Megavoltage Imaging Research Articles

Mega Voltage Cone Beam Computed Tomography (MV- CBCT) using a Standard Medical Linear Accelerator and EPID: A feasibility study.

Introduction: The success of radiotherapy cancer treatment delivery depends on the accuracy of patient positioning for each treatment session. A number of kilovoltage x-ray volumetric imaging modalities with an additional source and detector have been developed to allow patient set-up verification based on the internal anatomy, but a significant portion of medical linacs are only equipped with electronics portal imaging device (EPID) as a planar imaging system. The aim of this study is investigating the feasibility of megavoltage cone beam computed tomography (MV-CBCT) using medical linac and EPID to obtain volumetric data for 3D representation of patient in treatment position. Materials and Methods: The MV-CBCT system consists of an amorphous silicon EPID with an active area of 41×41 cm2 attached to an Elekta Precise linac. Initially, the quality control tests were performed to evaluate the safety features, mechanical stability and quality of the megavoltage imaging system. The MV-CBCT scanning protocol was divided into three major parts. In the first part, Step and Shoot delivery mode was used to acquire projections of an anthropomorphic head phantom and quality assurance phantom in increments of 30 with 3 MU and x-rays of 6 MV energy over 3600 of gantry rotation. Afterwards, noise reduction was conducted by applying 2D adaptive wiener filter to projections. Furthermore, it is found that scatter produces cupping artifacts and increase uniformity variations across the reconstructed tomograms. Therefore, a scatter correction method which is based on a superposition of Monte Carlo generated scatter kernels was applied to the portal images. In the latest part of the research, the Feldkamp cone-beam algorithm used to reconstruct the 2D portal images to a 3D image, then, image quality characteristics such as; uniformity, contrast-to-noise ratio (CNR) and spatial resolution were investigated using the Gammex phantom Results: The safety interlocks were found to be functional. The EPID response uniformity was within 97% across the detector. Contrast resolution of the MV imaging system was found to agree with the recommended tolerance; and the value of f50 for spatial resolution was 0.41lp/mm for 6 MV. The supporting arm deviation was within ±1 mm, and corrections are required to realign the projections. Reconstructed slices from the spatial and contrast resolution modules of the Gammex phantom shown that the 4 lp/cm section was clearly resolvable, and also bone, air, polyethylene, and acrylic inserts can be clearly resolved, with bone and air having the greatest and polyethylene having the lowest contrast, which improved after scatter correction. Furthermore, the maximum uniformity variation was determined of 15.75%-pixel intensity units which reduced significantly after scatter correction. Conclusion: The result of this study has demonstrated that EPID can be used to acquire volumetric images with high enough quality to improve patient alignment prior to dose delivery.

Set-up Reproducibility for Deep Inspiration Breath Hold for Left Sided Breast Cancer Treatment Using Daily Mega Voltage Imaging-Integrated Network Study

