MV CBCT Research Articles

SU-GG-T-414: Equivalent Uniform Dose Optimization: A Step Closer to Dose-Guided and Adaptive Radiation Therapy?

Purpose: To investigate whether IMRT optimization based on generalized Equivalent Uniform Dose1 (gEUD) objectives can compensate for and further reduce the additional doses to organs at risk (OAR) when MV CBCT is used for daily IGRT method. One of the ultimate goals of CBCT is the adaptation to changes of patient shape and treatment volumes during the treatment course. Performing frequent CBCT, such as daily setup IGRT, delivers additional dose to the treatment volume which needs to be accounted for and possibly be optimized upon. Method and Materials: Three IMRT plans for H&N were prepared. The first plan was based on dose-volume (DV) objectives set by the physician. Then, an additional CBCT beam, simulated as an arc of 200°, was added to the plan to investigate the dose increase to all the volumes of interest. To decrease the dose to OARs, a second plan was devised using the same DV objectives, but incorporating the dose from CBCT2. Finally, a third plan that included the CBCT was produced, based on gEUD1, to attempt further reduction of dose to OARs while maintaining similar target volume coverage. Results: The gEUD-based optimization produced a superior plan that not only accounted for the additional dose that the CBCT deposited to the treatment volume, but further reduced the dose to the OARs compared to the DV-based plans. In other words, gEUD-based optimization reduced the lower doses to the sensitive tissues further than DV-based optimization. Conclusion: gEUD-based optimization can easily provide a “dose adaptation” to allow the introduction of CBCT in routine clinical practice and eventually allow for dose guidance and adaptive radiation therapy that will demand complex dose sculpting for the volumes of interest.

Calibration of megavoltage cone‐beam CT for radiotherapy dose calculations: Correction of cupping artifacts and conversion of CT numbers to electron density

Megavoltage cone-beam CT (MV CBCT) is used for three-dimensional imaging of the patient anatomy on the treatment table prior to or just after radiotherapy treatment. To use MV CBCT images for radiotherapy dose calculation purposes, reliable electron density (ED) distributions are needed. Patient scatter, beam hardening and softening effects result in cupping artifacts in MV CBCT images and distort the CT number to ED conversion. A method based on transmission images is presented to correct for these effects without using prior knowledge of the object's geometry. The scatter distribution originating from the patient is calculated with pencil beam scatter kernels that are fitted based on transmission measurements. The radiological thickness is extracted from the scatter subtracted transmission images and is then converted to the primary transmission used in the cone-beam reconstruction. These corrections are performed in an iterative manner, without using prior knowledge regarding the geometry and composition of the object. The method was tested using various homogeneous and inhomogeneous phantoms with varying shapes and compositions, including a phantom with different electron density inserts, phantoms with large density variations, and an anthropomorphic head phantom. For all phantoms, the cupping artifact was substantially removed from the images and a linear relation between the CT number and electron density was found. After correction the deviations in reconstructed ED from the true values were reduced from up to 0.30 ED units to 0.03 for the majority of the phantoms; the residual difference is equal to the amount of noise in the images. The ED distributions were evaluated in terms of absolute dose calculation accuracy for homogeneous cylinders of different size; errors decreased from 7% to below 1% in the center of the objects for the uncorrected and corrected images, respectively, and maximum differences were reduced from 17% to 2%, respectively. The presented method corrects the MV CBCT images for cupping artifacts and extracts reliable ED information of objects with varying geometries and composition, making these corrected MV CBCT images suitable for accurate dose calculation purposes.

Monte Carlo investigations of megavoltage cone-beam CT using thick, segmented scintillating detectors for soft tissue visualization.

