Purpose/Objective(s) Reirradiation in the abdomen poses a challenge due to both interfraction and intrafraction motion of nearby organs at risk (OARs). We hypothesize that magnetic resonance-guided stereotactic body radiotherapy (MRgSBRT) using daily online adaptation as well as real-time tumor tracking improves the safety of dose escalated treatment by minimizing grade 3+ gastrointestinal toxicities. Materials/Methods We performed a single-institution retrospective review of 44 patients who received a total of 53 courses of MRgSBRT as reirradiation within the abdomen on a 0.35-T MR linear accelerator from October 2019 to October 2023. Toxicities were assessed using CTCAE v5. Overall survival (OS) and local control (LC) were estimated using the Kaplan-Meier method. Results Median age at time of reirradiation was 67 years (Range: 24-88 years). Eighteen different primary histologies were treated, with the most common including renal (9), pancreatic (7), and prostate (7) cancers. The most common sites for reirradiation were lymph nodes (25), liver (8), and adrenal glands (6). A majority (74%) of MRgSBRT courses overlapped with prior SBRT, while the remainder overlapped with prior conventional (19%) or palliative (7%) RT doses. The median BED10 for the prior course was 72 Gy (R: 28-132 Gy). The median time interval between courses was 11 months (R: 0-156 months). The median dose for MRgSBRT reirradiation was 40 Gy (R: 25-54 Gy); 89% of MRgSBRT reirradiation courses were prescribed to a dose of at least 35 Gy. A single course of 54 Gy was delivered in 3 fractions, while all others were treated in 5 fractions. All cases involved online adaptive replanning. The median follow-up following MRgSBRT re-irradiation was 12 months (R: 1-38 months). One-year LC and OS were 87% and 72%, respectively. Grade 3+ toxicity was seen in two (5%) patients. One patient developed self-limited gastrointestinal bleeding following three courses of SBRT (first CT-guided SBRT, remainder MRgSBRT) for pancreatic cancer without a source identified on endoscopy. A second patient who received two courses of MRgSBRT for a Klatskin tumor developed fatal biliary obstruction in the setting of global progression; this was favored to be due to local tumor growth, but we are unable to exclude treatment toxicity. Conclusion MRgSBRT allows for dose escalation and good local control in cases of abdominal reirradiation with acceptable toxicity. This retrospective study supports the development of a prospective clinical trial of MRgSBRT reirradiation in the abdomen.

Purpose/Objective(s) The on-board kV imager of therapeutic linear accelerators is essential in modern radiotherapy, which requires highly precise patient positioning before and during treatment. Planar imaging is often used for initial alignment and during treatment to monitor the patient's position. however, these images can often be challenging to interpret correctly due to overlapping anatomy, increasing the uncertainty on target alignment requiring larger margins. We have developed a novel dual-layer kV imager, which inherently provides spectral imaging, allowing for image enhancement techniques to be used. Here we evaluate the improvements in visualization of key anatomy on planar imaging for setup verification, using patient images collected with the DLI installed on one of our clinically operational LINACs. Materials/Methods The DLI is constructed using two detector layers, the top layer is equivalent to the current commercial imager and the bottom is a slightly modified version of the top. The data from each layer is read out simultaneously and saved for post-imaging analysis. The top layer image alone is used for clinical imaging, leaving the original patient workflow unchanged. Patient images were collected from a variety of treatment sites, including prostate, lung and head and neck. Combining the layers of the planar images collected using a weighted-log subtraction technique for material decomposition, we evaluate the reduction in bone visibility for the lung patients and improvement in fiducial visibility for the prostate patients. Results Visually comparing the single layer image and bone subtracted image (using weighted-log subtraction) acquired of a lung patient, the appearance of the ribs is greatly reduced, with the structures of the lung becoming more defined without obstruction of the bone. Quantifying this by comparing the contrast of the bone against surrounding tissue, the appearance of rib was reduced by 20%. Comparing the appearance of fiducials on the images acquired for the prostate patient, particularly for lateral projections, their visibility was also improved on the weighed-log subtraction images, where we note an increased visibility of 19%. Conclusion This preliminary evaluation of the first clinically operational dual-layer kV imager demonstrates the feasibility of using spectral imaging to improve the visibility of anatomy of interest. The opportunity to use image enhancement techniques could pave the way for reliable, accurate setup using planar imaging, reducing the reliance on CBCT and minimizing patient imaging dose.

