brainlab ExacTrac on Linac Unique. The aim of this comunication is to compare the two different system, to know which is the most appropriate ad finally to justify why a new tecnology such as Exactrac is installed on a less performing linac.

Accuracy of Dose Calculation with CBCT Images under Different Scanning Conditions

The treatment plans were designed based on planning CT of Catphan phantom's homogenic and heterogenetical module, respectively. With the OBI system, Catphan phantom was scanned under different scanning conditions. The dose was recalculated by applying treatment plans based on planning CT to the CBCT images with its individual hounsfield unit-electron density calibration curve. The dose distributions were compared with those of the original plan, the results of HI showed a good agreement. CIs in the homogenic module were superior to those in the heterogenetical one. There is a good dose distribution for CBCT images under different scanning conditions. It is helpful for CBCT images directly used for dose re-calculation in adaptive radiation therapy (ART).

Evaluation of dose from kV cone-beam computed tomography during radiotherapy: a comparison of methodologies

Three alternative methodologies to the Computed-Tomography Dose Index for the evaluation of Cone-Beam Computed Tomography dose are compared, the Cone-Beam Dose Index, IAEA Human Health Report No. 5 recommended methodology and the AAPM Task Group 111 recommended methodology. The protocols were evaluated for Pelvis and Thorax scan modes on Varian® On-Board Imager and Truebeam kV XI imaging systems. The weighted planar average dose was highest for the AAPM methodology across all scans, with the CBDI being the second highest overall. A 17.96% and 1.14% decrease from the TG-111 protocol to the IAEA and CBDI protocols for the Pelvis mode and 18.15% and 13.10% decrease for the Thorax mode were observed for the XI system. For the OBI system, the variation was 16.46% and 7.14% for Pelvis mode and 15.93% to the CBDI protocol in Thorax mode respectively.

SU‐F‐J‐14: Kilovoltage Cone‐Beam CT Dose Estimation of Varian On‐Board Imager Using GMctdospp Monte Carlo Framework

Purpose:Although cone‐beam CT (CBCT) imaging became popular in radiation oncology, its imaging dose estimation is still challenging. The goal of this study is to assess the kilovoltage CBCT doses using GMctdospp ‐ an EGSnrc based Monte Carlo (MC) framework.Methods:Two Varian OBI x‐ray tube models were implemented in the GMctpdospp framework of EGSnrc MC System. The x‐ray spectrum of 125 kVp CBCT beam was acquired from an EGSnrc/BEAMnrc simulation and validated with IPEM report 78. Then, the spectrum was utilized as an input spectrum in GMctdospp dose calculations. Both full and half bowtie pre‐filters of the OBI system were created by using egs_prism module. The x‐ray tube MC models were verified by comparing calculated dosimetric profiles (lateral and depth) to ion chamber measurements for a static x‐ray beam irradiation to a cuboid water phantom. An abdominal CBCT imaging doses was simulated in GMctdospp framework using a 5‐year‐old anthropomorphic phantom. The organ doses and effective dose (ED) from the framework were assessed and compared to the MOSFET measurements and convolution/superposition dose calculations.Results:The lateral and depth dose profiles in the water cuboid phantom were well matched within 6% except a few areas ‐ left shoulder of the half bowtie lateral profile and surface of water phantom. The organ doses and ED from the MC framework were found to be closer to MOSFET measurements and CS calculations within 2 cGy and 5 mSv respectively.Conclusion:This study implemented and validated the Varian OBI x‐ray tube models in the GMctdospp MC framework using a cuboid water phantom and CBCT imaging doses were also evaluated in a 5‐year‐old anthropomorphic phantom. In future study, various CBCT imaging protocols will be implemented and validated and consequently patient CT images will be used to estimate the CBCT imaging doses in patients.

Accounting for patient size in the optimization of dose and image quality of pelvis cone beam CT protocols on the Varian OBI system.