Deep inspiration breath hold (DIBH) technique has been effective to reduce cardiac dose for left sided breast cancer treatment. However there are limited data on the setup reproducibility with DIBH technique. Herein we report the daily setup reproducibility and variation of DIBH using daily mega voltage (MV) imaging based on a large network of data. In total, 109 left breast cancer patients from 10 hospital sites across the UPMC Hillman Cancer Center network were treated using DIBH technique with daily MV imaging from May 2016 to July 2017. All patients had visual coaching via iPod device as well as voice coaching for DIBH for CT simulation and treatment. The same procedure and technique for DIBH CT simulation and treatment planning have been shared between physicists and physicians across the network. For each patient, BMI, age, surgery type (lumpectomy or mastectomy), regional and internal mammary nodes (IMN) involvement, MV imaging procedure used, number of total portal images and inter-fraction directional shifts were collected. For the statistical analyses, we separated all patients into two groups in each of the following three categories; 1) obese (BMI ≥ 30) vs. non- obese, 2) surgery type: mastectomy vs. lumpectomy, 3) internal mammary node (IMN) treatment vs. no IMN treatment. A total of 1833 treatments and 2818 MV portal images were analyzed. Minimum mean distance from heart to the tangential field edge was 2.5 mm (ranges 0-12 mm). The group mean inter-fraction directional shifts were as following: 1) 0.7 mm (superior; SD = 2.26), 0.8 mm (inferior; SD = 2.21), 2) 0.65 mm (left; SD =1.60), 0.64 mm (right; SD = 1.62), 3) 0.89 mm (anterior; SD = 1.96), 0.83 mm (posterior; SD=2.03). However between hospitals, there was found a difference in mean inter-fraction directional shift (p < 0.001). Any inter-fraction directional shift for ≥ 2mm, ≥ 3mm, ≥ 4mm, ≥ 5mm, ≥ 10mm was found to be 52.9 %, 37.6 %, 30.9 %, 21.9 %, 3.7 % of total fractions, respectively. In the stratified analysis, obese patients had larger directional shifts (p < 0.05) and highly associated with number of fractions for ≥ 5mm in any directional shift compared to non-obese patients (29 % vs. 17 %; p =0.04). To the best of our knowledge, this is the first and the largest study that reports the daily setup reproducibility and variation using MV images and patient’s visual feedback. DIBH setup for left breast cancer treatment at our large cancer center network was reproducible with any mean directional shifts less than 1.0 mm, although there was some inter-fraction variation between hospital sites. Consideration of daily imaging for obese patients should be taken to ensure the benefit of DIBH for that population. Also, if the distance of heart to the edge of the tangential treatment field is relatively close, it would be more beneficial to have daily imaging because 53 % of patients still had ≥ 2 mm shift.

Blurring the lines for better visualisation

On-treatment imaging in radiotherapy has evolved over the last 60 years, bringing with it changes in the roles of radiographers, radiologists and oncologists. The ability to acquire and interpret high quality images (2D kilovoltage and megavoltage imaging and 3D CT and cone-beam CT) for radiotherapy planning and delivery requires therapy radiographers to have skills and knowledge that overlap with those of diagnostic radiographers. With the implementation of MRI-guided radiotherapy, treatment radiographers and clinical oncologists are exploring new territory, requiring truly collaborative working practices with their radiology partners. This short communication introduces the first images acquired using the hybrid MR Linac at our institution.

Independent validation of machine performance check for the Halcyon and TrueBeam linacs for daily quality assurance.

PurposeTo evaluate the ability of the machine performance check (MPC) on the Halcyon to detect errors, with comparison with the TrueBeam.Methods MPC is an automated set of quality assurance (QA) tests that use a phantom placed on the couch and the linac's imaging system(s) to verify the beam constancy and mechanical performance of the Halcyon and TrueBeam linacs. In order to evaluate the beam constancy tests, we inserted solid water slabs between the beam source and the megavoltage imager to simulate changes in beam output, flatness, and symmetry. The MPC results were compared with measurements, using two‐dimensional array under the same conditions. We then studied the accuracy of MPC geometric tests. The accuracies of the relative gantry offset and couch shift tests were evaluated by intentionally inserting phantom shifts, using a rotating or linear motion stage. The MLC offset and absolute gantry offset tests were assessed by miscalibrating these motions on a Halcyon linac.ResultsFor the Halcyon system, the average difference in the measured beam output between the IC Profiler and MPC, after intentional changes, was 1.3 ± 0.5% (for changes ≤5%). For Halcyon, the MPC test failed (i.e., prevented treatment) when the beam symmetry change was over 1.9%. The accuracy of the MLC offset test was within 0.05 mm. The absolute gantry offset test was able to detect an offset as small as 0.02°. The accuracy of the absolute couch shift test was 0.03 mm. The accuracy of relative couch shift test of Halcyon was measured as 0.16 mm.ConclusionWe intentionally inserted errors to evaluate the ability of the MPC to identify errors in dosimetric and geometric parameters. These results showed that the MPC is sufficiently accurate to be effectively used for daily QA of the Halcyon and TrueBeam treatment devices.

Enhancement of megavoltage electronic portal images for markerless tumor tracking.

