Purpose: Our purpose was to optimize an image guided radiation therapy (IGRT) workflow to achieve practical setup accuracy in spine stereotactic body radiation therapy (SBRT). We assessed the time-saving efficiencies gained from incorporating planar kV midimaging as a surrogate for cone beam computed tomography (CBCT) for intrafraction motion monitoring. Methods and Materials: We selected 5 thoracic spine SBRT patients treated in 5 fractions and analyzed patient shifts captured by a modified IGRT workflow using planar kV midimaging integrated with CBCT to maintain a tolerance of 1 mm and 1°. We determined the frequency at which kV midimaging captured intrafraction motion as validated on repeat CBCT and assessed the potential time and dosimetric advantages of our modified IGRT workflow. Results: Patient motion, detected as out-of-tolerance shifts on planar kV midimaging, occurred during 6 of 25 fractions (24%) and were validated on repeat CBCT 100% of the time. Observed intrafraction absolute shifts (mean ± standard deviation) for the 25 fractions were 0.39 ± 0.21, 0.56 ± 0.22, and 0.45 ± 0.21 mm for lateral-longitude-vertical translations and 0.38 ± 0.12°, 0.32 ± 0.09°, and 0.47 ± 0.14° for pitch-roll-yaw rotation, which if uncorrected, could have significantly affected target coverage and increased spinal cord dose. The average times for pretreatment imaging, midtreatment verification, and total treatment time were 8.94, 2.81, and 16.21 minutes. Our modified IGRT workflow reduced the total number of CBCTs required from 120 to 35 (70%) and imaging dose from 126.2 to 43.4 cGy (65.6%) while maintaining high fidelity for our patient population. Conclusions: Accurate patient positioning was effectively achieved with use of multiple 2-dimensional-3-dimensional kV images and an average of 1 verification CBCT scan per fraction. Integration of planar kV midimaging can effectively reduce treatment time associated with spine SBRT delivery and minimize the potential dosimetric effect of intrafraction motion on target coverage and spinal cord dose.

Cardiac metastases pose clinical challenges for radiation oncologists given the need to balance the benefit of local therapy against the risks of cardiac toxicity in the setting of cardiac motion, respiratory motion, and nearby organs at risk. Stereotactic magnetic resonance-guided adaptive radiation therapy has recently become more commonly used, conferring benefits in tumor visualization for setup, real-time motion management monitoring, and enabling plan adaptation for daily changes in tumor and/or normal tissues. Given these benefits, we developed and implemented a workflow for local treatment of metastatic disease within the heart using stereotactic magnetic resonance-guided adaptive radiation therapy.

MR-guided linear accelerators have enabled the routine adaption of radiation therapy plans to account for daily anatomy changes. However, the practical implementation of daily radiation replanning raises new challenges for the traditional radiation oncology treatment model. We report our initial experiences in designing and implementing an online adaptive radiation therapy program using a 0.35T MR-guided treatment machine. We developed a treatment model and workflow tailored for adaptive stereotactic body radiotherapy in which every fraction is intended to be adapted. This includes a team-based cross-coverage model between a set of adaptive specialist physicians who partner with disease-site specialists. We additionally implemented a pre-adaptive procedure timeout procedure and patient-specific adaptive treatment checklists that guides adaptive re-contouring and plan review. We present a treatment workflow developed for adaptive SBRT, and report on the first 50 patients treated in our adaptive SBRT model during our first four months of clinical operation. Disease sites treated included pancreas (19), lymph nodes (16), liver (4), lung (4), kidney/adrenal (3), prostate (2), umbilicus (1) and heart (1). Each treatment was delivered over 5 fractions, with a prescribed total dose range of 25-55 Gy, and image-based gating for motion was used in all cases. A total of 250 fractions were delivered, of which 62 (25%) used adaptation primarily to improve target coverage relative to the initial plan, and 165 (66%) used adaptation primarily for sparing organs at risk. In 23 fractions (9%), no improvement was noted from plan adaptation. The average fraction total treatment time was 1h 9min (stdev = 17min) with an average of 33 minutes spent on workflow steps. related to treatment adaptation. We have demonstrated a treatment model designed for adaptive radiation therapy that incorporates multiple elements that are not part of a traditional radiotherapy workflow.