Megavoltage cone-beam computed tomography (MV CBCT) is a highly promising technique for providing volumetric patient position information in the radiation treatment room. Such information has the potential to greatly assist in registering the patient to the planned treatment position, helping to ensure accurate delivery of the high energy therapy beam to the tumor volume while sparing the surrounding normal tissues. Presently, CBCT systems using conventional MV active matrix flat-panel imagers (AMFPIs), which are commonly used in portal imaging, require a relatively large amount of dose to create images that are clinically useful. This is due to the fact that the phosphor screen detector employed in conventional MV AMFPIs utilizes only approximately 2% of the incident radiation (for a 6 MV x-ray spectrum). Fortunately, thick segmented scintillating detectors can overcome this limitation, and the first prototype imager has demonstrated highly promising performance for projection imaging at low doses. It is therefore of definite interest to examine the potential performance of such thick, segmented scintillating detectors for MV CBCT. In this study, Monte Carlo simulations of radiation energy deposition were used to examine reconstructed images of cylindrical CT contrast phantoms, embedded with tissue-equivalent objects. The phantoms were scanned at 6 MV using segmented detectors having various design parameters (i.e., detector thickness as well as scintillator and septal wall materials). Due to constraints imposed by the nature of this study, the size of the phantoms was limited to approximately 6 cm. For such phantoms, the simulation results suggest that a 40 mm thick, segmented CsI detector with low density septal walls can delineate electron density differences of approximately 2.3% and 1.3% at doses of 1.54 and 3.08 cGy, respectively. In addition, it was found that segmented detectors with greater thickness, higher density scintillator material, or lower density septal walls exhibit higher contrast-to-noise performance. Finally, the performance of various segmented detectors obtained at a relatively low dose (1.54 cGy) was compared with that of a phosphor screen similar to that employed in conventional MV AMFPIs. This comparison indicates that for a phosphor screen to achieve the same contrast-to-noise performance as the segmented detectors approximately 18 to 59 times more dose is required, depending on the configuration of the segmented detectors.

Improved Image Quality and Beam Stability for a High Contrast Imaging Beam Line Used for Megavoltage Cone-beam CT

Purpose/Objective(s): To determine preferred system components and settings to improve image quality and beam stability of an imaging beam line which utilizes the beam of a clinical linear accelerator (linac) incident on a low-Z target used for megavoltage cone-beam CT (MV CBCT). Materials/Methods: Image quality is determined from MV CBCT using the EMMA phantom. This phantom contains a low contrast module consisting of 2 cm thick water equivalent with cylindrical inserts of variable contrast and sizes. The diameters of the cylinders are 3 mm, 5 mm, 7 mm, 10 mm, and 20 mm. There are four groups of inserts with variable densities of 1%, 3%, 5%, and 7% relative to water. The following system components and settings were varied to improve the imaging beam line: beam energy, target material, cranio-caudal image length and the detector buildup plate. The 6 MV treatment beam is stable. Lower energies, desired for improved image quality, push the linac toward a less stable beam. The 6 MV treatment beam with tungsten target and stainless steel flattener was used as well as unflattened beams generated from a 1.3 cm thick industrial diamond target at 4 and 6 MV and a 1.3 cm thick graphite target at 4 MV only. The graphite target thickness, limited by available physical space, was not thick enough to stop primary electron leakage at 6 MV. The diamond target is much higher density and absorbs the primary electron beam at 6 MV. Flat panel buildup plates of different thicknesses of copper, aluminum, brass, and no plate, were assessed with a standard CT phantom. The effect of scatter reduction on image quality was assessed at field lengths of 2 cm, 5 cm, 15 cm, 27.4 cm. The phantom used was a head-size water cylinder with tissue equivalent materials from the CT phantom placed in it. All images are taken with a dose of 10 cGy delivered at the depth of maximum dose at the isocenter. Results: For the tungsten target at 6 MV, 5% inserts down to 3 cm diameter are resolved. With both the graphite and the diamond target at 4 MV, 3% inserts are visible at all sizes. A small difference was observed between the diamond target at 4 MV and 6 MV, indicating that image quality is affected more by target Z than beam energy. Different buildup plates did not play a significant role in soft tissue contrast. In the scatter reduction study, only a weak dependence of contrast to noise ratio on field length was observed. Improved spatial resolution was observed with the graphite and diamond targets compared to the tungsten target. Evaluation of 2D projections shows an increase in f50 value from 0.5 lp/mm to 0.6 lp/mm. This improvement may be attributed to the absence of the flattening filter, which acts as a scatter source. This improvement did not directly translate to CBCT images due to limited number of projections used and the resolution of reconstructed volume (256x256x270). Conclusions: A diamond target offers the possibility of running MV CBCT at a more stable energy of 5-6 MV with image quality comparable to graphite at 4 MV. A narrow field and lower-Z (or no) build-up plate separately yield a modest improvement in image quality. Patient imaging is scheduled for the summer, 2007. It is anticipated that MV CBCT images taken for the same patient with the treatment and imaging beams will be available to compare and show at the meeting.

