Impact of a novel multilayer imager on metal artifacts in MV-CBCT.

Objective. Megavoltage cone-beam computed tomography (MV-CBCT) imaging offers several advantages including reduced metal artifacts and accurate electron density mapping for adaptive or emergent situations. However, MV-CBCT imaging is limited by the poor efficiency of current detectors. Here we examine a new MV imager and compare CBCT reconstructions under clinically relevant scenarios.Approach. A multilayer imager (MLI), consisting of four vertically stacked standard flat-panel imagers, was mounted to a clinical linear accelerator. A custom anthropomorphic pelvis phantom with replaceable femoral heads was imaged using MV-CBCT and kilovoltage CBCT (kV-CBCT). Bone, aluminum, and titanium were used as femoral head inserts. 8 MU 2.5 MV scans were acquired for all four layers and (as reference) the top layer. Prostate and bladder were contoured on a reference CT and transferred to the other scans after rigid registration, from which the structural similarity index measure (SSIM) was calculated. Prostate and bladder were also contoured on CBCT scans without guidance, and Dice coefficients were compared to CT contours.Main results. kV-CBCT demonstrated the highest SSIMs with bone inserts (prostate: 0.86, bladder: 0.94) and lowest with titanium inserts (0.32, 0.37). Four-layer MV-CBCT SSIMs were preserved with bone (0.75, 0.80) as compared to titanium (0.67, 0.74), outperforming kV-CBCT when metal is present. One-layer MV-CBCT consistently underperformed four-layer results across all phantom configurations. Unilateral titanium inserts and bilateral aluminum insert results fell between the bone and bilateral titanium results. Dice coefficients trended similarly, with four-layer MV-CBCT reducing metal artifact impact relative to KV-CBCT to provide better soft-tissue identification.Significance. MV-CBCT with a four-layer MLI showed improvement over single-layer MV scans, approaching kV-CBCT quality for soft-tissue contrast. In the presence of artifact-producing metal implants, four-layer MV-CBCT scans outperformed kV-CBCT by eliminating artifacts and single-layer MV-CBCT by reducing noise. MV-CBCT with a novel multi-layer imager may be a valuable alternative to kV-CBCT, particularly in the presence of metal.

Improvements in beam's eye view fiducial tracking using a novel multilayer imager.

Purpose.Electronic portal image devices (EPIDs) have been investigated previously for beams-eye view (BEV) applications such as tumor tracking but are limited by low contrast-to-noise ratio and detective quantum efficiency. A novel multilayer imager (MLI), consisting of four stacked flat-panels was used to measure improvements in fiducial tracking during liver stereotactic body radiation therapy (SBRT) procedures compared to a single layer EPID.Methods.The prototype MLI was installed on a clinical TrueBeam linac in place of the conventional DMI single-layer EPID. The panel was extended during volumetric modulated arc therapy SBRT treatments in order to passively acquire data during therapy. Images were acquired for six patients receiving SBRT to liver metastases over two fractions each, one with the MLI using all 4 layers and one with the MLI using the top layer only, representing a standard EPID. The acquired frames were processed by a previously published tracking algorithm modified to identify implanted radiopaque fiducials. Truth data was determined using respiratory traces combined with partial manual tracking. Results for 4- and 1-layer mode were compared against truth data for tracking accuracy and efficiency. Tracking and noise improvements as a function of gantry angle were determined.Results. Tracking efficiency with 4-layers improved to 82.8% versus 58.4% for the 1-layer mode, a relative improvement of 41.7%. Fiducial tracking with 1-layer returned a root mean square error (RMSE) of 2.1 mm compared to 4-layer RMSE of 1.5 mm, a statistically significant (p < 0.001) improvement of 0.6 mm. The reduction in noise correlated with an increase in successfully tracked frames (r = 0.913) and with increased tracking accuracy (0.927).Conclusion. Increases in MV photon detection efficiency by utilization of a MLI results in improved fiducial tracking for liver SBRT treatments. Future clinical applications utilizing BEV imaging may be enhanced by including similar noise reduction strategies.

