Frameless SRS Research Articles

Post SRS Target Diffusion Metrics for Response Prognostication in Idiopathic Trigeminal Neuralgia

Diffusion tensor imaging (DTI) is used to measure the movement of water molecules in biological tissue, which permits a thorough characterization of white matter microstructure through quantitative diffusivity metrics. The most commonly used diffusivity metric, fractional anisotropy (FA), provides insight into the overall white matter microstructural properties, whereas radial diffusivity (RD), axial diffusivity (AD), and mean diffusivity (MD) suggest changes in myelination, axonal integrity & underlying neuro-inflammation and edema respectively. Study purpose is to find whether trigeminal nerve diffusivity metrics at a single, early time point after frameless SRS is predictive of long-term pain relief in patients with TN.Study cohort comprised of 44 pts with inclusion criteria were: 1) diagnosis of classic, type 1 TN 2) SRS treatment with no prior surgical procedures for TN; 3) 3 tesla MRI scans at 6 months posttreatment (range 5-7 months); and 4) at least 12 months of clinical follow-up. Patients with TN secondary to multiple sclerosis, cranial tumors, or vertebrobasilar dolichoectasia compressing the brainstem were excluded from the study. For each patient, diffusivity metrics (FA, MD, RD, and AD) were extracted from both the affected and unaffected trigeminal nerves at 6 months posttreatment 6 months diffusivity metrics were compared with long-term clinical outcome. Patients were identified as long-term responders if they achieved at least 75% reduction in preoperative pain for 12 months or longer following SRS. Average microstructural diffusivity differences (within group - pre and post treatment; across group - responder & non-responders) were analyzed using paired / Independent T test.Trigeminal nerve diffusivity at 6 months post-SRS was predictive of long-term clinical effectiveness, where long-term responders (n = 36) showed significantly lower fractional anisotropy at the radiosurgical target of their affected nerve compared to their contralateral, unaffected nerve and to nonresponders. Radial diffusivity and mean diffusivity, correlates of myelin alterations and inflammation, were also significantly higher in the affected nerve of long-term responders compared to their unaffected nerve. Nonresponders (n = 8) did not exhibit any characteristic diffusivity changes after SRS.DTI, FA specifically, appears to be a strong, objective measure to assess the effects of radiation on a nerve. Incorporating early DTI assessment can provide prognostic information that supplements clinical measures.

Post SRS Reversal of Masticator Muscles Atrophy in Idiopathic Trigeminal Neuralgia

The trigeminal nerve conducts both sensory and motor impulses through sensory root (50 fascicles) and motor root of wrisberg (about 6-7 fascicles). Separate superior and inferior motor roots typically emerge from the pons just antero-superomedial to the entry point of the sensory root. Motor nerve root of trigeminal nerve receives considerable radiation dose during SRS. Purpose of the study is to evaluate MRI the changes in the muscles of mastication before and after SRS in patients with idiopathic Trigeminal neuralgia MATERIALS/METHODS: Data analysis was limited to 23 patients over the age of 65 years who had undergone a first-time frameless SRS for the treatment of idiopathic TN with at least 1 year of follow-up between 2015 and 2019. Standardized collection of preoperative demographics, radiosurgical dosimetric data, and postoperative outcomes was performed. Detailed CISS / 3D TOF sequences were acquired to study the neurovascular conflict zones. In addition, MRI the thicknesses of the temporalis, pterygoid, and masseter muscles were measured on both pre- and post-SRS MRI in a blinded fashion. Changes in the muscles like fatty infiltration, MRI signal, or atrophy were noted. Logistic regression analysis was used to define the parameters like site of contact of offending vessels, number of motor roots involved, pain distribution zones and preop BNI score which mostly influenced masticator muscle atrophy.The median follow-up was 26 months (range 13- 60 months). On MR imaging while sensory roots were identified in 100% pts, in 13 pts (56%) single motor root, 8 pts (35%) had two motor roots, and remaining pts (9%) had three motor roots identified by neuroradiologist. Among the 23 patients eligible for inclusion in the study, 46 temporalis muscles, 46 medial pterygoid muscles, 46 lateral pterygoid muscles, and 46 masseter muscles were assessed. Pre-SRS atrophy of the masseter, medial pterygoid, lateral pterygoid, and temporalis muscles was seen in 7 (31%), 6 (26%), 4 (17%), and 6 (26%) patients, respectively. Logistic regression analysis showed that distribution of pain in the involvement of at least 2 motor nerve roots (P = 0.01, OR 5.34, 95% CI 1.8 - 19.12), superior cerebellar artery contact (P = 0.01, OR 5.43, 95% CI 1.46-20.12), V3 territory (P = 0.01, OR 5.27, 95% CI 1.21-20.67) and significant pain on chewing (P = 0.02, OR 5.32, 95% CI 1.25-22.48) were predictive of pre-SRS atrophy. Reversal of atrophy of these muscles occurred after SRS in a 22 of 23 patients post radiosurgery as evident in 1 year follow-up scan. The incidence of new-onset permanent post-SRS muscle atrophy correlated with one patient who had 3 roots and received marginal dose of 82gy.While a subset of patients with TN with significant pain on chewing have pre-SRS disuse atrophy of the muscles of mastication, a reversal of the atrophy occurs in a majority of the patients following SRS. New-onset motor neuropathy post-SRS was rare suggesting to fact that motor roots are very sturdy to surgical doses of radiation.