The purpose of this study was to develop size-based radiotherapy kilovoltage cone beam CT (CBCT) protocols for the pelvis. Image noise was measured in an elliptical phantom of varying size for a range of exposure factors. Based on a previously defined "small pelvis" reference patient and CBCT protocol, appropriate exposure factors for small, medium, large and extra-large patients were derived which approximate the image noise behaviour observed on a Philips CT scanner (Philips Medical Systems, Best, Netherlands) with automatic exposure control (AEC). Selection criteria, based on maximum tube current-time product per rotation selected during the radiotherapy treatment planning scan, were derived based on an audit of patient size. It has been demonstrated that 110 kVp yields acceptable image noise for reduced patient dose in pelvic CBCT scans of small, medium and large patients, when compared with manufacturer's default settings (125 kVp). Conversely, extra-large patients require increased exposure factors to give acceptable images. 57% of patients in the local population now receive much lower radiation doses, whereas 13% require higher doses (but now yield acceptable images). The implementation of size-based exposure protocols has significantly reduced radiation dose to the majority of patients with no negative impact on image quality. Increased doses are required on the largest patients to give adequate image quality. The development of size-based CBCT protocols that use the planning CT scan (with AEC) to determine which protocol is appropriate ensures adequate image quality whilst minimizing patient radiation dose.

SU‐E‐J‐22: A Feasibility Study On KV‐Based Whole Breast Radiation Patient Setup

Purpose:In room kilovoltage x‐ray (kV) imaging provides higher contrast than Megavoltage (MV) imaging with faster acquisition time compared with on‐board cone‐beam computed tomography (CBCT), thus improving patient setup accuracy and efficiency. In this study we evaluated the clinical feasibility of utilizing kV imaging for whole breast radiation patient setup.Methods:For six breast cancer patients with whole breast treatment plans using two opposed tangential fields, MV‐based patient setup was conducted by aligning patient markers with in room lasers and MV portal images. Beam‐eye viewed kV images were acquired using Varian OBI system after the set up process. In house software was developed to transfer MLC blocks information overlaying onto kV images to demonstrate the field shape for verification. KV‐based patient digital shift was derived by performing rigid registration between kV image and the digitally reconstructed radiography (DRR) to align the bony structure. This digital shift between kV‐based and MV‐based setup was defined as setup deviation.Results:Six sets of kV images were acquired for breast patients. The mean setup deviation was 2.3mm, 2.2mm and 1.8mm for anterior‐posterior, superior‐inferior and left‐right direction respectively. The average setup deviation magnitude was 4.3±1.7mm for six patients. Patient with large breast had a larger setup deviation (4.4–6.2mm). There was no strong correlation between MV‐based shift and setup deviation.Conclusion:A preliminary clinical workflow for kV‐based whole breast radiation setup was established and tested. We observed setup deviation of the magnitude below than 5mm. With the benefit of providing higher contrast and MLC block overlaid on the images for treatment field verification, it is feasible to use kV imaging for breast patient setup.

SU‐E‐T‐714: Study On Set‐Up Accuracy of Three Different Immobilization Methods in Radiotherapy for Thoracic Cancer

Purpose:Using VARIAN Clinac ix cone beam CT to evaluate the different immobilization methods in thoracic cancer radiotherapy positioning accuracy.Methods:Ninety thoracic cancer patients were selected and were divided into 3 group randomly according to immobilization method, with Body fixed film, vacuum mat, Arm bracket joint vacuum mat. All of the patients in the first three times before radiotherapy with OBI system KV CBCT scanning,plan to get the volume of the image was treated with CT image comparison,two image space position deviation. After treatment of CBCT position verification once a week.Results:Body fixed way setup errors X, Y, Z axis deviation mean respectively (2.038±1.416)mm, (2.267±1.568)mm, (1.412±1.408)mm. vacuum mat fixed way respectively (2.395±1.576)mm, 2.721±1.764)mm,(2.419±1.516)mm. Arm bracket joint vacuum mat fixed way respectively (2.325±1.419)mm,(1.837±0.814)mm, (1.674±1.213)mm. Body fixed group Compared with the vacuum mat group P<0.05. Arm bracket joint vacuum mat fixed group Compared with the vacuum mat group P<0.05. Arm bracket joint vacuum mat fixed group Compared with the Body fixed group P<0.05.Conclusion:Thoracic tumor radiotherapy using Arm bracket joint vacuum mat fixed patients can significantly reduce the position error,improve the positioning accuracy,precise radiotherapy to provide quality assurance

SU‐C‐207‐05: A Comparative Study of Noise‐Reduction Algorithms for Low‐Dose Cone‐Beam Computed Tomography