Clinical translation of a new flat-panel detector for beam's-eye-view imaging.

Electronic portal imaging devices (EPIDs) lend themselves to beams-eye view clinical applications, such as tumor tracking, but are limited by low contrast and detective quantum efficiency (DQE). We characterize a novel EPID prototype consisting of multiple layers and investigate its suitability for use under clinical conditions. A prototype multi-layer imager (MLI) was constructed utilizing four conventional EPID layers, each consisting of a copper plate, a Gd2O2S:Tb phosphor scintillator, and an amorphous silicon flat panel array detector. We measured the detector's response to a 6 MV photon beam with regards to modulation transfer function, noise power spectrum, DQE, contrast-to-noise ratio (CNR), signal-to-noise ratio (SNR), and the linearity of the detector's response to dose. Additionally, we compared MLI performance to the single top layer of the MLI and the standard Varian AS-1200 detector. Pre-clinical imaging was done on an anthropomorphic phantom, and the detector's CNR, SNR and spatial resolution were assessed in a clinical environment. Images obtained from spine and liver patient treatment deliveries were analyzed to verify CNR and SNR improvements. The MLI has a DQE(0) of 9.7%, about 5.7 times the reference AS-1200 detector. Improved noise performance largely drives the increase. CNR and SNR of clinical images improved three-fold compared to reference. A novel MLI was characterized and prepared for clinical translation. The MLI substantially improved DQE and CNR performance while maintaining the same resolution. Pre-clinical tests on an anthropomorphic phantom demonstrated improved performance as predicted theoretically. Preliminary patient data were analyzed, confirming improved CNR and SNR. Clinical applications are anticipated to include more accurate soft tissue tracking.

Automated MV markerless tumor tracking for VMAT.

Tumor tracking during radiotherapy treatment can improve dose accuracy, conformity and sparing of healthy tissue. Many methods have been introduced to tackle this challenge utilizing multiple imaging modalities, including a template matching based approach using the megavoltage (MV) on-board portal imager demonstrated on 3D conformal treatments. However, the complexity of treatments is evolving with the introduction of VMAT and IMRT, and successful motion management is becoming more important due to a trend towards hypofractionation. We have developed a markerless lung tumor tracking algorithm, utilizing the electronic portal imager (EPID) of the treatment machine. The algorithm has been specifically adapted to track during complex treatment deliveries with gantry and MLC motion. The core of the algorithm is an adaptive template matching method that relies on template stability metrics and local relative orientations to perform multiple feature tracking simultaneously. Only a single image is required to initialize the algorithm and features are automatically added, modified or removed in response to the input images. This algorithm was evaluated against images collected during VMAT arcs of a dynamic thorax phantom. Dynamic phantom images were collected during radiation delivery for multiple lung SBRT breathing traces and an example patient data set. The tracking error was 1.34 mm for the phantom data and 0.68 mm for the patient data. A multi-region, markerless tracking algorithm has been developed, capable of tracking multiple features simultaneously without requiring any other a priori information. This novel approach delivers robust target localization during complex treatment delivery. The reported tracking error is similar to previous reports for 3D conformal treatments.

Comprehensive quality assurance phantom for the small animal radiation research platform (SARRP).

PURPOSE: To develop and test the suitability and performance of a comprehensive quality assurance (QA) phantom for the Small Animal Radiation Research Platform (SARRP). METHODS AND MATERIALS: A QA phantom was developed for carrying out daily, monthly and annual QA tasks including: imaging, dosimetry and treatment planning system (TPS) performance evaluation of the SARRP. The QA phantom consists of 15 (60 × 60 × 5 mm(3)) kV-energy tissue equivalent solid water slabs. The phantom can incorporate optically stimulated luminescence dosimeters (OSLD), Mosfet or film. One slab, with inserts and another slab with hole patterns are particularly designed for image QA. RESULTS: Output constancy measurement results showed daily variations within 3%. Using the Mosfet in phantom as target, results showed that the difference between TPS calculations and measurements was within 5%. Annual QA results for the Percentage depth dose (PDD) curves, lateral beam profiles, beam flatness and beam profile symmetry were found consistent with results obtained at commissioning. PDD curves obtained using film and OSLDs showed good agreement. Image QA was performed monthly, with image-quality parameters assessed in terms of CBCT image geometric accuracy, CT number accuracy, image spatial resolution, noise and image uniformity. CONCLUSIONS: The results show that the developed QA phantom can be employed as a tool for comprehensive performance evaluation of the SARRP. The study provides a useful reference for development of a comprehensive quality assurance program for the SARRP and other similar small animal irradiators, with proposed tolerances and frequency of required tests.

