Isocenter Accuracy Research Articles

Characterization of Ethos therapy systems for adaptive radiation therapy: A multi-machine comparison.

The recently released Ethos therapy system (Varian Medical Systems) allows for online CBCT-guided adaptive radiation therapy (RT). The clinical introduction of multiple systems requires machine characterization and machine variation quantification to allow patient interchangeability between systems. Despite several clinical introductions, limited vendor-independent information on machine performance is available. Our aim was to determine the relevant dosimetric and mechanical characteristics of individual machines and to quantify machine variations. Six Ethos treatment machines, equipped with a 6-MV FFF beam including dual-layer MLC and kV-CBCT system, were recently introduced clinically after extensive machine characterization and pre-configured beam model verification. Point doses and profiles were measured and compared to vendor-provided reference data and dose calculations. Also, dose calculations were verified based on point measurements for non-standard fields and dose distributions for optimized treatment plans. Agreements between dose profiles (dose distributions) were quantified using 1D (3D) γ-analysis. Additionally, we quantified leaf transmission, dosimetric leaf gap (DLG) and couch attenuation, determined isocenter accuracy and kV-MV isocenter coincidence and verified the kV-CBCT system. Machine variations were quantified for all dosimetric and mechanical characteristics. For all machines, distinct agreements were found between measurements and vendor-provided reference data as well as measurements and dose calculations. Mean γ1%/1mm values for all profiles were below 0.30. All profiles, point measurements and dose distributions matched well among the six machines. Minimal machine variations were found in terms of DLG (0.05mm), leaf transmission (0.001%), isocenter accuracy (0.08mm), kV-MV isocenter coincidence (0.15mm), couch attenuation (0.69%), and CBCT imaging dose (0.29mGy). This study demonstrates excellent agreement between individual Ethos therapy systems and vendor-provided reference data as well as a pre-configured beam model. Furthermore, our results show good consistency among all machines and provide valuable insights on relevant machine characteristics. The systematically obtained results provide benchmark data for future clinical introduction of Ethos therapy systems.

AlignRT®, Catalyst™ and RPM™ in locoregional radiotherapy of breast cancer with DIBH. Is IGRT still needed?

In locoregional radiotherapy of breast cancer with deep inspiration breath hold (DIBH), setup accuracy may depend on hospital protocol. At present, comparison between different positioning devices is challenging due to differing hospital protocols. The aim of this study was to evaluate the setup accuracy obtained with surface-guided radiation therapy (SGRT; AlignRT®, Catalyst™) or with lasers and real-time position management (RPM™) in DIBH. A total of 1692 image pairs were analyzed in three groups: positioning using AlignRT® surface guidance system (Group A, n = 45), Catalyst™ (Group C, n = 50) and conventional lasers and tattoos (Group L, n = 46). We evaluated residual errors for the bony chest wall, th1 and humeral head in kV images with laser- or SGRT-based setup with and without daily image-guided radiation therapy (IGRT). Less isocenter variance was found in Group A than in Group L or C (p ≤ 0.05) and in Group C than in L (p = 0.02-0.6). With SGRT only, the smallest random rotation error was found in Group A (p = 0.01). With daily IGRT, only a small difference was found for residual errors between the groups. Setup with SGRT improves the isocenter reproducibility compared to lasers and RPM™. Only small differences were found in setup accuracy between the SGRT devices. Due to improved isocenter accuracy, daily orthogonal IGRT is suggested in all the groups.

Optimized Camera-Based Patient Positioning in CT: Impact on Radiation Exposure.