Purpose:To study different noise‐reduction algorithms and to improve the image quality of low dose cone beam CT for patient positioning in radiation therapy.Methods:In low‐dose cone‐beam CT, the reconstructed image is contaminated with excessive quantum noise. In this study, three well‐developed noise reduction algorithms namely, a) penalized weighted least square (PWLS) method, b) split‐Bregman total variation (TV) method, and c) compressed sensing (CS) method were studied and applied to the images of a computer–simulated “Shepp‐Logan” phantom and a physical CATPHAN phantom. Up to 20% additive Gaussian noise was added to the Shepp‐Logan phantom. The CATPHAN phantom was scanned by a Varian OBI system with 100 kVp, 4 ms and 20 mA. For comparing the performance of these algorithms, peak signal‐to‐noise ratio (PSNR) of the denoised images was computed.Results:The algorithms were shown to have the potential in reducing the noise level for low‐dose CBCT images. For Shepp‐Logan phantom, an improvement of PSNR of 2 dB, 3.1 dB and 4 dB was observed using PWLS, TV and CS respectively, while for CATPHAN, the improvement was 1.2 dB, 1.8 dB and 2.1 dB, respectively.Conclusion:Penalized weighted least square, total variation and compressed sensing methods were studied and compared for reducing the noise on a simulated phantom and a physical phantom scanned by low‐dose CBCT. The techniques have shown promising results for noise reduction in terms of PSNR improvement. However, reducing the noise without compromising the smoothness and resolution of the image needs more extensive research.

Validation of a technique for estimating organ doses for kilovoltage cone-beam CT of the prostate using the PCXMC 2.0 patient dose calculator

The use of cone beam CT in common radiotherapy treatments is increasing with the growth of image guided radiotherapy. Whilst the benefits that this technology offers are clear, such as improved patient positioning prior to treatment, it is always important to consider the implications of such intensive imaging regimes on the patient, especially when considering the fundamental radiation protection requirements for justification and optimisation.The purpose of this study was to develop a technique that uses readily available dose calculation software (PCXMC 2.0) to estimate the organ and effective doses that result from these types of examination in prostate treatments on the Varian OBI system. It has been shown that by separating these types of examinations into 28 different projections, with a range of x-ray beam qualities, it is possible to reproduce the complex geometry that is used on these imaging systems in PCXMC i.e. asymmetric radiation field with a half bowtie filter rotating 360° around the patient.This new technique has been validated with thermo-luminescent dosimeter measurements in the Rando anthropomorphic phantom, and has been shown to give excellent agreement with this established method (R2 = 0.995). This technique will prove to be valuable to radiotherapy departments that are looking to optimise their CBCT imaging protocols as it allows a rapid evaluation of the impact of any changes on patient dose. It also serves to further highlight the levels of dose that these types of patient are subject to when having daily CBCT scans as part of the treatment, which further reinforces the need for optimisation of both patient dose and image quality on these systems.

Optimization of heart block in the left-sided whole breast radiation treatments.

Purpose: Blocks have been used to protect heart from potential radiation damage in left-sided breast treatments. Since cardiac motion pattern may not be fully captured on conventional 3DCT or 4DCT simulation scans, this study was intended to investigate the optimization of the heart block design taking the cardiac motion into consideration.Materials and Methods: Whole breast treatment plans using two opposed tangential fields were designed based on 4DCT simulation images for 10 left-sided breast cancer patients. Using an OBI system equipped to a Varian Linac, beam-eye viewed fluoroscopy images were acquired for each of the treatment beams after patient treatment setup, and the MLC heart blocks were overlaid onto the fluoroscopy images with an in-house software package. A non-rigid image registration and tracking algorithm was utilized to track the cardiac motion on the fluoroscopy images with minimal manual delineation for initialization, and the tracked cardiac motion information was used to optimize the heart block design to minimize the radiation damage to heart while avoiding the over-shielding that may lead to underdosing certain breast tissues.Results: Twenty-three sets of fluoroscopy images were acquired on 23 different days of treatment for the 10 patients. As expected, heart moved under the influences of both respiratory and cardiac motion. It was observed that for 16 out of the 23 treatments, heart moved beyond the planed heart block into treatment fields and MLC had to be adjusted to fully block heart. The adjustment was made for all but one patient. The number of the adjusted MLC leaves ranged from 1 to 16 (mean = 10), and the MLC leaf position adjustment ranged from 2 to 10 mm (mean = 6 mm). The added heart block areas ranged from 3 to 1230 mm2 (mean = 331 mm2).Conclusion: In left-sided whole breast radiation treatments, simulation CT (and 4DCT) based heart block design may not provide adequate heart protection for all the treatments. A fluoroscopy-based method has been developed to adaptively optimize the heart MLC block to achieve optimal heart protection.