3D fluoroscopic image estimation using patient-specific 4DCBCT-based motion models.

3D fluoroscopic images represent volumetric patient anatomy during treatment with high spatial and temporal resolution. 3D fluoroscopic images estimated using motion models built using 4DCT images, taken days or weeks prior to treatment, do not reliably represent patient anatomy during treatment. In this study we developed and performed initial evaluation of techniques to develop patient-specific motion models from 4D cone-beam CT (4DCBCT) images, taken immediately before treatment, and used these models to estimate 3D fluoroscopic images based on 2D kV projections captured during treatment. We evaluate the accuracy of 3D fluoroscopic images by comparison to ground truth digital and physical phantom images. The performance of 4DCBCT-based and 4DCT-based motion models are compared in simulated clinical situations representing tumor baseline shift or initial patient positioning errors. The results of this study demonstrate the ability for 4DCBCT imaging to generate motion models that can account for changes that cannot be accounted for with 4DCT-based motion models. When simulating tumor baseline shift and patient positioning errors of up to 5 mm, the average tumor localization error and the 95th percentile error in six datasets were 1.20 and 2.2 mm, respectively, for 4DCBCT-based motion models. 4DCT-based motion models applied to the same six datasets resulted in average tumor localization error and the 95th percentile error of 4.18 and 5.4 mm, respectively. Analysis of voxel-wise intensity differences was also conducted for all experiments. In summary, this study demonstrates the feasibility of 4DCBCT-based 3D fluoroscopic image generation in digital and physical phantoms and shows the potential advantage of 4DCBCT-based 3D fluoroscopic image estimation when there are changes in anatomy between the time of 4DCT imaging and the time of treatment delivery.

The use of theranostic gadolinium-based nanoprobes to improve radiotherapy efficacy.

A new efficient type of gadolinium-based theranostic agent (AGuIX®) has recently been developed for MRI-guided radiotherapy (RT). These new particles consist of a polysiloxane network surrounded by a number of gadolinium chelates, usually 10. Owing to their small size (<5 nm), AGuIX typically exhibit biodistributions that are almost ideal for diagnostic and therapeutic purposes. For example, although a significant proportion of these particles accumulate in tumours, the remainder is rapidly eliminated by the renal route. In addition, in the absence of irradiation, the nanoparticles are well tolerated even at very high dose (10 times more than the dose used for mouse treatment). AGuIX particles have been proven to act as efficient radiosensitizers in a large variety of experimental in vitro scenarios, including different radioresistant cell lines, irradiation energies and radiation sources (sensitizing enhancement ratio ranging from 1.1 to 2.5). Pre-clinical studies have also demonstrated the impact of these particles on different heterotopic and orthotopic tumours, with both intratumoural or intravenous injection routes. A significant therapeutical effect has been observed in all contexts. Furthermore, MRI monitoring was proven to efficiently aid in determining a RT protocol and assessing tumour evolution following treatment. The usual theoretical models, based on energy attenuation and macroscopic dose enhancement, cannot account for all the results that have been obtained. Only theoretical models, which take into account the Auger electron cascades that occur between the different atoms constituting the particle and the related high radical concentrations in the vicinity of the particle, provide an explanation for the complex cell damage and death observed.

Low-Z linac targets for low-MV gold nanoparticle radiation therapy.