Hypofractionated Brain Radiosurgery With Hundreds of Isocenters

To report our initial clinical implementation of a hypofractionated brain radiosurgery (hSRS) approach via packing hundreds of isocenters for treatment of a large target with a substantial GTV expansion in order to reduce dose hot spots in eloquent brain.A high-density isocenter packing (HDIP) technique similar to "onion peeling" was developed for hSRS, where a large target volume of 14.1 cc was sequentially shrunk or "peeled" by a margin of 2 to 4-mm depending on its size. The isocenters were densely packed inside each layer or each "peel" of the target, especially within the GTV-to-PTV margin. Once packed, the isocenter location, shapes and weights of the beams were inversely optimized and manually adjusted to achieve an optimal dose distribution. This technique was clinically implemented on a frameless SRS unit for the purpose of expediting the workflow in treating brain lesions with GTV-to-PTV expansion of at least 5 mm.HDIP enabled the target dose to be prescribed to the 80-90% isodose line (normalized to the dose maximum), thus reducing dose hot spots greater than 120% of prescription dose inside the GTV expansion volume comprised mostly of normal brain tissue. Compared to the conventional approach of using only 1/3 to 1/4 as many isocenters, HDIP achieved identical dose coverage (> 98%), dose conformality (< = 2.8) and Paddick gradient indices (< = 3.5) for the PTV. An important caveat in implementing HDIP was that when globally increasing the total number of isocenters, the dose contributions from some isocenters decreased rapidly, rendering their beam-on time to fall below the machine limit of 0.01 min. To overcome such a problem, manual blocking of beamlets associated with the affected isocenters was implemented to effectively lower the dose rate, propelling the treatment time at these isocenters > 0.01 min to enable clinical implementation. This strategy negligibly impacted the total treatment time of 20-30 min of a typical HDIP treatment with a dose rate of 2.0 Gy/min of our LGKI unit.HDIP is technically feasible and successfully implemented for the first time at our institution, enabling us to expedite the hSRS workflow to care for patients with targets in eloquent brain requiring substantial GTV-to-CTV/PTV margin expansion.N. Cho: Employee; Thermo Fisher Scientific. D. Raleigh: None. B.P. Ziemer: None. T. Nano: None. P.V. Theodosopoulos: None. P.K. Sneed: None. L. Boreta: None. S.E. Braunstein: Advisory Board; Radiation Oncology Questions, LLC.L. Ma: Patent/License Fees/Copyright; University of California Regents.

Efficacy of Frameless Radiosurgical Entorhino-Amygdalohippocampectomy for Pharmacoresistant Medial Temporal Lobe Epilepsy

<h3>Purpose/Objective(s)</h3> Evaluate the efficacy and safety of Linac based surgery in the treatment of pharmaco-resistant epilepsies of mesial temporal lobe origin due to medial temporal sclerosis. <h3>Materials/Methods</h3> Retrospective single institution study comprised of 12 MTLE pts. (10 pts. with MT Sclerosis on Dominant side) who underwent SRS between August 2015 to Jan 2019. The epileptogenic substrate was localized using invasive video-EEG monitoring with foramen ovale electrodes, magnetic resonance imaging, metabolic (PET) imaging. All patients underwent detailed neuropsychological testing and the ESI-55 quality-of-life questionnaire. The radiosurgical target included Anterior part of parahippocampal region; entorhinal area adjacent to the collateral sulcus & rhinal sulcus; hippocampus - head & Anterior part and Amygdalofugal part of the amygdaloid complex. Non-coplanar Volumetrically modulated plans or non-isocentric CK plans using variable collimator were generated for prescribed mean dose of 22gy (range: 20-24 Gy), without any dose heterogeneity. Normal structures doses were within acceptable limits. Radiosurgical treatment was performed by using the C-Arm Linac with hexapod robotics (9 pts.) or frameless robotic radiosurgery system with skull tracking (3pts.). A minimum 2-year follow-up period included clinical, neuropsychological, ophthalmic (Visual field charting) & radiologic evaluations. <h3>Results</h3> Median age at diagnosis is 28 yrs (Range: 17-42yrs). At each 6-month follow-up evaluation, the frequency of seizures was significantly smaller than that at the previous visit. The median seizure frequency of 10.25 the month before treatment was reduced to 0.33 at 2 years after treatment. At 2 years, 67% of the patients (8 of 12) were seizure free - 4 Engel class 1A, 3 Engel class 1B, 1 Engel class 2A responses. The median latent interval from SRS to seizure cessation was 9.5 months (range 6–20 months). One patient was immediately seizure free. The median latency in aura cessation was 14 months (range 9–22 months). Morphological changes, edema, on magnetic resonance imaging were visible at 10 months (median) after SRS (range 7–22 months) in nearly all patients. During the onset period of these radiological changes, three patients experienced headache associated, in two cases, with nausea and vomiting. In these three patients the signs resolved immediately after short course of low doses of steroids. No cases of permanent neurological deficit (except three cases of monosymptomatic visual field deficit), or morbidity, or mortality were observed <h3>Conclusion</h3> Initial experience of Frameless SRS, as non-invasive technique, for MTLE provides benefits especially in young patients requiring high social skills with dominant lobe involvement in the traditional realms of safety with short- to middle-term efficacy & superior neuropsychological outcomes, similar to published series in GKS studies but with appreciated realms of patient acceptance, comfort and costs. Further long-term follow up is required.

