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Online dose calculations before radiation treatment have applications in dose delivery verification, plan adaptation, and treatment planning. We propose a novel CBCT imaging pipeline to enhance accuracy. Our approach aims to improve HU accuracy in CBCT images for more precise dose calculations. A quantitative CBCT pipeline was implemented, combining data correction strategies and scatter rejection, achieving high CT number accuracy. We evaluated the pipeline's effect using pelvis anatomy phantoms and found that dosimetric errors in quantitative CBCT-based dose calculations were minimal. In contrast, clinical CBCT and high-performance ASG CBCT-based plans showed significant errors. The proposed quantitative CBCT pipeline offers comparable dose calculation accuracy to the gold-standard planning CT, eliminating the need for density overrides and enabling precise dose delivery monitoring or online plan adaptations in radiation therapy.

Poor tissue visualization and quantitative accuracy in CBCT is a major barrier in expanding the role of CBCT imaging from target localization to quantitative treatment monitoring and plan adaptations in radiation therapy sessions. To further improve image quality in CBCT, 2D antiscatter grid based scatter rejection was combined with a raw data processing pipeline and iterative image reconstruction. The culmination of these steps was referred as quantitative CBCT, qCBCT. qCBCT data processing steps include 2D antiscatter grid implementation, measurement based residual scatter, image lag, and beam hardening correction for offset detector geometry CBCT with a bow tie filter. Images were reconstructed with iterative image reconstruction to reduce image noise. To evaluate image quality, qCBCT acquisitions were performed using a variety of phantoms to investigate the effect of object size and its composition on image quality. qCBCT image quality was benchmarked against clinical CBCT and MDCT images. Addition of image lag and beam hardening correction to scatter suppression reduced HU degradation in qCBCT by 10 HU and 40 HU, respectively. When compared to gold standard MDCT, mean HU errors in qCBCT and clinical CBCT were 10 HU and 27 HU, respectively. HU inaccuracy due to change in phantom size was 22 HU and 85 HU in qCBCT and clinical CBCT images, respectively. With iterative reconstruction, contrast to noise ratio improved by a factor of 1.25 when compared to clinical CBCT protocols. Robust artifact and noise suppression in qCBCT images can reduce the image quality gap between CBCT and MDCT, improving the promise of qCBCT in fulfilling the tasks that demand high quantitative accuracy, such as CBCT based dose calculations and treatment response assessment in image guided radiation therapy.

The concept of using kilovoltage (kV) and megavoltage (MV) beams concurrently has potential applications in cone beam computed tomography (CBCT) guided radiation therapy, such as single breath hold scans, metal artifact reduction, and simultaneous imaging during MV treatment delivery. However, MV cross-scatter generated during MV beam delivery degrades CBCT image quality. To address this, a 2D antiscatter grid and cross scatter correction method were investigated. a 3D printed, tungsten 2D antiscatter grid prototype was utilized to reduce MV cross-scatter fluence in kV projections during concurrent MV beam delivery. Remaining cross-scatter was corrected by using the 2D grid itself as a cross-scatter intensity sampling device, referred as Grid-based Scatter Sampling. To test this approach, kV CBCT acquisitions were performed while delivering 6 and 10 MV beams, mimicking high dose rate treatment delivery scenarios. MV cross-scatter suppression performance of the proposed approach was evaluated in projections and CBCT images of phantoms. 2D grid reduced the intensity of MV cross-scatter in kV projections by a factor of 3 on the average. Remaining MV cross-scatter estimated by Grid-based Scatter Sampling was within 7% of measured reference intensity values. CBCT image quality was improved substantially during concurrent kV-MV beam delivery. Median Hounsfield Unit (HU) inaccuracy was up to 191 HU without our methods, and it was reduced to 3 HU with our 2D grid and scatter correction approach. Our methods provided a factor of 2-6 improvement in contrast-to-ratio. Results indicate that our approach can successfully minimize the effects of high energy cross-scatter in concurrent kV CBCT imaging and megavoltage treatment delivery.