The aim of this study was to evaluate whether a 3-dimensional (3D) camera can outperform highly trained technicians in precision of patient positioning and whether this transforms into a reduction in patient exposure. In a single-center study, 3118 patients underwent computer tomography (CT) scans of the chest and/or abdomen on a latest generation single-source CT scanner supported with an automated patient positioning system by 3D camera. One thousand five hundred fifty-seven patients were positioned laser-guided by a highly trained radiographer (camera off) and 1561 patients with 3D camera (camera on) guidance. Radiation parameters such as effective dose, organ doses, CT dose index, and dose length product were analyzed and compared. Isocenter accuracy and table height were evaluated between the 2 groups. Isocenter positioning was significantly improved with the 3D camera ( P < 0.001) as compared with visual laser-guided positioning. Absolute table height differed significantly ( P < 0.001), being higher with camera positioning (165.6 ± 16.2 mm) as compared with laser-guided positioning (170.0 ± 20.4 mm). Radiation exposure decreased using the 3D camera as indicated by dose length product (321.1 ± 266.6 mGy·cm; camera off: 342.0 ± 280.7 mGy·cm; P = 0.033), effective dose (3.3 ± 2.7 mSv; camera off: 3.5 ± 2.9; P = 0.053), and CT dose index (6.4 ± 4.3 mGy; camera off: 6.8 ± 4.6 mGy; P = 0.011). Exposure of radiation-sensitive organs such as colon ( P = 0.015) and red bone marrow ( P = 0.049) were also lower using the camera. The introduction of a 3D camera improves patient positioning in the isocenter of the scanner, which results in a lower and also better balanced dose reduction for the patients.

RSC: Gel dosimetry as a tool for clinical implementation of image-guided radiotherapy

The implementation of new image-guided radiotherapy (IGRT) treatment techniques requires the development of new quality assurance (QA) methods including geometric and dosimetric validation of the applied dose in 3D. Polymer gels (PG) provide a promising tool to perform such tests. However, to be used in a large variety of clinical applications, the PG must be flexibly applicable. In this work, we present a variety of phantoms used in clinical routine to perform both hardware and workflow tests in IGRT. This includes the validation of isocenter accuracy in magnetic resonance (MR)-guided RT (MRgRT) and end-to-end tests of online adaptive treatment techniques for inter- and intra-fraction motion management in IGRT. The phantoms are equipped with one or more PG containers of different materials including 3D printed containers to allow for 3D dosimetry in arbitrarily shaped structures. The proposed measurement techniques and phantoms provide a flexible application and show a clear benefit of PG for 3D dosimetry in combination with end-to-end tests in many clinical QA applications.

Radiation dose reduction, improved isocenter accuracy and CT scan time savings with automatic patient positioning by a 3D camera

PurposeTo compare CT isocenter accuracy, patient dose, and scan time in adults imaged with and without use of a 3D camera. Method571 CT examinations utilizing a 3D camera for initial patient positioning (optional radiographer isocenter adjustment) and 504 examinations scanned without the camera between 10/1/2018 and 3/19/2019 were retrospectively identified. All exams were chest or abdominopelvic CTs. The isocenters of these exams were compared with the true isocenters defined as the manually delineated centroid of the body in the CT volume. The size specific dose estimate (SSDE) (mGy) of radiation dose obtained from departmental software for the 4 most common protocols on one CT scanner was compared before and after implementation of the 3D camera. The times required for the entire scan and just the topogram “scout” were compared with and without the 3D camera for noncontrast chest and abdominopelvic CT enterography protocols. 2-tailed t-tests and Mann-Whitney U tests were used (P < 0.05 indicated statistical significance). ResultsThe deviation from true isocenter was 6.8 ± 6.1 mm (P = 0.043) and 16.3 ± 14.0 mm (P < 0.01) with and without the 3D camera, respectively (P < 0.01). CT radiographers accepted isocenter location without alteration in 93 % of examinations. Average SSDE savings with the 3D camera ranged 1.0–2.4 mGy (21–31 %) for the 4 most commonly performed protocols (p < 0.01). Median scout time savings was 32 s (Camera vs. No-Camera cohorts) (P < 0.01). Average noncontrast chest CT and CT enterography scan time savings were 19 s and 17 s with the 3D camera, respectively (P < 0.01). ConclusionsThe 3D camera improved accuracy of patient positioning while reducing radiation dose and examination time. Implementation of a 3D camera helps standardize workflow in a busy clinical practice.

Characterization and longitudinal assessment of daily quality assurance for an MR-guided radiotherapy (MRgRT) linac.

