kV Projections Research Articles

Six Degrees-of-Freedom Prostate and Lung Tumor Motion Measurements Using Kilovoltage Intrafraction Monitoring

Tumor positional uncertainty has been identified as a major issue that deteriorates the efficacy of radiation therapy. Tumor rotational movement, which is not well understood, can result in significant geometric and dosimetric inaccuracies. The objective of this study was to measure 6 degrees-of-freedom (6 DoF) prostate and lung tumor motion, focusing on the more novel rotation, using kilovoltage intrafraction monitoring (KIM). Continuous kilovoltage (kV) projections of tumors with gold fiducial markers were acquired during radiation therapy for 267 fractions from 10 prostate cancer patients and immediately before or after radiation therapy for 50 fractions from 3 lung cancer patients. The 6 DoF motion measurements were determined from the individual 3-dimensional (3D) marker positions, after using methods to reject spurious and smooth noisy data, using an iterative closest point algorithm. There were large variations in the magnitude of the tumor rotation among different fractions and patients. Various rotational patterns were observed. The average prostate rotation angles around the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) axes were 1.0 ± 5.0°, 0.6 ± 3.3°, and 0.3 ± 2.0°, respectively. For 35% of the time, the prostate rotated more than 5° about the LR axis, indicating the need for intrafractional adaptation during radiation delivery. For lung patients, the average LR, SI, and AP rotation angles were 0.8 ± 4.2°, -0.8 ± 4.5°, and 1.7 ± 3.1°, respectively. For about 30% of the time, the lung tumors rotated more than 5° around the SI axis. Respiration-induced rotation was detected in 2 of the 3 lung patients. The prostate and lung tumors were found to undergo rotations of more than 5° for about a third of the time. The lung tumor data represent the first 6 DoF tumor motion measured by kV images. The 6 DoF KIM method can enable rotational and translational adaptive radiation therapy and potentially reduce treatment margins.

Feasibility of X-Ray Acoustic Computed Tomography as a Tool for Noninvasive Volumetric In Vivo Dosimetry

motion XZ 6.3 (range: 5.9-7.1) mm, YZ 10.4 (9.6-11.2) mm, and ZZ 7.1 (6.6-7.7) mm. Root Mean Square (RMS) error between TT-tracked tumor motion and RPM was <0.9mm in all directions. Tumor B (4cm): TT could not track the position of the tumor at all angles as half-fan CBCT captured the tumor in only half of the images and high density and central airway structures obscured the target. Therefore, analysis was only performed for 100 (300-40 ) kV-source rotation. 4DCTmotion was XZ 1.8, YZ 7.5 and ZZ 8.6 mm. 100 TTon clinical kV projection series showed mean motion XZ 3.7 (2.5-5.8) mm, YZ 7.3 (5.3-9.2) mm, and ZZ 7.8 (5.5-10.6) mm. RMS error between TT and RPM was <1.1mm. Benchmarking tumor A: 360 TT showed mean motion X Z 3.5, Y Z 11.6 and Z Z 5.2 mm with standard deviation (SD) <0.7mm. Correlation coefficients of TT-tracked tumormotionwith center ofmass displacement on 4DCTwere 0.64, 0.95 and 0.84 (X, Y, Z). For tumor B: 2 sets of TT over 100 (300-40 /120-220 ) showed mean motion XZ 1.9, YZ 7.7 and ZZ 8.4mm (SD<0.8mm) and correlation coefficients of 0.63, 0.94 and 0.91 (X, Y, Z). Conclusions: TT software can track small lung tumors when they are visible in kV projections. Tumor motion during CBCT can vary from the planning 4DCT. TT and RPM motion were closely associated. Benchmarking TT against known tumor displacements showed good agreement. Author Disclosure: J.R. van Sornsen de Koste: I. Travel Expenses; has received travel support of Varian Medical Systems. M. Dahele: I. Travel Expenses; has received travel support/honoraria from Varian Medical Systems and travel support from Brainlab. S. Senan: I. Travel Expenses; has received travel support and honoraria from Varian Medical Systems. B. Slotman: I. Travel Expenses; has received travel support/honoraria from Varian Medical Systems. W. Verbakel: I. Travel Expenses; has received travel/honoraria support from Varian Medical Systems.