TH‐A‐18C‐11: An Investigation of KV CBCT Image Quality and Dose Reduction for Volume‐Of‐Interest Imaging Using Dynamic Collimation

Purpose:The focus of this work was to investigate the improvements in image quality and dose reduction for volume‐of‐interest (VOI) kV‐CBCT using dynamic collimation.Methods:A prototype iris aperture was used to track a VOI during a CBCT acquisition. The current aperture design is capable of one‐dimensional translation as a function of gantry angle and dynamic adjustment of the iris radius. The aperture occupies the location of the bow‐tie filter on a Varian OBI system. CBCT and planar image quality was investigated as a function of aperture radius, while maintaining the same dose to the VOI, for a 20 cm diameter cylindrical water phantom with a 9 mm diameter bone insert centered on isocenter. Corresponding scatter‐to‐primary ratios (SPR) were determined at the detector plane with Monte Carlo simulation using EGSnrc. Dose distributions for various anatomical sites were modeled using a dynamic BEAMnrc library and DOSXYZnrc. The resulting VOI dose distributions were compared to full‐field distributions.Results:SPR was reduced by a factor of 8.4 when decreasing iris diameter from 21.2 cm to 2.4 cm (at isocenter). Similarly, this change in iris diameter corresponds to a factor increase of approximately 1.4 and 1.5 in image contrast for CBCT and planar images, respectively, and similarly a factor decrease in image noise of approximately 1.7 and 1.5. This results in a measured gain in contrast‐to‐noise ratio of a factor of approximately 2.3 for both CBCT and planar images. Depending upon the anatomical site, dose was reduced to 10%–70% of the full field value along the central axis plane and down to 2% along the axial planes, while maintaining the same dose to the VOI compared to full‐field techniques.Conclusion:The presented VOI technique offers improved image quality for image‐guided radiotherapy while sparing the surrounding volume of unnecessary dose compared to full‐field techniques.

SU-E-J-244: Validation of a 6D-Robotic-Couch and Image Guidance Radiation Therapy (IGRT) System for Localization in World's First Single-Room Proton Therapy System

Purpose: The Mevion S250 proton therapy unit is equipped with a 6D-robotic couch and IGRT system (Verity). The patient alignment process allows corrections in six degrees of freedom: translation {x,y,z}, pitch, roll, and yaw {θ,ϑ,ψ}. Geometric accuracy of couch corrections and imaging vs. radiation isocenter coincidence were quantified before clinical implementation. Methods: A commercial phantom with sixteen 2mm tungsten BBs was rigidly couch-mounted and imaged with CT. Seventeen rigid translations/rotations of known magnitude were digitally applied to the original CT image using commercial software, validated with Varian OBI system. For each altered image, phantom was mounted on robotic couch in original position, then Verity 2D:2D match (PA-LLAT) was performed using DRRs from altered images. Corrections were recorded and applied, phantom was imaged a second time and residual corrections recorded. Physical measurements verified that applied couch corrections coincided with both physical couch shifts/rotations and known CT image translations/rotations. Additionally, image vs. radiation isocenter coicidence was quantified over couch treatment angles (±90° from setup position) using radiochromic film and an image-guided couch star-shot. Posterior-anterior and left-lateral kV radiographs were taken before each beam was delivered to verify imaging/radiation isocentricity. Results: Verity suggested couch corrections and known CT shifts/rotations agreed within ±1mm (average: Δ lat=0.5mm; Δ vert=0.4mm; Δ long=0.3mm) and ± 0.4° (average: Δ pitch=0.24° Δ roll=0.01°; Δ yaw=0.10°). Physical couch measurements and Verity applied corrections agreed within ± 1mm (average: Δlat=0.5mm; Δvert=0.4mm; Δlong=0.2mm) and ±0.2° (average: Δpitch=0.03°; Δ roll=0.04°; Δ yaw=0.04°). The directionality of all translations and rotations were qualitatively verified. The image vs. radiation isocenter coincidence was <1mm and radiation-isocenter precision was <1mm over the 180° of couch motion, as indicated by film analysis. Conclusion: The Verity IGRT software and 6D-couch combination on the Mevion S250 was verified as accurate within 1mm and 0.5°. This complies with the TG-142 standards for a stereotactic radiotherapy IGRT system. Rob Cessac is employed as Product Manager for Mevion Medical Systems.