PurposeThe poor quality of megavoltage (MV) images from electronic portal imaging device (EPID) hinders visual verification of tumor targeting accuracy particularly during markerless tumor tracking. The aim of this study was to investigate the effect of a few representative image processing treatments on visual verification and detection capability of tumors under auto tracking.MethodsImages of QC‐3 quality phantom, a single patient's setup image, and cine images of two‐lung cancer patients were acquired. Three image processing methods were individually employed to the same original images. For each deblurring, contrast enhancement, and denoising, a total variation deconvolution, contrast‐limited adaptive histogram equalization (CLAHE), and median filter were adopted, respectively. To study the effect of image enhancement on tumor auto‐detection, a tumor tracking algorithm was adopted in which the tumor position was determined as the minimum point of the mean of the sum of squared pixel differences (MSSD) between two images. The detectability and accuracy were compared.ResultsDeblurring of a quality phantom image yielded sharper edges, while the contrast‐enhanced image was more readable with improved structural differentiation. Meanwhile, the denoising operation resulted in noise reduction, however, at the cost of sharpness. Based on comparison of pixel value profiles, contrast enhancement outperformed others in image perception. During the tracking experiment, only contrast enhancement resulted in tumor detection in all images using our tracking algorithm. Deblurring failed to determine the target position in two frames out of a total of 75 images. For original and denoised set, target location was not determined for the same five images. Meanwhile, deblurred image showed increased detection accuracy compared with the original set. The denoised image resulted in decreased accuracy. In the case of contrast‐improved set, the tracking accuracy was nearly maintained as that of the original image.ConclusionsConsidering the effect of each processing on tumor tracking and the visual perception in a limited time, contrast enhancement would be the first consideration to visually verify the tracking accuracy of tumors on MV EPID without sacrificing tumor detectability and detection accuracy.

Quality control methods for linear accelerator radiation and mechanical axes alignment

The delivery accuracy of highly conformal dose distributions generated using intensity modulation and collimator, gantry, and couch degrees of freedom is directly affected by the quality of the alignment between the radiation beam and the mechanical axes of a linear accelerator. For this purpose, quality control (QC) guidelines recommend a tolerance of ±1mm for the coincidence of the radiation and mechanical isocenters. Traditional QC methods for assessment of radiation and mechanical axes alignment (based on pointer alignment) are time consuming and complex tasks that provide limited accuracy. In this work, an automated test suite based on an analytical model of the linear accelerator motions was developed to streamline the QC of radiation and mechanical axes alignment. The proposed method used the automated analysis of megavoltage images of two simple task-specific phantoms acquired at different linear accelerator settings to determine the coincidence of the radiation and mechanical isocenters. The sensitivity and accuracy of the test suite were validated by introducing actual misalignments on a linear accelerator between the radiation axis and the mechanical axes using both beam steering and mechanical adjustments of the gantry and couch. The validation demonstrated that the new QC method can detect sub-millimeter misalignment between the radiation axis and the three mechanical axes of rotation. A displacement of the radiation source of 0.2mm using beam steering parameters was easily detectable with the proposed collimator rotation axis test. Mechanical misalignments of the gantry and couch rotation axes of the same magnitude (0.2mm) were also detectable using the new gantry and couch rotation axis tests. For the couch rotation axis, the phantom and test design allow detection of both translational and tilt misalignments with the radiation beam axis. For the collimator rotation axis, the test can isolate the misalignment between the beam radiation axis and the mechanical collimator rotation axis from the impact of field size asymmetry. The test suite can be performed in a reasonable time (30-35min) due to simple phantom setup, prescription-based beam delivery, and automated image analysis. As well, it provides a clear description of the relationship between axes. After testing the sensitivity of the test suite to beam steering and mechanical errors, the results of the test suite were used to reduce the misalignment errors of the linac to less than 0.7-mm radius for all axes. The proposed test suite offers sub-millimeter assessment of the coincidence of the radiation and mechanical isocenters and the test automation reduces complexity with improved efficiency. The test suite results can be used to optimize the linear accelerator's radiation to mechanical isocenter alignment by beam steering and mechanical adjustment of gantry and couch.

Radiosurgical Treatment Verification Using Removable Megavoltage Radiation Detectors.