Purpose: We have been investigating 2D antiscatter grids (2D ASG) to reduce scatter fluence and improve image quality in cone beam computed tomography (CBCT). In this work, two different aspects of 2D ASGs, their scatter rejection and correction capability, were investigated in CBCT experiments. To correct residual scatter transmitted through the 2D ASG, it was used as a scatter measurement device with a novel method: grid-based scatter sampling. Methods: Three focused 2D ASG prototypes with grid ratios of 8, 12, and 16 were developed for linac-mounted CBCT geometry. In the first phase, 2D ASGs were used as a scatter rejection device, and the effect of grid ratio on CT number accuracy and contrast-to-noise ratio (CNR) evaluated in CBCT images. In the second phase, the grid-based scatter sampling method was implemented. The percent change in CT numbers was measured by changing the phantom size from head to pelvis configuration.Results: When 2D ASG was used as a scatter rejection device, CT number variation due to change in phantom dimensions was reduced from 23% to 2 - 6%. A grid ratio of 16 yielded the lowest CT number variation. All three 2D ASGs yielded improvement in CNR, up to a factor of two in pelvis-sized phantoms. When 2D ASG prototypes were used for both scatter rejection and correction, CT number variations were reduced further, to 1.3 - 2.6%. Conclusions: When 2D ASG was used solely as a scatter rejection device, substantial improvement in CT number accuracy could be achieved by increasing the grid ratio. 2D ASGs could also provide significant CNR improvement even at lower grid ratios. When 2D ASG was used in conjunction with the grid-based scatter sampling method, it provided further improvement in CT number accuracy, irrespective of the grid ratio, while preserving 2D ASG's capacity to improve CNR.

Tumor motion plays a key role in the safe delivery of Stereotactic Body Radiotherapy (SBRT) for pancreatic cancer. The purpose of this study was to use tumor motion data measured in patients to establish limits on motion magnitude for safe delivery of pancreatic SBRT. Using 91 sets of pancreatic tumor motion data measured in patients, we calculated motion-convolved dose for 25 pancreatic cancer patients, and established the maximum amount of motion allowable while satisfying error thresholds on key dose metrics. In our patient cohort, the mean [min-max] allowable motion for 33/40/50 Gy to the PTV was 11.9 [6.3-22.4], 10.4 [5.2-19.1] and 9.0 [4.2-16.0] mm, respectively. Maximum allowable motion decreased as dose was escalated, and was smaller in patients with larger tumors. The effects of motion on pancreatic SBRT are highly variable between patients and there is potential to allow more motion in certain patients, even in dose-escalated scenarios. In our dataset, a conservative limit of 6.3 mm would ensure safe treatment of all patients treated to 33 Gy in 5 fractions., Comment: 9 pages, 5 figures, supplement included. Submitted to Practical Radiation Oncology

Stereotactic body radiation therapy (SBRT) for pancreatic cancer requires a skillful approach to deliver ablative doses to the tumor while limiting dose to the highly sensitive duodenum, stomach, and small bowel. Here, we develop knowledge-based artificial neural network dose models (ANN-DMs) to predict dose distributions that would be approved by experienced physicians. Using dose distributions calculated by a commercial treatment planning system (TPS), physician-approved treatment plans were used to train ANN-DMs that could predict physician-approved dose distributions based on a set of geometric parameters (vary from voxel to voxel) and plan parameters (constant across all voxels for a given patient). Differences between TPS and ANN-DM dose distributions were used to evaluate model performance. ANN-DM design, including neural network structure and parameter choices, were evaluated to optimize dose model performance. Mean dose errors were less than 5% at all distances from the PTV, and mean absolute dose errors were on the order of 5%, but no more than 10%. Dose-volume histogram errors demonstrated good model performance above 25 Gy, but much larger errors were seen at lower doses. ANN-DM dose distributions showed excellent overall agreement with TPS dose distributions, and accuracy was substantially improved when each physician's treatment approach was taken into account by training their own dedicated models. In this manner, one could feasibly train ANN-DMs that could predict the dose distribution desired by a given physician for a given treatment site., Comment: 11 pages, 6 figures

Purpose: Implanted fiducial markers are often used in radiotherapy to facilitate accurate visualization and localization of tumors. Typically, such markers are used to aid daily patient positioning and to verify the target's position during treatment. This work introduces a novel, automated method for identifying fiducial markers in planar x-ray imaging. Methods: In brief, the method consists of automated filtration and reconstruction steps that generate 3D templates of marker positions. The normalized cross-correlation was the used to identify fiducial markers in projection images. To quantify the accuracy of the technique, a phantom study was performed. 75 pre-treatment CBCT scans of 15 pancreatic cancer patients were analyzed to test the automated technique under real life conditions, including several challenging scenarios for tracking fiducial markers. Results: In phantom and patient studies, the method automatically tracked visible marker clusters in 100% of projection images. For scans in which a phantom exhibited 0D, 1D, and 3D motion, the automated technique showed median errors of 39 $\mu$m, 53 $\mu$m, and 93 $\mu$m, respectively. Human precision was worse in comparison. Automated tracking was performed accurately despite the presence of other metallic objects. Additionally, transient differences in the cross-correlation score identified instances where markers disappeared from view. Conclusions: A novel, automated method for producing dynamic templates of fiducial marker clusters has been developed. Production of these templates automatically provides measurements of tumor motion that occurred during the CBCT scan that was used to produce them. Additionally, using these templates with intra-fractional images could potentially allow for more robust real-time target tracking in radiotherapy., Comment: 12 pages, 7 figures