Stereotactic magnetic resonance (MR)-guided online adaptive radiotherapy (SMART) combines real-time MR imaging with RT delivery, allowing for online adaptive planning and continuous motion management. However, uncertainty remains in optimal patient positioning, image quality standards, and contouring guidelines for SMART. To determine the optimal combination of MR sequence parameters, coil and patient positioning, and treatment immobilization, we conducted a healthy volunteer (HV) study utilizing a 0.35T MR-Linac (MRL). We hypothesized that with optimization of these parameters, image quality and SMART workflows for HVs would be acceptable across a range of clinically relevant organs, facilitating subsequent patient imaging. The HV study was approved by the IRB. Adults of all races and genders were included. MR simulation and simulated SMART were performed in HVs on an MRL. For each scan, the following parameters were assessed on a scale from 1-5 (5 = extremely high, 1 = extremely poor): visibility of target, visibility of nearby normal organs, and overall image quality. Adaptive recontouring time was recorded. With IRB approval, we retrospectively reviewed recontouring times for the first fraction for the first 18 patients treated clinically with SMART. HV and clinical times were compared with a t-test (alpha = 0.05). 46 HVs consented to the study. 18 underwent simulated treatments that included simulation of all aspects of SMART clinical workflow, except RT delivery, and targeted the following organs: adrenal, prostate, pancreas, lung, kidney, and brain. Average visibility of target and nearby organs were 4.58 and 4.62, respectively, and average overall image quality was 4.62 with a range of 4-5 for all measures. The average recontouring time was 15 minutes (range 4-35) in the HVs, comparable to an average of 16.1 minutes (range 7-33) in patients (p = 0.886). The clinical cases targeted adrenal, prostate, pancreas, lung, and pelvic and abdominal nodes. HV scanning on an MRL resulted in acceptable image quality and target visibility for a range of organs. Contouring time, the longest step in our workflow, was well-simulated in HVs, with similar times achieved clinically. The organs targeted in HVs correlated well with the organs treated clinically. The HV study provided a non-clinical environment to optimize all aspects of SMART, except for RT delivery. Further study is needed to determine the optimal number of HV scans required to prepare for clinical implementation of SMART.

PURPOSE:Among patients with brain metastases, hippocampal avoidance whole brain radiation (HA-WBRT) preserves neurocognitive function relative to conventional WBRT but the feasibility of hippocampal sparing in patients with metastases in/near the hippocampus is unknown. We identified the incidence of hippocampal/perihippocampal metastases and evaluated the feasibility of HA-WBRT in such patients.MATERIALS/METHODS:Dosimetric data from 34 patients randomized to HA-WBRT (30 Gy/10 fractions) in a phase III trial (NCT03075072) comparing HA-WBRT to stereotactic radiation in patients with 5 to 20 brain metastases were analyzed. Patients with metastases in/near the hippocampi received HA-WBRT with prioritization of tumor coverage over hippocampal avoidance. Target coverage and hippocampal sparing metrics were compared between patients with targets in/near the hippocampus versus not.RESULTS:In total, 9 of 34 (26%) patients had targets in the hippocampus and an additional 5 of 34 (15%) patients had targets in the hippocampal avoidance zone (HAZ, hippocampus plus 5 mm expansion) but outside the hippocampus. Patients with targets within the hippocampus and those with targets in the HAZ but outside the hippocampus were spared 34% and 73% of the ipsilateral mean biologically equivalent prescription dose, respectively. Of the latter cohort, 88% and 25% met conventional hippocampal sparing metrics of Dmin ≤ 9 Gy and Dmax ≤ 16 Gy, respectively. Among 11 patients with unilateral hippocampal/perihippocampal involvement, the uninvolved/contralateral hippocampus was limited to Dmin ≤ 9 Gy and Dmax ≤ 17 Gy in all cases.CONCLUSIONS:In this study, a substantial percentage of patients with 5 to 20 brain metastases harbored metastases in/near the hippocampus. In such cases, minimizing hippocampal dose while providing tumor coverage was feasible and may translate to neurocognitive protection.

Purpose: MR-linacs (MRLs) have enabled the use of stereotactic magnetic resonance (MR) guided online adaptive radiotherapy (SMART) across many cancers. As data emerges to support SMART, uncertainty remains regarding optimal technical parameters, such as optimal patient positioning, immobilization, image quality, and contouring protocols. Prior to clinical implementation of SMART, we conducted a prospective study in healthy volunteers (HVs) to determine optimal technical parameters and to develop and practice a multidisciplinary SMART workflow. Methods: HVs 18 years or older were eligible to participate in this IRB-approved study. Using a 0.35 T MRL, simulated adaptive treatments were performed by a multi-disciplinary treatment team in HVs. For each scan, image quality parameters were assessed on a 5-point scale (5 = extremely high, 1 = extremely poor). Adaptive recontouring times were compared between HVs and subsequent clinical cases with a t-test. Results: 18 simulated treatments were performed in HVs on MRL. Mean parameters for visibility of target, visibility of nearby organs, and overall image quality were 4.58, 4.62, and 4.62, respectively (range of 4–5 for all measures). In HVs, mean ART was 15.7 min (range 4–35), comparable to mean of 16.1 (range 7–33) in the clinical cases (p = 0.8963). Using HV cases, optimal simulation and contouring guidelines were developed across a range of disease sites and have since been implemented clinically. Conclusions: Prior to clinical implementation of SMART, scans of HVs on an MRL resulted in acceptable image quality and target visibility across a range of organs with similar ARTs to clinical SMART. We continue to utilize HV scans prior to clinical implementation of SMART in new disease sites and to further optimize target tracking and immobilization. Further study is needed to determine the optimal duration of HV scanning prior to clinical implementation.