PurposeTo describe and characterize daily machine quality assurance (QA) for an MR‐guided radiotherapy (MRgRT) linac system, in addition to reporting a longitudinal assessment of the dosimetric and mechanical stability over a 7‐month period of clinical operation.MethodsQuality assurance procedures were developed to evaluate MR imaging/radiation isocenter, imaging and patient handling system, and linear accelerator stability. A longitudinal assessment was characterized for safety interlocks, laser and imaging isocenter coincidence, imaging and radiation (RT) isocentricity, radiation dose rate and output, couch motion, and MLC positioning. A cylindrical water phantom and an MR‐compatible A1SL detector were utilized. MR and RT isocentricity and MLC positional accuracy was quantified through dose measured with a 0.40 cm2 x 0.83 cm2 field at each cardinal angle. The relationship between detector response to MR/RT isocentricity and MLC positioning was established through introducing known errors in phantom position.ResultsCorrelation was found between detector response and introduced positional error (N = 27) with coefficients of determination of 0.9996 (IEC‐X), 0.9967 (IEC‐Y), 0.9968 (IEC‐Z) in each respective shift direction. The relationship between dose (DoseMR/RT+MLC) and the vector magnitude of MLC and MR/RT positional error (Errormag) was calculated to be a nonlinear response and resembled a quadratic function: DoseMR/RT+MLC[%] = −0.0253 Errormag [mm]2 − 0.0195 Errormag [mm]. For the temporal assessment (N = 7 months), safety interlocks were functional. Laser coincidence to MR was within ±2.0 mm (99.6%) and ±1.0 mm (86.8%) over the 7‐month assessment. IGRT position–reposition shifts were within ±2.0 mm (99.4%) and ±1.0 mm (92.4%). Output was within ±3% (99.4%). Mean MLC and MR/RT isocenter accuracy was 1.6 mm, averaged across cardinal angles for the 7‐month period.ConclusionsThe linac and IGRT accuracy of an MR‐guided radiotherapy system has been validated and monitored over seven months for daily QA. Longitudinal assessment demonstrated a drift in dose rate, but temporal assessment of output, MLC position, and isocentricity has been stable.

Measurement of isocenter alignment accuracy and image distortion of an 0.35 T MR-Linac system

For hybrid devices combining magnetic resonance (MR) imaging and a linac for radiation treatment, the isocenter accuracy as well as image distortions have to be checked. This study presents a new phantom to investigate MR-Linacs in a single measurement in terms of (i) isocentricity of the irradiation and (ii) alignment of the irradiation and imaging isocenter relative to each other using polymer dosimetry gel as well as (iii) 3-dimensional (3D) geometric MR image distortions. The evaluation of the irradiated gel was performed immediately after irradiation with the imaging component of the 0.35 T MR-Linac using a T2-weighted turbo spin-echo sequence. Eight plastic grid sheets within the phantom allow for measurement of geometric distortions in 3D by comparing the positions of the grid intersections (control points) within the MR-image with their nominal position obtained from a CT-scan. The distance of irradiation and imaging isocenter in 3D was found to be (0.8 ± 0.9) mm for measurements with 32 image acquisitions. The mean distortion over the whole phantom was (0.60 ± 0.28) mm and 99.8% of the evaluated control points had distortions below 1.5 mm. These geometrical uncertainties have to be considered by additional safety margins.

Feasibility of virtual starshot analysis providing submillimeter radiation isocenter accuracy: A long-term multi-institutional analysis.

