M. Haytmyradov

To evaluate markerless tumor tracking (MTT) using fast-kV switching dual-energy (DE) fluoroscopy on a bench top system.Fast-kV switching DE fluoroscopy was implemented on a bench top which includes a turntable stand, flat panel detector, and x-ray tube. The customized generator firmware enables consecutive x-ray pulses that alternate between programmed high and low energies (e.g., 60 and 120 kVp) with a maximum frame rate of 15 Hz. In-house software was implemented to perform weighted DE subtraction of consecutive images to create an image sequence that removes bone and enhances soft tissues. The weighting factor was optimized based on gantry angle. To characterize this system, a phantom was used that simulates the chest anatomy and tumor motion in the lung. Five clinically relevant tumor sizes (5-25 mm diameter) were considered. The targets were programmed to move in the inferior-superior direction of the phantom, perpendicular to the x-ray beam, using a cos4 waveform to mimic respiratory motion. Target inserts were then tracked with MTT software using a template matching method. The optimal computed tomography (CT) slice thickness for template generation was also evaluated. Tracking success rate and accuracy were calculated in regions of the phantom where the target overlapped ribs vs spine, to compare the performance of single energy (SE) and DE imaging methods.For the 5 mm target, a CT slice thickness of 0.75 mm resulted in the lowest tracking error. For the larger targets (≥10 mm) a CT slice thickness ≤2 mm resulted in comparable tracking errors for SE and DE images. Overall DE imaging improved MTT accuracy, relative to SE imaging, for all tumor targets in a rotational acquisition. Compared to SE, DE imaging increased tracking success rate of small target inserts (5 and 10 mm). For fast motion tracking, success rates improved from 23% to 64% and 74% to 90% for 5 and 10 mm targets inserts overlapping ribs, respectively. For slow moving targets success rates improved from 19% to 59% and 59% to 91% in 5 and 10 mm targets overlapping the ribs, respectively. Similar results were observed when the targets overlapped the spine. For larger targets (≥15 mm) tracking success rates were comparable using SE and DE imaging.This work presents the first results of MTT using fast-kV switching DE fluoroscopy. Using DE imaging has improved the tracking accuracy of MTT, especially for small targets. The results of this study will guide the future implementation of fast-kV switching DE imaging using the on-board imager of a linear accelerator.

To evaluate fast-kV switching (FS) dual energy (DE) cone beam computed tomography (CBCT) using the on-board imager (OBI) of a commercial linear accelerator to produce virtual monoenergetic (VM) and relative electron density (RED) images. Using an polynomial attenuation mapping model, CBCT phantom projections obtained at 80 and 140 kVp with FS imaging, were decomposed into equivalent thicknesses of aluminum (Al) and polymethyl methacrylate (PMMA). All projections were obtained with the titanium foil and bowtie filter in place. Basis material projections were then recombined to create VM images by using the linear attenuation coefficients at the specified energy for each material. Similarly, RED images were produced by replacing the linear attenuation values of Al and PMMA by their respective RED values in the projection space. VM and RED images were reconstructed using Feldkamp-Davis-Kress (FDK) and an iterative algorithm (iCBCT, Varian Medical Systems). Hounsfield units (HU), contrast-to-noise ratio (CNR) and RED values were compared against known values. The results after VM-CBCT production showed good material decomposition and consistent HUVM values, with measured root mean square errors (RMSE) from theoretical values, after FDK reconstruction, of 20.5, 5.7, 12.8 and 21.7 HU for 50, 80, 100 and 150 keV, respectively. The largest CNR improvements, when compared to polychromatic images, were observed for the 50 keV VM images. Image noise was reduced up to 28% in the VM-CBCT images after iterative image reconstruction. RED values measured for our method resulted in a mean percentage error of 0.0%  ±  1.8%. This study describes a method to generate VM-CBCT and RED images using FS-DE scans obtained using the OBI of a linac, including the effects of the bowtie filter. The creation of VM and RED images increases the dynamic range of CBCT images, and provides additional data that may be used for adaptive radiotherapy, and on table verification for radiotherapy treatments.

