Search

Purpose: To commission and assess the clinical performance of a new commercial surface imaging (SI) system by analyzing intra‐fraction motion from the initial cohort of patients treated with frameless stereotactic radiosurgery (fSRS). Methods: The IDENTIFYTM SI system was commissioned for clinical use on an Edge (Varian Medical Systems, Palo Alto, CA) linear accelerator. All patients who received intracranial radiotherapy with HyperArcTM (Varian Medical Systems, Palo Alto, CA) were immobilized with the EncompassTM (Qfix, Avondale, PA) thermoplastic mask and monitored for intra‐fraction motion with SI. IDENTIFYTM log files were correlated with trajectory log files to correlate treatment parameters with SI‐reported offsets. IDENTIFYTM reported offsets were correlated with gantry and couch angles to assess system performance for obstructed and clear camera field of view. Data were stratified by race to evaluate performance differences due to skin tone. Results: All commissioning data were found to meet recommended tolerances. IDENTIFYTM was used to monitor intra‐fraction motion on 1164 fractions from 386 patients. The median magnitude of translational SI reported offsets at the end of treatment was 0.27 mm. SI reported offsets were shown to increase when camera pods are blocked by the gantry with larger increases seen at non‐zero couch angles. With camera obstruction, the median magnitude of the SI reported offset was 0.50 and 0.80 mm for White and Black patients, respectively. Conclusions: IDENTIFYTM performance during fSRS is comparable to other commercially available SI systems where offsets are shown to increase at non‐zero couch angles and during camera pod blockage. [ABSTRACT FROM AUTHOR]

Purpose: To validate surface imaging (SI)‐reported offsets using a six degree‐of‐freedom couch and an anthropomorphic phantom for commissioning and routine quality assurance of an SI system used for stereotactic radiosurgery (SRS). Methods: An anthropomorphic phantom with a radiopaque ball bearing (BB) placed either anterior, midline, or posterior, was tracked with SI with a typical SRS region of interest. Couch motion in all six degrees of freedom was programmed and delivered on a linac. SI system logs were synchronized with linac trajectory logs. Ten random couch positions were selected at couch 0°, 45°, 90°, 270°, 315° with megavolt (MV) images taken to account for couch walkout. The SI residual error (ε), the difference between SI reported offset and MV or trajectory log position, was calculated. Residual errors were measured with and without one SI pod blocked. Results: The median [range] of magnitude of translational ε was 0.13 [0.07, 0.21], 0.16 [0.11, 0.26], 0.61 [0.50, 0.68], 0.49 [0.42, 0.55], 0.55 [0.38, 0.72] mm for couch rotations of 0°, 45°, 90°, 270°, 315°, respectively, for the midline BB and no pod blocked. The range of all translational ε from all couch angles (with and without pod block) at different BB positions is [0.05, 0.96] mm. The absolute range of difference when changing BB position when no pod is blocked in median translational ε is [0.01, 0.40] mm with the maximum at BB posterior. The absolute range of difference when not changing BB positions with and without pod block in median translational ε is [0.01, 0.37] mm with the maximum at BB posterior and couch 315°. Conclusion: SI system and linac trajectory log analysis can be used to assess SI system performance with automated couch motion to validate SI accuracy. [ABSTRACT FROM AUTHOR]