PurposeWe developed a technique to calculate the offset between room lasers and the radiation isocenter using a digital Winston–Lutz (WL) test with a starshot technique. We have performed isocenter localization quality assurance (QA) with submillimeter accuracy for a long period. Here we evaluated the feasibility and accuracy of this virtual starshot (VS) analysis for isocenter localization QA.MethodsA 6‐MV photon beam with a square multileaf collimator field was used to irradiate a WL sphere positioned at the intersection of the room lasers. Images were acquired using an electronic portal imaging device. A four‐field WL test was performed, and the path of each beam was calculated from the offset between the beam and sphere. Virtual starshot analysis was used to analyze the radiation isocenter, which calculates the center of the beam paths by using a least‐squares method, similar to the starshot analysis. Then, eight coplanar and 12 noncoplanar beams were irradiated to evaluate isocenter localization accuracy.ResultsSeveral VS analyses, using different WL spheres, were performed at three institutions, and the calculated accuracies were within 0.1 mm at all institutions. Long‐term analysis showed that the isocenter localization accuracy was appropriately managed with three‐dimensional accuracy within ± 0.5 mm for 90 months after the first laser adjustments. The offset between each beam and the room laser was within 0.6 mm and within 1.0 mm for eight coplanar and 12 noncoplanar beams, respectively, for 90 months. Cone‐beam computed tomography images, acquired after verification beams, showed that the offset between the radiation isocenter and the imaging center was within 0.66 mm for 90 months. The isocenter localization accuracy within 1 mm was kept for long period at other four institutions.ConclusionsLong‐term analysis showed the feasibility of VS analysis for isocenter localization QA, including room laser re‐alignment, noncoplanar irradiation verification, and image guidance accuracy.

Polymer gel-based measurements of the isocenter accuracy in an MR-LINAC

As magnetic resonance-guided radiotherapy (MRgRT) is becoming increasingly important in clinical applications, the development of new quality assurance (QA) methods is needed. One important aspect is the alignment of the radiation and imaging isocenter. MR-visible polymer gels offer a way to perform such measurements online and additionally may allow for 3-dimensional (3D) evaluation. We present a star shot measurement irradiated and scanned with a 0.35 T MR-LINAC device evaluating the polyacrylamide gelatin (PAGAT) gel dosimeter immediately and 48 h after irradiation. The gel was additionally scanned at a 3 T MR device 5 h and 52 h after irradiation. The evaluation revealed an isocircle radius of 0.5 mm for both imaging devices and all image resolutions and time points after irradiation. The distance between radiation and imaging isocenter varied between 0.25 mm and 1.30 mm depending on the applied image resolution. This demonstrates that evaluation of a star shot measurement in a 0.35 T MR-LINAC is feasible, even immediately after irradiation.

SP-0474 Linac isocentric accuracy and its influence on treatment margins

SP-0474 Linac isocentric accuracy and its influence on treatment margins

Development and long-term stability of a comprehensive daily QA program for a modern pencil beam scanning (PBS) proton therapy delivery system.

PurposeThe main purpose of this study is to demonstrate the clinical implementation of a comprehensive pencil beam scanning (PBS) daily quality assurance (QA) program involving a number of novel QA devices including the Sphinx/Lynx/parallel‐plate (PPC05) ion chamber and HexaCheck/multiple imaging modality isocentricity (MIMI) imaging phantoms. Additionally, the study highlights the importance of testing the connectivity among oncology information system (OIS), beam delivery/imaging systems, and patient position system at a proton center with multi‐vendor equipment and software.MethodsFor dosimetry, a daily QA plan with spot map of four different energies (106, 145, 172, and 221 MeV) is delivered on the delivery system through the OIS. The delivery assesses the dose output, field homogeneity, beam coincidence, beam energy, width, distal‐fall‐off (DFO), and spot characteristics — for example, position, size, and skewness. As a part of mechanical and imaging QA, a treatment plan with the MIMI phantom serving as the patient is transferred from OIS to imaging system. The HexaCheck/MIMI phantoms are used to assess daily laser accuracy, imaging isocenter accuracy, image registration accuracy, and six‐dimensional (6D) positional correction accuracy for the kV imaging system and robotic couch.ResultsThe daily QA results presented herein are based on 202 daily sets of measurements over a period of 10 months. Total time to perform daily QA tasks at our center is under 30 min. The relative difference (Δrel) of daily measurements with respect to baseline was within ± 1% for field homogeneity, ±0.5 mm for range, width and DFO, ±1 mm for spots positions, ±10% for in‐air spot sigma, ±0.5 spot skewness, and ±1 mm for beam coincidence (except 1 case: Δrel = 1.3 mm). The average Δrel in dose output was −0.2% (range: −1.1% to 1.5%). For 6D IGRT QA, the average absolute difference (Δabs) was ≤0.6 ± 0.4 mm for translational and ≤0.5° for rotational shifts.ConclusionThe use of novel QA devices such as the Sphinx in conjunction with the Lynx, PPC05 ion chamber, HexaCheck/MIMI phantoms, and myQA software was shown to provide a comprehensive and efficient method for performing daily QA of a number of system parameters for a modern proton PBS‐dedicated treatment delivery unit.