<h2>Abstract</h2><h3>Purpose</h3> To describe and characterize fast-kV switching, dual-energy (DE) imaging implemented within the on-board imager of a commercial linear accelerator for markerless tumor tracking (MTT). <h3>Methods and Materials</h3> Fast-kV switching, DE imaging provides for rapid switching between programmed tube voltages (ie, 60 and 120 kVp) from one image frame to the next. To characterize this system, the weighting factor used for logarithmic subtraction and signal difference-to-noise ratio were analyzed as a function of time and frame rate. MTT was evaluated using a thorax motion phantom and fast kV, DE imaging was compared versus single energy (SE) imaging over 360 degrees of rotation. A template-based matching algorithm was used to track target motion on both DE and SE sequences. Receiver operating characteristics were used to compare tracking results for both modalities. <h3>Results</h3> The weighting factor was inversely related to frame rate and stable over time. After applying the frame rate–dependent weighting factor, the signal difference-to-noise ratio was consistent across all frame rates considered for simulated tumors ranging from 5 to 25 mm in diameter. An analysis of receiver operating characteristics curves showed improved tracking with DE versus SE imaging. The area under the curve for the 10-mm target ranged from 0.821 to 0.858 for SE imaging versus 0.968 to 0.974 for DE imaging. Moreover, the residual tracking errors for the same target size ranged from 2.02 to 2.18 mm versus 0.79 to 1.07 mm for SE and DE imaging, respectively. <h3>Conclusions</h3> Fast-kV switching, DE imaging was implemented on the on-board imager of a commercial linear accelerator. DE imaging resulted in improved MTT accuracy over SE imaging. Such an approach may have application for MTT of patients with lung cancer receiving stereotactic body radiation therapy, particularly for small tumors where MTT with SE imaging may fail.

We review the results on the bottomonium system from the CMS experiment at the Large Hadron Collider. Measurements have been carried out at different center-of-mass energies in proton collisions and in collisions involving heavy ions. These include precision measurements of cross sections and polarizations, shedding light on hadroproduction mechanisms, and the observation of quarkonium sequential suppression, a notable indication of quark-gluon plasma formation. The observation of the production of bottomonium pairs is also reported along with searches for new states. We close with a brief outlook of the future physics program.

Dual-energy (DE) imaging using planar imaging with an on-board imager (OBI) is being considered in radiotherapy. We describe here a custom phantom designed to optimize DE imaging parameters using the OBI of a commercial linear accelerator. The phantom was constructed of lung-, tissue- and bone-equivalent material slabs. Five simulated tumors located at two different depths were encased in the lung-equivalent materials. Two slabs with bone-equivalent material inserts were constructed to simulate ribs, which overlap the simulated tumors. DE bone suppression was performed using a weighted logarithmic subtraction based on an iterative method that minimized the contrast between simulated bone- and lung-equivalent materials. The phantom was subsequently used to evaluate different combinations of high-low kV x-ray pairs of images based on the signal-difference-to-noise ratio (SDNR) metric. The results show a strong correlation between tumor visibility and selected energy pairs, where higher energy separation leads to larger SDNR values. To evaluate the effect of image post-processing methods on tumor visibility, an anti-correlated noise reduction (ACNR) technique and adaptive kernel scatter correction method were applied to subsequent DE images. Application of the ACNR technique approximately doubled the SDNR values, hence increasing tumor visibility, while scatter correction had little effect on SDNR values. This phantom allows for quick image acquisition and optimization of imaging parameters and weighting factors. Optimized DE imaging increases soft tissue visibility and may allow for markerless motion tracking of lung tumors.

Template-based matching algorithms are currently being considered for markerless motion tracking of lung tumors. These algorithms use tumor templates derived from the planning CT scan, and track the motion of the tumor on single energy fluoroscopic images obtained at the time of treatment. In cases where bone may obstruct the view of the tumor, dual energy fluoroscopy may be used to enhance soft tissue contrast. The goal of this study is to predict which tumors will have a high degree of accuracy for markerless motion tracking based on radiomic features obtained from the planning CT scan, using peak-to-sidelobe ratio (PSR) as a surrogate of tracking accuracy. In this study, CT imaging data of 8 lung cancer patients were obtained and analyzed through the open source IBEX program to generate 2287 radiomic features. Agglomerative hierarchical clustering was used to narrow down these features into 145 clusters comprised of the highest correlation to PSR. The features among the clusters with the least inter-correlation were then chosen to limit redundancy in the data. The results of this study demonstrated a number of radiomic features that are positively correlated to PSR. The features with the highest degree of correlation included complexity, orientation and range. This approach may be used to determine patients for whom markerless motion tracking would be beneficial.