Frameless Multisession Radiosurgery for Symptomatic Circumscribed Choroidal Hemangioma

<h3>Purpose/Objective(s)</h3> Choroidal hemangioma (CH) is a benign vascular tumor that induces exudative retinal detachment and consequent visual symptoms. Current standard treatments for CH include cryotherapy, diathermy, photocoagulation, PDT, transpupillary thermotherapy, and Radiation. SRS is preferred in the treatment of CH because of its characteristic stiff dose-fall-off and accuracy. Current retrospective study evaluated tumor volume reductions and improvements to visual acuity following multisession Radiosurgery in the treatment of symptomatic circumscribed choroidal hemangioma. <h3>Materials/Methods</h3> Study comprised of Thirteen patients with symptomatic circumscribed CHs diagnosed between August 2012 to December 2019 (8 Male & 5 female). 3 pts. had Sturge-Weber syndrome. All patients underwent frameless SRS following retrobulbar block, Thermoplastic immobilization & Imaging (CT & MRI). A total of 15-18gy / 3 frcs was planned using a 5mm collimator, prescribed to 90% isodose and treatment delivered on a frameless robotic radiosurgery system with skull tracking or C arm Linac equipped with hexapod robotics. Best corrected visual acuity, funduscopic findings, basal tumor diameter and tumor thickness were recorded at baseline and at follow-up visits every 6 -month intervals. <h3>Results</h3> The mean age of patients was 33 years (range: 12-61 years) and the mean duration of clinical or radiological follow-up was 33 months (range: 16-80 months). The mean volume, Basal diameter, tumor height & tumor thickness at baseline were 533.5 mm³ (range: 124-1150 mm³); 8 mm (range: 5-12mm, ± 2.3); 2.8mm (range, 2.0 - 4.5mm); tumor thickness was 5.1 (range: 3.0-11.0, ± 2.7) mm respectively. 6 tumors were subfoveolar, 4 were juxtafoveolar, and 3 were extrafoveolar. Exudative retinal detachment resolved within a median of 5.5 months (response rate, 100%; 95% CI, 48%-100%). Median best-corrected visual acuity was 20/50 (range, 20/22-20/90) at diagnosis and 20/25 (range, 20/20-20/60) 18 months after treatment. The visual acuity of the affected eye had improved at the latest follow-up examination (<i>P</i> = .018) in all patients. Tumor height had decreased a median of 32% (range, 10-40%) by 6 months and 41% (range, 17%-59%) by 20 months. No recurrence of exudative RD occurred. Thinning of the CHs was observed in most patients; however, symptomatic radiation toxicity had not developed in any of the patients. <h3>Conclusion</h3> Multisession radiosurgery could be an acceptable alternative treatment for symptomatic circumscribed CH with resolution of exudative RD being achieved within 5-6mo.

Intra-Fraction Motion Analysis for Frameless Linac-Based Stereotactic Radiosurgery with Three-Dimension (3D) Optical Surface Imaging for Intra-Fractional Motion Management

There has been a rapid increase in the use of frameless SRS in place of the frame-based SRS for treatment of brain metastases (BM). Compared to the frame-based SRS, intra-fraction motion management is a major challenge for the frameless SRS. With advanced immobilization and patient monitoring technologies, intra-fraction motion can be reduced. The purpose of this study is to analyze intra-fraction motion with LINAC-based SRS using a frameless radiosurgery platform that uses an integrated 3D optical surface monitoring system for intra-fraction motion management. This retrospective study includes 97 LINAC-SRS treatments delivered to 60 patients of BM with an integrated 3D surface imaging software for intra-fraction monitoring. Every treatment was delivered with a 6MV FFF beam at a dose-rate of 1400MU/min. Each treatment had at least 3 non-coplanar arcs with a minimum of 3 couch rotations. The patients were immobilized using an open-faced rigid SRS-thermoplastic mask with an opening encompassing the eyes, nose, and part of their forehead. Each patient was first aligned using a high-quality cone-beam CT (CBCT) with 1mm slice thickness and positioned using a 6 degrees of freedom couch. The patient’s face was monitored during treatment delivery through the surface monitoring system. During delivery, beam holds occurred if patient motion exceeded pre-defined tolerance (1mm translation/1° rotation) for > 5 seconds. An additional CBCT was performed following the delivering of two arcs to assess for any intra-fraction motion. Each CBCT was then registered to the planning CT using the automated intensity-based registration algorithm, followed by confirmation with a manual check. The intra-fraction shifts (RL, AP, and SI) and rotation (yaw, roll, and pitch) data were retrospectively recorded and analyzed. The total magnitude of the patient intra-fraction motion for all ninety-seven SRS treatments as recorded through shifts were 0.4 mm (mean) ± 0.3 mm (stv). Translational shifts were 0.0±0.3 mm in RL, 0.0±0.3 mm in AP, 0.0±0.3 mm in SI, while the rotational variations were 0.0±0.4° in yaw, 0.0±0.2° in roll, and 0.0±0.2° in pitch. In total, 93.8%(n=91/97) delivered treatments had intra-fraction motion that was ≤1mm and ≤1 degree. Minimal intra-fraction motion was observed during frameless-based LINAC-SRS treatments when using an integrated 3D surface image tracking; Only 6% percent of treatments experienced motion that was >1mm or >1 degree. A planning target margin of ≤1 mm may be considered adequate during the planning process to account for intra-fraction motion in the majority of all cases.