TU‐A‐BRA‐01: EPID, MVCT

The Mega‐Voltage Cone‐Beam CT (MVCBCT) system provides a 3D image of the patient anatomy in the actual treatment position that can be tightly aligned to the planning CT, allowing daily verification and correction of the patient position moments before the dose delivery. Integrated onto a linear accelerator, the system consists of a new a‐Si flat panel adapted for MV imaging and a workflow application allowing the automatic acquisition of projection images at low dose rate, CBCT image reconstruction, CT to CBCT image registration and couch position adjustment. MVCBCT provides accurate electron density and allows studies of dosimetrical impact of setup error, anatomical changes or in presence of implanted metallic objects.This lecture will provide an overview of the physics characteristics of MV and MV CBCT imaging, acquisition and reconstruction, image registration, alignment precision and quality assurance procedures. A description of the workflow and a survey of the clinical applications, indications and protocols will be presented. Finally, current challenges and future developments will be addressed.Educational objectives1. Understand the basics of MV Cone‐Beam CT imaging.2. Understand the possibilities of daily 3D Imaging for patient alignment and other applications.3. Understand the clinical roles of Image‐Guided (IGRT) and Dose‐ Guided (DGRT) Radiation Therapy.This work was supported by Siemens Oncology Care Systems.

SU‐FF‐J‐61: Inter‐Observer Image Quality Analysis of Megavoltage Cone Beam‐CT Versus Megavoltage Fan Beam‐CT

Purpose: Use of strict scoring criteria to study the inter‐observer variability of image quality between megavoltage cone‐beam cat scan (MV CBCT) and megavoltage helical fan‐beam computed tomography (TomoTherapy). Method and Materials: A Siemens MVCB QA phantom was utilized for all image acquisitions. Sections 2 and 4 of this phantom contain low contrast objects, and section 3 contains a line pair array for spatial resolution determination. The phantom was imaged on a 20‐slice Siemens SOMATOM® Sensation Open (CT‐Sim) as the “gold standard” for image quality. The phantom was also imaged on a TomoTherapy machine utilizing the three available imaging modes of fine, normal, and coarse. The imaging was repeated on a Siemens OnCor Avant‐Garde linear accelerator utilizing the MVCB‐CT option. Five observers were asked to score each image modality. A scoring scheme and rules were developed to allow all of the observers to use the same criteria to judge these image attributes. Each image was processed using a constant region of interest (ROI) and fixed window and level to reduce scoring bias. Results: The outcome was consistent among the observers concerning spatial resolution. There was a decrease in perceived resolution in the z‐axis compared to the x‐y directions for all three imaging units The Siemens MVCB scored higher compared to TomoTherapy in the z‐axis, but scored lower in the x‐y plane. The observer scoring for low contrast detection was most consistent in the phantom containing high‐density objects (1.56–1.02 g/cc) compared to lower density (1.09–1.02 g/cc). A comparison was made between low contrast scoring and quantitative contrast ratio. Conclusion: The results show consistent scoring of image spatial resolution by multiple observers regardless of modality. The largest variation occurred when scoring the lowest density objects. A noticeable trend was established between contrast ratio and observer scoring. Conflict of Interest: Partially supported by Siemens.

