Frameless SRS Research Articles

SU‐E‐T‐336: Transition From Frame Based SRS to Frameless SRS Procedures: A Community Based Hospital Experience

Purpose:The project intent is to transit from frame based SRS to frameless SRS procedure and workflow in a community hospital setting.Methods:The clinic has strong experience in a wide verity of frame based SRS procedures using Novalis 600N linac equipped with BrainLab Micro‐MLC and ExacTrac systems. The addition of new Novalis TX system has opened an opportunity to explore and implement frameless SRS treatment procedure. Phantom End to End studies were performed for this purpose. A calibrated Ionization Chamber was inserted into the phantom and the phantom was positioned on the CT scan couch and SRS immobilization mask was made. A CT scan of the Immobilized phantom was generated with the localization box in place. The CT scan was localized and the ionization Chamber active volume was contoured as a target on the IPlan planning system. A 5 Dynamic Conformal Arcs plan was generated on the Iplan planning system. The Phantom was positioned on the treatment Couch with the SRS Mask and frameless array system in place, and the initial target treatment ISO position was achieved. The chamber was then connected to an electrometer. The two Orthogonal KV images were taken and the final treatment position was achieved within 1mm and 1 degree using ExacTrac. The Five arcs were delivered and the electrometer reading was noted. In addition, RPC SRS head phantom study was performed following RPC procedure.Results:The average absolute dose delivered to the in‐house phantom target was in agreement within 1.5% of the planning dose. The RPC SRS phantom results were within the criteria set by RPC.Conclusion:This work gave the confidence to our clinic to move from invasive frame based to non invasive and efficient frameless SRS and SRT treatments. The clinic has now frameless SRS and SRT treatment procedures and workflow.

WE-B-19A-01: SRT II: Uncertainties in SRT

SRS delivery has undergone major technical changes in the last decade, transitioning from predominantly frame-based treatment delivery to imageguided, frameless SRS. It is important for medical physicists working in SRS to understand the magnitude and sources of uncertainty involved in delivering SRS treatments for a multitude of technologies (Gamma Knife, CyberKnife, linac-based SRS and protons). Sources of SRS planning and delivery uncertainty include dose calculation, dose fusion, and intra- and inter-fraction motion. Dose calculations for small fields are particularly difficult because of the lack of electronic equilibrium and greater effect of inhomogeneities within and near the PTV. Going frameless introduces greater setup uncertainties that allows for potentially increased intra- and interfraction motion, The increased use of multiple imaging modalities to determine the tumor volume, necessitates (deformable) image and contour fusion, and the resulting uncertainties introduced in the image registration process further contribute to overall treatment planning uncertainties. Each of these uncertainties must be quantified and their impact on treatment delivery accuracy understood. If necessary, the uncertainties may then be accounted for during treatment planning either through techniques to make the uncertainty explicit, or by the appropriate addition of PTV margins. Further complicating matters, the statistics of 1-5 fraction SRS treatments differ from traditional margin recipes relying on Poisson statistics. In this session, we will discuss uncertainties introduced during each step of the SRS treatment planning and delivery process and present margin recipes to appropriately account for such uncertainties. Learning Objectives: 1. To understand the major contributors to the total delivery uncertainty in SRS for Gamma Knife, CyberKnife, and linac-based SRS. 2. Learn the various uncertainties introduced by image fusion, deformable image registration, and contouring variation. 3. Learn a variety of strategies for dealing with uncertainty, including margin recipes and explicit visualization of uncertainty. Understand how the assessment of PTV margins differs from regular fractionation (van Herk recipe) for 1–5 fraction deliveries.