Evaluation of the Geometric and Dosimetric Accuracy of Synthetic Computed Tomography Images for Magnetic Resonance Imaging-only Stereotactic Radiosurgery.

IntroductionStereotactic radiosurgery (SRS) plans created using synthetic computed tomography (CT) images derived from magnetic resonance imaging (MRI) data may offer the advantage of inhomogeneity correction by convolution algorithms, as is done for CT-based plans. We sought to determine and validate the clinical significance and accuracy of synthetic CT images for inhomogeneity correction in MRI-only stereotactic radiosurgery plans for treatment of brain tumors.MethodsIn this retrospective study, data from two patients with brain metastases and one with meningioma who underwent imaging with multiple modalities and received frameless SRS treatment were analyzed. The SRS plans were generated using a convolution algorithm to account for brain inhomogeneity using CT and synthetic CT images and compared with the original clinical TMR10 plans created using MRI images.ResultsSynthetic CT-derived SRS plans are comparable with CT-based plans using convolution algorithm, and for some targets, based on location, they provided better coverage and a lower maximum dose.ConclusionsThe results suggest similar dose delivery results for CT and synthetic CT-based treatment plans. Synthetic CT plans offered a noticeable improvement in target dose coverage and a more gradual dose fall-off relative to TMR10 MRI-based plans. The major disadvantage is a slightly increased dose (by 0.37%) to nearby healthy tissue (brainstem) for synthetic CT-based plans relative to those created using clinical MRI images, which may be a problem for patients undergoing high-dose treatment.

EP-1964 Setup verification and Intrafraction motion monitoring with Optical Surface Imaging for frame-less SRS

EP-1964 Setup verification and Intrafraction motion monitoring with Optical Surface Imaging for frame-less SRS

Comparision of 3D Printed Headrest for Pediatric Patients and SRS Standard Headrest in Radiation Therapy

The SRS standard headrest provided by frameless SRS system was too big for pediatric patients. We applied 3D printing technology to produce a novel headrest for pediatric patients in radiotherapy.Our study aimed to compare the setup errors and transmittance between 3D printed headrest for pediatric patients and SRS standard headrest in radiotherapy. Thirty pediatric patients aged from 3 to 7 years old with cranial tumors were randomly selected from August 2016 to November 2017. These patients were divided into two groups equally. First group of patients was immobilized by 3D printed headrest with masks. Second group of patients were immobilized by SRS standard headrest with masks. The transmittance in both groups were recorded by dosimeter with cc13 detector. The setup errors in VRT, LAT and AP directions were recorded by IGRT device with 6D couch before radiotherapy. The average of transmittance was 98.891455% in group 1 and 98.51455% in group 2. The setup errors in VRT, LAT and AP directions were (0.040±0.055) mm, (0.040±0.055) mm and (0.10±0.07) mm in group 1, as compared with (0.160±0.055) mm, (0.08±0.11) mm and (0.10±0.10) mm in group 2. 3D printed headrest reduced setup errors in VRT direction (P<0.05) with no significant difference in the other two directions (P>0.05). Compared with SRS standard headrest, 3D printed headrest for pediatric patients showed higher transmittance and reduces the setup errors in VRT direction.

Intrafraction Verification Using ExacTrac 6D Imaging for Frameless Cranial Radiosurgery

Background: Radiosurgery has been an integral component in the management of intracranial tumors. It involves the administration of a high dose to the tumor. An essential aspect of this kind of treatment delivery is accurate localization of the tumor target that is precisely reproducible during treatment. To achieve this, the methods of positioning and immobilization of the patient play a critical role. Conventionally, the patient has been immobilized with invasive head frame and the location of internal anatomy deduced by external coordinates. This method has some drawbacks. - It works on the assumption that the external coordinates represent the isocentre location correctly. - It depends on the spatial relationships between the frame and the skull. Slippage and deformity of the frame, although rare, can cause an error in positioning. - From the patient's perspective, the placement of the frame is a source of discomfort. It also puts them at risk for bleeding and infection. Frameless SRS has come into use in recent times. This is a noninvasive method. However, giving up the use of an invasive head frame leads to increased intrafraction motion. To overcome this, we require constant verification of the patient's positioning at every step of the treatment. Aim: The objective of this study is to compare the precision and accuracy between Brainlab ExacTrac imaging and CBCT and evaluate the intrafraction motion of patient undergoing intracranial SRT. Methods: For this study we included 10 patients with intracranial tumors who are eligible to undergo SRS. A frameless stereotactic head frame mask was prepared to immobilize the head during SRS. During treatment the patients were immobilized with the same frame and the 6D positional accuracy was checked for every single field before the treatment execution by taking verification image using 6D ExacTrac and Onboard Imagers. The 6D ExacTrac tolerance were set to rotation &lt; 1.00 and translational shift &lt; 0.7 mm. We make sure of intrafarction movement by taking frequent ExacTrac images during the treatment of every 30 s. After the treatment the registration between the planning CT image and CBCT image taken before treatment were cross verified 6 dimensionally using offline review. The difference in shift between ExacTrac and CBCT are evaluated by means of RMS. Results: The RMS of the difference in the translational dimensions between the ExacTrac System and CBCT was &lt; 1.0 for online matching and &lt; 1.10 for offline matching. Furthermore, the RMS of the difference in the rotational dimensions between the ExacTrac System and CBCT were &lt; 0.81 for online matching and &lt; 0.96 for offline matching. Conclusion: It is concluded that the discrepancies in residual set-up error between the ExacTrac 6D x-ray and the CBCT were minor. ExacTrac 6D imaging provides additional advantage of patient intrafraction movement control by means of imaging during treatment.

