On-board Imaging Research Articles

Feasibility of MV CBCT-based treatment planning for urgent radiation therapy: dosimetric accuracy of MV CBCT-based dose calculations.

Unlike scheduled radiotherapy treatments, treatment planning time and resources are limited for emergency treatments. Consequently, plans are often simple 2D image‐based treatments that lag behind technical capabilities available for nonurgent radiotherapy. We have developed a novel integrated urgent workflow that uses onboard MV CBCT imaging for patient simulation to improve planning accuracy and reduce the total time for urgent treatments. This study evaluates both MV CBCT dose planning accuracy and novel urgent workflow feasibility for a variety of anatomic sites. We sought to limit local mean dose differences to less than 5% compared to conventional CT simulation. To improve dose calculation accuracy, we created separate Hounsfield unit–to–density calibration curves for regular and extended field‐of‐view (FOV) MV CBCTs. We evaluated dose calculation accuracy on phantoms and four clinical anatomical sites (brain, thorax/spine, pelvis, and extremities). Plans were created for each case and dose was calculated on both the CT and MV CBCT. All steps (simulation, planning, setup verification, QA, and dose delivery) were performed in one 30 min session using phantoms. The monitor units (MU) for each plan were compared and dose distribution agreement was evaluated using mean dose difference over the entire volume and gamma index on the central 2D axial plane. All whole‐brain dose distributions gave gamma passing rates higher than 95% for 2%/2 mm criteria, and pelvic sites ranged between 90% and 98% for 3%/3 mm criteria. However, thoracic spine treatments produced gamma passing rates as low as 47% for 3%/3 mm criteria. Our novel MV CBCT‐based dose planning and delivery approach was feasible and time‐efficient for the majority of cases. Limited MV CBCT FOV precluded workflow use for pelvic sites of larger patients and resulted in image clearance issues when tumor position was far off midline. The agreement of calculated MU on CT and MV CBCT was acceptable for all treatment sites.PACS numbers: 87.55.D‐, 87.57.Q‐

Proposal for a Simple and Efficient Monthly Quality Management Program Assessing the Consistency of Robotic Image-Guided Small Animal Radiation Systems.

Modern pre-clinical radiation therapy (RT) research requires high precision and accurate dosimetry to facilitate the translation of research findings into clinical practice. Several systems are available that provide precise delivery and on-board imaging capabilities, highlighting the need for a quality management program (QMP) to ensure consistent and accurate radiation dose delivery. An ongoing, simple, and efficient QMP for image-guided robotic small animal irradiators used in pre-clinical RT research is described. Protocols were developed and implemented to assess the dose output constancy (based on the AAPM TG-61 protocol), cone-beam computed tomography (CBCT) image quality and object representation accuracy (using a custom-designed imaging phantom), CBCT-guided target localization accuracy and consistency of the CBCT-based dose calculation. To facilitate an efficient read-out and limit the user dependence of the QMP data analysis, a semi-automatic image analysis and data representation program was developed using the technical computing software MATLAB. The results of the first 6-mo experience using the suggested QMP for a Small Animal Radiation Research Platform (SARRP) are presented, with data collected on a bi-monthly basis. The dosimetric output constancy was established to be within ±1 %, the consistency of the image resolution was within ±0.2 mm, the accuracy of CBCT-guided target localization was within ±0.5 mm, and dose calculation consistency was within ±2 s (±3%) per treatment beam. Based on these results, this simple quality assurance program allows for the detection of inconsistencies in dosimetric or imaging parameters that are beyond the acceptable variability for a reliable and accurate pre-clinical RT system, on a monthly or bi-monthly basis.

Development of an integrated spacecraft Guidance, Navigation, & Control subsystem for automated proximity operations

