kV On-board Imaging Research Articles

Deep learning-based target decomposition for markerless lung tumor tracking in radiotherapy.

In radiotherapy, real-time tumor tracking can verify tumor position during beam delivery, guide the radiation beam to target the tumor, and reduce the chance of a geometric miss. Markerless kV x-ray image-based tumor tracking is challenging due to the low tumor visibility caused by tumor-obscuring structures. Developing a new method to enhance tumor visibility for real-time tumor tracking is essential. To introduce a novel method for markerless kV image-based tracking of lung tumors via deep learning-based target decomposition. We utilized a conditional Generative Adversarial Network (cGAN), known as Pix2Pix, to build a patient-specific model and generate the synthetic decomposed target image (sDTI) to enhance tumor visibility on the real-time kV projection images acquired by the onboard kV imager equipped on modern linear accelerators. We used 4DCT simulation images to generate the digitally reconstructed radiograph (DRR) and DTI image pairs for model training. We augmented the training dataset by randomly shifting the 4DCT in the superior-inferior, anterior-posterior, and left-right directions during the DRR and DTI generation process. We performed real-time 2D tumor tracking via template matching between the DTI generated from the CT simulation and the sDTI generated from the real-time kV projection images. We validated the proposed method using nine patients' datasets with implanted beacons near the tumor. The sDTI can effectively improve the image contrast around the lung tumors on the kV projection images for the nine patients. With the beacon motion as ground truth, the tracking errors were on average 0.8±0.7mm in the superior-inferior (SI) direction and 0.9±0.8mm in the in-plane left-right (IPLR) direction. The percentage of successful tracking, defined as a tracking error less than 2mm in the SI direction, is 92.2% on the 4312 tested images. The patient-specific model took approximately 12h to train. During testing, it took approximately 35ms to generate one sDTI, and 13ms to perform the tumor tracking using template matching. Our method offers the potential solution for nearly real-time markerless lung tumor tracking. It achieved a high level of accuracy and an impressive tracking rate. Further development of 3D lung tumor tracking is warranted.

Towards Clinical Proton Minibeam Radiation Therapy (pMBRT): Development of Clinical pMBRT System Prototype and pMBRT-Specific Treatment Planning Method

Proton minibeam radiation therapy (pMBRT) is an emerging spatially fractionated RT (SFRT) modality that can provide very high therapeutic index compared to conventional radiotherapy methods and clinically-available SFRT methods (GRID and LATTICE). The biological data collected thus far encourage the preparation of clinical trials in pMBRT. This work is to facilitate the clinical translation of pMBRT by developing (1) the first clinical pMBRT system prototype worldwide, readily available for both small-animal biology studies and large-animal pMBRT trials; (2) pMBRT-specific treatment planning method with peak-valley dose ratio (PVDR) optimization capability for large animal and patient pMBRT trials, which is currently unavailable. The pMBRT system is based on clinically-used pencil-beam-scanning proton machine equipped with large-field clinical-size pMBRT collimator, pMBRT-dedicated treatment planning system, and KV/CBCT imaging guidance. The multi-slit brass collimator has 10 × 10 cm field size, 0.4mm width per slit and 4 mm center-to-center distance. The divergence of slits is tailed to the divergence of the proton beam. A unique universal collimator design is implemented, so that we can keep the outer fitting to the snout and conveniently inter-change collimators as needed. The pMBRT-specific treatment planning method jointly optimizes PVDR and dose objectives, to meet a minimal PVDR threshold and maximize PVDR, to avoid the situations where meeting dose objectives can compromise PVDR when PVDR were not optimized. In addition, the survival fraction for organs at risk is also optimized. The dose calculation engine is based on the Monte Carlo method using TOPAS. The optimization algorithm utilizes total variation and L1 sparsity regularization to maximize PVDR and iterative convex relaxation method to solve the optimization problem. Monte Carlo simulations via TOPAS were performed to design this large-field multi-slit collimator, using the beam structure and the beam data specific to our proton system, with mean dose rate of 8 Gy/min under clinical condition with the collimator in place. The feasibility of using this pMBRT system for small-animal studies has been demonstrated, with customized 3D printed holder for immobilizing small animals, on-board KV imaging system for accurate small-animal positioning, and the GAFchromic film for verifying radiation dose and PVDR. On the other hand, the efficacy of pMBRT-specific treatment planning method (with PVDR optimization capability) to improve PVDR has been demonstrated using retrospective patient planning studies in comparison with standard proton treatment planning method (without PVDR optimization capability). The initial development of a clinical pMBRT system prototype and pMBRT-specific treatment planning method of PVDR optimization capability has been completed with ongoing efforts to make this system ready for large-animal pMBRT studies.