SU‐E‐J‐18: A Rapid Joint Therapist‐Physicist QA Procedure for Sub‐Mm Verification of KV Imaging System‐Radiation Isocenter Alignment for Frameless SRS

Purpose: To assess the feasibility of a rapid and accurate joint therapist‐physicist QA procedure to verify the accuracy of the alignment kV imaging system with the MV radiation Isocenter prior to frameless SRS treatment. Methods: A Winston‐Lutz phantom was CT scanned with a slice spacing of .75 mm. The isocenter was placed to within 0.1 mm of the center of the sphere. A plan was created containing 4 static 2 × 2 cm MLC fields with corresponding MV portal images, and 4 kv setup fields both spaced at 90d gantry angles and CBCT setup field. Prior to the frameless SRS treatment, therapists place the WL phantom approximately at isocenter, then acquire all images planned. A physicist extracts the images from the R&V system and performs an automated image analysis to compute the systematic error between the radiation isocenter and the origin of the radiographic and CBCT systems. Spatial accuracy was assessed by placing a steel sphere on a micrometer stage and making known dispalcements.The therapists will do 3D‐3D match between acquired CBCT and planned CBCT and record the couch shifts. The planar MV and KV images are analyzed automatically offline to determine the systematic error between the kV imaging origin and the radiation isocenter before SRS treatment. Results: The localization uncertainty of the sphere, for both the KV and MV systems was ± 0.1mm. Uncertainty on the systematic error between the radiation isocenter and the imaging origin was found to be ±0.2 mm. The time required to verify the spatial imaging accuracy required about 15 minutes of therapist time and about 10 minutes of physicist time. Conclusion: The designed QA procedure can be performed easily by the therapists and physicists to speed up SRS room QA before patient treatments.

TH-A-137-06: Development of a 4DOF Robotic Frameless SRS System for Both Translational and Rotational Head Motion Cancellation

Purpose: Frameless techniques have not been able to match or achieve spatial target accuracies better than traditional frame based SRS. This is due to the fact that decreased invasiveness will lead to decreased accuracy. Here, relaxation of the immobilization parameter by replacement of the head ring (strong immobilization) with a more comfortable immobilization device (weak immobilization) will lead to expression of a patient's natural intrinsic small sub‐millimeter involuntary motions. Current frameless techniques do not offer a means for correcting this intrinsic motion. To address this issue, we propose a novel non‐immobilization SRS approach based on continuous cancellation of a patient's intrinsic head motion throughout the radiation delivery process. The work presents for the first time a system where both translational (XYZ) and rotational (pitch) head motion cancellation is performed. Methods: A 4D (xyz+pitch) robotic motion cancellation device was constructed using 4 stepper motors and a 4 axis motor controller (PCI‐7344, National Instruments). Real‐time 6D head motion was optically monitored using either IR markers (Polaris, NDI) or a 3D surface camera (AlignRT, VisionRT). The measured head motion was inputted into a Labview algorithm based on a proportional‐integrative‐derivative (PID) controller used to guide the motors for performing real‐time head motion cancellation. Results: The real‐time (15 fps) 6D head motion of a human volunteer was monitored with and without the use of robotic cancellation. For all axes where motion cancellation was applied (XYZ+pitch), the mean translational and rotational motion never exceeded 0.001mm and 0.01deg., respectively (essentially reduced to small fluctuations around zero). Without cancellation, this increased to 0.8mm and 0.4deg., respectively. Conclusion: It is found that substantial reductions in both translational and rotational head motion can be realized through real‐time robotic motion cancellation. This has potential to allow frameless SRS to be performed at accuracies equal or greater than frame based SRS.

SU‐E‐T‐417: Registration with and Without the Cranium When Using the ExacTrac System for Frameless Intracranial SRS