Observation of Interfractional Variations in Lung Tumor Position Using Respiratory Gated and Ungated Megavoltage Cone-Beam Computed Tomography

To evaluate the use of megavoltage cone-beam computed tomography (MV CBCT) to measure interfractional variation in lung tumor position. Eight non-small-cell lung cancer patients participated in the study, 4 with respiratory gating and 4 without. All patients underwent MV CBCT scanning at weekly intervals. Contoured planning CT and MV CBCT images were spatially registered based on vertebral anatomy, and displacements of the tumor centroid determined. Setup error was assessed by comparing weekly portal orthogonal radiographs with digitally reconstructed radiographs generated from planning CT images. Hypothesis testing was performed to test the statistical significance of the volume difference, centroid displacement, and setup uncertainty. The vertebral bodies and soft tissue portions of tumor within lung were visible on the MV CBCT scans. Statistically significant systematic volume decrease over the course of treatment was observed for 1 patient. The average centroid displacement between simulation CT and MV CBCT scans were 2.5 mm, -2.0 mm, and -1.5 mm with standard deviations of 2.7 mm, 2.7 mm, and 2.6 mm in the right-left, anterior-posterior and superior-inferior directions. The mean setup errors were smaller than the centroid shifts, while the standard deviations were comparable. In most cases, the gross tumor volume (GTV) defined on the MV CBCT was located on average at least 5 mm inside a 10 mm expansion of the GTV defined on the planning CT scan. The MV CBCT technique can be used to image lung tumors and may prove valuable for image-guided radiotherapy. Our conclusions must be verified in view of the small patient number.

Dose comparison of megavoltage cone‐beam and orthogonal‐pair portal images

The technique of megavoltage cone‐beam computed tomography (MV CBCT) is available for image‐guided radiation therapy to improve the accuracy of patient setup and tumor localization. However, development of strategies to efficiently and effectively implement this technique or to replace the current orthogonal portal images technique remains challenging in the clinical environment. It is useful to compare the difference in absorbed dose between the MV CBCT technique and the orthogonal portal images technique, the current standard practice for treatment verification. Our study analyzed the doses generated from these two imaging techniques for six treatment sites (pelvis, abdomen, lung, head and neck, breast, prostate). The analysis was made by simulating the MV CBCT technique with an arc beam and a beam‐on time of 9 monitor units (MUs), and the orthogonal pair technique with a double‐exposure anterior–posterior and lateral pair and a beam‐on time of 4 MUs. The results are presented as dose per MU (cGy/MU) and absolute dose (cGy). The isocenter doses, integral doses, maximum doses, and mean doses to tumor and critical organs, and the two‐dimensional isodose distributions and dose–volume histograms of each critical organ were investigated. The absolute dose difference between MV CBCT and orthogonal pair at the isocenter was 4.02±0.59 cGy. Major differences were seen between the two techniques in critical organs whose locations are away from the tumor. These organs, such as the contralateral breast (difference: 0.17±0.10 cGy/MU) and lung (difference: 0.15±0.20 cGy/MU), receive a higher dose from MV CBCT images than from orthogonal portal images. Additionally, higher doses and larger dose areas involving more normal tissues were observed for MV CBCT images than for orthogonal portal images in our analysis methodology, which used 200 beam projections delivered from various angles for the MV CBCT simulation and from just two perpendicular angles for the orthogonal pair simulation. In our selected clinical cases, the high‐dose area from the orthogonal pair technique was always located inside the tumor; with MV CBCT, the high‐dose area will most likely be outside the tumor. Therefore, the potentially higher doses to critical organs from MV CBCT images should be properly analyzed to ensure that they do not exceed the tolerance dose when therapy is delivered using that technique. On the other hand, to obtain good image quality, the higher MUs with MV CBCT images may be necessary. The absorbed dose for the tumor and for other critical organs should be calculated accordingly in the treatment plans. Images by MV CBCT are a great tool for three‐dimensional verification of patient treatment position. The trade‐off is that the MV CBCT technique for patient treatment verification might have a higher chance of increasing the dose to normal tissue during image acquisition.PACS number: 87.53.Oq