Setting Action Levels for Abdominal Stereotactic Body Radiation Therapy Monitored With Frequent kV Imaging Surveillance

We hypothesize that intra-fractional motion monitoring with frequent kV imaging surveillance together with an appropriate intervention action level ensures adequate tumor dose to abdominal cancer patients with breathing amplitude restricted by compression who are treated with stereotactic body radiation therapy on a Linac. We retrospectively studied the treatment plans and intra-fractional images of 10 upper-abdominal cancer patients treated on a Linac with hypofractionated IMRT. For all patients, one or more radio-opaque markers were implanted near the tumor. Abdominal compression reduced respiratory marker motion below 6 mm as evaluated by fluoroscopy before each treatment. The PTV was defined by a 5 mm 3D expansion of the GTV. Intra-fractional kV radiographs were automatically acquired at 5-6 second intervals during beam delivery, triggered by the RPM system’s response to a motion phantom mounted above the patient. Planned marker positions are overlaid on each radiograph and displayed on the treatment console; therapists check compression and correct setup if persistent deviation from planned position is seen. kV images were retrospectively analyzed using in-house software to determine marker shifts from their planned positions. To estimate dosimetric effects, the MLC position at each control point is shifted according to the superior-inferior component of the fiducial trajectory, and the resulting dose to GTV and surrounding organs at risk (OAR) is reconstructed. The relationship between the degradation of GTV D95, dose to OAR, mean and maximal fiducial displacement is explored. The allowable mean and maximal marker motion excursion is determined to obey objectives such as GTV D95 degrades less than 5% and no OAR constraints are violated for this patient population. The marker motion excursion ranged from 2 to 6 mm on pre-treatment fluoroscopy, but from 2 to 23 mm on intra-fractional kV projections. The correlation between the two is 0.44. The weak correlation and bigger amplitude argue for intra-fractional motion monitoring and management. The relative difference between the planned GTV D95 and that reconstructed accounting for intra-fractional motion ranges from -8.8% to 4.4%, and averages -1.4%. Without additional intra-fractional intervention, the 5 mm margin around GTV resulted in 10% of fractions violating the objectives. However, the objectives can be met with tools to enforce marker-based action levels that limit mean marker motion to [-2mm 2mm] and maximal excursion to <12 mm. The combination of compression belt, intra-fractional kV imaging surveillance, and advanced intervention methods can ensure patients receive improved dose coverage.

Patient Setup Management for Pancreatic Stereotactic Body Radiation Therapy: Interfraction Adjustment Between Spine Alignment and the Pancreatic Fiducial During Breath-Hold kV Imaging

Active Breathing Coordinator (ABC) is a widely used motion management strategy for the radiation therapy of mobile tumors. During treatment, aligning the patient based on spine alone does not automatically lead to an acceptable alignment with respect to soft tissue targets and adjustment is necessary. Determining the magnitude of such an adjustment is an important quality assurance measure that will guide expectations for each treatment site. It will also allow creation of guidelines to determine acceptable shifts between the spine and a given target (fiducial) for a given site. Estimating the magnitude of this adjustment for pancreatic cancer stereotactic body radiation therapy (SBRT) was the purpose of this work. This was a retrospective study of nineteen (n = 19) pancreatic SBRT patients, average age 65y (range 47 - 88y), treated between 12/2012 - 6/2013. Radio-opaque fiducial markers were implanted by endoscopic guidance into or adjacent to the pancreatic tumor to provide a target for accurate localization. For each fraction, a free-breathing CBCT was registered to a reference breath-hold CT for alignment to spine. Then, two perpendicular breath-hold kV projection images were acquired and compared with corresponding reference digitally reconstructed radiographs (DRR) to further fine-tune the alignment with the fiducial marker. By comparing the breath-hold kV projection images from subsequent treatment fractions with those from fraction 1, 3D variability of the fiducial position was derived. A 2D-to-2D image registration code was written that registered each day’s 2D projection image to the projection image from day 1. We applied normalized cross-correlation for this registration where the template was chosen at the spine. After fusing the two images at the spine region, the inter-fraction variability of the anatomical breath-hold state was then derived by analyzing the relative position of the fiducials from day n (n>1) compared to day 1 images. We observed an average inter-fraction reproducibility of 1.73 ± 0.84 mm, 1.98 ± 1.42 mm, and 3.19 ± 2.49 mm in the LR, AP and SI directions, respectively. The average excursion from free breathing spine alignment to breath-hold fiducial alignment was 1.51 ± 1.38 mm, 2.05 ± 1.91 mm, and 3.01 ± 2.04 mm in the LR, AP and SI directions, respectively. The range of excursions which can be interpreted as the average expected adjustment required after spine alignment was observed as 0.18 - 5.07 mm, 0.07 - 5.89mm, and 0.61 - 7.82 mm in the LR, AP and SI directions respectively. Our study shows that inter-fraction shifts from free-breathing spine to ABC breath-hold target can be as high as 8 mm. Values that deviate significantly from this limit should be investigated to rule out potential problems such as bowel gas or fiducial shift prior to proceeding with treatment.