3D printing for dosimetric optimization and quality assurance in small animal irradiations using megavoltage X-rays

PurposeNew therapeutic options in radiotherapy (RT) are often explored in preclinical in-vivo studies using small animals. We report here on the feasibility of modern megavoltage (MV) linear accelerator (LINAC)-based RT for small animals using easy-to-use consumer 3D printing technology for dosimetric optimization and quality assurance (QA). MethodsIn this study we aimed to deliver 5×2Gy to the half-brain of a rat using a 4MV direct hemi-field X-ray beam. To avoid the beam's build-up in the target and optimize dosimetry, a 1cm thick, customized, 3D-printed bolus was used. A 1:1 scale copy of the rat was 3D printed based on the CT dataset as an end-to-end QA tool. The plan robustness to HU changes was verified. Thermoluminescent dosimeters (TLDs), for both MV irradiations and for kV imaging doses, and a gafchromic film were placed within the phantom for dose delivery verifications. The phantom was designed using a standard treatment planning software, and was irradiated at the LINAC with the target aligned using kV on-board imaging. ResultsThe plan was robust (dose difference<1% for HU modification from 0 to 250). Film dosimetry showed a good concordance between planned and measured dose, with the steep dose gradient at the edge of the hemi-field properly aligned to spare the contralateral half-brain. In the treated region, the mean TLDs percentage dose differences (±2 SD) were 1.3% (±3.8%) and 0.9% (±1.7%) beneath the bolus. The mean (±2 SD) out-of-field dose measurements was 0.05Gy (±0.02Gy) for an expected dose of 0.04Gy. Imaging doses (2mGy) still spared the contralateral-brain. ConclusionsUse of consumer 3D-printers enables dosimetry optimization and QA assessment for small animals MV RT in preclinical studies using standard LINACS.

Utilizing the TrueBeam Advanced Imaging Package to monitor intrafraction motion with periodic kV imaging and automatic marker detection during VMAT prostate treatments.

BackgroundFiducial markers are frequently used before treatment for image‐guided patient setup in radiation therapy (RT), but can also be used during treatment for image‐guided intrafraction motion detection. This report describes our implementation of automatic marker detection with periodic kV imaging (TrueBeam v2.5) to monitor and correct intrafraction motion during prostate RT.MethodsWe evaluated the reproducibility and accuracy of software fiducial detection using a phantom with 3 implanted fiducial markers. Clinical implementation for patients with intraprostatic fiducials receiving volumetric modulated arc therapy (VMAT) utilized periodic on‐board kV imaging with 10 s intervals during treatment delivery. For each image, the software automatically identified fiducial locations and determined whether their distance relative to planned locations were within a 3 mm tolerance. Motion was corrected if either ≥2 fiducials in a single image or ≥1 fiducial in sequential images were out of tolerance.ResultsPhantom studies demonstrated poorer performance of linear fiducials compared to collapsible fiducials, and wide variability to accurately detect fiducials across eight software settings. For any given setting, results were relatively reproducible and precise to ~0.5 mm. Across 17 patients treated with a median of 20 fractions, the software recommended a shift in 44% of fractions, and a shift was actually implemented after visual confirmation of movement greater than the 3 mm threshold in 20% of fractions. Adjustment of our approach led to improved accuracy for the latter (n = 7) patient subset. On average, table repositioning added 3.0 ± 0.3 min to patient time on table. Periodic kV imaging increased skin dose by an estimated 1 cGy per treatment arc.ConclusionsPeriodic kV imaging with automatic detection of motion during VMAT prostate treatments is commercially available, and can be successfully implemented to mitigate effects of intrafraction motion with careful attention to software settings.

Continuous generation of volumetric images during stereotactic body radiation therapy using periodic kV imaging and an external respiratory surrogate

We present a technique for continuous generation of volumetric images during SBRT using periodic kV imaging and an external respiratory surrogate signal to drive a patient-specific PCA motion model. Using the on-board imager, kV radiographs are acquired every 3 s and used to fit the parameters of a motion model so that it matches observed changes in internal patient anatomy. A multi-dimensional correlation model is established between the motion model parameters and the external surrogate position and velocity, enabling volumetric image reconstruction between kV imaging time points. Performance of the algorithm was evaluated using 10 realistic eXtended CArdiac-Torso (XCAT) digital phantoms including 3D anatomical respiratory deformation programmed with 3D tumor positions measured with orthogonal kV imaging of implanted fiducial gold markers. The clinically measured ground truth 3D tumor positions provided a dataset with realistic breathing irregularities, and the combination of periodic on-board kV imaging with recorded external respiratory surrogate signal was used for correlation modeling to account for any changes in internal-external correlation. The three-dimensional tumor positions are reconstructed with an average root mean square error (RMSE) of 1.47 mm, and an average 95th percentile 3D positional error of 2.80 mm compared with the clinically measured ground truth 3D tumor positions. This technique enables continuous 3D anatomical image generation based on periodic kV imaging of internal anatomy without the additional dose of continuous kV imaging. The 3D anatomical images produced using this method can be used for treatment verification and delivered dose computation in the presence of irregular respiratory motion.