Dose-guided radiation therapy with megavoltage cone-beam CT

Recent advances in fractionated external beam radiation therapy have increased our ability to deliver radiation doses that conform more tightly to the tumour volume. The steeper dose gradients delivered in these treatments make it increasingly important to set precisely the positions of the patient and the internal organs. For this reason, considerable research now focuses on methods using three-dimensional images of the patient on the treatment table to adapt either the patient position or the treatment plan, to account for variable organ locations. In this article, we briefly review the different adaptive methods being explored and discuss a proposed dose-guided radiation therapy strategy that adapts the treatment for future fractions to compensate for dosimetric errors from past fractions. The main component of this strategy is a procedure to reconstruct the dose delivered to the patient based on treatment-time portal images and pre-treatment megavoltage cone-beam computed tomography (MV CBCT) images of the patient. We describe the work to date performed to develop our dose reconstruction procedure, including the implementation of a MV CBCT system for clinical use, experiments performed to calibrate MV CBCT for electron density and to use the calibrated MV CBCT for dose calculations, and the dosimetric calibration of the portal imager. We also present an example of a reconstructed patient dose using a preliminary reconstruction program and discuss the technical challenges that remain to full implementation of dose reconstruction and dose-guided therapy.

Low-dose megavoltage cone-beam computed tomography for lung tumors using a high-efficiency image receptor

We report on the capabilities of a low-dose megavoltage cone-beam computed tomography (MV CBCT) system. The high-efficiency image receptor consists of a photodiode array coupled to a scintillator composed of individual CsI crystals. The CBCT system uses the 6 MV beam from a linear accelerator. A synchronization circuit allows us to limit the exposure to one beam pulse [0.028 monitor units (MU)] per projection image. 150-500 images (4.2-13.9 MU total) are collected during a one-minute scan and reconstructed using a filtered backprojection algorithm. Anthropomorphic and contrast phantoms are imaged and the contrast-to-noise ratio of the reconstruction is studied as a function of the number of projections and the error in the projection angles. The detector dose response is linear (R2 value 0.9989). A 2% electron density difference is discernible using 460 projection images and a total exposure of 13 MU (corresponding to a maximum absorbed dose of about 12 cGy in a patient). We present first patient images acquired with this system. Tumors in lung are clearly visible and skeletal anatomy is observed in sufficient detail to allow reproducible registration with the planning kV CT images. The MV CBCT system is shown to be capable of obtaining good quality three-dimensional reconstructions at relatively low dose and to be clinically usable for improving the accuracy of radiotherapy patient positioning.

Mégavoltage cone-beam CT : récents développements et applications cliniques pour la radiothérapie conformationnelle avec modulation d'intensité

The Megavoltage cone-beam (MV CBCT) system consists of a new a-Si flat panel adapted for MV imaging and an integrated workflow application allowing the automatic acquisition of projection images, cone-beam CT image reconstruction, CT to CBCT image registration and couch position adjustment. This provides a 3D patient anatomy volume in the actual treatment position, relative to the treatment isocenter, moments before the dose delivery, that can be tightly aligned to the planning CT, allowing verification and correction of the patient position, detection of anatomical changes and dose calculation. In this paper, we present the main advantages and performance of this MV CBCT system and summarize the different clinical applications. Examples of the image-guided treatment process from the acquisition of the MV CBCT scan to the correction of the couch position and dose delivery will be presented for spinal and lung lesions and for head and neck, and prostate cancers.