Purpose To present a novel method, based on convolutional neural networks (CNN), to automate weighted log subtraction (WLS) for dual‐energy (DE) fluoroscopy to be used in conjunction with markerless tumor tracking (MTT). Methods A CNN was developed to automate WLS (aWLS) of DE fluoroscopy to enhance soft tissue visibility. Briefly, this algorithm consists of two phases: training a CNN architecture to predict pixel‐wise weighting factors followed by application of WLS subtraction to reduce anatomical noise. To train the CNN, a custom phantom was built consisting of aluminum (Al) and acrylic (PMMA) step wedges. Per‐pixel ground truth (GT) weighting factors were calculated by minimizing the contrast of Al in the step wedge phantom to train the CNN. The pretrained model was then utilized to predict pixel‐wise weighting factors for use in WLS. For comparison, the weighting factor was manually determined in each projection (mWLS). A thorax phantom with five simulated spherical targets (5–25 mm) embedded in a lung cavity, was utilized to assess aWLS performance. The phantom was imaged with fast‐kV dual‐energy (120 and 60 kVp) fluoroscopy using the on‐board imager of a commercial linear accelerator. DE images were processed offline to produce soft tissue images using both WLS methods. MTT was compared using soft tissue images produced with both mWLS and aWLS techniques. Results Qualitative evaluation demonstrated that both methods achieved soft tissue images with similar quality. The use of aWLS increased the number of tracked frames by 1–5% compared to mWLS, with the largest increase observed for the smallest simulated tumors. The tracking errors for both methods produced agreement to within 0.1 mm. Conclusions A novel method to perform automated WLS for DE fluoroscopy was developed. Having similar soft tissue quality as well as bone suppression capability as mWLS, this method allows for real‐time processing of DE images for MTT.

Dual energy (DE) imaging is a well-known technique that has been used in diagnostic radiology for a number of years. More recently, there has been increased interest and utilization of DE imaging in radiation therapy (RT). The applications of DE imaging include both planar and tomographic imaging. In planar imaging, DE is used to remove overlapping bony anatomy to improve the visualization of lung tumors. Such an approach has shown promising results for markerless tumor tracking (MTT). With respect to tomographic imaging, DE imaging has been shown to provide more accurate material composition for low-energy brachytherapy dose calculations, improve the estimate of proton relative stopping power (RSP), and enhance soft-tissue visualization for RT treatment planning. Future directions include implementation of DE approaches for MTT and cone beam computed tomography (CBCT) to provide this technology at the time of treatment.