Background: For many years, the standard of care for radiosurgical thalamotomy has been a frame-based treatment, either on Gamma Knife or a linear accelerator with cones. Our institution has recently completed enrollment of phase I/II prospective clinical evaluating the safety and efficacy of a linear-accelerator based treatment with thermoplastic mask utilizing the linear accelerator’s native high-definition collimator to generate a similar rapid-falloff spherical dose distribution. In this study, we assessed the in-vivo accuracy of our treatments. Methods: We treated patients on an IRB-approved clinical trial (ClinicalTrials.gov Identifier: NCT03305588) using a Varian Edge™ (Palo Alto, CA) linear accelerator equipped with high-definition multi-leaf collimator and 10MV flattening-filter free beam using our previously described virtual-cone approach. Optical surface guidance was used to monitor the patient position during treatment. For 15 patients having follow-up imaging, the planned dose distribution was exported in the pre-treatment MPRAGE image space. The pre-treatment MPRAGE was coregistered to the 6-month post-treatment post-contrast T1 SPACE image using a two-stage linear registration (rigid followed by affine). The lesion was segmented using a semi-automated, threshold-based method. The centerof-gravity (COG) of the lesion and of the planned 65 Gy isodose volume were calculated. Results: The mean distances between the center coordinates of the lesion and target were less than 0.5 mm in all axes (mean ± standard deviation between lesion and target: x = -0.1 ± 0.3; y = -0.1 ± 0.5; z = -0.3 ± 0.7 mm). The mean Euclidean distance between the target and lesion COG was 0.86 mm (95% confidence interval 0.58 to 1.13 mm). The distance was significantly correlated with imaging signal-to-noise ratio (SNR), suggesting patients with greater head motion and poorer quality imaging resulted in a greater discrepancy between target and lesion. Conclusion: The distance between the target position and lesion position is consistent with sub-millimeter accuracy for frameless SRS thalamotomy. Imaging quality is a limiting factor when assessing the targeting accuracy. [ABSTRACT FROM AUTHOR]

Purpose/objective(s): Stereotactic radiosurgery (SRS) to ventral intermediate nucleus (VIM) is a storied & successful yet underutilized technique for tremor. High doses & small targets have historically necessitated treatment with stereotactic frame & either a GK platform or cone-mounted linear accelerator. We developed a coneless technique to replicate the dose distributions previously only attributable to GK on a linear accelerator (LINAC). We deployed this technique & tested it in a prospective clinical trial of safety & efficacy of SRS thalamotomy for non-DBS candidate patients with medically refractory essential or Parkinsonian tremor. Materials/methods: We assessed tremor pre- & posttreatment & QOL FTM/PROMIS scores. We obtained MPRAGE, FGATIR, diffusion-weighted tractographic, & resting-state fMRI sequences. We identified the VIM via both thalamic parcellation & classical stereotactic reference location, & targeted to 130Gy dmax dosimetrically equivalently to 4 mm GK shot with static MLC’s. We ensured 25Gy isodose line did not overlap the posterior limb of capsule. We delivered treatment on a LINAC with high-definition HDMLC & intrafraction optical surface monitoring (OSMS) to ensure patient immobility. We surveilled post-treatment imaging & tremor scores. Results: We accrued 43 patients. 1 withdrew & 1 elected to pursue previously declined DBS. At submission, 40 patients had been treated & 35 patients had ≥6 month follow-up. Median total treatment time was 32 minute. 31/35 (89%) exhibited meaningful tremor reduction (≥20% FTM reduction). Median maximum pre- & posttremor reduction was 59.6% (range: 9.8 - 100%). Time to tremor improvement ranged from 0.3 to 15 months. 1 patient experienced Grade 2 and 1 experienced reversible Grade 3 toxicity. Conclusion: Frameless, coneless MLC-based SRS thalamotomy on the LINAC is a safe, effective alternative workflow that may be preferable for some patients due to its efficient delivery. Data continue to mature, but current results are congruent to those of historical gamma knife & cone-based linac treatments. We have expanded the trial, and are beginning to integrate tractographic & functional imaging into target selection for future patients. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study was 2-fold. One purpose was to develop an automated, streamlined quality assurance (QA) program for use by multiple centers. The second purpose was to evaluate machine performance over time for multiple centers using linear accelerator (Linac) log files and electronic portal images. The authors sought to evaluate variations in Linac performance to establish as a reference for other centers. Methods: The authors developed analytical software tools for a QA program using both log files and electronic portal imaging device (EPID) measurements. The first tool is a general analysis tool which can read and visually represent data in the log file. This tool, which can be used to automatically analyze patient treatment or QA log files, examines the files for Linac deviations which exceed thresholds. The second set of tools consists of a test suite of QA fields, a standard phantom, and software to collect information from the log files on deviations from the expected values. The test suite was designed to focus on the mechanical tests of the Linac to include jaw, MLC, and collimator positions during static, IMRT, and volumetric modulated arc therapy delivery. A consortium of eight institutions delivered the test suite at monthly or weekly intervals on each Linac using a standard phantom. The behavior of various components was analyzed for eight TrueBeam Linacs. Results: For the EPID and trajectory log file analysis, all observed deviations which exceeded established thresholds for Linac behavior resulted in a beam hold off. In the absence of an interlocktriggering event, the maximum observed log file deviations between the expected and actual component positions (such as MLC leaves) varied from less than 1% to 26% of published tolerance thresholds. The maximum and standard deviations of the variations due to gantry sag, collimator angle, jaw position, and MLC positions are presented. Gantry sag among Linacs was 0.336±0.072 mm. The standard deviation in MLC position, as determined by EPID measurements, across the consortium was 0.33 mm for IMRT fields. With respect to the log files, the deviations between expected and actual positions for parameters were small (<0.12 mm) for all Linacs. Considering both log files and EPID measurements, all parameters were well within published tolerance values. Variations in collimator angle, MLC position, and gantry sag were also evaluated for all Linacs. Conclusions: The performance of the TrueBeam Linac model was shown to be consistent based on automated analysis of trajectory log files and EPID images acquired during delivery of a standardized test suite. The results can be compared directly to tolerance thresholds. In addition, sharing of results from standard tests across institutions can facilitate the identification of QA process and Linac changes. These reference values are presented along with the standard deviation for common tests so that the test suite can be used by other centers to evaluate their Linac performance against those in this consortium. [ABSTRACT FROM AUTHOR]