Local control and overall survival after frameless radiosurgery: A single center experience

IntroductionStereotactic radiosurgery (SRS) has been increasingly advocated for 1–3 small brain metastases. The goal of this study was to evaluate the clinical results in patients with brain metastases treated with LINAC-based SRS using a thermoplastic mask (non-invasive fixation system) and Image-Guided Radiotherapy (IGRT). Material and MethodsIn this single-institution study 48 patients with 77 brain metastases were treated between February 2012 and January 2014. The prescribed dose was 20 Gy or 18 Gy as a single fraction. SRS was performed with a True Beam STX Novalis Radiosurgery LINAC (Varian Medical Systems). The verification of positioning was done using the BrainLAB ExacTrac ® X-ray 6D system and cone-beam CT. ResultsIn 69 of 77 treated brain metastases (90%) the follow-up was documented on MR imaging performed every 3 months. Mean follow-up time was 10.86 months. Estimated 1-year local control was 83%, using the Kaplan-Meier method. In 7/69 brain metastases (10%) local failure (LF) was diagnosed. Median progression free survival (PFS) was 3.73 months, largely due to distant brain relapse. A GTV of ≤2.0 cm3 was significantly associated with a better PFS than a GTV >2.0 cm3. Extracranial stable disease and GTV ≤2.5 cm³ were significant predictors of OS.We observed 2 cases of radiation necrosis diagnosed by histology after surgical resection. No other cases of severe side effects (CTACE ≥ 3) were observed. ConclusionLINAC-based frameless SRS with the BrainLAB Mask using the BrainLAB ExacTrac ® X-ray 6D system for patient positioning is well tolerated, safe and leads to favorable crude local control of 90%. In our experience, local control after frameless (ringless) SRS is as good as ring-based SRS reported in literature. Without invasive head fixation, radiotherapy is more comfortable for patients.

Frameless Volumetric Intracranial Stereotactic Radiosurgery with Non-coplanar Arcs: Clinical Experience, Accuracy, and Dosimetric Evaluation

In the last decades technological advances are increasing with the development of new treatment techniques and ways to deliver cranial radiosurgery(SRS), that is based into precision and accuracy, Volumetric Arc Therapy (VMAT) have been used in brain radio surgery in terms of non coplanar rotational arc beams with the aid of multi-leaf(MLC) to provide beam collimation. Using the traditional Leksell Stereotactic System, the time to do a treatment in average was around 6 hours; however the workflow using a frameless system decreases the mean treatment time in less than an hour, turning out to be a resources optimization for its realization, all this with the help of all accessories, quality assurance that should be made before deliver a proper treatment. The goal of this study is to show our clinical experience doing frameless SRS, with VMAT technique in the treatment of Arteriovenous Malformations(AVMs), Brain Metastases, Meningiomas, among others. The issues discussed in this study with regard to not only the clinical features, but also beam characteristics, and dosimetric features. 20 patients have been treated since May up to September 2016 with different diagnostics, doses given are related to each pathology treated, each prescription isodose was higher than 96%. Frameless system used was trUpoint ARCH™ SRS/SRT System, the Immobilization is accomplished utilizing multiple immobilization components - custom head support, bite tray, thermoplastic mask, and nasion cushion - allowing a truly custom hold from CIVCO, Patients were simulated in Computed tomography (CT) simulation General Electric(GE) Optima model, slides acquisition 1.25 mm; Magnetic Resonance was also done in a Siemens de 1.5T with 1 mm slices in contrast enhanced T1 MPR, T2 Flair, T2 Ciss, Diffusion, Perfusion, DTI Tractography, Angiography was done in GE Discovery; for image fusion for PTV and OAR contouring; calculation were done in Monaco® planning system version 5.10.02 with Monte Carlo algorithms; treatment delivery were make in a LINAC Elekta Infinity™ with Agility™ head with 160 interdigitating leaves with 0.5 cm width at isocenter; positioning verification XVI versión 4.5.1 b141. Treatment Delivery time 22 +/- 1.30 minutes for VMAT. Mean Monitor Units (MU) were 3795; For each patient IGRT is made with 3 XVI (Cone Beam CT), Pre and Post Treatment, what makes the procedure safe, confortable and accurate. With regard to CIRTOG a 0.9825 was obtained in average and QA was made using the OCTAVIUS® 1000 SRS with a Gamma Index 3mm/3% over 96%. Data found in this study with regard to the 20 patients treated; suggest that Frameless arch design provides a secure hold with no surgical attachment to the patient, displacements are less than 1 mm of patient movement permitted during intrafraction immobilization testing; also is safe, confortable and easy to use even allows us to rapid response for anxious patients with its release mechanism. Less treatment times and also excellent clinical outcomes.