SU‐E‐I‐56: Scan Angle Reduction for a Limited‐Angle Intrafraction Verification (LIVE) System

Purpose:To develop a novel adaptive reconstruction strategy to further reduce the scanning angle required by the limited‐angle intrafraction verification (LIVE) system for intrafraction verification.Methods:LIVE acquires limited angle MV projections from the exit fluence of the arc treatment beam or during gantry rotation between static beams. Orthogonal limited‐angle kV projections are also acquired simultaneously to provide additional information. LIVE considers the on‐board 4D‐CBCT images as a deformation of the prior 4D‐CT images, and solves the deformation field based on deformation models and data fidelity constraint. LIVE reaches a checkpoint after a limited‐angle scan, and reconstructs 4D‐CBCT for intrafraction verification at the checkpoint. In adaptive reconstruction strategy, a larger scanning angle of 30° is used for the first checkpoint, and smaller scanning angles of 15° are used for subsequent checkpoints. The onboard images reconstructed at the previous adjacent checkpoint are used as the prior images for reconstruction at the current checkpoint. As the algorithm only needs to reconstruct the small deformation occurred between adjacent checkpoints, projections from a smaller scan angle provide enough information for the reconstruction. XCAT was used to simulate tumor motion baseline drift of 2mm along sup‐inf direction at every subsequent checkpoint, which are 15° apart. Adaptive reconstruction strategy was used to reconstruct the images at each checkpoint using orthogonal 15° kV and MV projections.Results:Results showed that LIVE reconstructed the tumor volumes accurately using orthogonal 15° kV‐MV projections. Volume percentage differences (VPDs) were within 5% and center of mass shifts (COMS) were within 1mm for reconstruction at all checkpoints.Conclusion:It's feasible to use an adaptive reconstruction strategy to further reduce the scan angle needed by LIVE to allow faster and more frequent intrafraction verification to minimize the treatment errors in lung cancer treatments.Grant from Varian Medical System

SU‐C‐BRF‐04: Delivered 4D Dose Calculation for Lung‐VMAT Patients Using In‐ Treatment CBCT and LINAC Log Data

Purpose:To verify the delivered four‐dimensional (4D) dose distribution for the lung VMAT using in‐treatment 4D cone‐beam CT (CBCT) and LINAC log data.Methods:Three patients for lung stereotactic radiotherapy (SRT) were treated by single‐arc volumetric modulated arc therapy (VMAT) created by SmartArc in Pinnacle v9.2 (Philips, USA).Kilovoltage (kV) projection images were acquired during VMAT delivery using the x‐ray volumetric imager, XVI (Elekta, UK). LINAC log data were also recorded via iCom interface (Elekta, UK). Subsequently, the iCom data format was converted to Pinnacle data format, thereby allowing Pinnacle to read actual MLC positions, jaw positions, gantry angles and MUs. These parameters were categorized into four groups according to patient's respiratory phase. The patient respiratory phase was determined from kV‐projection images by in‐house phase recognizing software. By exporting these data back to Pinnacle, in‐treatment dose distributions for each respiratory phase were obtained by combining calculated dose having a particular phase. The 4D CBCT were reconstructed by in‐house program. The density in the structures on the CBCT was overridden by the average density in the corresponding structures on the planning CT for the dose calculation. The dose indexes, such as maximum, minimum, mean dose in the target were compared among the plan, in‐treatment 3D calculation, and in‐treatment 4D calculation.Results:The day‐to‐day variation of the delivered dose was observed for all three patients. The maximum difference was 5.9% for the minimum dose in the target. Difference for the mean dose was found to be small (1.1% at maximum).Conclusion:We have obtained in‐treatment dose distribution for each of the different respiratory phases using in‐treatment 4D CBCT and LINAC log data during lung VMAT delivery. Our result indicate a small but significant dose difference between the 3D and 4D calculations for the case of large target traveling.

SU-E-J-272: Long-Term Evaluation and Cross-Checking of Two Geometric Calibration Systems On Linacs

Purpose: Geometric or mechanical accuracy of kV and MV imaging systems of two Varian Truebeam linacs have been monitored by two geomertirc calibration systems, Varian IsoCal geometric calibration system and home-developed gQA system. Results of both systems are cross-checked and the long term geometric stabilities of linacs are evaluated. Methods: Two geometric calibration methodologies, the built-in IsoCal system (Varian Medical Systems) and the home-developed gQA system, have been used to assess kV and MV imaging systems and their coincidence periodically on two Truebeam linacs. Both systems analyze kV or MV projection images of special designed phantoms to retrieve geometric parameters of the imaging systems. The iso-centers are then calculated based on results of multiple projections from different angles. Calibration results from both systems are compared for cross-checking. Results: Special tests were performed by misaligning QA phantoms on purpose and both systems could detect the known deviations. The errors are within 1 mm. To date, 32 sessions of side-by-side calibrations has been performed by both systems on two Truebeam linacs and the study still continues. All the disagreements of iso-centers between two calibrations systems are less than 1 mm with 0.1 mm standard deviations. The most of large disagreements occurred in vertical direction (AP direction) with an averaged disagreement of 0.45 mm. The average disagreements of iso-centers are 0.09 mm in other directions. The calibrations have been lasted for about one year. The deviations of geometric parameters have been small and consistent for both Truebeam linacs. Conclusion: Both geometric calibration systems, IsoCal and gQA, are capable of detecting geometric deviations of kV and MV imaging systems of linacs. The long term evaluation also shows that the geometric accuracies of both linacs are very consistent. This project has been supported by a Varian Research Grant.