Purpose: Leaf positions for dynamic multileaf collimator (DMLC) intensity modulated radiation therapy must be closely synchronized with MU delivery. For the Varian C3 series MLC controller, if the planned trajectory (leaf position vs. MU) requires velocities exceeding the capability of the MLC, the leaves fall behind the planned positions, causing the controller to momentarily hold the beam and thereby introduce dosimetric errors. We investigated the merits of a new commercial linear accelerator, TrueBeam™, that integrates MLC control with prospective dose rate modulation. If treatment is delivered at dose rates so high that leaves would fall behind, the controller reduces the dose rate such that harmony between MU and leaf position is preserved. Methods: For three sets of DMLC leaf trajectories, point doses and two-dimensional dose distributions were measured in phantom using an ionization chamber and film, respectively. The first set, delivered using both a TrueBeam™ and a conventional C3 controller, comprised a single leaf bank closing at planned velocities of 2.4, 7.1, and 14 cm/s. The maximum achievable leaf velocity for both systems was 3 cm/s. The remaining two sets were derived from clinical fluence maps using a commercial treatment planning system for a range of planned dose rates and were delivered using TrueBeam™ set to the maximum dose rate, 600 MU/min. Generating trajectories using a planned dose rate that is lower than the delivery dose rate effectively increased the leaf velocity constraint used by the planning system for trajectory calculation. The second set of leaf trajectories was derived from two fluence maps containing regions of zero fluence obtained from representative beams of two different patient treatment plans. The third set was obtained from all nine fields of a head and neck treatment plan. For the head and neck plan, dose-volume histograms of the spinal cord and target for each planned dose rate were obtained. Results: For the single closing leaf bank trajectories, the TrueBeam™ control system reduced the dose rate such that the leaf velocity was less than the maximum. Dose deviations relative to the 2.4 cm/s trajectory were less than 3%. For the conventional controller, the leaves repeatedly fell behind the planned positions until the beam hold threshold was reached, resulting in deviations of up to 19% relative to the 2.4 cm/s trajectory. For the two clinical fluence maps, reducing the planned dose rate reduced the dose in the zero fluence regions by 15% and 24% and increased the delivery time by 5 s and 14 s. No significant differences were noted in the high and intermediate dose regions measured using film. The DVHs for the head and neck plan showed a 10% reduction in cord dose for 20 MU/min relative to 600 MU/min sequencing dose rate, which was confirmed by measurement. No difference in target DVHs were observed. The reduction in cord dose increased total treatment time by 1.8 min. Conclusions: Leaf sequencing algorithms for integrated control systems should be modified to reflect the reduced importance of maximum leaf velocity for accurate dose delivery. [ABSTRACT FROM AUTHOR]