Purpose/Objective(s)Biology-guided radiotherapy (BgRT) integrates PET detectors with a LINAC and delivers a real-time tracked dose of radiation to moving targets using a continuous stream of limited time-sampled (LTS) PET images as a biological fiducial. These LTS PET images are converted into fluences and delivered to the target that cumulatively sum to deliver the intended prescription dose to the planning target volume (PTV). Physical validation of BgRT directed at a moving target has not previously been reported.Materials/MethodsAn 18F-Fluorodeoxyglucose (FDG) fillable insert that fits inside the independent quality assurance tool cavity was used in these experiments. The insert included a 22mm diameter target enclosed within a Styrofoam mesh compartment, simulating a lung-like background. The insert was filled with FDG with a target-to-background ratio of 8:1, placed in the independent quality assurance tool cavity which was then mounted on a custom motion stage that could move in the axial direction (IEC-Y). Motion was simulated using a breathing waveform that had a large amplitude motion (from -14.3 mm to +14.4 mm with respect to the mean position), with random amplitude and period with a baseline shift over time, simulating breathing baseline shifts. A BgRT plan (prescription of 8Gy) was created using motion averaged PET images, acquired on a pre-commercial version of the RefleXion system using a 32 × 32 × 32mm (IEC-X, Y, Z) PTV - 5mm expansion over the clinical target volume (CTV). An internal tumor volume (ITV) based SBRT plan (prescription of 10Gy) with a 32 × 32 × 62mm PTV covering the motion extent was also created. Radiochromic EBT-XD film was inserted into a slot in the spherical target to measure the delivered dose. Using the same motion waveform, BgRT and SBRT plans were delivered to the moving target, with an additional 8Gy BgRT delivery (32 × 32 × 32mm PTV) to the stationary target serving as control. Dosimetric accuracy for moving targets was measured using the concept of coverage where 100% of the CTV receives at least 97% of the prescription dose and maximum dose in the CTV was = < 130% of the maximum planned dose.ResultsUnder motion, both SBRT (32 × 32 × 62mm PTV) and BgRT (32 × 32 × 32mm PTV) met CTV dose coverage. CTV dose ranges were a) Motion SBRT: measured = [10.4Gy,12.4Gy], plan = [9.7Gy, 17.2Gy], b) Motion BgRT: measured = [9.7Gy,11.2Gy], plan = [7.8Gy, 15.3Gy], c) Static BgRT: measured = [8.7Gy,10.1Gy], plan = [7.8Gy), 10.4Gy]. Motion coverage results indicate that BgRT with a PTV that is 48.4% smaller than the SBRT PTV, delivering a tracked dose to the moving target., indicating a more conformal delivery.ConclusionThis is the first report of BgRT delivery achieving a tracked dose distribution directed at a moving target. Because tracked distributions have better conformality and normal tissue sparing than free-breathing internal tumor volume approaches, BgRT may improve the toxic-therapeutic ratio for applications such as early-stage lung cancer. Biology-guided radiotherapy (BgRT) integrates PET detectors with a LINAC and delivers a real-time tracked dose of radiation to moving targets using a continuous stream of limited time-sampled (LTS) PET images as a biological fiducial. These LTS PET images are converted into fluences and delivered to the target that cumulatively sum to deliver the intended prescription dose to the planning target volume (PTV). Physical validation of BgRT directed at a moving target has not previously been reported. An 18F-Fluorodeoxyglucose (FDG) fillable insert that fits inside the independent quality assurance tool cavity was used in these experiments. The insert included a 22mm diameter target enclosed within a Styrofoam mesh compartment, simulating a lung-like background. The insert was filled with FDG with a target-to-background ratio of 8:1, placed in the independent quality assurance tool cavity which was then mounted on a custom motion stage that could move in the axial direction (IEC-Y). Motion was simulated using a breathing waveform that had a large amplitude motion (from -14.3 mm to +14.4 mm with respect to the mean position), with random amplitude and period with a baseline shift over time, simulating breathing baseline shifts. A BgRT plan (prescription of 8Gy) was created using motion averaged PET images, acquired on a pre-commercial version of the RefleXion system using a 32 × 32 × 32mm (IEC-X, Y, Z) PTV - 5mm expansion over the clinical target volume (CTV). An internal tumor volume (ITV) based SBRT plan (prescription of 10Gy) with a 32 × 32 × 62mm PTV covering the motion extent was also created. Radiochromic EBT-XD film was inserted into a slot in the spherical target to measure the delivered dose. Using the same motion waveform, BgRT and SBRT plans were delivered to the moving target, with an additional 8Gy BgRT delivery (32 × 32 × 32mm PTV) to the stationary target serving as control. Dosimetric accuracy for moving targets was measured using the concept of coverage where 100% of the CTV receives at least 97% of the prescription dose and maximum dose in the CTV was = < 130% of the maximum planned dose. Under motion, both SBRT (32 × 32 × 62mm PTV) and BgRT (32 × 32 × 32mm PTV) met CTV dose coverage. CTV dose ranges were a) Motion SBRT: measured = [10.4Gy,12.4Gy], plan = [9.7Gy, 17.2Gy], b) Motion BgRT: measured = [9.7Gy,11.2Gy], plan = [7.8Gy, 15.3Gy], c) Static BgRT: measured = [8.7Gy,10.1Gy], plan = [7.8Gy), 10.4Gy]. Motion coverage results indicate that BgRT with a PTV that is 48.4% smaller than the SBRT PTV, delivering a tracked dose to the moving target., indicating a more conformal delivery. This is the first report of BgRT delivery achieving a tracked dose distribution directed at a moving target. Because tracked distributions have better conformality and normal tissue sparing than free-breathing internal tumor volume approaches, BgRT may improve the toxic-therapeutic ratio for applications such as early-stage lung cancer.