Integrating respiratory gating into a megavoltage cone‐beam CT system

We have previously described a low-dose megavoltage cone beam computed tomography (MV CBCT) system capable of producing projection image using one beam pulse. In this study, we report on its integration with respiratory gating for gated radiotherapy. The respiratory gating system tracks a reflective marker on the patient's abdomen midway between the xiphoid and umbilicus, and disables radiation delivery when the marker position is outside predefined thresholds. We investigate two strategies for acquiring gated scans. In the continuous rotation-gated acquisition, the linear accelerator (LINAC) is set to the fixed x-ray mode and the gantry makes a 5 min, 360 degree continuous rotation, during which the gating system turns the radiation beam on and off, resulting in projection images with an uneven distribution of projection angles (e.g., in 70 arcs each covering 2 degrees). In the gated rotation-continuous acquisition, the LINAC is set to the dynamic arc mode, which suspends the gantry rotation when the gating system inhibits the beam, leading to a slightly longer (6-7 min) scan time, but yielding projection images with more evenly distributed projection angles (e.g., approximately 0.8 degrees between two consecutive projection angles). We have tested both data acquisition schemes on stationary (a contrast detail and a thoracic) phantoms and protocol lung patients. For stationary phantoms, a separate motion phantom not visible in the images is used to trigger the RPM system. Frame rate is adjusted so that approximately 450 images (13 MU) are acquired for each scan and three-dimensional tomographic images reconstructed using a Feldkamp filtered backprojection algorithm. The gated rotation-continuous acquisition yield reconstructions free of breathing artifacts. The tumor in parenchymal lung and normal tissues are easily discernible and the boundary between the diaphragm and the lung sharply defined. Contrast-to-noise ratio (CNR) is not degraded relative to nongated scans of stationary phantoms. The continuous rotation-gated acquisition scan also yields tomographic images with discernible anatomic features; however, streak artifacts are observed and CNR is reduced by approximately a factor of 4. In conclusion, we have successfully developed a gated MV CBCT system to verify the patient positioning for gated radiotherapy.

TU‐A‐224C‐01: Daily Localization II: EPID, MVCBCT

The Mega‐Voltage Cone‐Beam CT (MVCBCT) system provides a 3D image of the patient anatomy in the actual treatment position that can be tightly aligned to the planning CT, allowing daily verification and correction of the patient position moments before the dose delivery. Integrated onto a linear accelerator, the system consists of a new a‐Si flat panel adapted for MV imaging and a workflow application allowing the automatic acquisition of projection images at low dose rate, CBCT image reconstruction, CT to CBCT image registration and couch position adjustment. Moreover, MVCBCT provides accurate electron density and allows studies of dosimetrical impact of setup error, anatomical changes or presence of implanted metallic objects.This lecture will provide an overview of the physics characteristics of MV and MV CBCT imaging, acceptance testing and commissioning, acquisition and reconstruction, image registration, alignment precision and quality assurance procedures. An overview of the clinical applications will be provided. Finally, current challenges and future developments will be addressed.Educational Objectives:1. Understand the basics concepts of MV Cone‐Beam CT imaging.2. Understand the workflow and issues related to clinical applications of MV CBCT, including acquisition, reconstruction, registration and patient alignment.3. Understand the possibilities of 3D Imaging for patient alignment.This work was supported by Siemens Oncology Care Systems.

SU‐FF‐I‐11: Site‐Specific Image‐Gain Calibration for MV CBCT

Purpose: MV CBCT is an image guidance modality which yields a 3D dataset representative of the patient anatomy in treatment position. During the acquisition, a series of low‐dose projection images are generated by exposing a high detection efficiency flat panel to short bursts of the linear accelerator beam. In order to avoid image artifacts, the projection images need to be “gain” corrected for variations in pixel intensity unrelated to traversed patient anatomy. These variations arise from differences in individual pixel sensitivities, as well as from the spectral and the intensity non‐uniformity of the incident beam. In this work, we propose and validate two treatment site‐specific gain correction (GC) strategies. Method and Materials: The first GC approach employs an open‐field in‐air image of the beam. Each subsequently acquired image is then divided by this GC image (GCI) to remove the effects of differing pixel gains as well as of incident beam non‐uniformity. The second approach acquires a GCI using a flattened beam. A tray with a 1/2 inch uniform lead plate is inserted in the accessory tray holder prior the gain image acquisition. The presence of the lead plate in the beam path results in spectral and spatial homogeneity of the beam incident on the flat panel imager. Results: The clinical scope of the two correction approaches was investigated by clinical MV CBCT acquisitions. In‐air gain calibration is well suited for head and neck imaging since the underlying spatial and spectral properties of the beam remain unaltered. However, for prostate cases the second calibration approach significantly reduces image artifacts related to the coupled phenomena of beam hardening and profile flattening. Conclusion: Site‐specific acquisition protocols that employ GCIs generated under different conditions result in substantial MV CBCT image quality improvement. Conflict of Interest: Supported by Siemens.