This paper describes the development and validation process of a highly automated Guidance, Navigation, & Control subsystem for a small satellite on-orbit inspection application, enabling proximity operations without human-in-the-loop interaction. The paper focuses on the integration and testing of Guidance, Navigation, & Control software and the development of decision logic to address the question of how such a system can be effectively implemented for full automation. This process is unique because a multitude of operational scenarios must be considered and a set of complex interactions between subsystem algorithms must be defined to achieve the automation goal. The Prox-1 mission is currently under development within the Space Systems Design Laboratory at the Georgia Institute of Technology. The mission involves the characterization of new small satellite component technologies, deployment of the LightSail 3U CubeSat, entering into a trailing orbit relative to LightSail using ground-in-the-loop commands, and demonstration of automated proximity operations through formation flight and natural motion circumnavigation maneuvers. Operations such as these may be utilized for many scenarios including on-orbit inspection, refueling, repair, construction, reconnaissance, docking, and debris mitigation activities. Prox-1 uses onboard sensors and imaging instruments to perform Guidance, Navigation, & Control operations during on-orbit inspection of LightSail. Navigation filters perform relative orbit determination based on images of the target spacecraft, and guidance algorithms conduct automated maneuver planning. A slew and tracking controller sends attitude actuation commands to a set of control moment gyroscopes, and other controllers manage desaturation, detumble, thruster firing, and target acquisition/recovery. All Guidance, Navigation, & Control algorithms are developed in a MATLAB/Simulink six degree-of-freedom simulation environment and are integrated using decision logic to autonomously determine when actions should be performed. The complexity of this decision logic is the primary challenge of the automated process, and the Stateflow tool in Simulink is used to establish logical relationships and manage data flow between each of the individual hardware and software components. Once the integrated simulation is fully developed in MATLAB/Simulink, the algorithms are autocoded to C/C++ and integrated into flight software. Hardware-in-the-loop testing provides validation of the Guidance, Navigation, & Control subsystem performance.

Radiation dose measurements of an on-board imager X-ray unit using optically-stimulated luminescence dosimeters.

Cone beam computed tomography (CBCT) is now widely used to image radiotherapy patients prior to treatment for the purpose of accurate patient setup. However each CBCT image delivered to a patient increases the total radiation dose that they receive. The measurement of the dose delivered from the CBCT images is not readily performed in the clinic. In this study, we have used commercially available optically stimulated luminescence (OSLD) dosimeters to measure the dose delivered by the Varian OBI on a radiotherapy linear accelerator. Calibration of the OSLDs was achieved by using a therapeutic X-ray unit. The dose delivered by a head CBCT scan was found to be 3.2 ± 0.3 mGy which is similar in magnitude to the dose of a head computed tomography (CT) scan. The results of this study suggest that the radiation hazard associated with CBCT is of a similar nature to that of conventional CT scans. We have also demonstrated that the OSLDs are suitable for these low X-ray dose measurements.

Next Generation Spine Radiosurgery Using Volumetric Intensity Modulated Arc Treatment (VMAT)

Spine stereotactic radiosurgery (SRS) delivers an accurate and conformal high radiation dose to the tumor in 1–5 sessions. Volumetric intensity-modulated arc treatment (VMAT) is currently used by the authors for spine SRS while patient's position is continuously monitored and corrected using Exact Track on board imaging (OBI) of BrainLab Robotic Radiosurgery System. We aim to highlight the advantages of VMAT based SRS by means of dose distribution, treatment time and clinical outcomes. Thirty six patients with 57 lesions were treated in our medical center. Dosimetric analysis was conducted for 35 lesions (21 patients). Ten consecutive lesions were planned using both IMRT (iPLAN, BrainLab) with 7 co-planar static fields and VMAT (RapidArc, Eclipse, Varian) using 2-3 dynamic arcs with a prescribed dose of 16Gy. The plans were compared for conformity (Dice Similarity Coefficient - DSC), homogeneity (HI), treatment delivery time and safety (spinal cord dose).For additional 25 lesions, with a prescribed dose of 16-18Gy, we calculated the same parameters for the VMAT plans alone. Four of these patients had skip lesions in 2 separate vertebral levels which were treated simultaneously. Toxicity and local control will be reported with a median follow up of 165 days. All evaluated parameters favored the VMAT plan over the IMRT plans. The Dmin in the IMRT was significantly lower than in the VMAT plan (7.65Gy/10.88Gy p=0.00002), the DSC was found to be significantly better for the VMAT plans compared to the IMRT plans (0.77/0.58 respectively p =0.01), and an almost 50% reduction in the net treatment time was calculated for the VMAT compared to the IMRT plans (6.73min/12.96min p<0.0002).The DSC, HI cord safety and treatment time in the additional 25 lesions were comparable or better than the above VMAT data. Conformity, homogeneity and safety parameters for four plans of simultaneous treatment of two closely lying lesions were comparable as well. Thirty four of 36 patients had at least one follow up visit 3 months post treatment. Two patients reported transient dysphagia and two suffered vertebral fracture (both were previously treated to the same area with standard radiation therapy). Local control rate in our cohort is 96.5% with only 2 local recurrences (5m and 12m post treatment). VMAT provides better conformity homogeneity and safety profile compared to IMRT with excellent local control and toxicity rates. It allows simultaneous treatment of closely situated separate vertebral lesions without compromising accuracy and safety. The shorter treatment time is a major advantage and provides not only convenience to the painful patient but contributes to the precision of this high dose radiation therapy.