Carbon Fiducial Image Guidance Increases the Accuracy of Lumpectomy Cavity Localization in Radiation Therapy for Breast Cancer

PurposeWe investigated the feasibility and accuracy of using carbon fiducials to localize the lumpectomy cavity with 2-dimensional kV imaging for early stage breast cancer radiation therapy. Methods and MaterialsCarbon fiducials were placed intraoperatively in the periphery of the lumpectomy cavity. Nine patients received whole breast irradiation with a boost, and 2 patients received 3-dimensional conformal partial breast irradiation. A total of 89 fractions were assessed for setup errors relative to a predefined gold standard, cone beam computed tomography (CBCT) match to the lumpectomy cavity, using the following 4 setup methods: (1) Align skin tattoos with lasers; (2) match bone with 2-dimensional–2-dimensional (2D/2D) kV onboard imaging (OBI); (3) match the whole breast with CBCT; and (4) match carbon fiducials with 2D/2D kV OBI. The margin for the planning target volume (PTV) was calculated by 2 standard deviations of the setup errors, and compared among the 4 setup methods. Setup errors for patients treated with free breathing and patients with deep inspiration breath hold were also compared. ResultsThe carbon fiducials were sufficiently visible on OBI for matching and introduced minimal artifacts. Of the 4 alignment methods, 2D/2D OBI match to fiducials resulted in the smallest setup errors. The PTV margin was 12 mm for aligning skin tattoos using lasers, 9.2 mm for matching bone on OBI, 6.5 mm for matching breast on CBCT, and 3.5 mm for matching fiducials on 2D/2D OBI. Compared with free breathing, deep inspiration breath hold generally reduced the standard deviations of the setup errors, but further investigation would be needed. ConclusionsMatching to carbon fiducials increased the localization accuracy to the lumpectomy cavity. This reduces residual setup error and PTV margins, facilitating tissue sparing without diminishing treatment efficacy.

A Novel method to generate on-board 4D MRI using prior 4D MRI and on-board kV projections from a conventional LINAC for target localization in liver SBRT.

On-board MRI can provide superb soft tissue contrast for improving liver SBRT localization. However, the availability of on-board MRI in clinics is extremely limited. On the contrary, on-board kV imaging systems are widely available on radiotherapy machines, but its capability to localize tumors in soft tissue is limited due to its poor soft tissue contrast. This study aims to explore the feasibility of using an on-board kV imaging system and patient prior knowledge to generate on-board four-dimensional (4D)-MRI for target localization in liver SBRT. Prior 4D MRI volumes were separated into end of expiration (EOE) phase (MRIprior ) and all other phases. MRIprior was used to generate a synthetic CT at EOE phase (sCTprior ). On-board 4D MRI at each respiratory phase was considered a deformation of MRIprior . The deformation field map (DFM) was estimated by matching DRRs of the deformed sCTprior to on-board kV projections using a motion modeling and free-form deformation optimization algorithm. The on-board 4D MRI method was evaluated using both XCAT simulation and real patient data. The accuracy of the estimated on-board 4D MRI was quantitatively evaluated using Volume Percent Difference (VPD), Volume Dice Coefficient (VDC), and Center of Mass Shift (COMS). Effects of scan angle and number of projections were also evaluated. In the XCAT study, VPD/VDC/COMS among all XCAT scenarios were 10.16±1.31%/0.95±0.01/0.88±0.15mm using orthogonal-view 30° scan angles with 102 projections. The on-board 4D MRI method was robust against the various scan angles and projection numbers evaluated. In the patient study, estimated on-board 4D MRI was generated successfully when compared to the "reference on-board 4D MRI" for the liver patient case. A method was developed to generate on-board 4D MRI using prior 4D MRI and on-board limited kV projections. Preliminary results demonstrated the potential for MRI-based image guidance for liver SBRT using only a kV imaging system on a conventional LINAC.