SU-D-17A-02: Four-Dimensional CBCT Using Conventional CBCT Dataset and Iterative Subtraction Algorithm of a Lung Patient

Purpose: The Iterative Subtraction Algorithm (ISA) method generates retrospectively a pre-selected motion phase cone-beam CT image from the full motion cone-beam CT acquired at standard rotation speed. This work evaluates ISA method with real lung patient data. Methods: The goal of the ISA algorithm is to extract motion and no- motion components form the full reconstruction CBCT. The workflow consists of subtracting from the full CBCT all of the undesired motion phases and obtain a motion de-blurred single-phase CBCT image, followed by iteration of this subtraction process. ISA is realized as follows: 1) The projections are sorted to various phases, and from all phases, a full reconstruction is performed to generate an image CTM. 2) Generate forward projections of CTM at the desired phase projection angles, the subtraction of projection and the forward projection will reconstruct a CTSub1, which diminishes the desired phase component. 3) By adding back the CTSub1 to CTm, no motion CBCT, CTS1, can be computed. 4) CTS1 still contains residual motion component. 5) This residual motion component can be further reduced by iteration. The ISA 4DCBCT technique was implemented using Varian Trilogy accelerator OBI system. To evaluate the method, a lung patient CBCT dataset was used. The reconstruction algorithm is FDK. Results: The single phase CBCT reconstruction generated via ISA successfully isolates the desired motion phase from the full motion CBCT, effectively reducing motion blur. It also shows improved image quality, with reduced streak artifacts with respect to the reconstructions from unprocessed phase-sorted projections only. Conclusion: A CBCT motion de-blurring algorithm, ISA, has been developed and evaluated with lung patient data. The algorithm allows improved visualization of a single phase motion extracted from a standard CBCT dataset. This study has been supported by National Institute of Health through R01CA133539.

SU-E-J-214: Comparative Assessment On IGRT On Partial Bladder Cancer Treatment Between CT-On-Rails (CTOR) and KV Cone Beam CT (CBCT)

Purpose: Image-Guided radiation therapy(IGRT) depends on reliable online patient-specific anatomy information to address random and progressive anatomy changes. Large margins have been suggested to bladder cancer treatment due to large daily bladder anatomy variation. KV Cone beam CT(CBCT) has been used in IGRT localization prevalently; however, its lack of soft tissue contrast makes clinicians hesitate to perform daily soft tissue alignment with CBCT for partial bladder cancer treatment. This study compares the localization uncertainties of bladder cancer IGRT using CTon- Rails(CTOR) and CBCT. Methods: Three T2N0M0 bladder cancer patients (total of 66 Gy to partial bladder alone) were localized daily with either CTOR or CBCT for their entire treatment course. A total of 71 sets of CTOR and 22 sets of CBCT images were acquired and registered with original planning CT scans by radiation therapists and approved by radiation oncologists for the daily treatment. CTOR scanning entailed 2mm slice thickness, 0.98mm axial voxel size, 120kVp and 240mAs. CBCT used a half fan pelvis protocol from Varian OBI system with 2mm slice thickness, 0.98axial voxel size, 125kVp, and 680mAs. Daily localization distribution was compared. Accuracy of CTOR and CBCT on partial bladder alignment was also evaluated by comparing bladder PTV coverage. Results: 1cm all around PTV margins were used in every patient except target superior limit margin to 0mm due to bowel constraint. Daily shifts on CTOR averaged to 0.48, 0.24, 0.19 mms(SI,Lat,AP directions); CBCT averaged to 0.43, 0.09, 0.19 mms(SI,Lat,AP directions). The CTOR daily localization showed superior results of V100% of PTV(102% CTOR vs. 89% CBCT) and bowel(Dmax 69.5Gy vs. 78Gy CBCT). CTOR images showed much higher contrast on bladder PTV alignment. Conclusion: CTOR daily localization for IGRT is more dosimetrically beneficial for partial bladder cancer treatment than kV CBCT localization and provided better soft tissue PTV identification.