Daily patient setup error in prostate image guided radiation therapy with fiducial-based kilovoltage onboard imaging and conebeam computed tomography.

This study examined the interfraction setup error in patients undergoing prostate radiotherapy using fiducial markers and on-board imaging. Patients (n=53) were aligned to the treatment isocenter by laser followed by orthogonal kilovoltage (kV) radiographs to visualize bony anatomy and implanted fiducial markers. The magnitude and direction of couch shifts for isocenter correction required was determined by image registration for bony anatomy and fiducial markers. Twice weekly, 25 of the 53 patients also underwent conebeam computed tomography (CBCT) to measure any residual error in patient positioning. Based on individual coordinate shifts from CBCT, a net three-dimensional (3D) residual shift magnitude vector R was calculated. The average couch shifts were 0.26 and 0.40 cm in inferior direction and 0.25 and 0.33 cm in superior direction for alignments made with bony anatomy and fiducial markers, respectively (P<0.0001). There were no significant differences noted in the vertical or lateral planes between the two image registration methods. In subset of 25 patients, no residual shift from fiducial plain film set up was required with CBCT matching in 66.5%, 52.4% and 57.9% of fractions for longitudinal, vertical and lateral planes, respectively, with majority (79%) of patients having a net residual 3D shifts of <0.3 cm. The use of CBCT increased average treatment time by approximately 6 min compared to kV radiographs alone. The residual setup errors following daily kV image guided localization, as determined by CBCT, were small, which demonstrates high accuracy of kV localization when fiducial markers are present.

Characterization of the onboard imaging unit for the first clinical magnetic resonance image guided radiation therapy system.

To characterize the performance of the onboard imaging unit for the first clinical magnetic resonance image guided radiation therapy (MR-IGRT) system. The imaging performance characterization included four components: ACR (the American College of Radiology) phantom test, spatial integrity, coil signal to noise ratio (SNR) and uniformity, and magnetic field homogeneity. The ACR phantom test was performed in accordance with the ACR phantom test guidance. The spatial integrity test was evaluated using a 40.8 × 40.8 × 40.8 cm(3) spatial integrity phantom. MR and computed tomography (CT) images of the phantom were acquired and coregistered. Objects were identified around the surfaces of 20 and 35 cm diameters of spherical volume (DSVs) on both the MR and CT images. Geometric distortion was quantified using deviation in object location between the MR and CT images. The coil SNR test was performed according to the national electrical manufacturers association (NEMA) standards MS-1 and MS-9. The magnetic field homogeneity test was measured using field camera and spectral peak methods. For the ACR tests, the slice position error was less than 0.10 cm, the slice thickness error was less than 0.05 cm, the resolved high-contrast spatial resolution was 0.09 cm, the resolved low-contrast spokes were more than 25, the image intensity uniformity was above 93%, and the percentage ghosting was less than 0.22%. All were within the ACR recommended specifications. The maximum geometric distortions within the 20 and 35 cm DSVs were 0.10 and 0.18 cm for high spatial resolution three-dimensional images and 0.08 and 0.20 cm for high temporal resolution two dimensional cine images based on the distance-to-phantom-center method. The average SNR was 12.0 for the body coil, 42.9 for the combined torso coil, and 44.0 for the combined head and neck coil. Magnetic field homogeneities at gantry angles of 0°, 30°, 60°, 90°, and 120° were 23.55, 20.43, 18.76, 19.11, and 22.22 ppm, respectively, using the field camera method over the 45 cm DSV. The onboard imaging unit of the first commercial MR-IGRT system meets ACR, NEMA, and vendor specifications.

Daily QA of linear accelerators using only EPID and OBI.