Poster ‐ 44: Development and implementation of a comprehensive end‐to‐end testing methodology for linac‐based frameless SRS QA using a modified commercial stereotactic anthropomorphic phantom

Purpose:An end‐to‐end testing methodology was designed to evaluate the overall SRS treatment fidelity, incorporating all steps in the linac‐based frameless radiosurgery treatment delivery process. The study details our commissioning experience of the Steev (CIRS, Norfolk, VA) stereotactic anthropomorphic head phantom including modification, test design, and baseline measurements.Methods:Repeated MR and CT scans were performed with interchanging inserts. MR‐CT fusion accuracy was evaluated and the insert spatial coincidence was verified on CT. Five non‐coplanar arcs delivered a prescription dose to a 15 mm spherical CTV with 2 mm PTV margin.Following setup, CBCT‐based shifts were applied as per protocol. Sequential measurements were performed by interchanging inserts without disturbing the setup. Spatial and dosimetric accuracy was assessed by a combination of CBCT hidden target, radiochromic film, and ion chamber measurements. To facilitate film registration, the film insert was modified in‐house by etching marks.Results:MR fusion error and insert spatial coincidences were within 0.3 mm. Both CBCT and film measurements showed spatial displacements of 1.0 mm in similar directions. Both coronal and sagittal films reported 2.3 % higher target dose relative to the treatment plan. The corrected ion chamber measurement was similarly greater by 1.0 %. The 3 %/2 mm gamma pass rate was 99% for both filmsConclusions:A comprehensive end‐to‐end testing methodology was implemented for our SRS QA program. The Steev phantom enabled realistic evaluation of the entire treatment process. Overall spatial and dosimetric accuracy of the delivery were 1 mm and 3 % respectively.

SU‐F‐T‐562: Validation of EPID‐Based Dosimetry for FSRS Commissioning

Purpose:The prevailing approach to frameless SRS (fSRS) small field dosimetry is Gafchromic film. Though providing continuous information, its intrinsic uncertainties in fabrication, response, scan, and calibration often make film dosimetry subject to different interpretations. In this study, we explored the feasibility of using EPID portal dosimetry as a viable alternative to film for small field dosimetry.Methods:Plans prescribed a dose of 21 Gy were created on a flat solid water phantom with Eclipse V11 and iPlan for small static square fields (1.0 to 3.0 cm). In addition, two clinical test plans were computed by employing iPlan on a CIRS Kesler head phantom for target dimensions of 1.2cm and 2.0cm. Corresponding portal dosimetry plans were computed using the Eclipse TPS and delivered on a Varian TrueBeam machine. EBT‐XD film dosimetry was performed as a reference. The isocenter doses were measured using EPID, OSLD, stereotactic diode, and CC01 ion chamber.Results:EPID doses at the center of the square field were higher than Eclipse TPS predicted portal doses, with the mean difference being 2.42±0.65%. Doses measured by EBT‐XD film, OSLD, stereotactic diode, and CC01 ion chamber revealed smaller differences (except OSLDs), with mean differences being 0.36±3.11%, 4.12±4.13%, 1.7±2.76%, 1.45±2.37% for Eclipse and −1.36±0.85%, 2.38±4.2%, −0.03±0.50%, −0.27±0.78% for iPlan. The profiles measured by EPID and EBT‐XD film resembled TPS (Eclipse and iPlan) predicted ones within 3.0%. For the two clinical test plans, the EPID mean doses at the center of field were 2.66±0.68% and 2.33±0.32% higher than TPS predicted doses.Conclusion:We found that results obtained with EPID portal dosimetry were slightly higher (∼2%) than those obtained with EBT‐XD film, diode, and CC01 ion chamber with the exception of OSLDs, but well within IROC tolerance (5.0%). Therefore, EPID has the potential to become a viable real‐time alternative method to film dosimetry.