IntroductionAccurate dose delivery is critical to the success of stereotactic radiosurgery. Unfortunately, verification of the accuracy of treatment delivery remains a challenging problem. Existing radiosurgery delivery paradigms are limited in their ability to verify the accurate delivery of radiation beams using data sampled from the beam after it has traversed the patient. The Zap-X Radiosurgery System (Zap Surgical Systems, San Carlos, CA) addresses this issue by implementing a fully integrated treatment delivery system that utilizes a factory commissioned megavoltage (MV) imager to measure the transmitted beam. The measured intensity is then compared with an expected value in order to confirm that treatment is proceeding as expected. The purpose of this study was to evaluate a prototype system and investigate the accuracy of an attenuation model used in generating the expected transmitted intensity values.MethodsA prototype MV imager was used to measure transmitted beam intensities at various exposure levels and through several thicknesses of solid water. The data were used to evaluate imager linearity and model accuracy.ResultsExperimental results indicate that a quadratic attenuation model is appropriate for predicting beam attenuation and that the imager exhibits excellent dose linearity.ConclusionsThe MV imager system is shown to be capable of accurately acquiring the data needed to confirm treatment validity.

A novel multilayer MV imager computational model for component optimization

A novel Megavoltage (MV) multilayer imager (MLI) design featuring higher detective quantum efficiency and lower noise than current conventional MV imagers in clinical use has been recently reported. Optimization of the MLI design for multiple applications including tumor tracking, MV-CBCT and portal dosimetry requires a computational model that will provide insight into the physics processes that affect the overall and individual components' performance. The purpose of the current work was to develop and validate a comprehensive computational model that can be used for MLI optimization. The MLI model was built using the Geant4 Application for Tomographic Emission (GATE) application. The model includes x-ray and charged-particle interactions as well as the optical transfer within the phosphor. A first prototype MLI device featuring a stack of four detection layers was used for model validation. Each layer of the prototype contains a copper buildup plate, a phosphor screen and photodiode array. The model was validated against measured data of Modulation Transfer Function (MTF), Noise-Power Spectrum (NPS), and Detective Quantum Efficiency (DQE). MTF was computed using a slanted slit with 2.3° angle and 0.1mm width. NPS was obtained using the autocorrelation function technique. DQE was calculated from MTF and NPS data. The comparison metrics between simulated and measured data were the Pearson's correlation coefficient (r) and the normalized root-mean-square error (NRMSE). Good agreement between measured and simulated MTF and NPS values was observed. Pearson's correlation coefficient for the combined signal from all layers of the MLI was equal to 0.9991 for MTF and 0.9992 for NPS; NRMSE was 0.0121 for MTF and 0.0194 for NPS. Similarly, the DQE correlation coefficient for the combined signal was 0.9888 and the NRMSE was 0.0686. A comprehensive model of the novel MLI design was developed using the GATE toolkit and validated against measured MTF, NPS, and DQE data acquired with a prototype device featuring four layers. This model will be used for further optimization of the imager components and configuration for clinical radiotherapy applications.

Megavoltage 2D topographic imaging: An attractive alternative to megavoltage CT for the localization of breast cancer patients treated with TomoDirect

PurposeTo show the usefulness of topographic 2D megavoltage images (MV2D) for the localization of breast cancer patients treated with TomoDirect (TD), a radiotherapy treatment technique with fixed-angle beams performed on a TomoTherapy system. MethodsA method was developed to quickly localize breast cancer patients treated with TD by registering the MV2D images produced before a TD treatment with reference images reconstructed from a kilovoltage CT simulation scanner and by using the projection of the beam-eye-view TD treatment field. Dose and image quality measurements were performed to determine the optimal parameters for acquiring MV2D images. A TD treatment was simulated on a chest phantom equipped with a breast attachment. MVCT and MV2D images were performed for 7 different shifted positions of the phantom and registered by 10 different operators with the simulation kilovoltage CT images. ResultsCompared to MVCT, MV2D imaging reduces the dose by a factor of up to 45 and the acquisition time by a factor of up to 49. Comparing the registration shift values obtained for the phantom images obtained with MVCT in the coarse mode to those obtained with MV2D, the mean difference is 1.0±1.1mm, −1.1mm±1.1, and −0.1±2.2mm, respectively, in the lateral, longitudinal, and vertical directions. ConclusionsWith dual advantages (very fast imaging and a potentially reduced dose to the heart and contralateral organs), MV2D topographic images may be an attractive alternative to MVCT for the localization of breast cancer patients treated with TomoDirect.