The potential failure risk of the cone-beam computed tomography-based planning target volume margin definition for prostate image-guided radiotherapy based on a prospective single-institutional hybrid analysis

BackgroundThe purpose of this study was to evaluate the impact of markerless on-board kilovoltage (kV) cone-beam computed tomography (CBCT)-based positioning uncertainty on determination of the planning target volume (PTV) margin by comparison with kV on-board imaging (OBI) with gold fiducial markers (FMs), and to validate a methodology for the evaluation of PTV margins for markerless kV-CBCT in prostate image-guided radiotherapy (IGRT).MethodsA total of 1177 pre- and 1177 post-treatment kV-OBI and 1177 pre- and 206 post-treatment kV-CBCT images were analyzed in 25 patients who received prostate IGRT with daily localization by implanted FMs. Intrafractional motion of the prostate was evaluated between each pre- and post-treatment image with these two different techniques. The differences in prostate deviations and intrafractional motions between matching by FM in kV-OBI (OBI-FM) and matching by soft tissues in kV-CBCT (CBCT-ST) were compared by Bland-Altman limits of agreement. Compensated PTV margins were determined and compensated by references.ResultsMean differences between OBI-FM and CBCT-ST in the anterior to posterior (AP), superior to inferior (SI), and left to right (LR) directions were − 0.43 ± 1.45, − 0.09 ± 1.65, and − 0.12 ± 0.80 mm, respectively, with R2 = 0.85, 0.88, and 0.83, respectively. Intrafractional motions obtained from CBCT-ST were 0.00 ± 1.46, 0.02 ± 1.49, and 0.15 ± 0.64 mm, respectively, which were smaller than the results from OBI-FM, with 0.43 ± 1.90, 0.12 ± 1.98, and 0.26 ± 0.80 mm, respectively, with R2 = 0.42, 0.33, and 0.16, respectively. Bland-Altman analysis showed a significant proportional bias. PTV margins of 1.5 mm, 1.4 mm, and 0.9 mm for CBCT-ST were calculated from the values of CBCT-ST, which were also smaller than the values of 3.15 mm, 3.66 mm, and 1.60 mm from OBI-FM. The practical PTV margin for CBCT-ST was compensated with the values from OBI-FM as 4.1 mm, 4.8 mm, and 2.2 mm.ConclusionsPTV margins calculated from CBCT-ST might be underestimated compared to the true PTV margins. To determine a reliable CBCT-ST-based PTV margin, at least the systemic error Σ and the random error σ for on-line matching errors need to be investigated by supportive preliminary FM evaluation at least once.

The need for, and implementation of, image guidance in radiation therapy.

The introduction of image guidance in radiation therapy and its subsequent innovations have revolutionised the delivery of cancer treatment. Modern imaging systems can supplement and often replace the historical practice of relying on external landmarks and laser alignment systems. Rather than depending on markings on the patient's skin, image-guided radiation therapy (IGRT), using techniques such as computed tomography (CT), cone beam CT, MV on-board imaging (OBI), and kV OBI, allows the patient to be positioned based on the internal anatomy. These advances in technology have enabled more accurate delivery of radiation doses to anatomically complex and temporally changing tumour volumes, while simultaneously sparing surrounding healthy tissues. While these imaging modalities provide excellent bony anatomy image quality, magnetic resonance imaging (MRI) surpasses them in soft tissue image contrast for better visualisation and tracking of soft tissue tumours with no additional radiation dose to the patient. However, the introduction of MRI into a radiotherapy facility has a number of complications, including the influence of the magnetic field on the dose deposition, as well as the effects it can have on dosimetry systems. The development and introduction of these new IGRT techniques will be reviewed, and the benefits and disadvantages of each will be described.

Rapid acceptance testing of modern linac using on-board MV and kV imaging systems.

The purpose of this study was to develop a novel process for using on-board MV and kV Electronic Portal Imaging Devices (EPIDs) to perform linac acceptance testing (AT) for two reasons: (a) to standardize the assessment of new equipment performance, and (b) to reduce the time to clinical use while reducing physicist workload. In this study, Varian TrueBeam linacs equipped with amorphous silicon-based EPID (aS1000) were used. The conventional set of AT tests and tolerances were used as a baseline guide. A novel methodology was developed or adopted from published literature to perform as many tests as possible using the MV and kV EPIDs. The developer mode on Varian TrueBeam linacs was used to automate the process. In the EPID-based approach, most of mechanical tests were conducted by acquiring images through a custom phantom and software tools were developed for quantitative analysis to extract different performance parameters. The embedded steel-spheres in a custom phantom provided both visual and radiographic guidance for beam geometry testing. For photon beams, open field EPID images were used to extract inline/crossline profiles to verify the beam energy, flatness and symmetry. EPID images through a double wedge phantom were used for evaluating electron beam properties via diagonal profile. Testing was augmented with a commercial automated application (Machine Performance Check) which was used to perform several geometric accuracy tests such as gantry, collimator rotations, and couch rotations/translations. The developed process demonstrated that the tests, which required customer demonstration, were efficiently performed using EPIDs. The AT tests that were performed using EPIDs were fully automated using the developer mode on the Varian TrueBeam system, while some tests, such as the light field versus radiation field congruence, and collision interlock checks required user interaction. On-board imagers are quite suitable for both geometric and dosimetric testing of linac system involved in AT. Electronic format of the acquired data lends itself to benchmarking, transparency, as well as longitudinal use of AT data. While the tests were performed on a specific model of a linear accelerator, the proposed approach can be extended to other linacs.