Current evidence suggests patients who have oligometastatic disease or have need for re-irradiation may benefit from stereotactic body radiation therapy (SBRT), however delivery of this modality is limited by the efficiency of current motion management systems. In patients with non-spine bone metastases which rely on computed tomography (CT) to assess correct anatomic positioning during SBRT delivery, there is risk of extending treatment time and radiation exposure to patients that may move due to prolonged discomfort because of this inefficiency. This analysis aims to determine if an Optical Surface Monitoring System (OSMS) can minimize time spent imaging and subsequent adjustments for non-spine SBRT patients, and to determine if OSMS is a valid replacement for intrafraction imaging. Seven iliac SBRT patients, treated November 2019 with 2 or 3 arc VMAT plans, were retrospectively evaluated under IRB approval. Immobilization included Civco SBRT/extremity board with an indexed vaclok, aquaform behind the knee, and aquaplast over the knee. Patient skin was exposed inferior to the umbilicus and genitals were covered with a small cloth. The OSMS ROI was set to cover the thighs and pelvis, excluding the cloth. Setup time was compared with seven iliac SBRT patient setups without OSMS treated between April and November 2019. Setup accuracy was assessed by aligning the patient to the planning CT surface, then confirming the position with 2D3D and CBCT imaging. CBCT alignment was considered to be ground truth. Motion detection was assessed by comparing OSMS offsets to mid-arc 2D3D offsets. Differences between OSMS and CBCT positioning were compared to differences between 2D3D and CBCT positioning. A total of 33 fractions were evaluated. Setup time was reduced on average from 10.3+/-3.1 minutes without OSMS to 8.1+/-1.7 minutes with OSMS. For setup accuracy, the standard-deviation of the OSMS offset distributions were 1.8, 5.2, and 2.8 mm for AP, SI, and LR directions and 1.4, 0.7 and 1.2 degrees in the rotation, roll and pitch directions respectively. For motion detection, offset distribution standard deviations were 1.1, 1.3, and 1.6 mm for AP, SI, and LR directions and 0.62, 0.45, and 0.71 degrees in the rotation, roll and pitch directions respectively. Error distribution standard deviations between 2D3D and CBCT were 0.4, 0.6, and 0.9 mm for AP, SI, and LR directions and 0.3, 0.4, and 0.4 degrees in the rotation, roll and pitch directions respectively. OSMS helped reduce setup time for iliac SBRT patients by eliminating the need for repeat setup imaging. Accuracy for OSMS was predictably lower than that for 2D3D imaging in setup and motion detection, however improved initial positioning and real time feedback makes OSMS an extremely valuable tool in this setting. Further work will show how these results are generalizable across palliative patients and could be of more utility for extremity PTV's or large single fraction treatments for metastatic pain relief.

IGRT is an integral aspect of spine SBRT treatment delivery, allowing for accurate patient positioning and high dose to tumor while sparing dose to critical nerves. Meanwhile, intrafraction imaging with kV CBCT introduces additional radiation dose to the peripheral tissues, bone, and spinal cord, which is unaccounted for in treatment planning, with implications particularly for pediatric patients or those at high risk of myelopathy in setting of reirradiation. IGRT to verify our institutional tolerance of 1 mm and 1° between arcs may prolong treatment time and introduce further setup uncertainty. To optimize the imaging workflow (prioritizing KV-2D-3D vs KV-CBCT if sufficient) for achieving practical set up accuracy while limiting the imaging dose received, the cumulative CBCT imaging dose to the spinal cord and the dose perturbation induced by patient setup error were investigated. CBCT imaging dose to spinal cord was derived by correlating our institutional CTDI QA assessment on 4 linear accelerators and 3 C-Series LINACs with the reference value provided in the current literature. The clinical pelvis protocol CTDI was measured with the ACR torso phantom with 10cm pencil chamber, 125kVp, 1080mAs on liner accelerators and 125kVp, 80mA, 13ms on C-Series, respectively. Dosimetry change from setup uncertainty was analyzed by shifting the VMAT plan isocenter by 1 mm in Y direction and 1° couch rotation from the original planning isocenter on 5 spine SBRT patients, prescribed a course of 30Gy in 5 fractions. Dose differences were evaluated in terms of the target volume coverage D95, the spinal cord D0.035cc, D0.1cc and Dmax by re-calculating the plan with the same beam geometry, MLC motion, and MUs as the original plans. Differences between measured and vendor specified CTDI was 14% ± 11%SD (n = 7). Correlating the CBCT imaging dose reference based on Rando phantom measurement and Monte Carlo simulation in the literature, imaging dose to the spinal cord in our clinic was estimated as 3 - 4cGy per pelvis protocol CBCT. Dosimetry change with respect to target D95 showed an average deviation of 0.2% prescription dose with 1mm and 1° positioning uncertainty from the planning intents (n = 5). In the worst-case, the spinal cord with 1mm expansion had a max dose (D0.035cc) increase by an average of 132cGy (4.4% Rx) and with a max of 245cGy (8.2% Rx). CBCT dose from multiple scans (conventionally initial, mid and post) contribute to the total dose of the spinal cord. Mid-CBCTs may also prolong the treatment and introduce additional uncertainty. Our current patient positioning accuracy, effectively achieved with use of multiple 2D-3D kV images and minimized CBCT verification scans per fraction, does not produce any significant change in patient dose distributions in the target or spinal cord. Customized CBCT protocols, optical monitoring system (OSMS), or use of MR-LINAC could potentially further reduce imaging dose related to intrafraction monitoring for spine SBRT.