WE-G-BRF-05: Feasibility of Markerless Motion Tracking Using Dual Energy Cone Beam Computed Tomography (DE-CBCT) Projections

Purpose: To compare markerless template-based tracking of lung tumors using dual energy (DE) cone-beam computed tomography (CBCT) projections versus single energy (SE) CBCT projections. Methods: A RANDO chest phantom with a simulated tumor in the upper right lung was used to investigate the effectiveness of tumor tracking using DE and SE CBCT projections. Planar kV projections from CBCT acquisitions were captured at 60 kVp (4 mAs) and 120 kVp (1 mAs) using the Varian TrueBeam and non-commercial iTools Capture software. Projections were taken at approximately every 0.53° while the gantry rotated. Due to limitations of the phantom, angles for which the shoulders blocked the tumor were excluded from tracking analysis. DE images were constructed using a weighted logarithmic subtraction that removed bony anatomy while preserving soft tissue structures. The tumors were tracked separately on DE and SE (120 kVp) images using a template-based tracking algorithm. The tracking results were compared to ground truth coordinates designated by a physician. Matches with a distance of greater than 3 mm from ground truth were designated as failing to track. Results: 363 frames were analyzed. The algorithm successfully tracked the tumor on 89.8% (326/363) of DE frames compared to 54.3% (197/363) of SE frames (p<0.0001). Average distance between tracking and ground truth coordinates was 1.27 +/− 0.67 mm for DE versus 1.83+/−0.74 mm for SE (p<0.0001). Conclusion: This study demonstrates the effectiveness of markerless template-based tracking using DE CBCT. DE imaging resulted in better detectability with more accurate localization on average versus SE. Supported by a grant from Varian Medical Systems

A limited‐angle intrafraction verification (LIVE) system for radiation therapy

Currently, no 3D or 4D volumetric x-ray imaging techniques are available for intrafraction verification of target position during actual treatment delivery or in-between treatment beams, which is critical for stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) treatments. This study aims to develop a limited-angle intrafraction verification (LIVE) system to use prior information, deformation models, and limited angle kV-MV projections to verify target position intrafractionally. The LIVE system acquires limited-angle kV projections simultaneously during arc treatment delivery or in-between static 3D/IMRT treatment beams as the gantry moves from one beam to the next. Orthogonal limited-angle MV projections are acquired from the beam's eye view (BEV) exit fluence of arc treatment beam or in-between static beams to provide additional anatomical information. MV projections are converted to kV projections using a linear conversion function. Patient prior planning CT at one phase is used as the prior information, and the on-board patient volume is considered as a deformation of the prior images. The deformation field is solved using the data fidelity constraint, a breathing motion model extracted from the planning 4D-CT based on principal component analysis (PCA) and a free-form deformation (FD) model. LIVE was evaluated using a 4D digital extended cardiac torso phantom (XCAT) and a CIRS 008A dynamic thoracic phantom. In the XCAT study, patient breathing pattern and tumor size changes were simulated from CT to treatment position. In the CIRS phantom study, the artificial target in the lung region experienced both size change and position shift from CT to treatment position. Varian Truebeam research mode was used to acquire kV and MV projections simultaneously during the delivery of a dynamic conformal arc plan. The reconstruction accuracy was evaluated by calculating the 3D volume percentage difference (VPD) and the center of mass (COM) difference of the tumor in the true on-board images and reconstructed images. In both simulation and phantom studies, LIVE achieved substantially better reconstruction accuracy than reconstruction using PCA or FD deformation model alone. In the XCAT study, the average VPD and COM differences among different patient scenarios for LIVE system using orthogonal 30° scan angles were 4.3% and 0.3 mm when using kV+BEV MV. Reducing scan angle to 15° increased the average VPD and COM differences to 15.1% and 1.7 mm. In the CIRS phantom study, the VPD and COM differences for the LIVE system using orthogonal 30° scan angles were 6.4% and 1.4 mm. Reducing scan angle to 15° increased the VPD and COM differences to 51.9% and 3.8 mm. The LIVE system has the potential to substantially improve intrafraction target localization accuracy by providing volumetric verification of tumor position simultaneously during arc treatment delivery or in-between static treatment beams. With this improvement, LIVE opens up a new avenue for margin reduction and dose escalation in both fractionated treatments and SRS and SBRT treatments.