Purpose/Objective(s)Biology-guided radiotherapy (BgRT), utilizes hardware that incorporates a PET detection system into a ring-gantry LINAC for real-time tracking delivery. For this study, we focus on the BgRT delivery performance in the case of static targets for dose accuracy measurement. During active beam delivery, the system operates by aiming beamlets of therapeutic radiation at malignant tumors in response to outgoing PET emissions with sub-second latency. Over a treatment fraction, these beamlets sum to the total intended dose prescribed by the physician. Physical demonstration of BgRT has not previously been reported. Here we report phantom experiments validating BgRT using static PET-avid targets of varying shapes and background PET environments.Materials/MethodsAn FDG-fillable insert with different shaped targets was developed to mimic different potential radiotherapy targets. Spherical and C-shaped targets were filled with 18F-flurodeoxyglucose (FDG) to represent tumors and/or organs-at-risk. The background material in the insert was either a homogenous water medium or a water filled heterogeneous medium with a Styrofoam mesh simulating lung tissue surrounding the target. Targets and OARs were filled with FDG to achieve a target/OAR: background ratio of 8:1, while the background concentration varied from 4.5-10 kBq/ml to simulate typical patient background activity concentrations. For our set-ups were investigated: Spherical target in homogenous background, spherical target in heterogeneous background, C-shaped target in homogenous background, and spherical target in homogenous background with nearby PET-avid, C-shaped OAR. For each combination, SBRT and BgRT treatment plans were created on the RefleXion treatment planning system (TPS). Next, SBRT and BgRT plans were delivered on a pre-commercial version of the RefleXion system and dosimetric accuracy was measured using the AC phantom. Gamma criteria (3%/3mm) were used to compare delivered and calculated dose. Paired Student's t-test assessed differences in SBRT and BgRT gamma pass results.ResultsAll BgRT deliveries met 3%/3mm gamma pass criteria greater than 95% (Range, 95.2% to 98.9%). Additionally, gamma passing rates were not significantly different than the companion SBRT plans performed on the same targets (P = 0.16).ConclusionDose accuracy results indicate that BgRT plans can be delivered accurately over a variety of test conditions (different target shapes, attenuating media and activity concentration levels) to FDG-avid targets, with performance similar to SBRT treatments. Biology-guided radiotherapy (BgRT), utilizes hardware that incorporates a PET detection system into a ring-gantry LINAC for real-time tracking delivery. For this study, we focus on the BgRT delivery performance in the case of static targets for dose accuracy measurement. During active beam delivery, the system operates by aiming beamlets of therapeutic radiation at malignant tumors in response to outgoing PET emissions with sub-second latency. Over a treatment fraction, these beamlets sum to the total intended dose prescribed by the physician. Physical demonstration of BgRT has not previously been reported. Here we report phantom experiments validating BgRT using static PET-avid targets of varying shapes and background PET environments. An FDG-fillable insert with different shaped targets was developed to mimic different potential radiotherapy targets. Spherical and C-shaped targets were filled with 18F-flurodeoxyglucose (FDG) to represent tumors and/or organs-at-risk. The background material in the insert was either a homogenous water medium or a water filled heterogeneous medium with a Styrofoam mesh simulating lung tissue surrounding the target. Targets and OARs were filled with FDG to achieve a target/OAR: background ratio of 8:1, while the background concentration varied from 4.5-10 kBq/ml to simulate typical patient background activity concentrations. For our set-ups were investigated: Spherical target in homogenous background, spherical target in heterogeneous background, C-shaped target in homogenous background, and spherical target in homogenous background with nearby PET-avid, C-shaped OAR. For each combination, SBRT and BgRT treatment plans were created on the RefleXion treatment planning system (TPS). Next, SBRT and BgRT plans were delivered on a pre-commercial version of the RefleXion system and dosimetric accuracy was measured using the AC phantom. Gamma criteria (3%/3mm) were used to compare delivered and calculated dose. Paired Student's t-test assessed differences in SBRT and BgRT gamma pass results. All BgRT deliveries met 3%/3mm gamma pass criteria greater than 95% (Range, 95.2% to 98.9%). Additionally, gamma passing rates were not significantly different than the companion SBRT plans performed on the same targets (P = 0.16). Dose accuracy results indicate that BgRT plans can be delivered accurately over a variety of test conditions (different target shapes, attenuating media and activity concentration levels) to FDG-avid targets, with performance similar to SBRT treatments.