Using 3D dosimetry to quantify the Electron Return Effect (ERE) for MR-image-guided radiation therapy (MR-IGRT) applications

Image-guided radiation therapy (IGRT) using computed tomography (CT), cone-beam CT, MV on-board imager (OBI), and kV OBI systems have allowed for more accurate patient positioning prior to each treatment fraction. While these imaging modalities provide excellent bony anatomy image quality, MRI surpasses them in soft tissue image contrast for better visualization and tracking of soft tissue tumors with no additional radiation dose to the patient. A pre-clinical integrated 1.5 T magnetic resonance imaging and 7 MV linear accelerator system (MR-linac) allows for real-time tracking of soft tissues and adaptive treatment planning prior to each treatment fraction. However, due to the presence of a strong magnetic field from the MR component, there is a three dimensional (3D) change in dose deposited by the secondary electrons. Especially at nonhomogeneous anatomical sites with tissues of very different densities, dose enhancements and reductions can occur due to the Lorentz force influencing the trajectories of secondary electrons. These dose changes at tissue interfaces are called the electron return effect or ERE. This study investigated the ERE using 3D dosimeters.

Highly Conformal Hybrid Forward and Inverse Planned Intensity Modulated Radiation Therapy for Supine Craniospinal Irradiation (CSI): Long-term Results and Implications for Pencil Beam Proton Planning

Proton pencil beam scanning allows for highly conformal radiation therapy making target volume definition and treatment localization increasingly important. Since 2009 we have utilized a highly conformal, supine, hybrid forward and inverse IMRT technique for photon craniospinal irradiation (CSI). We report clinical outcomes utilizing our CSI technique and implications for pencil beam proton planning. Charts of 35 patients who received supine, hybrid IMRT CSI from 2009-2014 were retrospectively reviewed. Craniospinal CTV was defined as intracranial contents on bone windows with special attention to cribriform plate, without intentional inclusion of optic nerves or globes; and thecal sac defined as spinal canal including nerve roots terminating at the lateral border of the vertebral body. PTV was expanded 2 mm at cribriform plate/orbit, 5 mm for remaining intracranial CTV, 3-5 mm ant/post, and 5-8 mm right/left/inferior for thecal sac depending on age and size of patient. Dose was prescribed such that >95% of PTV received > 95% of prescription dose and >99% of CTV received >99% of prescription with no attempt to include vertebral bodies. Lateral fields were utilized at the cranium and upper cervical spine; inferior spine fields were either single posterior, 2-3 obliques, or VMAT depending on patient age and depth to CTV. Plans were generated with a hybrid of forward and inverse planned IMRT. Inferior borders of the brain fields and the superior border of the lower spine field were designed with 6-10 cm long, gradual dose gradients by sequential closing of MLC leaves using forward planned multiple static segment IMRT delivery. The sliding window upper spine IMRT field was created by the inverse planning system to match gradients of the brain and lower spine fields. Onboard kV imaging of cranial fields and a single posterior spine image confirmed daily patient position. Subsequent shifts were calculated based on couch coordinates and planned isocenter shifts. Histologies included 19 medulloblastomas, 9 primitive neuroectodermal tumors, 4 germ cell tumors, 1 pineoblastoma, 1 anaplastic ganglioglioma, and 1 atypical teratoid/rhabdoid tumor. Median CSI dose was 36 Gy (range: 18-39.6 Gy) and median boost dose was 55.8 Gy (range: 30.6-60 Gy). At a median follow up of 51 months (range: 3.2-89.2 months), there were no isolated recurrences or spinal myelopathies at field gradients or CTV margins. Fourteen patients had recurrence, all of which were intracranial. Our hybrid forward and inverse planned IMRT supine CSI technique did not result in any isolated recurrences or myelopathies at field gradients. Furthermore, the safety and efficacy of this highly conformal technique suggests our target volumes and blended gradients are appropriate for incorporation into spot scanning proton therapy plans.