Generation of virtual monochromatic CBCT from dual kV/MV beam projectionsa)

To develop a novel on-board imaging technique which allows generation of virtual monochromatic (VM) cone-beam CT (CBCT) with a selected energy from combined kilovoltage (kV)∕megavoltage (MV) beam projections. With the current orthogonal kV∕MV imaging hardware equipped in modern linear accelerators, both MV projections (from gantry angle of 0°-100°) and kV projections (90°-200°) were acquired as gantry rotated a total of 110°. A selected range of overlap projections between 90° to 100° were then decomposed into two material projections using experimentally determined parameters from orthogonally stacked aluminum and acrylic step-wedges. Given attenuation coefficients of aluminum and acrylic at a predetermined energy, one set of VM projections could be synthesized from two corresponding sets of decomposed projections. Two linear functions were generated using projection information at overlap angles to convert kV and MV projections at nonoverlap angles to approximate VM projections for CBCT reconstruction. The contrast-to-noise ratios (CNRs) were calculated for different inserts in VM CBCTs of a CatPhan phantom with various selected energies and compared with those in kV and MV CBCTs. The effect of overlap projection number on CNR was evaluated. Additionally, the effect of beam orientation was studied by scanning the CatPhan sandwiched with two 5 cm solid-water phantoms on both lateral sides and an electronic density phantom with two metal bolt inserts. Proper selection of VM energy [30 and 40 keV for low-density polyethylene (LDPE), polymethylpentene, 2 MeV for Delrin] provided comparable or even better CNR results as compared with kV or MV CBCT. An increased number of overlap kV and MV projection demonstrated only marginal improvements of CNR for different inserts (with the exception of LDPE) and therefore one projection overlap was found to be sufficient for the CatPhan study. It was also evident that the optimal CBCT image quality was achieved when MV beams penetrated through the heavy attenuation direction of the object. A novel technique was developed to generate VM CBCTs from kV∕MV projections. This technique has the potential to improve CNR at selected VM energies and to suppress artifacts at appropriate beam orientations.

An optimization algorithm for 3D real‐time lung tumor tracking during arc therapy using kV projection images

To develop a real-time markerless 3D tumor tracking using kilovoltage (kV) cone-beam CT (CBCT) projection images during volumetric modulated arc therapy (VMAT) treatment of lung tumors. The authors have developed a method to identify the position of lung tumors during VMAT treatment, where the current mean 3D position is detected and subsequently the real time 3D position is obtained. The mean position is evaluated by iteratively minimizing an observation error function between the tumor coordinate detected in the imaging plane and the coordinate of the corresponding projection of the estimated mean position. The 3D trajectory is reconstructed using the same optimization formalism, where an observation error function is minimized for tumor positions confined within a predefined amplitude bin as determined from the superior-inferior tumor motion. Dynamic phantom experiments were performed and image data acquired during patient treatment were analyzed to characterize the reconstruction ability of the proposed method. The proposed algorithm needs to acquire kV projection data until a certain gantry angle is passed through, termed the black-out angle, before accurate estimation mean 3D tumor position is possible. The black-out angle for the mean position method is approximately 20°, while for the 3D trajectory reconstruction an additional ≈ 15° is required. The mean 3D position and 3D trajectory reconstruction are accurate within ± 0.5 mm. The authors present a real-time tracking framework to locate lung tumors during VMAT treatment using an optimization algorithm applied to CBCT kV projection images taken concomitantly with the treatment delivery. The authors' technique does not introduce significant additional dose and can be used for real-time treatment monitoring.

Digital tomosynthesis (DTS) for verification of target position in early stage lung cancer patients

The ability to verify intrafraction tumor position is clinically useful for hypofractionated treatments. Short arc kV digital tomosynthesis (DTS) could facilitate more frequent target verification. The authors used DTS combined with triangulation to determine the mean temporal position of small-volume lung tumor targets treated with stereotactic radiotherapy. DTS registration results were benchmarked against online clinical localization using registration between free-breathing cone-beam computed tomography (CBCT) and the average intensity projection (AvIP) of the planning 4DCT. In this retrospective study, 76 sets of kV-projection images from online CBCT scans of 13 patients were used to generate DTS image slices (CB-DTS) with nonclinical research software (DTS Toolkit, Varian Medical Systems). Three-dimensional tumor motion was 1.3-4 mm in six patients and 6.1-25.4 mm in seven patients on 4DCT (significant difference in the mean of the groups, P < 0.01). The 4DCT AvIP was used to digitally reconstruct the Reference-DTS. DTS registration and DTS registration combined with triangulation were investigated. Progressive shortening of total DTS arc lengths from 95° to 35° around 0° gantry position was evaluated for different scenarios: DTS registration using the entire arc; DTS registration plus triangulation using two nonoverlapping arcs; and for 55° and 45° total gantry rotation, DTS registration plus triangulation using two overlapping arcs. Finally, DTS registration plus triangulation performed at eight gantry angles, each separated by 45° was evaluated using full fan kV projection data for one patient with an immobile tumor and five patients with mobile tumors. For DTS registration alone, shortening arc length did not influence accuracy in X- and Y-directions, but in Z-direction, mean deviations from online CBCT localization systematically increased for shorter arc length (P < 0.05). For example, using a 95° arc mean DTS-CBCT difference was 0.8 mm (1 SD = 0.6 mm) and for a 35° arc the mean was 2.4 mm (1 SD = 1.7 mm). DTS plus triangulation using nonoverlapping-arcs increased accuracy in Z-direction for tested arc lengths ≤55° (P < 0.01). Overlapping arcs increased accuracy in Y-direction for tumors with motion >4 mm (P < 0.02) but increased Z-direction accuracy was only observed with 55° total gantry rotation. The 95th percentile deviations with this overlapping technique in X-, Y-, and Z-directions were 1.3, 2.0, and 2.5 mm, respectively. For the five patients with mobile tumors where DTS + triangulation was performed with 45° intervals, the pooled deviation from online CBCT correction showed, for X-, Y-, and Z-directions, mean of 1.1 mm, standard deviations (SD) of 0.9, 1.0, and 0.9 mm, respectively. The mean + 2 SD was <3 mm for each direction. Short-arc DTS verification of time averaged lung tumor position is feasible using free-breathing kV projection data and the AvIP of the 4DCT as a reference. Observed differences between DTS and online CBCT registration with AvIP were ≤3 mm (mean + 2 SD), however, the increased temporal resolution of DTS + triangulation also identified short period deviations from the average target position on the CBCT. Short-arc DTS appears promising for intrafraction tumor position monitoring during stereotactic lung radiotherapy delivered with a rotational technique.