As treatment delivery becomes more complex, there is a pressing need for robust quality assurance (QA) tools to improve efficiency and comprehensiveness while simultaneously maintaining high accuracy and sensitivity. This work aims to present the hardware and software tools developed for comprehensive QA of linear accelerator (LINAC) using only electronic portal imaging devices (EPIDs) and kV flat panel detectors. A daily QA phantom, which includes two orthogonally positioned phantoms for QA of MV-beams and kV onboard imaging (OBI) is suspended from the gantry accessory holder to test both geometric and dosimetric components of a LINAC and an OBI. The MV component consists of a 0.5 cm water-equivalent plastic sheet incorporating 11 circular steel plugs for transmission measurements through multiple thicknesses and one resolution plug for MV-image quality testing. The kV-phantom consists of a Leeds phantom (TOR-18 FG phantom supplied by Varian) for testing low and high contrast resolutions. In the developed process, the existing LINAC tools were used to automate daily acquisition of MV and kV images and software tools were developed for simultaneous analysis of these images. A method was developed to derive and evaluate traditional QA parameters from these images [output, flatness, symmetry, uniformity, TPR20/10, and positional accuracy of the jaws and multileaf collimators (MLCs)]. The EPID-based daily QA tools were validated by performing measurements on a detuned 6 MV beam to test its effectiveness in detecting errors in output, symmetry, energy, and MLC positions. The developed QA process was clinically commissioned, implemented, and evaluated on a Varian TrueBeam LINAC (Varian Medical System, Palo Alto, CA) over a period of three months. Machine output constancy measured with an EPID (as compared against a calibrated ion-chamber) is shown to be within ±0.5%. Beam symmetry and flatness deviations measured using an EPID and a 2D ion-chamber array agree within ±0.5% and ±1.2% for crossline and inline profiles, respectively. MLC position errors of 0.5 mm can be detected using a picket fence test. The field size and phantom positioning accuracy can be determined within 0.5 mm. The entire daily QA process takes ∼15 min to perform tests for 5 photon beams, MLC tests, and imaging checks. The exclusive use of EPID-based QA tools, including a QA phantom and simultaneous analysis software tools, has been demonstrated as a viable, efficient, and comprehensive process for daily evaluation of LINAC performance.

Evaluation of lens absorbed dose with Cone Beam IGRT procedures

The purpose of this work is to evaluate the absorbed dose to the eye lenses due to the cone beam computed tomography (CBCT) system used to accurately position the patient during head-and-neck image guided procedures. The on-board imaging (OBI) systems (v.1.5) of Clinac iX and TrueBeam (Varian) accelerators were used to evaluate the imparted dose to the eye lenses and some additional points of the head. All CBCT scans were acquired with the Standard-Dose Head protocol from Varian. Doses were measured using thermoluminescence dosimeters (TLDs) placed in an anthropomorphic phantom. TLDs were calibrated at the beam quality used to reduce their energy dependence. Average dose to the lens due to the OBI systems of the Clinac iX and the TrueBeam were 0.71 ± 0.07 mGy/CBCT and 0.70 ± 0.08 mGy/CBCT, respectively. The extra absorbed dose received by the eye lenses due to one CBCT acquisition with the studied protocol is far below the 500 mGy threshold established by ICRP for cataract formation (ICRP 2011 Statement on Tissue Reactions). However, the incremental effect of several CBCT acquisitions during the whole treatment should be taken into account.

Radiothérapie adaptative en routine : point de vue de l’oncologue radiothérapeute

Adaptive radiotherapy is defined as all processes leading to the modification of a treatment plan on the basis of patient-specific variations observed during the course of a treatment. This concept is currently of particular relevance due to the development of onboard volumetric imaging systems, which allow for daily viewing of variations in both tumour and organs at risk in terms of position, shape or volume. However, its application in routine clinical practice is limited due to the demanding nature of the processes involved (re-delineation and replanning) and increased dependence on available human resources. Even if “online” strategies, based on deformable image registration (DIR) algorithms, could lead to a reduction in both work and calculation time, for the moment their use is limited to the research field due to uncertainties surrounding the validity of results gathered. Other strategies without DIR can be used as “offline” or “hybrid offline–online” strategies that seem to offer a compromise between time consumption and therapeutic gain for the patient.

An investigation of kV CBCT image quality and dose reduction for volume-of-interest imaging using dynamic collimation.

The focus of this work was to investigate the improvements in image quality and dose reduction for volume-of-interest (VOI) kilovoltage-cone beam CT (CBCT) using dynamic collimation. A prototype iris aperture was used to track a VOI during a CBCT acquisition. The current aperture design is capable of 1D translation as a function of gantry angle and dynamic adjustment of the iris radius. The aperture occupies the location of the bow-tie filter on a Varian On-Board Imager system. CBCT and planar image quality were investigated as a function of aperture radius, while maintaining the same dose to the VOI, for a 20 cm diameter cylindrical water phantom with a 9 mm diameter bone insert centered on isocenter. Corresponding scatter-to-primary ratios (SPR) were determined at the detector plane with Monte Carlo simulation using EGSnrc. Dose distributions for various sizes VOI were modeled using a dynamic BEAMnrc library and DOSXYZnrc. The resulting VOI dose distributions were compared to full-field distributions. SPR was reduced by a factor of 8.4 when decreasing iris diameter from 21.2 to 2.4 cm (at isocenter). Depending upon VOI location and size, dose was reduced to 16%-90% of the full-field value along the central axis plane and down to 4% along the axis of rotation, while maintaining the same dose to the VOI compared to full-field techniques. When maintaining constant dose to the VOI, this change in iris diameter corresponds to a factor increase of approximately 1.6 in image contrast and a factor decrease in image noise of approximately 1.2. This results in a measured gain in contrast-to-noise ratio by a factor of approximately 2.0. The presented VOI technique offers improved image quality for image-guided radiotherapy while sparing the surrounding volume of unnecessary dose compared to full-field techniques.