PURPOSE: To investigate the potential of low-Z/low-MV (low-Z) linac targets for gold nanoparticle radiotherapy (GNPT) and to determine the microscopic dose enhancement ratio (DER) due to GNP for the alternative beamlines. In addition, to evaluate the degradation of dose enhancement arising from the increased attenuation of x rays and larger skin dose in water for the low-MV beams compared to the standard linac. METHODS: Monte Carlo simulations were used to compute dose and DER for various flattening-filter-free beams (2.5, 4, 6.5 MV). Target materials were beryllium, diamond, and tungsten-copper high-Z target. Target thicknesses were selected based on 20%, 60%, 70%, and 80% of the continuous slowing down approximation electron ranges for a given target material and energy. Evaluation of the microscopic DER was carried out for 100 nm GNP including the degradation factors due to beam attenuation. RESULTS: The greatest increase in DER compared to the standard 6.5 MV linac was for a 2.5 MV Be-target (factor of ∼ 2). Skin dose ranged from ∼ 10% (Be, 6.5 MV-80%) to ∼ 85% (Be, 2.5 MV-20%) depending on the target case. Attenuation of 2.5 MV beams at 22 cm was higher by ∼ 75% compared with the standard beam. Taking into account the attenuation at 22 cm depth, the effective dose enhancement was up to ∼ 60% above the DER of the high-Z target. For these cases the effective DER ranged between ∼ 1.6 and 6 compared with the standard linac. CONCLUSIONS: Low-Z (2.5 MV) GNPT is possible even after accounting for greater beam attenuation for deep-seated tumors (22 cm) and the increased skin dose. Further, it can lead to significant sparing of normal tissue while simultaneously escalating the dose in the tumor cells.

High throughput film dosimetry in homogeneous and heterogeneous media for a small animal irradiator.

PURPOSE: We have established a high-throughput Gafchromic film dosimetry protocol for narrow kilovoltage beams in homogeneous and heterogeneous media for small-animal radiotherapy applications. The kV beam characterization is based on extensive Gafchromic film dosimetry data acquired in homogeneous and heterogeneous media. An empirical model is used for parameterization of depth and off-axis dependence of measured data. METHODS: We have modified previously published methods of film dosimetry to suit the specific tasks of the study. Unlike film protocols used in previous studies, our protocol employs simultaneous multi-channel scanning and analysis of up to nine Gafchromic films per scan. A scanner and background correction were implemented to improve accuracy of the measurements. Measurements were taken in homogeneous and inhomogeneous phantoms at 220 kVp and a field size of 5 × 5 mm(2). The results were compared against Monte Carlo simulations. RESULTS: Dose differences caused by variations in background signal were effectively removed by the corrections applied. Measurements in homogeneous phantoms were used to empirically characterize beam data in homogeneous and heterogeneous media. Film measurements in inhomogeneous phantoms and their empirical parameterization differed by about 2%-3%. The model differed from MC by about 1% (water, lung) to 7% (bone). Good agreement was found for measured and modelled off-axis ratios. CONCLUSIONS: EBT2 films are a valuable tool for characterization of narrow kV beams, though care must be taken to eliminate disturbances caused by varying background signals. The usefulness of the empirical beam model in interpretation and parameterization of film data was demonstrated.

Markerless EPID image guided dynamic multi-leaf collimator tracking for lung tumors.

Compensation of target motion during the delivery of radiotherapy has the potential to improve treatment accuracy, dose conformity and sparing of healthy tissue. We implement an online image guided therapy system based on soft tissue localization (STiL) of the target from electronic portal images and treatment aperture adaptation with a dynamic multi-leaf collimator (DMLC). The treatment aperture is moved synchronously and in real time with the tumor during the entire breathing cycle. The system is implemented and tested on a Varian TX clinical linear accelerator featuring an AS-1000 electronic portal imaging device (EPID) acquiring images at a frame rate of 12.86 Hz throughout the treatment. A position update cycle for the treatment aperture consists of four steps: in the first step at time t = t0 a frame is grabbed, in the second step the frame is processed with the STiL algorithm to get the tumor position at t = t0, in a third step the tumor position at t = ti + δt is predicted to overcome system latencies and in the fourth step, the DMLC control software calculates the required leaf motions and applies them at time t = ti + δt. The prediction model is trained before the start of the treatment with data representing the tumor motion. We analyze the system latency with a dynamic chest phantom (4D motion phantom, Washington University). We estimate the average planar position deviation between target and treatment aperture in a clinical setting by driving the phantom with several lung tumor trajectories (recorded from fiducial tracking during radiotherapy delivery to the lung). DMLC tracking for lung stereotactic body radiation therapy without fiducial markers was successfully demonstrated. The inherent system latency is found to be δt = (230 ± 11) ms for a MV portal image acquisition frame rate of 12.86 Hz. The root mean square deviation between tumor and aperture position is smaller than 1 mm. We demonstrate the feasibility of real-time markerless DMLC tracking with a standard LINAC-mounted (EPID).