Recent studies suggest that 4DCT is unable to accurately measure respiratory-induced pancreatic tumor motion. In this work, we assessed the daily motion of pancreatic tumors treated with SBRT, and developed adaptive strategies to predict and account for this motion. The daily motion trajectory of pancreatic tumors during CBCT acquisition was calculated using a model which reconstructs the instantaneous 3D position in each 2D CBCT projection image. We developed a metric (termed "Spectral Coherence," SC) based on the Fourier frequency spectrum of motion in the SI direction, and analyzed the ability of SC to predict motion-based errors and classify patients according to motion characteristics. The amplitude of daily motion exceeded the predictions of pre-treatment 4DCT imaging by an average of 3.0 mm, 2.3 mm, and 3.5 mm in the AP, LR, and SI directions. SC was correlated with daily motion differences and tumor dose coverage. In a simulated adaptive protocol, target margins were adjusted based on SC, resulting in significant increases in mean target D95, D99, and minimum dose. Our Fourier-based approach differentiates between consistent and inconsistent motion characteristics of respiration and correlates with daily motion deviations from pre-treatment 4DCT. The feasibility of an SC-based adaptive protocol was demonstrated, and this patient-specific respiratory information was used to improve target dosimetry by expanding coverage in inconsistent breathers while shrinking treatment volumes in consistent breathers., Comment: 8 pages, 5 figures in Radiother Oncol (2015)

Purpose: Cone-beam CT (CBCT) projection images provide anatomical data in real-time over several respiratory cycles, forming a comprehensive picture of tumor movement. We developed and validated a method which uses these projections to determine the trajectory of and dose to highly mobile tumors during each fraction of treatment. Methods: CBCT images of a respiration phantom were acquired, the trajectory of which mimicked a lung tumor with high amplitude (up to 2.5 cm) and hysteresis. A template-matching algorithm was used to identify the location of a steel BB in each CBCT projection, and a Gaussian probability density function for the absolute BB position was calculated which best fit the observed trajectory of the BB in the imager geometry. Two modifications of the trajectory reconstruction were investigated: first, using respiratory phase information to refine the trajectory estimation (Phase), and second, using the Monte Carlo (MC) method to sample the estimated Gaussian tumor position distribution. Results: With all methods, the mean position of the BB was determined with accuracy better than 0.1 mm, and trajectory errors averaged 3.8$\pm$1.1% of the marker amplitude. Dosimetric calculations using Phase methods were more accurate, with mean absolute error less than 0.5%, and with error less than 1% in the highest-noise trajectory. MC-based trajectories prevent the over-estimation of dose, but when viewed in an absolute sense, add a small amount of dosimetric error (<0.1%). Conclusions: Marker trajectory and target dose-of-the-day were accurately calculated using CBCT projections. This technique provides a method to evaluate highly-mobile tumors using ordinary CBCT data, and could facilitate better strategies to mitigate or compensate for motion during SBRT., Comment: Submitted to Medical Physics journal, 11 pages, 5 figures

During Stereotactic Body Radiotherapy (SBRT) for the treatment of prostate cancer, an inflatable endorectal balloon (ERB) may be used to reduce motion of the target and reduce the dose to the posterior rectal wall. This work assessed the dosimetric impact of manual interventions on ERB position in patients receiving prostate SBRT and investigated the impact of ERB interventions on prostate shape. Daily kilovoltage (kV) cone-beam computed tomography (CBCT) imaging was performed to localize the PTV, and an automated fusion with the planning images yielded displacements required for PTV re-localization. When the ERB volume and/or position were judged to yield inaccurate repositioning, manual adjustment (ERB re-inflation and/or repositioning) was performed. Based on all 59 CBCT image sets acquired, a deformable registration algorithm was used to determine the dose received by, displacement of, and deformation of the prostate, bladder, and anterior rectal wall. This dose tracking methodology was applied to images taken before and after manual adjustment of the ERB (intervention), and the delivered dose was compared to that which would have been delivered in the absence of intervention. Interventions occurred in 24 out of 35 (69%) of the treated fractions. The direct effect of these interventions was an increase in the prostate radiation dose that included 95% of the PTV (D95) and an increase in prostate coverage. Additionally, ERB interventions reduced prostate deformation in the anterior-posterior (AP) direction, reduced errors in the sagittal rotation of the prostate, and increased the similarity in shape of the prostate to the radiotherapy plan. Image-guided interventions in ERB volume and/or position during prostate SBRT were necessary to ensure the delivery of the dose distribution as planned., Comment: 10 pages, 7 figures