Purpose: To determine if the inclusion or exclusion of the cranium during image registration affects the delivery accuracy when using Brainlab Novalis ExacTrac (ExacTrac) for frameless intracranial SRS. Methods: Tests performed on a Varian TrueBeam™ STX linac with the 6D robotic couch ExacTrac system (0.6‐mm 3D mechanical isocenter accuracy established at acceptance). A head phantom with a 7‐mm dia. steel ball insert was the experimental subject. The hidden target was localized with the Brainlab H&N localizer. 1.5‐cm square fields were planned to treat the hidden target using the Brainlab planning system. The target was initially positioned at isocenter using the ExacTrac IR system. Then, stereoscopic kV images were acquired. The images were registered, using the bony anatomy, to the DRRs in two ways: with and without the cranium. Fifty‐one auto‐registration actions were performed to determine the residual registration uncertainty. Translational and rotational couch shifts were applied. Radiographic film captured the treatment field images at gantry angles of 0°, 90°, 180°, and 270°, and couch rotations of 0°, 45°, and 90°. The experiment was repeated with the cranium digitally removed from the registration using the ROI tool of the ExacTrac software. The treatment field hidden target images were analyzed using RIT113 software. Results: The residual uncertainties of translational and rotational shifts were ±0.13 mm and ±0.13°, respectively. The center of the hidden target was within 1.0 mm of radiation isocenter for all gantry angles and couch rotations (range 0.15 to 1.0 mm) whether the cranium was included or excluded from the registration. However, registration without the cranium produced small but significant improvements in accuracy of treatment delivery based on two‐tailed student t‐test analysis (p = 0.05). Conclusion: Removing the cranium with the ROI tool may help improve the treatment delivery accuracy when using the ExacTrac system for intracranial lesions.

The Role of Frameless Stereotactic Radiosurgery in the Management of Pediatric Central Nervous System Tumors

Recurrent malignant primary and metastatic central nervous system (CNS) tumors in pediatric patients are difficult to treat and impossible to cure, and despite significant advances in cancer care over several decades, efforts to improve local tumor control, prolong survival, and decrease morbidity for these patients have been disappointing. Irradiation to previously treated sites, especially in children, significantly raises the risk of detrimental toxicity. Thus, alternatives to current practices for recurrence and for metastatic disease to the brain must improve long-term local control, while limiting dose to normal tissue. It must also be accurate, safe, convenient, and tolerable by children. Frameless SRS represents a potentially beneficial and underused mode of treatment in children. We reviewed four cases of recurrent primary and metastatic brain tumors treated with optically-image-guided systems of frameless SRS between 2008 and 2011. We examined safety and feasibility, and analyzed dosimetric data, local tumor control, and adverse effects in this review. Treatment was well tolerated. No acute or delayed grade 3 or 4 toxicities were identified. We found no in-field failures for the duration of follow-up; however due to distant progression, three patients eventually required craniospinal irradiation or whole-brain radiation 6, 38, and 45 months after initial course of SRS. Frameless SRS represents a safe, effective option for local control of recurrent and metastatic CNS disease allowing the delay of whole brain or craniospinal irradiation and major toxicity risks associated with irradiation of more tissues. It is convenient, safe for patients, and allows for hypofractionation and substantial shortening of treatment course. Frameless SRS should be considered in management of patients with recurrent primary or metastatic brain tumors.

SU-E-T-409: A Comparison of 3D Optical Surface and IR Marker Tracking for Head Motion Compensated Frameless SRS.

Both infrared marker and 3D facial surface tracking were performed simultaneously on both a head phantom and healthy volunteers. Accuracies of the two 6D real-time tracking systems were quantitatively assessed both spatially and temporally. The suitability of 3D facial surface tracking for providing real-time head positioning to guide a robotic head motion compensation stage was investigated. A xyz stage was programmed to move a head phantom to approximately 200 points in 10×10×10 mm3 and 100×100×100 mm3 cubic volumes, respectively. Both 3D surface and IR marker tracking were performed simultaneously on the head phantom to allow direct comparison of spatial accuracy. Sinusoidal motion was performed along all three axis directions and recorded under the same clock cycle. Spatial and temporal accuracies on actual deformable facial surfaces were assessed by simultaneous 3D surface and IR marker tracking on human volunteers. 3D surface tracking both with and without the use of robotic head motion compensation was evaluated. For 10 samples at each position, the 10 mm cube had RMSE of 0.30 mm and 0.08 mm for the AlignRT and Polaris, respectively. Temporal lag of the AlignRT system was found to be approximately twice the fps time (t_{lag} ∼ 2/fps). The fps value was found to be strongly correlated with region-of-interest (ROI) surface size and could vary from 0.5 - 1.6 fps, leading to a lag time of 1 - 4 s. The Polaris output lagged behind the reference by 0.06 s in all cases. Both 3D surface and IR marker devices demonstrate high spatial accuracy for phantom based studies, however, a slight dimensional warping along the SI direction was observed to occur with the AlignRT. The current ROI dependent lag time of the AlignRT could also pose an issue for image guidance techniques requiring quick response times.