A fast analytic dose calculation method for arc treatments for kilovoltage small animal irradiators.

Arc treatments require calculation of dose for collections of discrete gantry angles. The sampling of angles must balance between short computation time of small angle sets and the better calculation reliability of large sets. In this paper, an analytical formula is presented that allows calculation of dose delivered during continuous rotation of the gantry. The formula holds valid for continuous short arcs of up to about 30° and is derived by integrating a dose formula over gantry angles within a small angle approximation. Doses for longer arcs may be obtained in terms of doses for shorter arcs. The formula is derived with an empirical beam model in water and extended to inhomogeneous media. It is validated with experimental data obtained by applying arc treatment using kV small animal irradiator to a phantom of solid water and lung-equivalent material. The results are a promising step towards efficient 3D dose calculation and inverse planning purposes. In principle, this method also applies to VMAT dose calculation and optimization but requires extensions.

Application of a spring-dashpot system to clinical lung tumor motion data.

PURPOSE: The treatment efficacy of radiation therapy for lung tumors can be increased by compensating for breath-induced tumor motion. In this study, we quantitatively examine a mathematical model of pseudomechanical linkages between an external surrogate signal and lung tumor motion. METHODS: A spring-dashpot system based on the Voigt model was developed to model the correlation between abdominal respiratory motion and tumor motion during lung radiotherapy. The model was applied to clinical data obtained from 52 treatments ("beams") from 10 patients, treated on the Mitsubishi Real-Time Radiation Therapy system, Sapporo, Japan. In Stage 1, model parameters were optimized for individual patients and beams to determine reference values and to investigate how well the model can describe the data. In Stage 2, for each patient the optimal parameters determined for a single beam were applied to data from other beams to investigate whether a beam-specific set of model parameters is sufficient to model tumor motion over a course of treatment. RESULTS: In Stage 1, the baseline root mean square (RMS) residual error for all individually optimized beam data was 0.90 ± 0.40 mm (mean ± 1 standard deviation). In Stage 2, patient-specific model parameters based on a single beam were found to model the tumor position closely, even for irregular beam data, with a mean increase with respect to Stage 1 values in RMS error of 0.37 mm. On average, the obtained model output for the tumor position was 95% of the time within an absolute bound of 2.0 and 2.6 mm in Stages 1 and 2, respectively. The model was capable of dealing with baseline, amplitude and frequency variations of the input data, as well as phase shifts between the input abdominal and output tumor signals. CONCLUSIONS: These results indicate that it may be feasible to collect patient-specific model parameters during or prior to the first treatment, and then retain these for the rest of the treatment period. The model has potential for clinical application during radiotherapy treatment of lung tumors.

SU-E-J-174: ITV Variations as a Function of CT Geometry and Scan Time : A Simulation Study Using Patient Data.