Accuracy of UTE-MRI-based patient setup for brain cancer radiation therapy.

Radiation therapy simulations solely based on MRI have advantages compared to CT-based approaches. One feature readily available from computed tomography (CT) that would need to be reproduced with MR is the ability to compute digitally reconstructed radiographs (DRRs) for comparison against on-board radiographs commonly used for patient positioning. In this study, the authors generate MR-based bone images using a single ultrashort echo time (UTE) pulse sequence and quantify their 3D and 2D image registration accuracy to CT and radiographic images for treatments in the cranium. Seven brain cancer patients were scanned at 1.5 T using a radial UTE sequence. The sequence acquired two images at two different echo times. The two images were processed using an in-house software to generate the UTE bone images. The resultant bone images were rigidly registered to simulation CT data and the registration error was determined using manually annotated landmarks as references. DRRs were created based on UTE-MRI and registered to simulated on-board images (OBIs) and actual clinical 2D oblique images from ExacTrac™. UTE-MRI resulted in well visualized cranial, facial, and vertebral bones that quantitatively matched the bones in the CT images with geometric measurement errors of less than 1 mm. The registration error between DRRs generated from 3D UTE-MRI and the simulated 2D OBIs or the clinical oblique x-ray images was also less than 1 mm for all patients. UTE-MRI-based DRRs appear to be promising for daily patient setup of brain cancer radiotherapy with kV on-board imaging.

WE-AB-303-10: The Use of On-Board KV Imaging During Respiratory-Gated VMAT Delivery to Determine the Correlation and Phase Shift Between External Marker Motion and Internal Tumour Motion

Purpose: To determine the correlation between internal tumor motion and external surrogate motion and any potential phase shifts during respiratory-gated VMAT delivery using on-board kV imaging. Methods: A respiratory motion phantom was used to trace the motion of an internal target, embedded with a 3 cm moving target, and an external surrogate. On-board kV projections were acquired at 11 frames per second during VMAT delivery on a TrueBeam v.1.6 (Varian Medical Systems, Palo Alto, CA) operating in developer mode. Full motion encompassed treatment delivery using sinusoidal motion with 4 second period and 2 cm peak-to-peak amplitude, and real-patient breathing (RPB) motion was programmed to the phantom. Respiratory-gated treatment deliveries (30–60% gating window) on five additional RPB waveforms were programmed to investigate correlation within the gating window. To investigate the capability of during treatment kV imaging to detect phase shifts during gated delivery, a second phantom with independent motion was used. Eight controlled phase shifts in intervals of 0.4 seconds were added to the sinusoidal motion. For each phase shift, the Pearson linear correlation coefficient statistical test was performed for each set of respiratory traces. The phase shift was calculated by shifting the external trace, in time, until maximum correlation was obtained. Results: A strong internal and external correlation was obtained for both the free-breathing (sinusoidal R2=0.993 and RPB R2=0.990) and respiratory-gated cases (range from R2=0.986–0.996). There was no significant difference between the programmed shift and that acquired using on-board kV imaging (p=0.899, R-squared = 0.997). Conclusion: During treatment kV imaging has the capability to verify intrafractional anatomical position. It can be accurately used as a tool to quantify internal and external correlation, and determine phase shifts within the gating window, and ultimately, the accuracy of respiratory-gating treatment delivery.

SU‐C‐304‐02: Robust and Efficient Process for Acceptance Testing of Varian TrueBeam Linacs Using An Electronic Portal Imaging Device (EPID)

Purpose:The purpose of this project was to develop a process that utilizes the onboard kV and MV electronic portal imaging devices (EPIDs) to perform rapid acceptance testing (AT) of linacs in order to improve efficiency and standardize AT equipment and processes.Methods:In this study a Varian TrueBeam linac equipped with an amorphous silicon based EPID (aSi1000) was used. The conventional set of AT tests and tolerances was used as a baseline guide, and a novel methodology was developed to perform as many tests as possible using EPID exclusively. The developer mode on Varian TrueBeam linac was used to automate the process. In the current AT process there are about 45 tests that call for customer demos. Many of the geometric tests such as jaw alignment and MLC positioning are performed with highly manual methods, such as using graph paper. The goal of the new methodology was to achieve quantitative testing while reducing variability in data acquisition, analysis and interpretation of the results. The developed process was validated on two machines at two different institutions.Results:At least 25 of the 45 (56%) tests which required customer demo can be streamlined and performed using EPIDs. More than half of the AT tests can be fully automated using the developer mode, while others still require some user interaction. Overall, the preliminary data shows that EPID‐based linac AT can be performed in less than a day, compared to 2–3 days using conventional methods.Conclusions:Our preliminary results show that performance of onboard imagers is quite suitable for both geometric and dosimetric testing of TrueBeam systems. A standardized AT process can tremendously improve efficiency, and minimize the variability related to third party quality assurance (QA) equipment and the available onsite expertise.Research funding provided by Varian Medical Systems. Dr. Sasa Mutic receives compensation for providing patient safety training services from Varian Medical Systems, the sponsor of this study.