Image-guided radiation therapy (IGRT): practical recommendations of Italian Association of Radiation Oncology (AIRO)

The use of imaging to maximize precision and accuracy throughout the entire process of radiation therapy (RT) delivery has been called "Image-guided RT" (IGRT). RT has long been image guided: in fact, historically, the portal films and later electronic megavoltage images represented an early form of IGRT. A broad range of IGRT modalities is now available and adopted. The target location may be defined for each treatment fraction by several methods by localizing surrogates, including implanted fiducial markers, external surface markers or anatomical features (through planar imaging, fluoroscopy, KV or MV computed tomography, magnetic resonance imaging, ultrasound and X-ray imaging, electromagnetic localization, optical surface imaging, etc.). The aim of the present review is to define practical recommendations for IGRT.

Characterization of a 2.5 MV inline portal imaging beam.

A new megavoltage (MV) energy was recently introduced on Varian TrueBeam linear accelerators for imaging applications. This work describes the experimental characterization of a 2.5 MV inline portal imaging beam for commissioning, routine clinical use, and quality assurance purposes. The beam quality of the 2.5 MV beam was determined by measuring a percent depth dose, PDD, in water phantom for 10×10 cm2 field at source‐to‐surface distance 100 cm with a CC13 ion chamber, plane parallel Markus chamber, and GafChromic EBT3 film. Absolute dosimetric output calibration of the beam was performed using a traceable calibrated ionization chamber, following the AAPM Task Group 51 procedure. EBT3 film measurements were also performed to measure entrance dose. The output stability of the imaging beam was monitored for five months. Coincidence of 2.5 MV imaging beam with 6 MV therapy beam was verified with hidden‐target cubic phantom. Image quality was studied using the Leeds and QC3 phantom. The depth of maximum dose, dmax, and percent dose at 10 cm depth were, respectively, 5.7 mm and 51.7% for CC13, 6.1 mm and 51.9% for Markus chamber, and 5.1 mm and 51.9% for EBT3 film. The 2.5 MV beam quality is slightly inferior to that of a 60Co teletherapy beam; however, an estimated kQ of 1.00 was used for output calibration purposes. The beam output was found to be stable to within 1% over a five‐month period. The relative entrance dose as measured with EBT3 films was 63%, compared to 23% for a clinical 6 MV beam for a 10×10 cm2 field. Overall coincidence of the 2.5 MV imaging beam with the 6 MV clinical therapy beam was within 0.2 mm. Image quality results for two commonly used imaging phantoms were superior for the 2.5 MV beam when compared to the conventional 6 MV beam. The results from measurements on two TrueBeam accelerators show that 2.5 MV imaging beam is slightly softer than a therapeutic 60Co beam, it provides superior image quality than a 6 MV therapy beam, and has excellent output stability. These 2.5 MV beam characterization results can serve as reference for clinics planning to commission and use this novel energy‐image modality.PACS number(s): 87.57.‐s, 87.59.‐e, 06.20.fb, 87.53.Bn

Modern radiotherapy using image guidance for unresectable non-small cell lung cancer can improve outcomes in patients treated with chemoradiation therapy.

To investigate the impact of advances in image-guided radiotherapy (IGRT) on the outcomes of patients with non-small cell lung cancer (NSCLC) treated with chemoradiation therapy (CRT). We retrospectively reviewed 91 patients with NSCLC treated with definitive CRT using image guidance with daily orthogonal kilovoltage (kV) imaging compared to standard weekly megavoltage (MV) portal verifications. Kaplan-Meier curves for overall survival and locoregional failure were computed and stratified by image guidance techniques. Log-rank tests were used to compare strata. Cox Proportional Hazards models were used to identify risk factors for worse mortality and locoregional control. Fifty-four percent (n=49) of patients received weekly MV portal imaging, while 46% (n=42) underwent IGRT using daily orthogonal kV imaging. kV IGRT was associated with longer median survival (36.4 months) compared to MV imaging (14.9 months; P=0.01). kV imaging was also marginally associated with lower risk of locoregional failure. Median time to local progression in patients imaged with kV was 21.4 months compared to 10.9 months (P=0.065) for those treated with MV portal imaging. Daily kV imaging appears to be marginally associated with better survival and disease control when compared to MV imaging. Given the small study size and the numerable factors tested, these finding require additional confirmation.