Monte Carlo-calculated patient organ doses from kV-cone beam CT in image-guided radiation therapy

This study evaluated patient specific organ doses from kV-cone beam computed tomography (kV-CBCT) for the Varian on-board imager (OBI) and the Elekta x-ray volumetric imager (XVI) used in image-guided radiation therapy using Monte Carlo (MC) simulations. The beam modeling for both kV-CBCT systems was performed with the EGSnrc/BEAMnrc user-code. The patient dose distributions were calculated from the modeled kV-CBCT beams by using planning CT data sets for five anatomical regions of head, ‘head and neck’, chest, abdomen, and pelvis. Two default acquisition modes, full scan mode for chest, abdomen, and pelvis, and half scan mode for head and ‘head and neck’, were used for both kV-CBCT systems. The MC-calculated dose distributions were converted into absorbed doses by Farmer chamber measurements in body- and head-type phantoms. A body-type phantom (30 cm diameter and 51 cm length) and a head-type phantom (16 cm diameter and 33 cm length) were irradiated with the full scan mode and the half scan mode, respectively. Finally, the patient specific organ doses were quantitatively evaluated from dose-volume histograms. The mean organ doses for soft tissue in chest, abdomen, and pelvis were 1.52-4.13 cGy, 1.30-2.56 cGy, and 2.32-3.81 cGy for OBI, and 0.82-1.60 cGy, 0.66-1.04 cGy, and 0.97-1.41 cGy for XVI, respectively. In the full scan mode, organ doses for XVI were one-half to one-third of those for OBI. The organs located at the anterior surfaces like heart and testes were higher doses than central regions. Meanwhile, the mean doses for soft tissue in the head and ‘head and neck’ were 0.09-0.43 cGy and 0.09-0.45 cGy for OBI, and 0.13-0.39 cGy and 0.09-0.31 cGy for XVI, respectively. In the half scan mode, the higher doses were observed at the posterior surface for OBI and at the anterior surface for XVI, depending on the scan direction.

Comparison of onboard low-field magnetic resonance imaging versus onboard computed tomography for anatomy visualization in radiotherapy

ABSTRACTBackground. Onboard magnetic resonance imaging (OB-MRI) for daily localization and adaptive radiotherapy has been under development by several groups. However, no clinical studies have evaluated whether OB-MRI improves visualization of the target and organs at risk (OARs) compared to standard onboard computed tomography (OB-CT). This study compared visualization of patient anatomy on images acquired on the MRI-60Co ViewRay system to those acquired with OB-CT.Material and methods. Fourteen patients enrolled on a protocol approved by the Institutional Review Board (IRB) and undergoing image-guided radiotherapy for cancer in the thorax (n = 2), pelvis (n = 6), abdomen (n = 3) or head and neck (n = 3) were imaged with OB-MRI and OB-CT. For each of the 14 patients, the OB-MRI and OB-CT datasets were displayed side-by-side and independently reviewed by three radiation oncologists. Each physician was asked to evaluate which dataset offered better visualization of the target and OARs. A quantitative contouring study was performed on two abdominal patients to assess if OB-MRI could offer improved inter-observer segmentation agreement for adaptive planning.Results. In total 221 OARs and 10 targets were compared for visualization on OB-MRI and OB-CT by each of the three physicians. The majority of physicians (two or more) evaluated visualization on MRI as better for 71% of structures, worse for 10% of structures, and equivalent for 14% of structures. 5% of structures were not visible on either. Physicians agreed unanimously for 74% and in majority for > 99% of structures. Targets were better visualized on MRI in 4/10 cases, and never on OB-CT.Conclusion. Low-field MR provides better anatomic visualization of many radiotherapy targets and most OARs as compared to OB-CT. Further studies with OB-MRI should be pursued.

Use of TrueBeam developer mode for imaging QA.