Template-based matching algorithms are currently being considered for markerless motion tracking (MMT) of lung tumors using single energy (SE) fluoroscopy. A limitation of these algorithms is that tracking may not be accurate when there is significant overlap of tumor/bone on planar images. In these cases, dual energy (DE) imaging may be used to create a soft tissue only image, potentially improving the accuracy of MMT. The goal of this study is to evaluate the accuracy of MMT using a fast-kV-switching DE imaging system. A fast-kV-switching real-time DE imaging system, emulating a clinical on-board imager, was implemented on a bench top that included an x-ray tube, amorphous silicon (a-Si) flat panel detector, and x-ray generator with custom firmware and software. A programmable motion phantom, consisting of a torso with embedded ribs and spine along with a simulated tumor in a lung-type cavity, was used. SE and DE images were obtained at fixed angles as well as over 360 degrees of rotation allowing the tumor projection to overlap with varying amounts of bone. Soft tissue images obtained from the DE fluoroscopic sequences were created using a frame-by-frame weighted logarithmic subtraction. Separately, MMT requires a template that was derived from the contoured simulated tumor obtained from a CT scan of the phantom. A template-based matching algorithm was then used to track tumor motion throughout the DE and SE fluoroscopy sequences. This algorithm shifts the template across the image and calculates the normalized cross correlation (NCC) between the template and image, resulting in a match score surface. The offset at which the NCC has a maximum value represents the potential target position. The strength of this peak relative to NCC values away from the peak, called side lobe values, is quantified by the peak-to-side lobe ratio (PSR). Previous studies showed higher PSR values are correlated with tracking accuracy. For fixed gantry imaging, MMT was able to track 296/380 (78%) of SE images. Tracking failure was due to tumor/bone overlap. The removal of bone through DE imaging increased the success rate to 379/380 (99%) (p < 0.0001). The average PSR was 4.28 +/- 1.04 and 6.25 +/-1/41 for SE and DE, respectively (p < 0.0001). For the rotational acquisition, the number of images that the algorithm was able to track was 414/450 (92%) vs. 419/450 (93%) for SE and DE, respectively (p=0.61). However, the PSR values were 5.06 +/- 1.13 and 6.30 +/- 1.46 for SE and DE, respectively (p < 0.0001). Tracking accuracy was determined by comparing the ability of MMT to reproduce the programmed waveform. The root mean square error (RMSE) of tracked vs. the programmed waveform was 0.24 +/- 0.11 mm for SE vs. 0.15 +/- 0.04 mm for DE (p = 0.03). A fast-kV-switching real-time DE prototype has been implemented and utilized to provide DE imaging for MMT. The principle advantage of this approach is the ability to accurately track tumors where MMT fails on SE fluoroscopy due to overlapping bony anatomy.