Assessment of the accuracy and stability of frameless gamma knife radiosurgery.

The aim of this study was to assess the accuracy and stability of frameless gamma knife radiosurgery (GKRS). The accuracies of the radiation isocenter and patient couch movement were evaluated by film dosimetry with a half‐year cycle. Radiation isocenter assessment with a diode detector and cone‐beam computed tomography (CBCT) image accuracy tests were performed daily with a vendor‐provided tool for one and a half years after installation. CBCT image quality was examined twice a month with a phantom. The accuracy of image coregistration using CBCT images was studied using magnetic resonance (MR) and computed tomography (CT) images of another phantom. The overall positional accuracy was measured in whole procedure tests using film dosimetry with an anthropomorphic phantom. The positional errors of the radiation isocenter at the center and at an extreme position were both less than 0.1 mm. The three‐dimensional deviation of the CBCT coordinate system was stable for one and a half years (mean 0.04 ± 0.02 mm). Image coregistration revealed a difference of 0.2 ± 0.1 mm between CT and CBCT images and a deviation of 0.4 ± 0.2 mm between MR and CBCT images. The whole procedure test of the positional accuracy of the mask‐based irradiation revealed an accuracy of 0.5 ± 0.6 mm. The radiation isocenter accuracy, patient couch movement accuracy, and Gamma Knife Icon CBCT accuracy were all approximately 0.1 mm and were stable for one and a half years. The coordinate system assigned to MR images through coregistration was more accurate than the system defined by fiducial markers. Possible patient motion during irradiation should be considered when evaluating the overall accuracy of frameless GKRS.

Feasibility of polymer gel-based measurements of radiation isocenter accuracy in magnetic fields

For conventional irradiation devices, the radiation isocenter accuracy is determined by star shot measurements on films. In magnetic resonance (MR)-guided radiotherapy devices, the results of this test may be altered by the magnetic field and the need to align the radiation and imaging isocenter may require a modification of measurement procedures. Polymer dosimetry gels (PG) may offer a way to perform both, the radiation and imaging isocenter test, however, first it has to be shown that PG reveal results comparable to the conventionally applied films. Therefore, star shot measurements were performed at a linear accelerator using PG as well as radiochromic films. PG were evaluated using MR imaging and the isocircle radius and the distance between the isocircle center and the room isocenter were determined. Two different types of experiments were performed: i) a standard star-shot isocenter test and (ii) a star shot, where the detectors were placed between the pole shoes of an experimental electro magnet operated either at 0 T or 1 T. For the standard star shot, PG evaluation was independent of the time delay after irradiation (1 h, 24 h, 48 h and 216 h) and the results were comparable to those of film measurements. Within the electro magnet, the isocircle radius increased from 0.39 ± 0.01 mm to 1.37 ± 0.01 mm for the film and from 0.44 ± 0.02 mm to 0.97 ± 0.02 mm for the PG-measurements, respectively. The isocenter distance was essentially dependent on the alignment of the magnet to the isocenter and was between 0.12 ± 0.02 mm and 0.82 ± 0.02 mm. The study demonstrates that evaluation of the PG directly after irradiation is feasible, if only geometrical parameters are of interest. This allows using PG for star shot measurements to evaluate the radiation isocenter accuracy with comparable accuracy as with radiochromic films.