GW24-e2918 Quantitative myocardial CT perfusion with rapid kV switching dual energy CT: a microspheres validation study

ObjectivesWe validated the usefulness of beam hardening (BH) reduction with a rapid kV switching dual energy CT (DECT) protocol in quantitative myocardial perfusion (MP) imaging against microspheres measurement of MP.MethodsNormal...

WE-G-134-01: A Novel Method to Correct Scatter and Metal Artifacts in Kilovoltage CBCT Using Megavoltage Projections

Purpose: To improve quantitative accuracy of CT numbers in kV‐CBCT, we developed a method to correct kV projections using MV images acquired during CBCT acquisition. This method exploits inherent characteristics of MV image signal, such as relatively lower scatter fraction and lower sensitivity to elemental composition, to both correct for scatter and suppress metal artifacts in kV‐CBCT. Methods: To correct for scatter in kV‐CBCT, two water‐based mapping functions were developed to correct beam hardening in MV projections, and to transform them to kV‐equivalent MV projections in Radon space. At a given gantry angle, the difference in log‐attenuation values between kV‐equivalent MV and raw kV projections served as the estimate of scatter distribution. Smoothed scatter distributions were subtracted from the raw kV projections to obtain corrected kV projections. In addition, the difference between kV‐equivalent MV and scatter‐corrected kV projections were used to identify image pixels that exhibited nonlinearities due to metal objects in raw KV projections. Such pixels were replaced by the corresponding pixel values in the kV‐equivalent MV projections. Standard filter‐backprojection algorithm was utilized to reconstruct the CBCT image from corrected KV projections. Results: Varian TrueBeam linac operated in research mode was utilized to acquire kV‐MV projections. In a torso phantom image, shading artifacts due to scatter was reduced from 300 HU to less than 100 HU after corrections. Two steel rods placed on the surface of the phantom lead to more than 600 HU image streaks in the uncorrected kV‐CBCT image. After metal artifact corrections, variation in CT numbers was reduced below 100 HU. Conclusion: Scatter and metal artifacts can be significantly reduced in kV‐CBCT using our approach. Improved quantitative accuracy of CT numbers may enable CBCT image based dose calculations in radiation therapy, and be particularly useful to reduce range uncertainties in particle therapy for patients with metallic implants.

MO‐F‐WAB‐03: Trajectory Imaging: Exploring Options of Kilovoltage Digital Tomosynthesis During Intensity Modulated Radiation Therapy

Purpose: To integrate novel kilovoltage (kV) digital tomosynthesis (DTS) sequences into intensity modulated radiation therapy (IMRT) to determine 3D fiducial positions for intrafractional motion monitoring. Method: In one method, we converted a static gantry IMRT beam into a series of arcs in which dose index and multileaf collimator positions for all control points were kept intact, but gantry angles were modified to oscillate +/−3° around the original angle. kV projections were acquired continuously throughout delivery and reconstructed to provide a series of 6° arc DTS images which were used to evaluate in‐plane positions of gold fiducials embedded in an anthropomorphic phantom. To obtain out‐of‐plane positions via triangulation, a 20° gantry rotation with beam held‐off was inserted during delivery to produce a pair of 6° DTS images separated by 14°. In a second method, the gantry remained stationary, but both kV source and detector moved over a 15° longitudinal arc via pitch and translational adjustment of the robotic arms. Evaluation of localization accuracy in phantom during simulated intrafractional motion used programmed couch translations from customized scripts. In‐house software was used to reconstruct DTS images, register them to reference DTS images and calculate 3D fiducial positions. Result No significant dose difference (&lt;0.5%) was found between the original and converted IMRT beams. For a typical hypofractionated spine treatment, 200 single DTS (6° arc) and 10 paired DTS (20° arc) were acquired for each IMRT beam, providing in‐plane (IP) and out‐of‐plane (OP) monitoring every 1.5 and 30 seconds, respectively. Mean (standard deviation) error in predicted position was −0.3(0.2)mm, −0.1(0.1)mm IP, and 0.2(0.4)mm OP with rotational gantry, −0.1(0.1)mm, 0.7(0.3)mm IP and 1.9(1.1)mm OP with translational source/detector. Conclusion: Acquiring 3D fiducial positions from kV‐DTS during IMRT delivery is technically feasible. Enabling movement of kV‐source and detector during delivery could further reduce imaging time overhead. Part of the research is sponsored by Varian Medical System.