TH‐D‐ValB‐05: Evaluation of Image Quality in Megavoltage Digital Tomosynthesis

Purpose: We report on the characteristics of Megavoltage Cone Beam Digital Tomosynthesis (MVCB DTS) and its potential clinical application for imaging of pulmonary lesions. Method and Materials: MVCB CT refers to the reconstruction of a 3D image from a set of 2D projections, acquired using a medical linear accelerator equipped with an electronic portal imaging device (EPID). A typical MVCB CT scan is acquired over a 200 degrees arc. In the case of MVCB DTS, the angular range is limited to reduce the acquisition time. This limited angular range affects the image quality of the reconstructed tomograms. To study the image quality as a function of the angular range, phantom measurements were performed and data from a head and neck patient were analyzed. The image quality was analyzed in terms of effective slice thickness, shape distortion and contrast sensitivity. MVCB DTS of the lung was performed on patients, with localized and diffuse lesions. Results: The image quality and the capability to distinguish overlaid structures decreases with decreasing angular range: a 20 degrees arc DTS results in a slice thickness of 2.7cm (vs. 1mm), a ratio of the vertical to lateral diameter of a sphere of 0.15, and a reduced contrast sensitivity. The acquisition is faster than MVCB CT. It takes 5–10 seconds for arcs of 20–40 degrees, compared to 45 seconds for a 200 degrees arc. In lung images, the faster acquisition results in a reduced blur due to the respiratory motion. Conclusion: This study indicates some potential advantages of DTS for imaging lung patients in the treatment position. Compared with EPID, DTS provides 3D information and better soft tissue contrast. Compared with CBCT, DTS allows shorter acquisition times, compatible with breath holding.Conflict of Interest: Research sponsored by Siemens OCS.

Megavoltage cone-beam CT to complement CT-based treatment planning for HDR brachytherapy

Purpose: This study investigates the feasibility of using Megavoltage Cone-Beam CT (MV CBCT) images to complement CT-based High-dose-rate (HDR) brachytherapy treatment planning in the presence of non-removable metallic objects.

Megavoltage cone-beam CT: System description and clinical applications

In this article, we describe a clinical mega-voltage cone-beam computed tomography (MV CBCT) system, present the image acquisition and patient setup procedure, discuss the positioning accuracy and image quality, and illustrate its potential use for image-guided radiation therapy (IGRT) through selected clinical examples. The MV CBCT system consists of a standard linear accelerator equipped with an amorphous-silicon flat panel electronic portal-imaging device adapted for mega-electron volt (MeV) photons. An integrated computer workspace provides automated acquisition of projection images, image reconstruction, CT to CBCT image registration, and couch shift calculation. The system demonstrates submillimeter localization precision and sufficient soft-tissue resolution to visualize structures such as the prostate. In our clinic, we have used the MV CBCT system to detect nonrigid spinal cord distortions, monitor tumor growth and shrinkage, and locate and position stationary tumors in the lung. MV CBCT has also greatly improved the delineation of structures in CT images that suffer from metal artifacts. MV CBCT has undergone significant development in the last few years. Current image quality has already proven sufficient for many IGRT applications. Moreover, we expect the range of clinical applications for MV CBCT to grow as imaging technology continues to improve.