EP-2171: Feasibility study for polymer gel-based radiation isocenter accuracy test of MR-Linac-devices

EP-2171: Feasibility study for polymer gel-based radiation isocenter accuracy test of MR-Linac-devices

A preliminary investigation on long-term consistency of MPC as a quick daily QA application

Purpose: The purpose of this study was to establish Machine performance check (MPC) application as a comprehensive daily QA program in a clinical setting for a True Beam 2.0 system and investigate the first ten months (195 days) daily QA data generated by the MPC. Methods: An automated daily quality assurance (QA) application named machine performance check (MPC) was recently launched by Varian Medical Systems with their TrueBeam 2.0 linear accelerator (linac) system. MPC performs all the essential machine tests such as Beam Constancy Check, and Geometry Check with the use of an IsoCal phantom. There is no systematic published study on long-term consistency and validation of MPC in a clinical set-up for its acceptance as an alternative QA application. In the present study, we collected data with the MPC for over ten months (195 days) on a TrueBeam 2.0 system. The data was analysed for reproducibility and also compared with the data collected with other statndard QA devices at the time of commissioning of the TrueBeam system for validation. Results: The results showed that the reproducibility of MPC was at least an order of magnitude less than the tolerance values for the respective parameters and also the average measured values for all QA parameters studied. The MPC measured isocenter accuracy, and output values were close to the Winston-Lutz test (within 0.1 mm) and the ion-chamber measurements (within 0.1%), respectively. Conclusion: With our long term result, it is evident that the MPC could be an alternative daily QA tool. A comprehensive and long-term validation of the MPC measured values with the other standard QA methods over the ten month period will be needed before accepting MPC as a reliable QA tool.

A robotic C-arm cone beam CT system for image-guided proton therapy: design and performance.

A ceiling-mounted robotic C-arm cone beam CT (CBCT) system was developed for use with a 190° proton gantry system and a 6-degree-of-freedom robotic patient positioner. We report on the mechanical design, system accuracy, image quality, image guidance accuracy, imaging dose, workflow, safety and collision-avoidance. The robotic CBCT system couples a rotating C-ring to the C-arm concentrically with a kV X-ray tube and a flat-panel imager mounted to the C-ring. CBCT images are acquired with flex correction and maximally 360° rotation for a 53 cm field of view. The system was designed for clinical use with three imaging locations. Anthropomorphic phantoms were imaged to evaluate the image guidance accuracy. The position accuracy and repeatability of the robotic C-arm was high (<0.5 mm), as measured with a high-accuracy laser tracker. The isocentric accuracy of the C-ring rotation was within 0.7 mm. The coincidence of CBCT imaging and radiation isocentre was better than 1 mm. The average image guidance accuracy was within 1 mm and 1° for the anthropomorphic phantoms tested. Daily volumetric imaging for proton patient positioning was specified for routine clinical practice. Our novel gantry-independent robotic CBCT system provides high-accuracy volumetric image guidance for proton therapy. Advances in knowledge: Ceiling-mounted robotic CBCT provides a viable option than CT on-rails for partial gantry and fixed-beam proton systems with the added advantage of acquiring images at the treatment isocentre.

Dosimetric Evaluation of a Gamma Ray System for Advanced Radiation Therapy Treatments