SU‐E‐J‐04: On‐Treatment Volumetric Imaging During Respiratory Gated VMAT

Purpose: To obtain on‐treatment volumetric patient anatomy during respiratory gated VMAT. Methods: On‐board imaging device integrated with Linacs offers a viable tool for obtaining patient anatomy during radiation treatment delivery.In this study, we acquired beam‐level kV images during gated VMAT treatments using a Varian TrueBeam\texttrademark STx Linac. These kV projection images are triggered by a respiratory gating signal and can be acquired immediately before treatment MV beam on at every breathing cycle during delivery. Because the kV images are acquired with an on‐board imaging device during a rotational arc therapy, they provide the patient anatomical information from many different angles or projection views (typically 20– 40). To reconstruct the volumetric image representing patient anatomy during the VMAT treatment, we used a compressed sensing method with a fast first‐order optimization algorithm. The conventional FDK reconstruction was also used for comparison purposes. The method was tested on a dynamic anthropomorphic physical phantom as well as a lung patient. Results: The reconstructed volumetric images for a dynamic anthropomorphic physical phantom and a lung patient showed clearly visible soft‐tissue target as well as other anatomical structures,with the proposed compressed sensing‐based image reconstruction method. Compared with FDK, the compressed sensing method leads to a ∼2‐and 3fold increase in contrast‐to‐noise ratio around the target area in the phantom and patient case,respectively. Conclusion: The proposed technique provides on‐treatment volumetric patient anatomy, with only a fraction (&lt;10%) of the imaging dose used in conventional CBCT procedures. This anatomical information may be valuable for geometric verification and treatment guidance, and useful for verification of treatment dose delivery, accumulation, and adaptation in the future.

SU‐C‐141‐06: Registration Accuracy with Four‐Dimensional Cone‐Beam CT for Lung Cancer Treatment

Purpose: A four‐dimensional (4D) cone‐beam CT (CBCT) is widely implemented in the image‐guided radiation therapy (IGRT) for lung cancer treatment. Recently, we established a verifying system using in‐treatment 4D CBCT acquired during Volumetric Modulated Arc Therapy (VMAT). With this system, we assess the registration accuracy with pre‐treatment 4D CBCT for IGRT. Methods: Fifteen lung cancer patients treated with single‐arc VMAT were selected in this study. The breathing condition of all patients was constrained with a stereotactic body frame (ELEKTA) accompanied by abdominal compression plate. The gross tumor volumes were contoured on 10 CT datasets from 4D CT (TOSHIBA and ANZAI) and merged into internal target volumes (ITVs). The planning target volume (PTV) margin was isotropically taken as 5 mm. The patient registration was performed by an Elekta x‐ray volume imaging functionality version 4.5 (XVI ver.4.5), which allows to use 4D CBCT respiratory sorting for registration. The VMAT treatment was performed with simultaneous acquisition of the kV projection images. The in‐treatment 4D CBCT was reconstructed by in‐house software. The reconstructed images were exported to a treatment planning system. To assess set‐up errors and respiratory motion, the displacement of tumors between the planning ITV and the shadow of the tumor in in‐treatment 4D CBCT images was measured in CC, left‐right (LR), and anterior‐posterior (AP) directions. Results: The average displacements between the ITV and the actual tumor location during treatment were 0.24 +/− 0.45, 0.19 +/− 0.46, and 0.42 +/− 0.43 mm for the CC, LR, and AP directions, respectively. The discrepancy in each phase did not exceed 3 mm in any direction. Conclusion: With in‐treatment 4D CBCT, which is a direct method for quantitatively assessing the intrafractional location, we confirmed that the required PTV margins were 3 mm when the registration was performed using pre‐treatment 4D CBCT. This work was supported by JSPS KAKENHI 22791176.