Introduction: Stereotactic body radioablation (SBRT) is emerging as a treatment for ventricular tachycardia(VT) refractory to catheter ablation. However, its effect on ischemic scar remains poorly ...

Linac and MLC-based stereotactic radiosurgery (SRS) using single-isocenter-multiple-target (SIMT) VMAT has become increasingly popular in the management of multi-focal cranial metastases. However, significant geometrical and dosimetric challenges exist due to the typically small target volumes and in most cases, non-isocentric locations. To the best of our knowledge, there hasn't been a study in the optimization of MLC parameters, in the context of SIMT SRS, to ensure TPS calculation accuracy. In this work, we set out to optimize the dosimetric leaf gap (DLG) for the HD MLC installed on dedicated stereotactic Varian STx systems using a diverse group of 21 clinical SRS and SBRT plans. These plans featured a broad range of target sizes and target-to-isocenter distances that are typical of the stereotactic cases treated on these systems. Dose discrepancies between TPS calculations and verification measurements using a previously validated diode array Delta4 (ScandiDos) were minimized in a balanced manner to accommodate the variety of stereotactic plans. A DLG of 0.6 mm was found to be 'optimal' for the HD MLC and for the 'typical' plans treated on our STx systems. The finding was independently verified using commercially available 3D polymer gel dosimeter CrystalBallTM (MGS Research Inc.). 3D verification for 6 SIMT SRS plans, consisted of 5 to 15 targets, achieved an average gamma score of 97.3% (σ = 2.0%) on 3%/2 mm criteria with a cutoff isodose level of 20%. We further examined the practice of routine dosimetric verifications including the selection of appropriate detectors and optimal gamma parameters. We found that the commonly used standard 3%/3 mm criteria would have resulted in all but 4 (out of 2840) clinical plans achieving a gamma score of 95% or better, and therefore, losing sensitivity to detect potential dosimetric discrepancies. Based on the characteristics of stereotactic plans, a more stringent distance-to-agreement parameter is needed.

SBRT has become increasingly applied to non-spine bone oligometastases or in the setting of re-irradiation; however, treatment guidelines are not well-defined. The purpose of this study was to determine the planning target volume expansion necessary to adequately compensate for intra-fraction patient motion of non-spine bone SBRT targets in the femur and iliac.

The work described here compared the available technical solutions for the treatment of ventricular tachycardia with stereotactic body radiation therapy. Due to the complexity of target motion during cardiac and pulmonary motion as well as the several proximate radiosensitive structures of the tracheobronchial tree and esophagogastrointestinal tract, four potential candidates for this treatment were identified: Accuray CyberKnife (Accuray Incorporated, Sunnyvale, California, United States), Varian TrueBeam (Varian Medical Systems, Palo Alto, California, USA), Elekta Infinity (Elekta, Stockholm, Sweden), and Varian Edge (Varian Medical Systems, Palo Alto, California, USA). All four treatment modalities were evaluated for their ability to deliver a conformal, homogeneous dose to most of the target volume, to spare nearby and distant critical and sensitive anatomical structures as well as for treatment efficiency. It was found that conventional linear accelerator technology was superior in their ability to spare distant critical structures and deliver treatments efficiently while the CyberKnife showed superiority in sparing nearby critical structures more aggressively by creating larger dose gradients at the periphery of the target volume. Both treatment modalities were similar in their ability to cover the entire target with the prescription dose, conform that dose to the target volume, and deliver a homogeneous dose.