PURPOSE: The internal target volume (ITV) accounts for uncertainties in tumor position and shape and is defined from images acquired with 4DCT. In this work, the locations of gold fiducial markers implanted in lung tumors of seven patients are used to represent tumor motion and investigate the role of the CT scanner geometry and scan duration on ITV definition. METHODS: All of the simulations are geometric simulations performed in MATLAB (The Mathworks, Nattick,MA) and variations in image reconstruction are not considered. The CT scanner geometries simulated include a 4-slice CT scanner (with an axial field of view of 10 mm) and an 'ideal' CT scanner geometry that covers all potential tumor positions in the same bed position (axial field of view of 100 mm). The scan durations were varied from 6 seconds per bed position to 30 seconds per bed position. For all of the simulations, the tumor was modeled as a sphere of diameter 25 mm. The ITVs were compared by calculating the mean ITV for each patient and scan parameters. RESULTS: For all of the patients, the mean ITV increased as the scan duration increased. The maximum increase in mean ITV was 25%. CONCLUSIONS: This work highlights the potential benefit of modified CT protocols to determine the ITV. One example of a modified protocol would be to increase the scan duration (to decrease ITV uncertainties) while decreasing the mA of the scanner (to avoid giving extra radiation dose). The largest factor influencing the ITV is the scan duration (and not the scanner geometry).

SU-E-J-126: Generation of Fluoroscopic 3D Images Using Single X-Ray Projections on Realistic Modified XCAT Phantom Data.

PURPOSE: To simulate the process of generating fluoroscopic 3D treatment images from 4DCT and measured 2D x-ray projections using a realistic modified XCAT phantom based on measured patient 3D tumor trajectories. METHODS: First, the existing XCAT phantom is adapted to incorporate measured patient lung tumor trajectories. Realistic diaphragm and chest wall motion are automatically generated based on input tumor motion and position, producing synchronized, realistic motion in the phantom. Based on 4DCT generated with the XCAT phantom, we derive patient-specific motion models that are used to generate 3D fluoroscopic images. Patient-specific models are created in two steps: first, the displacement vector fields (DVFs) are obtained through deformable image registration of each phase of 4DCT with respect to a reference image (typically peak-exhale). Each phase is registered to the reference image to obtain (n-1) DVFs. Second, the most salient characteristics in the DVFs are captured in a compact representation through principal component analysis (PCA). Since PCA is a linear decomposition method, all the DVFs can be represented as linear combinations of eigenvectors. Fluoroscopic 3D images are obtained using the projection image to determine optimal weights for the eigenvectors. These weights are determined through iterative optimization of a cost function relating the projection image to the 3D image via the PCA lung motion model and a projection operator. Constructing fluoroscopic 3D images is thus reduced to finding optimal weights for the eigenvectors. RESULTS: Fluoroscopic 3D treatment images were generated using the modified XCAT phantom. The average relative error of the reconstructed image over 30 sec is 0.0457 HU and the standard deviation is 0.0063. CONCLUSIONS: The XCAT phantom was modified to produce realistic images by incorporating patient tumor trajectories. The modified XCAT phantom can be used to simulate the process of generating fluoroscopic 3D treatment images from 4DCT and 2D x-ray projections.

SU-E-J-210: Lung Tumor Target Volume Contours on EPID Cine Mode Images.

PURPOSE: While real time imaging of treatment through an electronic portal imaging device (EPID) is a powerful tool to monitor treatment, limited field of view and lower contrast from an MV beam can make assessment difficult for physicians. This work will develops a method to register and project contour outlines for the internal target volume (ITV) and planning target volume (PTV) of lung tumor cases onto cine mode EPID images to help physicians in interpretation during treatment. METHODS: A sequence of EPID images, acquired during treatment, was registered to treatment planning computed tomography (CT) by machine geometry and patient setup with cone-beam computed tomography (CBCT). The planning CT was converted from Hounsfield scale to electron density by calibration curves of our CT simulator and digitally reconstructed radiographs (DRRs) were produced to match the EPID geometry, pixel for pixel. ITV and PTV structures as defined on the planning CT were then projected onto the DRRs. The DRRs were registered to the EPID images using cross correlation of a single template defined within the treatment aperture of each image. Once registered, the contours from the DRR were transferred to the EPID. RESULTS: We were able to successfully register MV DRRs to EPID images and display the projected target volumes. Without introduced motion, geometric registration and CBCT guided patient setup up were sufficient to register the contours within a single pixel, as normalized cross correlations produced no additional shift. We expect the DRR/EPID registration to be an important step when looking at cases with substantial tumor movement. CONCLUSIONS: The visualization of target volumes provides a tool for physicians to interpret EPID images and assess treatment, especially in cases with tumor movement. The methods developed will serve as the basis for a clinical tool providing real time contours.