Purpose/Objective(s) Dual-energy imaging confers potential advantages including reduced artifacts and material decomposition – e.g. allowing material enhancement for contrast or material suppression to view underlying anatomy better. Spectral separation may be achieved via a dual-layer detector, with beam hardening by the first layer providing the shift in spectra. This approach may offer unique advantages relative to other on-board spectral imaging concepts, in terms of dose savings, motion artifact elimination, and innate view registration. Materials/Methods The first layer of the on-board dual-layer imager (DLI) was designed to be identical to existing detector construction, with a CsI scintillator followed by an aSi TFT photodiode array. This enabled seamless clinical implementation, as standard onboard image guidance was maintained, using the top layer only. The bottom layer has a slightly thicker scintillator to aid in photon detection efficiency given the reduced fluence received after the top layer. The DLI prototype was constructed and underwent rigorous safety testing by a commercial partner prior to clinical integration. The DLI was installed on a clinical Linac with a novel imaging chain in which the top layer only is used for clinical tasks and both layers are read out to a research computer for retrospective analysis. Modulation transfer function (MTF), noise power spectrum, and detective quantum efficiency (DQE) were measured for the top, bottom, and combined layers. Detector imaging performance was further characterized by Leeds, Catphan, and anthropomorphic phantoms. To date, data collection has been performed for more than 20 patients of diverse disease sites. Results Clinical installation of the prototype DLI was completed successfully without any interruption to routine workflow. Phantom measurements confirmed that the top layer MTF and DQE were similar to the commercial single-layer imager which had been replaced. Spatial resolution for the combined-layer images were slightly lower than the top layer only but benefited from increased photon detection efficiency. The patients imaged so far include head & neck, pelvis, extremity, thorax, and CNS. All routine treatment imaging and delivery proceeded as usual without interference from the study. Virtual monoenergetic images were generated from the dual-layer CBCT data using a U-net convolutional neural network. In 2D imaging, log-weighted subtraction of two layers successfully removed bone and metal hardware from resulting images, enabling better tumor visualization. Conclusion A prototype kV DLI was constructed and clinically translated for study under protocol. Combining the layers yields a higher photon detection efficiency with a small loss in resolution. Preliminary clinical results show promise for spectral imaging applications, such as removing ribs to enable better lung tumor imaging. The dual-layer design may be an effective method for adding spectral imaging capabilities to a Linac.

Purpose/Objective(s) While radiotherapy depends critically on contouring anatomical structures in CT, poor soft tissue contrast and metal artifacts reduce CT contour accuracy. As a remedy to these constraints, photon-counting CT (PCCT) holds promise to provide improved soft tissue contrast and metal artifact reduction (MAR). In this work, we examine these parameters; quantifying PCCT soft tissue contrast, anatomical visibility, and contouring accuracy in a detailed, anthropomorphic pelvis phantom with interchangeable bone, aluminum (Al), and titanium (Ti) femoral head inserts. Materials/Methods Ground truths cannot typically be defined for patient bladder and prostate contours, to avoid this constraint, an anthropomorphic pelvis phantom was used in this study, the phantom was scanned using a high-resolution commercial PCCT system (140 kVp, slice thickness 0.2 mm) and an energy-integrating CT (EICT) system (140 kVp, slice thickness 1.25 mm). PCCT images with bilateral and unilateral metal hip inserts were reconstructed using standard and iterative MAR methods, respectively. The bladder and prostate were contoured manually in all images and the contrast to noise ratio (CNR) was measured in regions of interest (ROIs) in the bladder and prostate. PCCT images were binned to have slice thicknesses of 1.2 mm for CNR measurements. For each set of contours, Dice coefficients, Hausdorff distances, and structural similarity index measures (SSIMs) were calculated relative to the no metal ground truth. Fine detail of the images was analyzed through delineation of the border between the prostate and rectum in sagittal slices. Results PCCT showed similar soft tissue CNR and a qualitative reduction of metal artifacts compared to EICT, however, the reduction in metal artifacts did not correlate to better contours. Dice coefficients, Hausdorff distances and SSIMs for the bladder and prostate (Table 1.) were increased, remained similar, and decreased using MAR for the bilateral Ti, unilateral Ti, and bilateral Al hip inserts, respectively. With the MAR algorithm removing metal artifacts but also blurring soft tissue boundaries. EICT CNRs in the bladder and prostate ROIs (13.8 and 20.3) were slightly higher than PCCT (13.6 and 17.8). PCCT images showed delineation of the boundary between the prostate and the rectum in three sagittal slices where the boundary was unresolved in EICT. Conclusion Overall, we find some evidence that PCCT could provide improvement to current workflows in radiation oncology through delineation of soft-tissue boundaries, while PCCT with the vendor supplied MAR shows benefits for contouring anatomy near bilateral Ti hip implants but detriment for contouring anatomy near unilateral Ti or bilateral Al hip implants.