Markerless tumor tracking (MTT) using template matching is currently being considered for real-time lung tumor tracking using single energy (SE) fluoroscopy. A limitation of MTT is that the tumor has to be clearly visible on images. In cases where the tumor is obscured by bony anatomy, dual energy (DE) imaging may be used to create a soft tissue only image, improving tumor visibility as well as the accuracy of MTT. The goal of this study is to evaluate MTT with fast-kV switching DE imaging using the on-board imager (OBI) of a commercial linear accelerator. Fast-kV switching DE fluoroscopy was implemented on the OBI of a commercial linac. This technique produces x-ray pulses that alternate between programmed tube voltages (ie, 120 and 60 kVp) at a rate of 15 Hz. To evaluate MTT with DE fluoroscopy, a motion phantom, consisting of a torso with embedded ribs and spine along with a cavity having lung-equivalent density, was used. Simulated tumors (5-25 mm diameter) were placed inside the lung equivalent compartment of the phantom, and then programmed to simulate breathing. Three categories of motion with varying periods were considered: static (ST), slow-motion (SM) - 5 sec period and fast-motion (FM) - 2.5 sec period. While the target was moving, fast-kV switching DE images were acquired over 360° of rotation allowing the tumor projection to overlap with varying amounts of bone. Weighted logarithmic subtraction was performed on consecutive high-low projections to produce soft tissue images. A template-based matching algorithm was then used to track target motion on both DE and SE sequences. Successful matching was defined as any instance in which the algorithm matched the template on the image. Tumor tracking coordinates were also evaluated against ground truth motion using the root-mean-squared error (RMSE). For the 5 mm target, the success rates ranged from 11-17% for SE vs. 36-63% for DE imaging, for the three types of motion considered. Because of the low success rates, RMSE were not evaluated for this target. The 10 mm target demonstrated success rates of 66-73% and 87-90% for SE and DE, respectively. The corresponding RMSE for SE were 2.7, 2.9 and 3.4 mm, while for DE they were 0.2, 1.0 and 0.5 mm, for ST, SM and FM, respectively. For the 15 mm target, SE success rates ranged from 84-89% vs. 90-94% for DE. RMSE for SE was 0.5, 2.0 and 1.4 mm vs. 0.2, 0.5, and 0.2 mm for DE for ST, SM and FM, respectively. For the larger tumors (20 and 25 mm), the success rates were > 90% for both SE and DE. RMSE for SE and DE were <1.1 mm and <0.4 mm, respectively, for all motion categories. Fast-kV switching DE imaging has been implemented on the OBI of a commercial linac. Overall, MTT with DE imaging results in higher success rates and lower RMSE than using SE imaging alone. The approach allows for the accurate tracking of small tumors where MTT fails on SE imaging due to overlapping bony anatomy.

To present a feasibility study for using dual-energy (DE) cone beam computed tomography (CBCT) to produce relative electron density (RED), effective atomic number (Zeff) and proton relative stopping power (RSP) images, using the on-board imager (OCI) of a commercial linear accelerator. Low- and high-energy (80 kVp and 130 kVp, respectively) CBCT scans of the Catphan 604 phantom were acquired sequentially using the OBI of a commercial Linac. Each acquired projection pair was subsequently decomposed into basis material equivalent thicknesses using a calibration obtained from an aluminum (Al) and polymethyl methacrylate (PMMA) wedge phantom. The Al and PMMA thicknesses projections where then recombined using the linear attenuation coefficient (μ) for each basis material at the desired energy and reconstructed to produce virtual monoenergetic (VM) CBCT images. RED values for each insert of the phantom were obtained by converting the measured μ values for 150 keV VM-CBCT images directly to RED values based on a theoretical μ-RED linear correlation at that energy. The Zeff numbers for each insert were generated using the correlation between the ratio of 40 and 150 keV μ values and the reference Zeff. After obtaining RED and Zeff images, RSP images were obtained using the parametric method proposed by Hunemohr et al. Our theoretical analysis showed good linear correlation (0.9937) between 150 keV μ-values with the reference RED for elements with Z from 6 to 20, with a mean prediction error of 0.000 ± 0.073.The mean RED percentage error for phantom inserts was -0.24% ± 0.75%. The 50% bone insert had the largest percentage error (-1.49%). A logarithmic fit that transforms the ratio of 150/40 keV μ theoretical values to Zeff values from 6 to 20 was also defined. This fit resulted in a correlation coefficient of 0.9979, with a mean prediction error of 0.000 ± 0.021. Applying this data to the phantom inserts, the mean percentage error found for Zeff was 0.08% ± 2.09%. The Teflon insert had the largest percentage error for Zeff estimation (3.44%). The mean percentage RSP error value was -0.19% ± 1.78% with the 50% bone insert having the highest percentage error (-3.74.%). The presented method successfully obtained RED, Zeff and RSP images from VM-CBCT images produced using a sequential DE approach by scanning the phantom directly with the OBI of a commercial linear accelerator. Such an approach may allow for direct verification of RSP values at the time of treatment for proton systems that are equipped with an OBI.