SU-E-J-157: Simulation and Design of a Real-Time 6D Head Motion Compensation Platform Based on a Stewart Platform Approach.

Real-time sub-millimeter head motion compensation during frameless SRS delivery has the potential to achieve the accuracy of frame-based SRS while being significantly less invasive. Previously, we demonstrated real-time 6D head motion monitoring using an optical camera, however, at the time we were limited to only 3D (x-y-z) of head motion correction due to mechanical restrictions of the head platform. In this work we investigate the feasibility of using a compact 6D robotic Stewart platform (hexapod) placed under the patient's head to perform both translational and rotational motion compensation in real-time. Benefits of a hexapod approach over a conventional serial kinematics stage include less flex, compactness, high force to weight ratio, and fast response times. A hexapod is a parallel robotics device consisting of two platforms connected by six linear actuators oriented at particular angles. To provide accurate motion in 6D, the desired position of the top platform (head) was ascertained using inverse kinematics. MATLAB was used to simulate the six actuator positions for performing motion along x-y-z-phi -theta-psi. Prior recorded 6D human volunteer head motion data was used as an input for simulation of motion compensation. Six Firgelli L12-P linearservo actuators, together with a PCI-7344 motion controller and Labview software, were used for initial construction of a hexapod prototype. The necessary actuator lengths over time were computed for this data, simulating the required 6D movement of the hexapod for motion correction. Simulations on previously collected volunteer data indicate a hexapod system is capable of responding to subject head motion with corrections of precise movements, and solutions to the linear system can be computed at near real-time speeds. Based on simulated results, it was successfully demonstrated that a hexapod device can compensate for small patient head motions along all six degrees of freedom.

SU-E-T-543: The Use of a Proportional-Integral-Derivative Design for Optimized Real-Time Head Motion Correction in Frameless SRS

Purpose: While less invasive, the immobilization devices used in frameless SRS techniques generally allow for more head movement than a frame, leading to a larger PTV. To maintain traditional SRS accuracies, we have developed a robotic 3D head motion stage to correct for these sub‐ millimeter head deviations occurring during treatment. To achieve optimal head motion management we report on our use of a proportional‐integral‐ derivative (PID) feedback loop to monitor and compensate for real‐time patient motion during frameless SRS. Methods: A PID controller consists of three corrective parameters: a proportional term to account for the error difference between the desired patient position and the actual position; an integral term that accelerates the position correction toward the desired setpoint value; and a derivative term that accounts for the instantaneous rate of change of the patient position error. These parameters were tuned according to manual and heuristic Zielger‐Nichols methods. The PID algorithm was integrated into existing Labview software designed to control patient motion via a computer‐controlled 3D stepper motor stage. For realtime head position feedback either a Polaris external marker tracker or VisionRT system was used. The PID correction was tested both on phantom and healthy volunteers. Results: With volunteers in a relaxed supine position, the PID method maintained head motion to under 0.25 mm as based on the optical monitoring system in all three directions for greater than 90% of the time. Artificial introduction of extreme 1–5 mm head shifts by manual couch moving were corrected 2–3 times faster by PID when compared to our prior constant motor velocity approach Conclusions: In the event of patient head deviations, a properly tuned PID controller can offer significant time sparing advantages. This would allow for quicker initial setup after coarse couch positioning and a higher duty cycle during treatment.