Purpose/Objective(s) Cone beam CT (CBCT) is common in radiation therapy image guidance. However, when metal prosthetics or implanted fiducials are present in the patient, CBCTs exhibit image artifacts which may impede targeting and registration with prior planning CTs. Likewise, metal artifacts hinder dose recalculation in adaptive radiotherapy (ART). Many artifact suppression algorithms rely on an initial delineation of the metal structures, either in the acquired 2D x-ray projection images or in their 3D reconstruction. Delineation in 2D presents challenges, since overlapping structures in the projections impede recognition of metal. Delineation in 3D is also challenging, since metal artifacts in initially uncorrected CBCT images obscure the boundaries of metal objects. In this work, we test an artifact reduction method employing novel dual-layer imager (DLI) data to facilitate 3D metal delineation. Materials/Methods The DLI is composed of two detector layers, each containing CsI scintillator sub-layers of different thicknesses, and an aSi photodiode sub-layer. It was mounted on a clinical linac and used to scan several patients, including a head and neck patient with dental implants and a lower abdominal patient with prostate fiducials. The x-ray projections acquired in each layer were reconstructed without spectral correction and subtracted to form a 3D subtraction image. Metal regions in the subtraction images were both prominent in intensity and sharply delineated, and hence easily thresholded to produce metal segmentation maps. Metal-affected areas of the projections were located through reprojection of the maps and overpainted using a Dirchlet boundary problem solver. Over-painted projections from both layers were averaged together and reconstructed to form the final CBCT images. Voxel standard deviations in regions of interests (ROIs) were used to quantify tissue uniformity in metal-affected locations and to quantify noise in other locations. Results In addition to a substantial visual reduction in artifacts, ROI measurements showed significant restoration of tissue uniformity vis-a-vis standard CBCT. Uniformity improved by 30% near the prostate fiducials and by a factor of 9 near the dental implants. Moreover, image noise was 30% lower near the center of the reconstructed lower abdomen, as compared to standard imaging. Conclusion These preliminary patient studies found the multi-spectral, geometrically aligned x-ray measurements provided by dual-layer CBCT to be an effective tool for locating and correcting for implanted metal structures. Moreover, the increased radiation capture efficiency of the DLI reduced image noise in large, lower abdominal anatomy. This stands to benefit registration accuracy with pre-treatment imaging as well as on-treatment dose recalculations in ART.