SU‐F‐T‐536: Contra‐Lateral Breast Study for Prone Versus Supine Patients

Purpose:There are several advantages to utilizing the prone technique for intact breast cancer patients. However, as the topography changes, accompanied by the influence of a supporting breast board and patient treatment couch, the question that arises is to whether there is a concern for contralateral breast dose for intact breast cancer patients being treated with this technique.Methods:An anthropomorphic phantom with breast mounds to duplicate intact breast cancer treatment was planned in prone and supine position. Two tangential beams were executed in the similar manner for as the radiotherapy planning system. For the prone setup, a breast dense foam board was used to support the phantom. A grid of 24 OSL nanodots was placed at 6cm, 4cm, and 2cm apart from the medial border for both prone and supine setups. The phantom was set up using megavoltage imaging and treated as per plan. Additional, a similar study was performed on a patient treated in prone position.Results:Overall, the contralateral breast dose was generally higher for prone setups at all locations especially when close to the medial border. The average mean dose was found to be 1.8%, 2.5% of the prescribed dose for supine respectively prone position. The average of the standard deviation is 1.04%, 1.38% for supine respectively prone position. As for patient treated in prone position average mean dose was found to be 1.165% of the prescribed dose and average of the standard deviation is 9.456%.Conclusion:There is minimal influence of scatter from the breast board. It appears that the volatility of the setup could lead to higher doses than expected from the planning system to the contralateral breast when the patient is in the prone position

WE‐DE‐BRA‐06: Evaluation of the Imaging Performance of a Novel Water‐Equivalent EPID

Purpose:To evaluate the megavoltage imaging performance of a novel, water‐equivalent electronic portal imaging device (EPID) developed for simultaneous imaging and dosimetry applications in radiotherapy.Methods:A novel EPID prototype based on active matrix flat panel imager technology has been developed by our group and previously reported to exhibit a water‐equivalent dose response. It was constructed by replacing all components above the photodiode detector in a standard clinical EPID (including the copper plate and phosphor screen) with a 15 × 15 cm2 array of plastic scintillator fibers. Individual fibers measured 0.5 × 0.5 × 30 mm3. Spatial resolution was evaluated experimentally relative to that of a standard EPID with the thin slit technique to measure the modulation transfer function (MTF) for 6 MV x‐ray beams. Monte Carlo (MC) EPID models were used to benchmark simulated MTFs against the measurements. The zero spatial frequency detective quantum efficiency (DQE(0)) was simulated for both EPID configurations and a preliminary optimization of the prototype was performed by evaluating DQE(0) as a function of fiber length up to 50 mm.Results:The MC‐simulated DQE(0) for the prototype EPID configuration was ∼7 times greater than that of the standard EPID. The prototype's DQE(0) also increased approximately linearly with fiber length, from ∼1% at 5 mm length to ∼11% at 50 mm length. The standard EPID MTF was greater than the prototype EPID's for all spatial frequencies, reflecting the trade off between x‐ray detection efficiency and spatial resolution with thick scintillators.Conclusion:This study offers promising evidence that a water‐equivalent EPID previously demonstrated for radiotherapy dosimetry may also be used for radiotherapy imaging applications. Future studies on optimising the detector design will be performed to develop a next‐generation prototype that offers improved megavoltage imaging performance, with the aim to at least match that of current clinical EPIDs.Funding for this project was provided by an Australian Research Council Linkage Project grant (2015) between The University of Sydney, South Western Sydney Local Health District and Perkin‐Elmer Pty Ltd.