Po-Poster - 02: Generation of MV cone beam CT - a feasibility study

The objective of this work was to study the feasibility of acquiring mega-voltage cone-beam CT (MV CBCT) images of sufficient quality for setup verification from 2 D projection images. The generation of mega-voltage cone-beam CT with a standard radiation therapy Primus linear accelerator and an amorphous silicon electronic portal imaging device has been attempted in this work. 2 D projection images of the head section of a Rando phantom were acquired at an interval of 2° from gantry angle of 258° to 102°. Thus 103 2D projection images of the Rando phantom were acquired with 6 MV beam with 1 MU/projection exposure at 300MU/minute dose rate. The estimated dose to the center of the phantom placed at the isocentre of the linac was about 75cGy. The cone angle was only 14.2° and hence for simplicity the images were reconstructed assuming a parallel beam geometry using the IRADON function implemented in MATLAB. High contrast objects are clearly seen on the reconstructed images thus showing the potential of MV CBCT for patient alignment during external beam radiotherapy. From the available image data set, reconstructions were also performed with fewer projections i.e with 52 and 26. Image quality was acceptable with 103 and 52 projections. Reconstruction with Feldkamp cone beam algorithm or iterative algorithm such as Algebraic Reconstruction Techniques (ART) could further improve the image quality.

SU‐FF‐J‐81: Clinical Integration of a MV Conebeam CT System for Image‐Guided Treatment

Purpose: To perform the integration of a newly developed image‐guidance system and to describe the main advantages and performance of the first Megavoltage Conebeam CT (MV CBCT) system. Method and Materials: The MV CBCT system, consisting of a new a‐Si flat panel adapted for MV imaging and an integrated workflow application allowing the automatic acquisition of projection images, conebeam CT image reconstruction, CT to CBCT image registration and couch position adjustment was recently introduced in clinic. Template protocols can be used for the acquisition of CBCT images at different dose ranging from 1 to 60 M.U. Geometrical calibration, gain image adjustment and defect pixels correction procedures are performed off‐line. Results: For a typical case, 200 projection portal images and a total exposure of 5 to 8 M.U. are acquired with the 6 MV beam in 45 seconds and the 256×256×256 MV CBCT image is reconstructed less than two minutes after the start of the acquisition. Examples of the image‐guided treatment process including the acquisition of projections images, the reconstruction of the MV CBCT image and its registration with the planning CT, followed by the couch position correction and dose delivery will be presented. Conclusion: MV CBCT provides a 3D patient anatomy volume in the actual treatment position, relative to the treatment isocenter, moments before the dose delivery, that can be tightly aligned to the planning CT, allowing verification and correction of the patient position. Research supported by Siemens Oncology Care Systems.

SU-FF-J-72: The Effect of MV Cone-Beam CT Cupping Artifacts On Dose Calculation Accuracy

Purpose: MV cone‐beam (CB) CTimaging can be used to position the patient prior to external‐beam radiotherapy. If calibrated, these images may also be used in dose calculations, opening the possibility to reconstruct the delivered dose or re‐optimize the treatment plan before delivery. This study investigates the effect of imaging artifacts on dosimetric accuracy. Method and Materials: Two water cylinders of 13 cm and 22.5 cm diameters were imaged using A) a MV CBCT system integrated with a Siemens accelerator and B) a conventional kV CTimager. Cupping trends observed in the MV CBCT were imposed on the kV CTimages to create artificial data with simulated artifacts. Using the Phillips Pinnacle planning system, a 6 MV 10 × 16 cm2 field was applied to 1) the original kV CT of the smaller cylinder and 2) the kV CT with simulated artifacts. Similarly, a 18 × 18 cm2 field was applied to the original and artificial kV CT of the larger cylinder. Results: For the smaller cylinder, the MV CBCTwater signal relative to air at the cylinder center differs from the radial and axial extremities by approximately 14% and 17% respectively. For the larger cylinder, the differences are 38% and 20% in radial and axial directions. The dose distributions for the artificial images show a systematic deviation which increases with depth. In the high‐dose, low‐gradient regions, the largest deviation for the smaller cylinder is 1% of the maximum delivered dose. For the larger cylinder, the largest deviation is 4%. Conclusion: Cupping artifacts in water phantoms produce dosimetric errors much smaller in magnitude than the cupping trend itself but as large as 4% for large phantoms. These results suggest that rough correction of MV CBCT artifacts may be sufficient for dosimetric accuracy. Conflict of Interest: Research supported by Siemens OCS.