WE‐E‐BRB‐06: An EPID Based Method for High Accuracy Calibration of an Optical External Marker Tracking Device with the LINAC Reference Frame

Purpose: The accuracy of an optical camera system used for patient motion monitoring is typically limited to 1–2 mm due to use of the light field crosshair and in‐room lasers for initial camera calibration setup making it unsuitable for highly accurate patient monitoring procedures such as frameless SRS. To provide submillimeter accuracy in calibrating a 3D stereoscopic infrared optical camera with the treatment frame of a LINAC, a semi‐automated calibration method was developed based on simultaneous viewing of external markers with both the MV treatment beam and optical camera system. Methods: A phantom comprised of 9 steel BBs covered in highly IR reflective tape was placed on the LINAC treatment couch and imaged with both a 3D stereoscopic IR imager, and the on board MV EPID imager at a variety of gantry angles. Using the relative 3D BB positions given by the camera, simulated annealing was used to optimize 9 parameters including source‐to‐imager distance, detector tilt, and deviations from nominal isocenter. Singular value decomposition was then used to calculate the transformation between different reference frames. Additionally, a traditional method of calibration was used for comparison. Results: Using modeled data, the simulated annealing process was able to determine the locations of the BBs with .2 mm accuracy. Using projection images acquired with an MV imager, initial preliminary results show the ability to determine locations of BBs with a .2 mm planar standard deviation and a 1 mm axial standard deviation. Conclusions: The method can be used to provide a highly accurate spatial registration between an external 3D camera frame and the LINAC frame. The real MV imager results, while not as precise as the simulated results, exceed current standards and are expected to improve with continued refinement.

Development of Frameless SRS using Real-Time 6D Facial Surface Monitoring and Continuous Adaptive Head Motion Correction

Development of Frameless SRS using Real-Time 6D Facial Surface Monitoring and Continuous Adaptive Head Motion Correction

SU‐GG‐J‐23: Optical Guidance System vs. CBCT for Phantom and Patient Setup

Purpose: To quantify the discrepancy between Varian optical guidance (OG) frameless localization system and Varian Trilogy on board imaging (OBI) system for setting up phantom and SRS patient. Materials and Methods: Two different phantoms were used in this study. One is a custom‐made phantom; the other is Penta‐Guide phantom. The bite‐tray used for frameless SRS localization is fixed on both phantoms. After CT scan, images were exported to Pinnacle and FastPlan treatment planning systems, where the same isocenter was identified and then the images were exported to Mosaiq and OG system respectively. On the Varian Trilogy, OG was used to position phantom. Then kV‐kV, conebeam CT (CBCT) and portal imager were used to image the phantom and calculate the isocenter shift. The same method has been used for four SRS frameless patients to check the OG setup, two patients' shifts were recorded and one patient's planning CT and CBCT images were fused and analyzed. Results & Discussion: For both phantom studies, the shift performed by CBCT, kV‐kV and MV were all within 1 mm. However, for actual patient setup, the shifts were greater than 2mm between OG and CBCT for two patients. The impact of bite‐tray fixation was studied by changing the angle of bite‐tray slightly; significant shift up to several mm was observed by OBI system. The isocenter change with the angle has been calculated based on a real patient's geometry. Conclusions: The discrepancy between CBCT and OG for setting up phantoms is less than 1mm, but can be greater for setting up SRS patients. The bite‐tray repositioning in patient's mouth is the major factor to cause this discrepancy.

Frameless, Image-Guided Stereotactic Radiosurgery

To trace the evolution from frame-based to frameless image-guided SRS, to discuss the basic radiobiological principle of fractionation, current clinical trial data, and procedural components of the treatment plan. Nursing and medical literature, neurosurgical textbooks, and select internet sites. The CyberKnife (Accuray, Sunnyvale, CA) is the newest machine added to the technologic armamentarium of patient care. Its capacities are only beginning to be explored and the possibilities are limitless, giving hope to countless persons. Technologic advances have necessitated a diversification of nursing roles. Coordination of patient care services requires nurses to advance their knowledge of frameless, image-guided SRS.