SU-E-J-271: A Feasibility Study to Utilize XR-QA2 Radiochromic Films as An Alternative Tool for Measuring Beam Parameters of Kilovoltage CBCT Systems

Purpose: The beam parameters of the x-ray beams used in image guidance are difficult to measure in water due to the low exposure. This study is to investigate if XR-QA2 radiochromic films can be used in obtaining beam parameters, such as beam profiles and outputs for kV-CBCT systems. Methods: The kV-CBCT beams were from a Varian OBI system. The films used were XR-QA2-GAFCHROMICTM films placed in the middle of two Plastic -Water slab phantoms at isocenter. The scanner used to analyze the films was an Epson Expression 1680 flat-bed scanner. Pre and 24 h-post-irradiated scans of films were obtained by using the reflective mode. The corresponding pixel-by-pixel dose in the films was obtained using a dose calibration equation as a function of net reflectance from the literature. Beam profiles of different scan protocols with and without bow-tie filters were measured. Monte Carlo simulations were used to compare the accuracy of the measured dose profiles. Results: The beam profiles measured from XR-QA2 films for different clinical default kV-CBCT scan protocols were benchmarked against Monte Carlo simulated profiles which have been validated by measurements performed in water phantom. We confirmed that a minimum post-irradiated time of 24 hours is necessary to pass to scan the films to account for light scatter stability. We found that the uncertainties of the results depend on the scan orientation of the film when analyzed in different directions. Conclusion: The XR-QA2 radiochromic films are sensitive to measure the x-ray beam profiles with irradiated dose ranging 1–5 cGy. The film measurements can be performed with an easy setup compared to ion chamber measurements in a water phantom. These films provide an alternative method to obtain the beam profiles for kV-CBCT beams.

Automatic image registration performance for two different CBCT systems; variation with imaging dose

The performance of an automatic image registration algorithm was compared on image sets collected with two commercial CBCT systems, and the relationship with imaging dose was explored. CBCT images of a CIRS Virtually Human Male Pelvis phantom (VHMP) were collected on Varian TrueBeam/OBI and Elekta Synergy/XVI linear accelerators, across a range of mAs settings. Each CBCT image was registered 100 times, with random initial offsets introduced. Image registration was performed using the grey value correlation ratio algorithm in the Elekta XVI software, to a mask of the prostate volume with 5 mm expansion. Residual registration errors were calculated after correcting for the initial introduced phantom set-up error. Registration performance with the OBI images was similar to that of XVI. There was a clear dependence on imaging dose for the XVI images with residual errors increasing below 4mGy. It was not possible to acquire images with doses lower than ~5mGy with the OBI system and no evidence of reduced performance was observed at this dose. Registration failures (maximum target registration error > 3.6 mm on the surface of a 30mm sphere) occurred in 5% to 9% of registrations except for the lowest dose XVI scan (31%). The uncertainty in automatic image registration with both OBI and XVI images was found to be adequate for clinical use within a normal range of acquisition settings.

Analysis of setup errors and margin for thoracic carcinoma radiotherapy with cone-beam CT-based image guidance

Objective To study the role of KV CBCT on the geometrical accuracy of threedimensional conformal radiotherapy (3DCRT) and to evaluate the margin of targets and peripheral OAR for thoracic carcinoma.Methods 34 patients with thoracic carcinoma were enrolled.Varian-IX liuac with OBI system was used to acquire CBCT scans before delivery in 3DCRT.The left-right (x),superior-inferior (y),anterior-posterior (z) setup errors of patients can be obtained from the tomography images automatically restructured by the system.Results According to 279 CBCT scans the systemic ± random error on x,y,z directions were (-0.16 ± 3.25) mm,(-1.36 ± 5.43) mm,(-2.43 ± 2.14) mm and (2.41 ± 2.18) mm,(4.27 ± 3.60) mm,(2.71 ± 1.77) mm respectively if we do not consider the direction of setup errors.The margins of targets were calculated as 2.68 mm,7.19 mm and 7.57 mm respectively.Conclusions Setup errors are unavoidable in thoracic carcinoma irradiation.We suggest a PTV margin of 2.68 mm,7.19 mm and 7.57 mm in the left-right,superior-inferior and anterior-posterior directions respectively in our department. Key words: Thoracic neopalsms; Image guidance radiotherapy; Tomography, X-ray computed, cone-beam; Setup error; Margin