TU‐D‐BRC‐00: The EPID Strikes Back

There has been increased deployment of gantry‐mounted linacs with onboard kilovoltage x‐ray imagers. Despite this trend, most clinics still employ beam's‐eye‐view (BEV) imaging using flat panel electronic portal imaging devices (EPIDs) as a highly useful means of verifying geometric localization pre‐ and post‐delivery. In recent years, there has been increased interest in the use of EPIDs for a variety of clinical and investigational applications ranging from megavoltage cone‐beam CT, in vivo dosimetry, patient‐specific QA to real‐time target localization for dynamic MLC‐based tumor tracking. In conjunction, there are several innovative EPID designs currently under investigation that hold great promise in bringing novel imaging and image‐guidance applications into clinical practice.This symposium is aimed at clinical and research‐oriented medical physicists who want to learn more about current and future megavoltage imaging technology and applications. In this symposium, we will examine the (i) interesting applications that make use of current EPID technology such as volumetric 4D image‐based models, real‐time position estimation for dynamic MLC tracking, etc., (ii) clinical applications such as in vivo EPID dosimetry and (iii) innovative high‐performance EPID designs and exciting future applications, including designs based on thick segmented scintillators, hybrid kV/MV imagers using segmented strip scintillators, and energy‐resolving EPIDs based on multilayer imagers and poly‐Silicon active matrix flat panel imagers.Learning Objectives – In this symposium, the attendees will learn Novel applications using currently available EPIDs Use of EPIDs to perform in vivo dosimetry Novel designs using high‐efficiency, energy resolving EPIDs and future applications of megavoltage imagingJ. Rottmann, Varian Medical SystemsP. Greer, Research Grant Funding from Radiation Oncology Institute (Grant ROI2013‐912), 2013–2014

SU‐F‐J‐179: Commissioning Dosimetric Data of a New 2.5 Megavoltage Imaging Beam from a TrueBeam Linear

Purpose:Recently a new 2.5 megavoltage imaging beam has become available in a TrueBeam linear accelerator for image guidance. There is limited information available related to the beam characteristics. Commissioning dosimetric data of the new imaging is necessary for configuration of the beam in a treatment planning system in order to calculate imaging doses to patients resulting from this new imaging beam. The purpose of this study is to provide measured commissioning data recommended for a beam configuration in a treatment planning system.Methods:A recently installed TrueBeam linear accelerator is equipped with a new low energy photon beam with a nominal energy of 2.5 MV which provides better image quality in addition to other therapeutic megavoltage beams. Dosimetric characteristics of the 2.5 MV are measured for commissioning. An ionization chamber was used to measure dosimetric data including depth‐dose curves and dose profiles at different depths for field sizes ranging from 5×5 cm2 to 40×40 cm2.Results:Although the new 2.5 MV beam is a flattening‐filter‐free (FFF) beam, its dose profiles are much flatter compared to a 6 MV FFF beam. The dose decrease at 20 cm away from the central axis is less than 30% for a 40×40 cm2 field. This moderately lower dose at off‐axis distances benefits the imaging quality. The values of percentage depth‐dose (PDD) curves are 53% and 63% for 10×10 cm2 and 40×40 cm2 fields respectively. The measured beam output is 0.85 cGy/MU for a reference field size at depth 5 cm obtained according to the AAPM TG‐51 protocol.Conclusion:This systematically measured commissioning data is useful for configuring the new imaging beam in a treatment planning system for patient imaging dose calculations resulting from the application of this 2.5 MV beam which is commonly set as a default in imaging procedures.

TU‐D‐BRC‐03: Future EPID Technology and Applications

There has been increased deployment of gantry‐mounted linacs with onboard kilovoltage x‐ray imagers. Despite this trend, most clinics still employ beam's‐eye‐view (BEV) imaging using flat panel electronic portal imaging devices (EPIDs) as a highly useful means of verifying geometric localization pre‐ and post‐delivery. In recent years, there has been increased interest in the use of EPIDs for a variety of clinical and investigational applications ranging from megavoltage cone‐beam CT, in vivo dosimetry, patient‐specific QA to real‐time target localization for dynamic MLC‐based tumor tracking. In conjunction, there are several innovative EPID designs currently under investigation that hold great promise in bringing novel imaging and image‐guidance applications into clinical practice.This symposium is aimed at clinical and research‐oriented medical physicists who want to learn more about current and future megavoltage imaging technology and applications. In this symposium, we will examine the (i) interesting applications that make use of current EPID technology such as volumetric 4D image‐based models, real‐time position estimation for dynamic MLC tracking, etc., (ii) clinical applications such as in vivo EPID dosimetry and (iii) innovative high‐performance EPID designs and exciting future applications, including designs based on thick segmented scintillators, hybrid kV/MV imagers using segmented strip scintillators, and energy‐resolving EPIDs based on multilayer imagers and poly‐Silicon active matrix flat panel imagers.Learning Objectives – In this symposium, the attendees will learn Novel applications using currently available EPIDs Use of EPIDs to perform in vivo dosimetry Novel designs using high‐efficiency, energy resolving EPIDs and future applications of megavoltage imagingJ. Rottmann, Varian Medical SystemsP. Greer, Research Grant Funding from Radiation Oncology Institute (Grant ROI2013‐912), 2013–2014