WE-G-213CD-05: Beam's-Eye-View Prostate Motion Detection during Arc Radiotherapy.

PURPOSE: To track prostate motion during clinical arc radiotherapy using a novel statistical motion tracking algorithm in both phantom and patient systems. METHODS: For the phantom study, a dynamic phantom with a custom tumor model and radiopaque fiducials was used. For the patient study, the clinical data from two prostate patients were acquired during volumetric modulated arc therapy (VMAT). For each patient, two gold fiducials (X-Mark, 1cm and 2cm in length) were implanted in the prostate. A total of 35 fractions with 2-field full arcs were delivered. Beam's-eye- view (BEV) images were acquired in cine mode every five fractions at a frame rate of 2 fps.The prostate motion was measured by two sequential steps: fiducial detection and motion estimation. In the first step, a wavelet- based block matching filter was used to suppress the noise and enhance the fiducials in the BEV images. A trajectory of detected fiducials was generated along the projection angles. Sequentially, the prostate motion was tracked using a statistical maximum a posteriori (MAP) algorithm. The 3-D fiducial position was estimated by maximizing the probability of displacement of 2-D projections. RESULTS: A total of 8,400 BEV images were processed for this study. The phantom experiments showed that the baseline accuracy of estimation is 0.69+/-0.56 mm. For patient 1, intra- fraction prostate motion was measured to be 0.98+/-0.51 mm for all fractions, and individual fraction motion less than 1 mm. For patient 2, intra- fraction motion was 1.12+/-0.65 mm for all fractions. However, an exceptionally large displacement was measured as 4.9+/-0.74mm in one of the fractions. CONCLUSIONS: Prostate motion during arc-therapy can be rigorously tracked to an accuracy of one millimeter. This technique is well suited for in-treatment room target tracking and post-therapy evaluation. Varian Medical Systems Inc.

SU-E-T-408: Enhancing Stereotactic Radiosurgery for Neovascular Age-Related Macular Degeneration, Using Gold Nanoparticles.

PURPOSE: Age-related macular degeneration (AMD) is the leading cause of irreversible vision loss for people over the age of 60 in the United States. In this study the dosimetric feasibility of using gold nanoparticles (AuNP) as radiosensitizers to enhance stereotactic radiosurgery for neovascular AMD is investigated. METHODS: Analytic calculations were carried out to estimate the nucleus dose enhancement factor (nDEF) due to photon-induced photo- /Auger electrons from AuNP targeting neovascular AMD endothelial cells (EC). The nDEF represents the ratio of the dose to the nucleus with and without the presence of AuNP. As in previous studies, the EC is modeled as a slab of 2 µm (thickness) × 10 µm (length) × 10 µm (width) containing a nucleus of 5 µm diameter and thickness of 0.5 - 1 µm. The targeted AuNP are attached to the exterior of the EC. The nDEF was calculated for a range of feasible AuNP local concentrations (1-7 mg/g) using the clinically applicable 100 kVp x-rays employed by the IRayTM system (Oraya Therapeutics Inc. Newark, CA), with total filtration of 0.75 mm Al and 0.8 mm Be. For comparison the nDEF for other energies: 80 kVp, 90 kVp, 110 kVp, and 120 kVp was also investigated. RESULTS: For 100 kVp x-rays, the results revealed nDEF values of 1.30 - 3.26 for the investigated concentration range of 1 - 7 mg/g, respectively. In comparison, for the same concentration range, nDEF values of 1.32 - 3.40, 1.31-3.33, 1.29 - 3.19, 1.28 - 3.12 were calculated for 80 kVp, 90 kVp, 110 kVp, and 120 kVp x-rays, respectively. CONCLUSIONS: The results predict substantial dose enhancement to the sensitive nucleus of neovascular endothelial cells, targeted by AuNP during kilovoltage stereotactic radiosurgery. This suggests that AuNP may be employed as radiosensitizers to enhance therapeutic efficacy during radiosurgery for neovascular AMD.