The purpose of this study was to automate regular Imaging QA procedures to become more efficient and accurate. Daily and monthly imaging QA for SRS and SBRT protocols were fully automated on a Varian linac. A three‐step paradigm where the data are automatically acquired, processed, and analyzed was defined. XML scripts were written and used in developer mode in a TrueBeam linac to automatically acquire data. MATLAB R013B was used to develop an interface that could allow the data to be processed and analyzed. Hardware was developed that allowed the localization of several phantoms simultaneously on the couch. 14 KV CBCTs from the Emma phantom were obtained using a TrueBeam onboard imager as example of data acquisition and analysis. The images were acquired during two months. Artifacts were artificially introduced in the images during the reconstruction process using iTool reconstructor. Support vector machine algorithms to automatically identify each artifact were written using the Machine Learning MATLAB R2011 Toolbox. A daily imaging QA test could be performed by an experienced medical physicist in 14.3±2.4 min. The same test, if automated using our paradigm, could be performed in 4.2±0.7 min. In the same manner, a monthly imaging QA could be performed by a physicist in 70.7±8.0 min and, if fully automated, in 21.8±0.6 min. Additionally, quantitative data analysis could be automatically performed by Machine Learning Algorithms that could remove the subjectivity of data interpretation in the QA process. For instance, support vector machine algorithms could correctly identify beam hardening, rings and scatter artifacts. Traditional metrics, as well as metrics that describe texture, are needed for the classification. Modern linear accelerators are equipped with advanced 2D and 3D imaging capabilities that are used for patient alignment, substantially improving IGRT treatment accuracy. However, this extra complexity exponentially increases the number of QA tests needed. Using the new paradigm described above, not only the bare minimum — but also best practice — QA programs could be implemented with the same manpower.PACS number: 87, 87.10.‐e

Investigation of practical approaches to evaluating cumulative dose for cone beam computed tomography (CBCT) from standard CT dosimetry measurements: a Monte Carlo study

A function called Gx(L) was introduced by the International Commission on Radiation Units and Measurements (ICRU) Report–87 to facilitate measurement of cumulative dose for CT scans within long phantoms as recommended by the American Association of Physicists in Medicine (AAPM) TG–111. The Gx(L) function is equal to the ratio of the cumulative dose at the middle of a CT scan to the volume weighted CTDI (CTDIvol), and was investigated for conventional multi-slice CT scanners operating with a moving table. As the stationary table mode, which is the basis for cone beam CT (CBCT) scans, differs from that used for conventional CT scans, the aim of this study was to investigate the extension of the Gx(L) function to CBCT scans. An On-Board Imager (OBI) system integrated with a TrueBeam linac was simulated with Monte Carlo EGSnrc/BEAMnrc, and the absorbed dose was calculated within PMMA, polyethylene (PE), and water head and body phantoms using EGSnrc/DOSXYZnrc, where the body PE body phantom emulated the ICRU/AAPM phantom. Beams of width 40–500 mm and beam qualities at tube potentials of 80–140 kV were studied. Application of a modified function of beam width (W) termed Gx(W), for which the cumulative dose for CBCT scans f (0) is normalized to the weighted CTDI (CTDIw) for a reference beam of width 40 mm, was investigated as a possible option. However, differences were found in Gx(W) with tube potential, especially for body phantoms, and these were considered to be due to differences in geometry between wide beams used for CBCT scans and those for conventional CT. Therefore, a modified function Gx(W)100 has been proposed, taking the form of values of f (0) at each position in a long phantom, normalized with respect to dose indices f 100(150)x measured with a 100 mm pencil ionization chamber within standard 150 mm PMMA phantoms, using the same scanning parameters, beam widths and positions within the phantom. f 100(150)x averages the dose resulting from a CBCT scan over the 100 mm length. Like the Gx(L) function, the Gx(W)100 function showed only a weak dependency on tube potential at most positions for the phantoms studied. The results were fitted to polynomial equations from which f (0) within the longer PMMA, PE, or water phantoms can be evaluated from measurements of f 100(150)x. Comparisons with other studies, suggest that these functions may be suitable for application to any CT or CBCT scan acquired with stationary table mode.