TU‐D‐BRC‐02: Free Information ‐ EPID‐Based In Vivo Dosimetry

There has been increased deployment of gantry‐mounted linacs with onboard kilovoltage x‐ray imagers. Despite this trend, most clinics still employ beam's‐eye‐view (BEV) imaging using flat panel electronic portal imaging devices (EPIDs) as a highly useful means of verifying geometric localization pre‐ and post‐delivery. In recent years, there has been increased interest in the use of EPIDs for a variety of clinical and investigational applications ranging from megavoltage cone‐beam CT, in vivo dosimetry, patient‐specific QA to real‐time target localization for dynamic MLC‐based tumor tracking. In conjunction, there are several innovative EPID designs currently under investigation that hold great promise in bringing novel imaging and image‐guidance applications into clinical practice.This symposium is aimed at clinical and research‐oriented medical physicists who want to learn more about current and future megavoltage imaging technology and applications. In this symposium, we will examine the (i) interesting applications that make use of current EPID technology such as volumetric 4D image‐based models, real‐time position estimation for dynamic MLC tracking, etc., (ii) clinical applications such as in vivo EPID dosimetry and (iii) innovative high‐performance EPID designs and exciting future applications, including designs based on thick segmented scintillators, hybrid kV/MV imagers using segmented strip scintillators, and energy‐resolving EPIDs based on multilayer imagers and poly‐Silicon active matrix flat panel imagers.Learning Objectives – In this symposium, the attendees will learn Novel applications using currently available EPIDs Use of EPIDs to perform in vivo dosimetry Novel designs using high‐efficiency, energy resolving EPIDs and future applications of megavoltage imagingJ. Rottmann, Varian Medical SystemsP. Greer, Research Grant Funding from Radiation Oncology Institute (Grant ROI2013‐912), 2013–2014

TU‐D‐BRC‐01: Novel Applications for Current EPID Technology

There has been increased deployment of gantry‐mounted linacs with onboard kilovoltage x‐ray imagers. Despite this trend, most clinics still employ beam's‐eye‐view (BEV) imaging using flat panel electronic portal imaging devices (EPIDs) as a highly useful means of verifying geometric localization pre‐ and post‐delivery. In recent years, there has been increased interest in the use of EPIDs for a variety of clinical and investigational applications ranging from megavoltage cone‐beam CT, in vivo dosimetry, patient‐specific QA to real‐time target localization for dynamic MLC‐based tumor tracking. In conjunction, there are several innovative EPID designs currently under investigation that hold great promise in bringing novel imaging and image‐guidance applications into clinical practice.This symposium is aimed at clinical and research‐oriented medical physicists who want to learn more about current and future megavoltage imaging technology and applications. In this symposium, we will examine the (i) interesting applications that make use of current EPID technology such as volumetric 4D image‐based models, real‐time position estimation for dynamic MLC tracking, etc., (ii) clinical applications such as in vivo EPID dosimetry and (iii) innovative high‐performance EPID designs and exciting future applications, including designs based on thick segmented scintillators, hybrid kV/MV imagers using segmented strip scintillators, and energy‐resolving EPIDs based on multilayer imagers and poly‐Silicon active matrix flat panel imagers.Learning Objectives – In this symposium, the attendees will learn Novel applications using currently available EPIDs Use of EPIDs to perform in vivo dosimetry Novel designs using high‐efficiency, energy resolving EPIDs and future applications of megavoltage imagingJ. Rottmann, Varian Medical SystemsP. Greer, Research Grant Funding from Radiation Oncology Institute (Grant ROI2013‐912), 2013–2014

The development of the MRI linac system for online MRI-guided radiotherapy: a clinical update.

n this clinical update, we describe the development and the expected clinical impact of the UMC Utrecht MRI linac (MRL) system. This combination of a radiotherapy accelerator with a diagnostic quality 1.5 T MRI scanner provides real-time imaging at the moment of treatment, thus providing unprecedented targeting accuracy.