SU‐G‐JeP1‐04: Characterization of a High‐Definition Optical Patient Surface Tracking System Across Five Installations

Purpose:To assess the performance of Varian's real‐time, Optical Surface Monitoring System (OSMS) by measuring relative regular and irregular surface detection accuracy in 6 degrees of motion (6DoM), across multiple installations.Methods:Varian's Intracranial SRS Package includes OSMS, which utilizes 3 HD camera/projector pods to map a patient surface, track intra‐fraction motion, and gate the treatment beam if motion exceeds a threshold. To evaluate motion‐detection accuracy of OSMS, we recorded shifts of a cube‐shaped phantom on a single Varian TrueBeam linear accelerator as known displacements were performed incrementally across 6DoM. A subset of these measurements was repeated on identical OSMS installations. Phantom motion was driven using the TrueBeam treatment couch, and incremented across ±2cm in steps of 0.1mm, 1mm, and 1cm in the cardinal planes, and across ±40° in steps of 0.1°, 1°, and 5° in the rotational (couch kick) direction. Pitch and Roll were evaluated across ±2.5° in steps of 0.1° and 1°. We then repeated this procedure with a frameless SRS setup with a head phantom in a QFix Encompass mask.Results:Preliminary data show OSMS is capable of detecting regular‐surfaced phantom displacement within 0.03±0.04mm in the cardinal planes, and within 0.01±0.03° rotation across all planes for multiple installations. In a frameless SRS setup, OSMS is accurate to within 0.10±0.07mm and 0.04±0.07° across 6DoM. Additionally, a reproducible “thermal drift” was observed during the first 15min of monitoring each day, and characterized by recording displacement of a stationary phantom each minute for 25min. Drift settled after 15min to an average delta of 0.26±0.03mm and 0.38±0.03mm from the initial capture in the Y and Z directions, respectively.Conclusion:For both regular surfaces and clinical SRS situations, OSMS exceeds quoted detection accuracy. To reduce error, a warm‐up period should be employed to allow camera/projector pod thermal stabilization.

RTRB-06TIME BETWEEN DIAGNOSTIC MRI AND FRAME-FIXED STEREOTACTIC RADIOSURGERY PLANNING MRI IS ASSOCIATED WITH BRAIN METASTASIS GROWTH BUT NOT WORSE LOCAL CONTROL

RTRB-06. TIME BETWEEN DIAGNOSTIC MRI AND FRAME-FIXED STEREOTACTIC RADIOSURGERY PLANNING MRI IS ASSOCIATED WITH BRAIN METASTASIS GROWTH BUT NOT WORSE LOCAL CONTROL Michael Garcia, Sai Duriseti, Shannon Fogh, Jean Nakamura, David Larson, Philip Theodosopoulos, Michael McDermott, Penny Sneed, and Steve Braunstein; University of California, San Francisco, San Francisco, CA, USA BACKGROUND: Frame-fixed stereotactic radiosurgery (FFSRS) and frameless SRS precisely target brain metastases (BM). Seymour et al. recently showed patients treated with frameless SRS planned on MRI completed ≥ 14 days before treatment delivery have worse local control. An advantage of FFSRS is acquisition of planning MRI on day of SRS. We sought to describe BM growth (BMG) before SRS by comparing the preceding diagnostic MRI (dMRI) and FFSRS MRI scans. METHODS: Patients treated with FFSRS for BM at our institution from 2010 through 2013 were retrospectively reviewed. We identified 256 patients (1302 BM) without upfront surgical resection who had a dMRI for comparison of BM size on FFSRS MRI. Post-FFSRS local failure free progression (LFFP) and overall survival (OS) were compared by log-rank tests. RESULTS: Median imaging follow up was 10 months. Mean size [95%CI] of largest BM was 1.4cm [1.2-1.5]. BMG was observed on FFSRS MRI compared to dMRI for 30% patients. Mean time interval between dMRI and FFSRS was 25 days, and time interval was associated with BMG (Pearson’s r 1⁄4 0.14, p 1⁄4 0.02). The frequencies of BMG for time intervals between dMRI and FFSRS of ,7, 7-13, 14-20, 21-27, and ≥ 28 days were 11%, 12%, 32%, 36% and 38% respectively (p 1⁄4 0.02, chi square). Melanoma patients more commonly had BMG (p 1⁄4 0.03), but chemotherapy, neurologic symptoms, and dMRI slice thickness were not associated with BMG. Patients with BMG at FFSRS had similar LFFP compared to patients without growth (82% versus 88% at 6 months, p 1⁄4 0.71), and OS did notdiffer (p 1⁄4 0.47). CONCLUSION: Timebetween diagnosticMRI and SRS is associated with BMG, but LFFP is not compromised when patients receive FFSRS. Over a third of patients have clear BMG at SRS delivered 14 days afterdMRI.For framelessSRS, BMGafterplanningMRI may lead tomarginal miss. Neuro-Oncology 17:v195–v200, 2015. doi:10.1093/neuonc/nov231.6 Published by Oxford University Press on behalf of the Society for Neuro-Oncology 2015.

Heterogeneity correction for intensity-modulated frameless SRS in pituitary and cavernous sinus tumors: a retrospective study.