SU-E-I-39: Combining Conventional Tomographic Imaging Strategy and Interior Tomography for Low Dose Dual-Energy CT (DECT)

Purpose: Dual-energy CT (DECT) affords quantitative information of tissue density and provides a new dimension for disease diagnosis and treatment planning. The technique, however, increases the imaging dose because of the doubled scans, and thus hinders its widespread clinical applications. The purpose of this work is to develop a novel hybrid DECT image acquisition and reconstruction strategy, in which one of the energies is dealt by interior tomography while the other one is obtained using conventional tomography approach. Methods: In the proposed hybrid imaging strategy, the projection data of one of the energies (e.g., high-energy) were acquired and processed in an interior scanning model, whereas the other energy in the conventional tomographic approach. It known that, if the ROI is piecewise constant or polynomial, the interior ROI can be reconstructed with TV or HOT minimization. Here we extend the TV based interior reconstruction method into dual-energy situation. The ROI images so obtained were overlaid in the context of conventional CT of the companion energy. A material based composition in ROI was used in the proposed reconstruction framework. Results: In the simulation experiment with a diagnostic DECT geometry and energies, we were able to derive the densities of soft-tissues and bonesmore » in the ROI with high fidelity. In the experimental CBCT study, both kV and MV data were collected using the on-board kV and MV imaging system. The MV data were truncated only across the ROI. Using the interior tomography reconstruction above, we were able to obtain the ROI images as that obtained using un-truncated MV data with known tissue densities. Conclusion: The proposed DECT imaging strategy provides an effective way to extract tissue density information in the ROI and in the context of anatomical images of CT imaging, with much reduced imaging dose.« less

SU-E-J-59: Feasibility of Markerless Tumor Tracking by Sequential Dual-Energy Fluoroscopy On a Clinical Tumor Tracking System

Purpose: To evaluate the feasibility of markerless tumor tracking through the implementation of a novel dual-energy imaging approach into the clinical dynamic tracking (DT) workflow of the Vero SBRT system. Methods: Two sequential 20 s (11 Hz) fluoroscopy sequences were acquired at the start of one fraction for 7 patients treated for primary and metastatic lung cancer with DT on the Vero system. Sequences were acquired using 2 on-board kV imaging systems located at ±45° from the MV beam axis, at respectively 60 kVp (3.2 mAs) and 120 kVp (2.0 mAs). Offline, a normalized cross-correlation algorithm was applied to match the high (HE) and low energy (LE) images. Per breathing phase (inhale, exhale, maximum inhale and maximum exhale), the 5 best-matching HE and LE couples were extracted for DE subtraction. A contrast analysis according to gross tumor volume was conducted based on contrast-to-noise ratio (CNR). Improved tumor visibility was quantified using an improvement ratio. Results: Using the implanted fiducial as a benchmark, HE-LE sequence matching was effective for 13 out of 14 imaging angles. Overlying bony anatomy was removed on all DE images. With the exception of two imaging angles, the DE images showed no significantly improved tumor visibility comparedmore » to HE images, with an improvement ratio averaged over all patients of 1.46 ± 1.64. Qualitatively, it was observed that for those imaging angles that showed no significantly improved CNR, the tumor tissue could not be reliably visualized on neither HE nor DE images due to a total or partial overlap with other soft tissue. Conclusion: Dual-energy subtraction imaging by sequential orthogonal fluoroscopy was shown feasible by implementing an additional LE fluoroscopy sequence. However, for most imaging angles, DE images did not provide improved tumor visibility over single-energy images. Optimizing imaging angles is likely to improve tumor visibility and the efficacy of dual-energy imaging. This work was in part sponsored by corporate funding from BrainLAB AG.(BrainLAB AG, Feldkirchen, Germany)« less

Influence of interfraction motion on margins for radiotherapy of gastric cancer.