Objective. Megavoltage cone-beam computed tomography (MV-CBCT) imaging offers several advantages including reduced metal artifacts and accurate electron density mapping for adaptive or emergent situations. However, MV-CBCT imaging is limited by the poor efficiency of current detectors. Here we examine a new MV imager and compare CBCT reconstructions under clinically relevant scenarios.Approach. A multilayer imager (MLI), consisting of four vertically stacked standard flat-panel imagers, was mounted to a clinical linear accelerator. A custom anthropomorphic pelvis phantom with replaceable femoral heads was imaged using MV-CBCT and kilovoltage CBCT (kV-CBCT). Bone, aluminum, and titanium were used as femoral head inserts. 8 MU 2.5 MV scans were acquired for all four layers and (as reference) the top layer. Prostate and bladder were contoured on a reference CT and transferred to the other scans after rigid registration, from which the structural similarity index measure (SSIM) was calculated. Prostate and bladder were also contoured on CBCT scans without guidance, and Dice coefficients were compared to CT contours.Main results. kV-CBCT demonstrated the highest SSIMs with bone inserts (prostate: 0.86, bladder: 0.94) and lowest with titanium inserts (0.32, 0.37). Four-layer MV-CBCT SSIMs were preserved with bone (0.75, 0.80) as compared to titanium (0.67, 0.74), outperforming kV-CBCT when metal is present. One-layer MV-CBCT consistently underperformed four-layer results across all phantom configurations. Unilateral titanium inserts and bilateral aluminum insert results fell between the bone and bilateral titanium results. Dice coefficients trended similarly, with four-layer MV-CBCT reducing metal artifact impact relative to KV-CBCT to provide better soft-tissue identification.Significance. MV-CBCT with a four-layer MLI showed improvement over single-layer MV scans, approaching kV-CBCT quality for soft-tissue contrast. In the presence of artifact-producing metal implants, four-layer MV-CBCT scans outperformed kV-CBCT by eliminating artifacts and single-layer MV-CBCT by reducing noise. MV-CBCT with a novel multi-layer imager may be a valuable alternative to kV-CBCT, particularly in the presence of metal.

BackgroundLiver tumors are often invisible on four-dimensional commuted tomography (4D-CT). Imperfect imaging surrogates are used to estimate the tumor motion. Here, we assessed multiple 4D magnetic resonance (MR) binning algorithms for directly visualizing liver tumor motion for radiotherapy planning.MethodsPatients were simulated using a 3 Tesla MR and CT scanner. Three prototype binning algorithms (phase, amplitude, and two-directional) were applied to the 4D-MRIs, and the image quality was assessed using a qualitative clarity score and quantitative sharpness score. Radiation plans were generated for internal target volumes (ITVs) derived using 4D-MRI and 4D-CT, and the dosimetry of targets were compared. Paired t-tests were used to compare sharpness scores and dosimetric data.ResultsTwelve patients with 17 liver tumors were scanned between May and November 2021. Compared to phase binning, two-directional demonstrated equal or better clarity and sharpness scores (end-expiration: 0.33 vs 0.38, p = 0.018, end-inspiration: 0.28 vs 0.31, p = 0.010). Compared to amplitude binning, two-directional binning captured hysteresis of ≥ 3 mm in 35 % of patients. Evaluation of dosimetry CT-optimized plans revealed that PTV coverage of MR-derived targets were significantly lower than CT-derived targets (PTV receiving 90 % of prescription: 75.56 % vs 89.38 %, p = 0.002).ConclusionUsing contrast-enhanced 4D-MRI is feasible for directly delineating liver tumors throughout the respiratory cycle. The current standard of using radiation plans optimized for 4D-CT-derived targets achieved lower coverage of directly visualized MRI targets, suggesting that adopting MRI for motion management may improve radiation treatment of liver lesions and reduce the risk of marginal misses.

Noise reduction and increased MV photon detection efficiency achieved by utilizing a multi-layer MV imager results in improved fiducial tracking for liver SBRT treatments. Future BEV clinical applications may be improved by pursuing similar noise reduction and photon detection enhancement strategies.