BackgroundFrameless immobilization allows for planning and quality assurance of intensity-modulated radiosurgery (IM-SRS) plans. We tested the hypothesis that IM-SRS planning with uniform tissue density corrections results in dose inaccuracy compared to heterogeneity-corrected algorithms.MethodsFifteen patients with tumors of the pituitary or cavernous sinus underwent frameless IM-SRS. Treatment planning CT and MRI scans were obtained and fused to delineate the tumor, optic nerves, chiasm, and brainstem. The plan was developed with static gantry IM-SRS fields using a pencil beam (PB), analytical anisotropic (AAA), and Acuros XB (AXB) algorithms. We evaluated measures of target coverage as well as doses to organs at risk (OAR) for each algorithm. We compared the results of each algorithm in the cases where PTV overlapped OAR (n = 10) to cases without overlapping OAR with PTV (n = 5). Utilizing film dosimetry, we measured the dose distribution for each algorithm through a uniform density target to a rando phantom with non-uniform density of air, tissue, and bone.ResultsThere was no difference in target coverage measured by DMaxPTV, DMinPTV, D95%PTV, or the isodose surface (IDS) covering 95 % of the PTV regardless of algorithm. However, there were differences in dose to OAR. PB predicted higher (p < 0.05) Dmax for the brainstem, chiasm, right optic nerve, and left optic nerve. In cases of PTV overlapping an optic nerve (n = 7), PB was unable to limit dose to 8Gy while achieving PTV coverage (PB 855 cGy vs. AAA 769 cGy, p = 0.05 vs. AXB 658 cGy, p = 0.03). Within the rando phantom, the PB and AAA algorithms over-estimated the dose delivered in the bone-tissue-air interface of the sinus (+17 %), while the AXB algorithm closely predicted the actual dose delivered through the inhomogeneous tissue (+/- 1 % max, p < 0.05).ConclusionsPatients undergoing frameless SRS benefit from heterogeneity corrected dose plans when the lesion lies in areas of widely varying tissue density and near critical normal structures such as the skull base. Film dosimetry confirms that the AXB dose calculation algorithm more accurately predicts actual dose delivered though tissues of varying densities than PB or AAA dose calculation algorithms.

SU‐E‐J‐163: A Biomechanical Lung Model for Respiratory Motion Study

Purpose:This work presents a biomechanical model to investigate the complex respiratory motion for the lung tumor tracking in radiosurgery by computer simulation.Methods:The models include networked massspring‐dampers to describe the tumor motion, different types of surrogate signals, and the force generated by the diaphragm. Each mass‐springdamper has the same mechanical structure and each model can have different numbers of mass‐spring‐dampers. Both linear and nonlinear stiffness parameters were considered, and the damping ratio was tuned in a range so that the tumor motion was over‐damped (no natural tumor oscillation occurs without force from the diaphragm). The simulation was run by using ODE45 (ordinary differential equations by Runge‐Kutta method) in MATLAB, and all time courses of motions and inputs (force) were generated and compared.Results:The curvature of the motion time courses around their peaks was sensitive to the damping ratio. Therefore, the damping ratio can be determined based on the clinical data of a high sampling rate. The peak values of different signals and the time the peaks occurred were compared, and it was found that the diaphragm force had a time lead over the tumor motion, and the lead time (0.1–0.4 seconds) depended on the distance between the tumor and the diaphragm.Conclusion:We reported a model based analysis approach for the spatial and temporal relation between the motion of the lung tumor and the surrogate signals. Due to the phase lead of the diaphragm in comparing with the lung tumor motion, the measurement of diaphragm motion (or its electromyography signal) can be used as a beam gating signal in radiosurgery, and it can also be an additional surrogate signal for better tumor motion tracking.The research is funded by the American Cancer Society (ACS) grant. The grant name is: Frameless SRS Based on Robotic Head Motion Cancellation. The grant number is: RSG‐13‐313‐01‐CCE

A multicenter retrospective study of frameless robotic radiosurgery for intracranial arteriovenous malformation.

Introduction: CT-guided, frameless radiosurgery is an alternative treatment to traditional catheter-angiography targeted, frame-based methods for intracranial arteriovenous malformations (AVMs). Despite the widespread use of frameless radiosurgery for treating intracranial tumors, its use for treating AVM is not-well described.Methods: Patients who completed a course of single fraction radiosurgery at The University of North Carolina or Georgetown University between 4/1/2005–4/1/2011 with single fraction radiosurgery and received at least one follow-up imaging study were included. All patients received pre-treatment planning with CTA ± MRA and were treated on the CyberKnife (Accuray) radiosurgery system. Patients were evaluated for changes in clinical symptoms and radiographic changes evaluated with MRI/MRA and catheter-angiography.Results: Twenty-six patients, 15 male and 11 female, were included in the present study at a median age of 41 years old. The Spetzler-Martin grades of the AVMs included seven Grade I, 12 Grade II, six Grade III, and one Grade IV with 14 (54%) of the patients having a pre-treatment hemorrhage. Median AVM nidal volume was 1.62 cm3 (0.57–8.26 cm3) and was treated with a median dose of 1900 cGy to the 80% isodose line. At median follow-up of 25 months, 15 patients had a complete closure of their AVM, 6 patients had a partial closure, and 5 patients were stable. Time since treatment was a significant predictor of response, with patients experience complete closure having on average 11 months more follow-up than patients with partial or no closure (p = 0.03). One patient experienced a post-treatment hemorrhage at 22 months.Conclusion: Frameless radiosurgery can be targeted with non-invasive MRI/MRA and CTA imaging. Despite the difficulty of treating AVM without catheter angiography, early results with frameless, CT-guided radiosurgery suggest that it can achieve similar results to frame-based methods at these time points.

Alignment Accuracy and Intrafraction Motion Analysis in Frameless Functional Image Guided Radiosurgery

Alignment Accuracy and Intrafraction Motion Analysis in Frameless Functional Image Guided Radiosurgery