SU‐E‐J‐37: Feasibility of Utilizing Carbon Fiducials to Increase Localization Accuracy of Lumpectomy Cavity for Partial Breast Irradiation

PurposeTo investigate the feasibility of utilizing carbon fiducials to increase localization accuracy of lumpectomy cavity for partial breast irradiation (PBI).MethodsCarbon fiducials were placed intraoperatively in the lumpectomy cavity following resection of breast cancer in 11 patients. The patients were scheduled to receive whole breast irradiation (WBI) with a boost or 3D‐conformal PBI. WBI patients were initially setup to skin tattoos using lasers, followed by orthogonal kV on‐board‐imaging (OBI) matching to bone per clinical practice. Cone beam CT (CBCT) was acquired weekly for offline review. For the boost component of WBI and PBI, patients were setup with lasers, followed by OBI matching to fiducials, with final alignment by CBCT matching to fiducials. Using carbon fiducials as a surrogate for the lumpectomy cavity and CBCT matching to fiducials as the gold standard, setup uncertainties to lasers, OBI bone, OBI fiducials, and CBCT breast were compared.ResultsMinimal imaging artifacts were introduced by fiducials on the planning CT and CBCT. The fiducials were sufficiently visible on OBI for online localization. The mean magnitude and standard deviation of setup errors were 8.4mm ± 5.3 mm (n=84), 7.3mm ± 3.7mm (n=87), 2.2mm ± 1.6mm (n=40) and 4.8mm ± 2.6mm (n=87), for lasers, OBI bone, OBI fiducials and CBCT breast tissue, respectively. Significant migration occurred in one of 39 implanted fiducials in a patient with a large postoperative seroma.ConclusionOBI carbon fiducial‐based setup can improve localization accuracy with minimal imaging artifacts. With increased localization accuracy, setup uncertainties can be reduced from 8mm using OBI bone matching to 3mm using OBI fiducial matching for PBI treatment. This work demonstrates the feasibility of utilizing carbon fiducials to increase localization accuracy to the lumpectomy cavity for PBI. This may be particularly attractive for localization in the setting of proton therapy and other scenarios in which metal clips are contraindicated.

SU‐C‐304‐04: A Compact Modular Computational Platform for Automated On‐Board Imager Quality Assurance

Purpose:Traditionally, the assessment of X‐ray tube output and detector positioning accuracy of on‐board imagers (OBI) has been performed manually and subjectively with rulers and dosimeters, and typically takes hours to complete. In this study, we have designed a compact modular computational platform to automatically analyze OBI images acquired with in‐house designed phantoms as an efficient and robust surrogate.Methods:The platform was developed as an integrated and automated image analysis‐based platform using MATLAB for easy modification and maintenance. Given a set of images acquired with the in‐house designed phantoms, the X‐ray output accuracy was examined via cross‐validation of the uniqueness and integration minimization of important image quality assessment metrics, while machine geometric and positioning accuracy were validated by utilizing pattern‐recognition based image analysis techniques.Results:The platform input was a set of images of an in‐house designed phantom. The total processing time is about 1–2 minutes. Based on the data acquired from three Varian Truebeam machines over the course of 3 months, the designed test validation strategy achieved higher accuracy than traditional methods. The kVp output accuracy can be verified within +/−2 kVp, the exposure accuracy within 2%, and exposure linearity with a coefficient of variation (CV) of 0.1. Sub‐millimeter position accuracy was achieved for the lateral and longitudinal positioning tests, while vertical positioning accuracy within +/−2 mm was achieved.Conclusion:This new platform delivers to the radiotherapy field an automated, efficient, and stable image analysis‐based procedure, for the first time, acting as a surrogate for traditional tests for LINAC OBI systems. It has great potential to facilitate OBI quality assurance (QA) with the assistance of advanced image processing techniques. In addition, it provides flexible integration of additional tests for expediting other OBI quality assurance tests, such as 2D/3D image quality, making completely automated QA possible.Research Funding from Varian Medical Systems Inc. . Dr. Sasa Mutic receives compensation for providing patient safety training services from Varian Medical Systems, the sponsor of this study.

SU-E-J-79: Evaluation of Prostate Volume Changes During Radiotherapy Using Implanted Markers and On-Board Imaging

Purpose: To evaluate prostate volume changes during radiation therapy using implanted gold markers and on-board imaging. Methods: Twenty-five patients were included who underwent an implantation of three gold markers. Cartesian coordinates of markers were assessed in kV-images. The coordinates of centers of two markers were measured on kV-images from the center of the marker at the apex which was reference. The distances between the markers were extrapolated from the coordinates using the Euclid formula. The radius of the sphere through markers was calculated using sinus theorem. The prostate volume for the first and last fraction was substituted with a sphere model and was calculated for each patient. The t-test was used for analysis. Results: The mean prostate volume for first and last fraction was 24.65 and 20.87 cc, respectively (p≤0.05). The prostate volume was smaller for 23 patients, whereas there was an expansion for 2 patients. Fifteen patients had androgen deprivation during radiotherapy (H group) and ten did not (NH group). The mean prostate volume for the first and last fraction for the NH group was 30.73 cc and 24.89 cc and for the H group 20.84 cc and 18.19 cc, respectively. There was a 15.8% volume change during treatmentmore » for the NH group and 12.2% for the H group, but the difference was not statistically significant. The radius difference of the theoretical sphere for the first and last fraction was 0.98 mm (range, 0.09–2.95 mm) and remained below 2 mm in 88% of measurements. Conclusion: There was a significant volume change during prostate radiotherapy. The difference between H group and NH group was not significant. The radius changes did not exceed 3 mm and it was below adaptive treatment requirements. Our results indicate that prostate volume changes during treatment should be taken into account during contouring and treatment planning.« less