SU-E-J-118: Quantifying Intrafractional Prostate Rotation From Cone-Beam Computed Tomography with Radiopaque Markers

Purpose: Rotation greater than one degree can severely compromise the accuracy of radiotherapy treatments. Prostate intrafractional rotation is not negligible because peristaltic motion has a time scale that is shorter than the delivery time of a single fraction. The objectives of this study were (a) to investigate a clinically feasible method to calculate tumor rotation with radiopaque markers; (b) to evaluate the accuracy of the method and (c) to quantify prostate intrafractional rotation in intensity‐modulated arc radiotherapy (IMAT). Methods: 2D kilovoltage (kV) cone‐beam computed tomography (CBCT) images were acquired for prostate cancer patients with three implanted gold fiducial markers as surrogates for the tumor position during fractionated IMAT. Post‐treatment, 3D marker trajectories were determined from the 2D projection images by maximum likelihood estimation of a 3D probability density function. Rotations about each axis (roll (right/left), pitch (superior/inferior) and yaw (anterior/posterior)) were then calculated based on the iterative closest point algorithm (ICP) method, with the origin set at the centroid of the three markers. Results: Intrafractional rotation was studied for10 patients, 268 fractions. Average roll, pitch and yaw were calculated as 0.85±1.16, 0.41±0.51 and 0.37±0.4 degrees, respectively. Roll was the predominant rotation among the three axes which is consistent with literature. Rotations showed a similar pattern among different fractions for the same patient. The pattern acquired at the first few fractions could be used as a prediction for the rest of the fractions. Conclusion: This study demonstrated a novel method to quantify intrafractional tumor rotation from 2D kV projections with radiopaque markers. Intrafractional rotation is small but significant for certain cases. Real‐time radiopaque marker‐based adaptive radiotherapy with rotation correction could make a positive impact on reducing treatment margins. This project is supported by an NHMRC Australia Fellowship and NHMRC project grant 1034060.

SU‐E‐J‐14: Evaluation of MV and KV Isocenter Displacement for An IGRT Linac with and Without An Add‐On Micro‐Multileaf Collimator

Purpose: The impact of the add‐on micro MLC (mMLC) on the MV and kV isocenters was investigated. Methods: Winston‐Lutz tests was performed with an add‐on mMLC, and a ball bearing (BB) was placed at the isocenter given by localization lasers. A square MV field was shaped by the mMLC, and the MV field center position relative to the BB position were measured by a portal imager with different gantry angles. A desirable BB position that minimizes the maximum deviation was obtained as an MV isocenter, and the BB center was aligned onto the revised MV isocenter. Using an on‐board kV imaging system, two‐dimensional BB image was acquired during gantry rotation. The displacement of the kV projection image center from the BB for each gantry angle was recorded as ‘flexmap’. After unmounting the mMLC, Winston‐Lutz test and the flexmap measurement were performed again. A phantom was placed on a couch and cone‐beam CT (CBCT) images were acquired with the two different flexmap data. The set‐up errors were calculated and the differences of required couch translations with each of the flexmap data were obtained. Results: After unmounting the mMLC, the MV field center and the kV image center were shifted approximately 0.4 mm and 0.05 mm upward, respectively. Assuming a perfect MV and kV isocenter coincidence with mMLC using the flexmap for CBCT reconstruction, the MV/kV isocenter discrepancy without the mMLC could be calculated by the difference of the above two values, 0.35 mm which satisfies AAPM TG179 guideline of 1 mm. The largest difference of required couch translations between the two flexmap data was 0.07 mm toward the vertical direction, which agreed well with the kV image center shift of 0.05 mm. Conclusion: The MV and kV isocenters defined with the add‐on mMLC are also applicable to image‐guided VMAT without the mMLC.

TU‐E‐141‐10: 3D Delivered Dose Assessment for SBRT Treatment Based On Estimated Volumetric Time‐Varying Images From a Single Projection Image

Purpose: To develop a method of verifying delivered dose based on time‐varying 3D treatment images estimated from single kV projection images and a 4DCT‐based patient‐specific motion model. Methods: Delivered dose calculation is performed in two steps. In the first step 3D fluoroscopic (time‐varying) images are generated from single 2D kV projection images. In the second step a set of 3D fluoroscopic images are used to calculate the delivered dose. 3D fluoroscopic images are estimate by creating a patient‐specific lung motion model through deformable registration of 4DCT, and then iteratively optimization the parameters of the motion model. Using a modified digital XCAT phantom, the accuracy of the delivered dose calculated with our method was compared to the dose that would be calculated during routine 4DCT‐based treatment planning and to the actual delivered dose. The planning dose was calculated on an average intensity projection (AIP) image. The actual delivered dose is calculated based on the defined motion of the digital XCAT phantom. The same treatment plan was delivered to all three phantoms. The treatment plan was developed from the AIP and designed as a 3×18 Gy SBRT plan. Results: The dose calculated on 3D fluoroscopic images closely matches to the actual delivered dose. Both the calculated and delivered doses vary from AIP‐based planned dose. D95 tumor dose values of planned, delivered and calculated dose are 54.0, 59.0 and 58.4 Gy. Corresponding V20 lung values are 4.5%, 4.8% and 4.8% respectively. Conclusions: We introduced a novel method for delivered dose calculation. Our algorithm successfully generates a 3D fluoroscopic image from a single kV images and then uses these time‐varying 3D images to accurately calculate delivered dose. The project described was supported, in part, by an RSNA Research Scholar Grant and Award Number R21CA156068 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the RSNA, National Cancer Institute or the National Institutes of Health.