Linear accelerator (linac) based CNS stereotactic radiosurgery (SRS) requires significant time resources. We hypothesized that CNS SRS using a flattening filter free (FFF) linac would reduce treatment time and improve clinical efficiency. A FFF linac was recently commissioned for CNS radiosurgery at the University of Alabama at Birmingham. The efficiency of this linac for CNS SRS was retrospectively reviewed. Beam on time (BOT), time in room (TIR), and clinical dose rate (CDR) were calculated using an integrated treatment planning, record, and verification software platform and are proposed as surrogates for treatment efficiency. Twenty-seven eligible CNS SRS cases consisting of 1-5 fractions of 5 Gy or more per fraction were reviewed. Mean BOT was 1:21 (minutes:seconds; range: 00:36-2:52) and mean TIR was 10:42 (minutes:seconds; range: 6:05-22:56). The mean CDR was 1820 MU/min (range: 872-2396). On regression analysis the number of alignment images, treatment arcs, targets, monitor units, and presence of intra-fraction imaging were factors significantly (p < 0.05) associated with prolonged TIR. Use of FFF mode in CNS SRS more than triples the CDR and results in shortened BOT and TIR compared to treatment at conventional dose rates. Reduction in clinical treatment times may translate to better target localization due to reduced opportunity for intrafraction motion. Linac-based CNS SRS can be completed in a normal time slot with a high output FFF linac. [ABSTRACT FROM AUTHOR]

Purpose: We report our initial clinical experience using IDENTIFY, an optical surface guidance system, for monitoring intra-fraction motion during stereotactic radiosurgery with HyperArcTM. Methods: The IDENTIFY system consists of three ceiling mounted camera pods. For each treatment fraction, a reference surface was captured after radiographic image guidance prior to the first beam on. Surface guidance and linear accelerator trajectory logs were used to evaluate the reported offsets relative to the reference surface. Offsets were obtained at the start of treatment (after radiographic image alignment), end of treatment, and during treatment at 4 gantry angles: 140 and 220 degrees, when all three camera pods had a clear view, and 50 and 310 degrees, when the view of one of the camera pods was obstructed. Results: 733 fractions of 295 treatment plans were evaluated. The average treatment time was 3.35 min (range 1.87 to 7.45 min). The mean (x,y,z) translation offset at the end of treatment was (0.00, -0.01, -0.07) mm, with median magnitude 0.27 mm and 95% of magnitudes were less than 0.97 mm. The mean reported offset during treatment was (-0.40, 0.23, -0.08), (-0.25, 0.02, -0.16), and (-0.16, 0.21, -0.05) mm at table angles 45, 90, and 315 degrees, respectively. The mean change in translation when a camera pod was blocked by the gantry was (0.01, 0.14, 0.02), (0.10, 0.23, 0.07), and (-0.02, 0.15, -0.00) mm at table angles 45, 90, and 315 degrees, respectively. The fraction of offsets exceeding 1 mm was 4.4%, 6.9%, and 2.5% when all three cameras had an unobstructed view, and was 13.4%, 1.9%, and 10.6% when a camera pod was blocked at table angles 45, 90, and 315 degrees, respectively. Conclusion: The IDENTIFY system can be used to monitor for intra-fraction motion with submillimeter accuracy; however, the accuracy is reduced when a camera is obstructed by the gantry and/or the couch is rotated. Action levels should be established accordingly to account for this behavior. [ABSTRACT FROM AUTHOR]