The aim of this study was to evaluate interfraction uncertainties using kilovoltage (kV) radiographs for patients with gastric cancer during chemoradiotherapy and to calculate the planning target volume (PTV) margins. 1284 measurements of set-up errors were analysed for treated patients. The measurements of craniocaudal (axis y), laterolateral (axis x) and anteroposterior (axis z) shifts in kV radiographs were performed. Interfraction clinical target volume (CTV)-PTV margins for all directions were calculated using the van Herk formula. The main shift for the y-axis was 0.7 mm [standard deviation (SD), ±7.6], for the x-axis was 0.4 mm (SD, ±3.7) and for the z-axis was 0.6 mm (SD, ±3.5). The CTV-PTV margin in the x, y and z directions was 8.3, 15.5 and 8.0 mm, respectively. We observed that the interfractional motion for patients increased approximately 0.0034 cm along the x direction with each subsequent fraction, whereas a 0.0058-cm reduction in length along the y-axis was observed. No time effects for the z direction were noticed. According to our experience, a PTV margin of 9 mm along the x-axis, 16 mm along the y-axis and 8 mm along the z direction should be considered in the absence of image-guided radiotherapy. This knowledge concerning PTV set-up margins could be particularly useful for centres without a kV on-board imaging system.

On-Board CBCT Imaging Using 2.5 MV Beam on a Radiation Therapy Linac for Metal Artifact Reduction

Purpose/Objective(s): Highly attenuating metallic objects such as dental fillings, hip/shoulder prosthesis, fiducial markers, cause severe streaking artifacts in on-board kV CBCT imaging and can lead to errors in imageguided patient setup. The purpose of this work is to reduce metal artifact in on-board CBCT imaging using new 2.5 MV beam available on a radiation therapy Linac. Materials/Methods: A head phantom with a copper bar inserted in the center was scanned with the 2.5 MV beam. With a full gantry rotation, 68 projection views were collected for a total of 20 MU. For comparison, a CBCT scan was also performed using the on-board kV imaging system. The peak voltage for kV scanning was 100 kVp. Due to the sparse projections of MV scanning, we performed iterative image reconstruction on both kV and MV data respectively. Several representative regions of interest were selected for evaluation of metal artifacts and contrast. Results: The mean CT numbers of the bright and dark metal artifacts in kV CBCT are 9000HU and -3000HU. In comparison, there are no bright metal artifacts and the mean CT number of dark metal artifacts is reduced to -460HU in MV CBCT. The mean CT numbers of the bone and metal are 1000HU, 5000HU for kV, and 380HU, 3600HU for MV. The mean CT numbers for three low contrast regions are -800HU, -600HU, -400HU for kV, and -460HU,-300HU, -100HU for MV. Conclusions: Using the new 2.5 MV beam for on-board CBCT imaging, we have significantly reduced metal artifacts compared with kV imaging, although with a decrease in soft-tissue contrast. Combining kV and MV imaging may lead to improved soft-tissue contrast with negligible metal artifact. Author Disclosure: Q. Xu: None. R. Li: None. Y. Yang: None. L. Xing: None.

SU-E-T-133: Dosimetric Impact of Scan Orientation Relative to Target Motion During Spot Scanning Proton Therapy

Purpose: This study seeks to evaluate the dosimetric effects of intra-fraction motion during spot scanning proton beam therapy as a function of beam-scan orientation and target motion amplitude. Method: Multiple 4DCT scans were collected of a dynamic anthropomorphic phantom mimicking respiration amplitudes of 0 (static), 0.5, 1.0, and 1.5 cm. A spot-scanning treatment plan was developed on the maximum intensity projection image set, using an inverse-planning approach. Dynamic phantom motion was continuous throughout treatment plan delivery.The target nodule was designed to accommodate film and thermoluminescent dosimeters (TLD). Film and TLDs were uniquely labeled by location within the target. The phantom was localized on the treatment table using the clinically available orthogonal kV on-board imaging device. Film inserts provided data for dose uniformity; TLDs provided a 3% precision estimate of absolute dose. An inhouse script was developed to modify the delivery order of the beam spots, to orient the scanning direction parallel or perpendicular to target motion.TLD detector characterization and analysis was performed by the Imaging and Radiation Oncology Core group (IROC)-Houston. Film inserts, exhibiting a spatial resolution of 1mm, were analyzed to determine dose homogeneity within the radiation target. Results: Parallel scanning and target motions exhibited reduced target dose heterogeneity, relative to perpendicular scanning orientation. The average percent deviation in absolute dose for the motion deliveries relative to the static delivery was 4.9±1.1% for parallel scanning, and 11.7±3.5% (p<<0.05) for perpendicularly oriented scanning. Individual delivery dose deviations were not necessarily correlated to amplitude of motion for either scan orientation. Conclusions: Results demonstrate a quantifiable difference in dose heterogeneity as a function of scan orientation, more so than target amplitude. Comparison to the analyzed planar dose of a single field hint that multiple-field delivery alters intra-fraction beam-target motion synchronization and may mitigate heterogeneity, though further study is warranted.