WE-AB-303-02: A Real-Time Homography-Based Algorithm for Markerless Deformable Lung Tumor Motion Tracking Using KV X-Ray Fluoroscopy

Purpose: To develop a fast and robust tracking algorithm that does not require implanting fiducial markers for tracking deformable respiration-induced lung tumor motion using kV x-ray fluoroscopy. Methods: Four kV X-ray image sequences were acquired in lung cancer treatment by using On-Board Imaging (OBI) system (Varian Medical Systems, Palo Alto, CA) at frame rate 15 Hz. Given an OBI image sequence, a histogram equalization processing was employed to enhance the tumor contrast in advance. A template image containing the tumor target was then delineated manually in the first frame. Deformable tumor motion in the subsequent frames was represented by a non-linear homographic transformation. The parameters of the homographic transformation were estimated by minimizing a sum-of-squared-difference (SSD) between the template image and the observed frame. To improve the computational efficiency, an efficient second-order minimization (ESM) method was utilized to minimize the SSD. Results: We evaluated the performance of the proposed method in terms of the tracking accuracy and computational time. For the clinical kV image sequence, the absolute means and standard deviations of tracking error in superior-inferior (SI) and left-right (LR) directions were 0.41±0.43 mm and 0.08±0.29 mm, respectively. The computational time is about 0.04 sec/frame. Experimental results also demonstrated that the proposed method is superior to conventional template matching-based methods both in tracking accuracy and computational cost. Conclusion: In this study, we developed a novel markerless tracking algorithm to track deformable lung tumor motion. By using the homographic transformation and ESM method, the proposed method is capable of robustly tracking deformable tumor motion in real-time. The experimental results conducted on clinical kV image sequences demonstrated the effectiveness of the proposed method. In addition, compared with conventional template matching-based methods, our method achieved a higher tracking accuracy and lower computational cost.

TH-CD-304-03: A Novel and Intelligent Strategy for Automated On-Board Imager Quality Assurance

Purpose: Traditionally, on-board imager (OBI) quality assurance (QA) tests are manually and subjectively performed for the kV/MV imaging system, lasting several hours. To improve test efficiency and accuracy, eliminate the inflexibility of using third party tools, and minimize potential human errors, we design a novel, automated, and intelligent QA strategy. Methods: The proposed strategy includes compact phantom design, automated EPID-image acquisition, and a modular computational platform for image analysis. For reduction of setup errors and human interaction, two compact phantoms are designed to encompass X-ray tube output, geometric, and image quality tests. QA tests and image acquisitions are automatically completed via XML controlled machine operation following the phantom setup. A compact modular computational platform is developed to automatically analyze the acquired images, generate reports, manage the QA database, and provide information for strategy self-optimization. Results: The entire strategy only requires one initial manual phantom setup, significantly minimizing the time of user intervention, avoiding potential setup errors, and eliminating the necessity of using various QA tools. Compared to traditional tests often requiring several hours, the proposed intelligent strategy considerably speeds up the QA process, requiring only 10–15 minutes. With the in-house implemented computational platform, the test results can be clearly reported in both graphical and textual representations within 2 minutes. The analysis of images acquired from three Varian Truebeam machines shows sub-millimeter detector positional accuracy; kVp accuracy of +/−2 kVp interval, and exposure accuracy and linearity within 2%. Conclusion: We introduce an automated and intelligent OBI QA strategy that enables cross-calibration, robust data analysis, and QA test validation. The strategy eliminates the inflexibility and incompatibility obstacles of using third party QA devices. It is capable of collecting long-term multi-machine/multi-institution QA data, and thus has great potential for making the self-optimized intelligent QA environment a reality. Research Funding from Varian Medical Systems Inc. . Dr. Sasa Mutic receives compensation for providing patient safety training services from Varian Medical Systems, the sponsor of this study.