EPID-guided 3D dose verification of lung SBRT.

PURPOSE: To investigate the feasibility of utilizing tumor tracks from electronic portal imaging device (EPID) images taken during treatment to verify the delivered dose. METHODS: The proposed method is based on a computation of the delivered fluence by utilizing the planned fluence and the tumor motion track for each field. A phantom study was designed to assess the feasibility of the method. The CIRS dynamic thorax phantom was utilized with a realistic soft resin tumor, modeled after a real patient tumor. The dose calculated with the proposed method was compared to direct measurements taken with 15 metal oxide semiconductor field effect transistors (MOSFETs) inserted in small fissures made in the tumor model. The phantom was irradiated with the tumor static and moved with different range of motions and setup errors. EPID images were recorded throughout all deliveries and the tumor model was tracked post-treatment with in-house developed software. The planned fluence for each field was convolved with the tumor motion tracks to obtain the delivered fluence. Utilizing the delivered fluence from each field, the delivered dose was calculated. The estimated delivered dose was compared to the dose directly measured with the MOSFETs. The feasibility of the proposed method was also demonstrated on a real lung cancer patient, treated with stereotactic body radiotherapy. RESULTS: The calculation of delivered dose with the delivered fluence method was in good agreement with the MOSFET measurements, with average differences ranging from 0.8% to 8.3% depending on the proximity of a dose gradient. For the patient treatment, the planned and delivered dose volume histograms were compared and verified the overall good coverage of the target volume. CONCLUSIONS: The delivered fluence method was applied successfully on phantom and clinical data and its accuracy was evaluated. Verifying each treatment fraction may enable correction strategies that can be applied during the course of treatment to ensure the desired dose coverage.

A multi-region algorithm for markerless beam's-eye view lung tumor tracking.

Methods that allow online lung tumor tracking during radiotherapy are desirable for a variety of applications that have the potential to vastly improve treatment accuracy, dose conformity and sparing of healthy tissue. Several publications have proposed the use of an on-board kV x-ray imager to assess the tumor location during treatment. However, there is some concern that this strategy may expose the patient to a significant amount of additional dose over the course of a typical radiotherapy treatment. In this paper we present an algorithm that utilizes the on-board portal imager of the treatment machine to track lung tumors. This does not expose the patient to additional dose, but is somewhat more challenging as the quality of portal images is inferior when compared to kV x-ray images. To quantify the performance of the proposed algorithm we retrospectively applied it to portal image sequences retrieved from a dynamic chest phantom study and an SBRT treatment performed at our institution. The results were compared to manual tracking by an expert. For the phantom data the tracking error was found to be smaller than 1 mm and for the patient data smaller than 2 mm, which was in the same range as the uncertainty of the gold standard.

Inference of hysteretic respiratory tumor motion from external surrogates: a state augmentation approach.

It is important to monitor tumor movement during radiotherapy. Respiration-induced motion affects tumors in the thorax and abdomen (in particular, those located in the lung region). For image-guided radiotherapy (IGRT) systems, it is desirable to minimize imaging dose, so external surrogates are used to infer the internal tumor motion between image acquisitions. This process relies on consistent correspondence between the external surrogate signal and the internal tumor motion. Respiratory hysteresis complicates the external/internal correspondence because two distinct tumor positions during different breathing phases can yield the same external observation. Previous attempts to resolve this ambiguity often subdivided the data into inhale/exhale stages and restricted the estimation to only one of these directions. In this study, we propose a new approach to infer the internal tumor motion from external surrogate signal using state augmentation. This method resolves the hysteresis ambiguity by incorporating higher-order system dynamics. It circumvents the segmentation of the internal/external trajectory into different phases, and estimates the inference map based on all the available external/internal correspondence pairs. Optimization of the state augmentation is investigated. This method generalizes naturally to adaptive on-line algorithms.