SU‐E‐J‐55: Influence of Patient Weight and Couch Rotation On the Accuracy of a Surface Tracking System for Stereotactic Radiosurgery

Purpose: To evaluate the influence of patient weight and couch angle on the treatment accuracy of brain SRS guided by the AlignRT system integrated with TrueBeam couch system. Method and Materials: A Winston Lutz phantom with a facemask on top was used to simulate a brain SRS treatment with surface monitoring. The phantom contained small fiducial markers that guided the initial setup using kV and CBCT imaging. A CT scan of the phantom was taken and used to form a plan that included 15 sets of different combinations of couch and gantry angles. Once the phantom was set up, a reference image for surface was acquired and motion monitoring was initiated. The gantry and couch were then moved to their respective positions and an EPID image was acquired. A motion report was generated on the AlignRT system. EPID image registration was performed using the OBI system with DRRimage and the shifts were recorded. The method was repeated for evaluation of patient weight with five different weights in the range of 20 and 120 kg. Results: Examination of the AlignRT data suggests that the accuracy of TrueBeam couch system is slightly compromised for the longitudinal direction (&lt; 0.9 mm) after applying couch rotation during the tracking. The accuracy did not seem to be affected by varying patient weight if couch sagging is compensated for initial setup. All of the deviations of the AlignRT record showed that the accuracy was within 1 mm to the deviations from image registration of isocenter. Conclusions: Our data suggests that TrueBeam couch system maintains geometric accuracy of up to 120 kg of weight, at various gantry and couch angle positions, suggesting that the system is reliable for use with Brain SRS treatments. Addition of AlignRT for intrafraction monitoring helps to ensure that the treatment remains accurate.

TU‐G‐103‐06: Evaluations of Cone Beam CT Dose in Image Guided Radiation Therapy for Brain Cancer Patients Via GPU‐Based Monte Carlo Simulations

Purpose: Cone beam CT (CBCT) has been widely used for patient setup in image guided radiation therapy (IGRT). The radiation dose from CBCT scans is a clinical concern. The purpose of this study is to quantitatively evaluate CBCT dose to brain cancer patients under IGRT using a GPU‐based Monte Carlo CBCT dose calculation package gCTD. Methods: We first performed dose measurements in a water phantom scanned under Varian OBI system and the measurement data are used to commission gCTD. Eight brain cancer patients under IGRT are studied. For each patient, dose distribution under a standard head scan protocol is computed, in which a 100 kVp x‐ray source rotates around the patient head in a 200‐degree range through the posterior side. Mean dose to key organs are computed. Mean dose to 2% voxels that have the highest dose is also computed to characterize dose inhomogeneity. Results: For each CBCT scan, dose to brain, brainstem, chiasm, eyes, and optical nerves ranges in 0.24–0.28cGy, 0.22–0.31cGy, 0.13–0.17cGy, 0.036–0.13cGy, and 0.061–0.17cGy, respectively, depending on the patient head size and isocenter position. The dose is not homogeneous and the mean doses to 2% voxels with the highest dose for these organs are 0.40‐0.58cGy, 0.30–0.43cGy, 0.18–1.02cGy, 0.063– 0.28cGy, and 0.093–0.83cGy, respectively. Although mean dose inside body is 0.26–0.41cGy, the maximum dose can be up to 3.3 cGy, which locates on the bone. It takes about 1 minutes on an Nvidia C2050 GPU card to simulate 1 billion source photons, yielding an average relative uncertainty less than 1%. Conclusion: For the standard head scan protocol, while average CBCT dose to each organ is relatively low, the dose distribution is not homogeneous and the maximum dose can be substantially high. gCTD provides an efficiency and accurate way to evaluate CBCT dose in IGRT.