Purpose. Electronic portal image devices (EPIDs) have been investigated previously for beams-eye view (BEV) applications such as tumor tracking but are limited by low contrast-to-noise ratio and detective quantum efficiency. A novel multilayer imager (MLI), consisting of four stacked flat-panels was used to measure improvements in fiducial tracking during liver stereotactic body radiation therapy (SBRT) procedures compared to a single layer EPID. Methods. The prototype MLI was installed on a clinical TrueBeam linac in place of the conventional DMI single-layer EPID. The panel was extended during volumetric modulated arc therapy SBRT treatments in order to passively acquire data during therapy. Images were acquired for six patients receiving SBRT to liver metastases over two fractions each, one with the MLI using all 4 layers and one with the MLI using the top layer only, representing a standard EPID. The acquired frames were processed by a previously published tracking algorithm modified to identify implanted radiopaque fiducials. Truth data was determined using respiratory traces combined with partial manual tracking. Results for 4- and 1-layer mode were compared against truth data for tracking accuracy and efficiency. Tracking and noise improvements as a function of gantry angle were determined. Results. Tracking efficiency with 4-layers improved to 82.8% versus 58.4% for the 1-layer mode, a relative improvement of 41.7%. Fiducial tracking with 1-layer returned a root mean square error (RMSE) of 2.1 mm compared to 4-layer RMSE of 1.5 mm, a statistically significant (p < 0.001) improvement of 0.6 mm. The reduction in noise correlated with an increase in successfully tracked frames (r = 0.913) and with increased tracking accuracy (0.927). Conclusion. Increases in MV photon detection efficiency by utilization of a MLI results in improved fiducial tracking for liver SBRT treatments. Future clinical applications utilizing BEV imaging may be enhanced by including similar noise reduction strategies.

Tumor tracking during radiotherapy treatment can improve dose accuracy, conformity and sparing of healthy tissue. Many methods have been introduced to tackle this challenge utilizing multiple imaging modalities, including a template matching based approach using the megavoltage (MV) on-board portal imager demonstrated on 3D conformal treatments. However, the complexity of treatments is evolving with the introduction of VMAT and IMRT, and successful motion management is becoming more important due to a trend towards hypofractionation. We have developed a markerless lung tumor tracking algorithm, utilizing the electronic portal imager (EPID) of the treatment machine. The algorithm has been specifically adapted to track during complex treatment deliveries with gantry and MLC motion. The core of the algorithm is an adaptive template matching method that relies on template stability metrics and local relative orientations to perform multiple feature tracking simultaneously. Only a single image is required to initialize the algorithm and features are automatically added, modified or removed in response to the input images. This algorithm was evaluated against images collected during VMAT arcs of a dynamic thorax phantom. Dynamic phantom images were collected during radiation delivery for multiple lung SBRT breathing traces and an example patient data set. The tracking error was 1.34 mm for the phantom data and 0.68 mm for the patient data. A multi-region, markerless tracking algorithm has been developed, capable of tracking multiple features simultaneously without requiring any other a priori information. This novel approach delivers robust target localization during complex treatment delivery. The reported tracking error is similar to previous reports for 3D conformal treatments.

Electronic portal imaging devices (EPIDs) lend themselves to beams-eye view clinical applications, such as tumor tracking, but are limited by low contrast and detective quantum efficiency (DQE). We characterize a novel EPID prototype consisting of multiple layers and investigate its suitability for use under clinical conditions. A prototype multi-layer imager (MLI) was constructed utilizing four conventional EPID layers, each consisting of a copper plate, a Gd2O2S:Tb phosphor scintillator, and an amorphous silicon flat panel array detector. We measured the detector's response to a 6 MV photon beam with regards to modulation transfer function, noise power spectrum, DQE, contrast-to-noise ratio (CNR), signal-to-noise ratio (SNR), and the linearity of the detector's response to dose. Additionally, we compared MLI performance to the single top layer of the MLI and the standard Varian AS-1200 detector. Pre-clinical imaging was done on an anthropomorphic phantom, and the detector's CNR, SNR and spatial resolution were assessed in a clinical environment. Images obtained from spine and liver patient treatment deliveries were analyzed to verify CNR and SNR improvements. The MLI has a DQE(0) of 9.7%, about 5.7 times the reference AS-1200 detector. Improved noise performance largely drives the increase. CNR and SNR of clinical images improved three-fold compared to reference. A novel MLI was characterized and prepared for clinical translation. The MLI substantially improved DQE and CNR performance while maintaining the same resolution. Pre-clinical tests on an anthropomorphic phantom demonstrated improved performance as predicted theoretically. Preliminary patient data were analyzed, confirming improved CNR and SNR. Clinical applications are anticipated to include more accurate soft tissue tracking.