Gamma ray systems have exhibited superior dosimetric advantages for stereotactic radiosurgery/radiotherapy (SRS/SRT) of intracranial tumors due to the use of non-coplanar multiple sources and the rapid dose fall-off outside the treatment target as a result of lower secondary electron energies. In this work we performed a dosimetric planning comparison study on a novel Gamma ray system to evaluate its potential clinical benefits for SRS of intracranial tumors and stereotactic body radiotherapy (SBRT) of other body sites. The new Gamma ray system (CybeRT, OUR United RT Group, China) consists of a ring gantry with two treatment heads: a focusing head with 16 cobalt-60 sources and an adjustable (1-2cm) cobalt-60 source equipped with a multileaf collimator (MLC). The MLC has 60 paired leaves, and the maximum field size is 40cmx40cm (40 pairs of 0.5cm central leaves, 20 pairs of 1cm outer leaves). The treatment heads can provide a total of 35° non-coplanar beam incidence. The treatment couch provides 6-degrees-of-freedom motion compensation and the kV cone-beam CT system has a spatial resolution of 0.4mm. Monte Carlo simulations were performed to obtain dose distributions and compare with measurements. A retrospective study of 27 previously treated patients was performed using the Prowess RT Pro (a prototype version), Accuray MultiPlan and Varian Eclipse TPS to compare cobalt plans with 6 MV x-ray plans on the CybeRT, CyberKnife and linac systems. Monte Carlo results confirmed the new Gamma ray system design parameters including output factors and 3D dose distributions. Its beam penumbra/dose gradient was similar to or better than that of 6MV photon beams with adjustable source sizes that also resulted in variable dose rates required for treatment optimization and VMAT delivery. CybeRT had a 0.3mm isocenter accuracy. The low-dose acquisition mode of the CBCT system provided fluoroscopy and 3D imaging at a dose level 3 times less than conventional CBCT systems. Since cobalt beams produced lower-energy secondary electrons the new Gamma ray system exhibited better dose properties in low-density lung tissues. Because of their rapid depth dose falloff, cobalt beams were favorable for peripheral lung tumors with partial-arc deliveries to spare the opposite lung and critical structures. The skin dose with CybeRT for head and neck treatments was similar to that with CyberKnife and conventional linacs and only 2-3% higher for abdominal and pelvic treatments. Superior treatment plans were obtained using the focusing Gamma treatment head for small (up to 50cc) tumor volumes and using the Gamma MLC treatment head for large (> 30cc) tumor volumes for intracranial, head and neck, breast, lung, liver and prostate patients investigated in this study. The unique dosimetric properties of cobalt beams and the accurate stereotaxy/dose delivery techniques make the new Gamma ray system an ideal RT system for advanced SRS/SBRT as well as conventional radiation therapy.

Stability assessment of radiation isocenter with the gimbaled linac system

Purpose: We report the results of our year-long radiation isocenter accuracy verification for daily quality assurance (QA) implementation on a Vero4DRT system. Methods: The radiation isocenter was calculated using a cube phantom with a steel ball of diameter 10 mm fixed to the center of the phantom. A single photon beam was set with a field size of 100 × 100 mm 2 . Coincidence of the centroid of the steel ball at kiloVolt X-ray imaging isocenter and megaVolt beam radiation isocenter at each gantry and ring angle was tested. This procedure was performed for gantry angles of 0°, 90°, 180°, and 270°, and ring angles of 0°, 20°, and 340°. The centroid of the steel ball and the center of the radiation field were calculated to analyze the radiation isocenter error. This analysis was automatically calculated using the Daily Check tool in the Vero4DRT system. This QA was implemented between 24 August 2015 and 23 August 2016. Results: The average and standard deviation for pan and tilt directions were 0.12 ± 0.10 mm and -0.20 ± 0.13 mm, respectively. The maximum radiation isocenter accuracy error was 0.50 mm in both directions. Conclusion: The radiation isocenter alignment for the one year duration of the experiment was performed with high accuracy.

SU‐F‐T‐647: Linac‐Based Stereotactic Radiosurgery (SRS) in the Treatment of Trigeminal Neuralgia: Detailed Description of SRS Procedural Technique and Reported Clinical Outcomes

Purpose:SRS is an effective non‐invasive alternative treatment modality with minimal‐toxicity used to treat patients with medically/surgically refractory trigeminal neuralgia root(TNR) or those who may not tolerate surgical intervention. We present our linac‐based SRS procedure for TNR treatment and simultaneously report our clinical outcomes.Methods:Twenty‐eight TNR‐patients treated with frame‐based SRS at our institution (2009–2015) with a single‐fraction point‐dose of 60‐80Gy to TNR were included in this IRB‐approved study. Experienced neurosurgeon and radiation oncologist delineated the TNR on 1.0mm thin 3D‐FIESTA‐MRI that was co‐registered with 0.7mm thin planning‐CT. Treatment plans were generated in iPlan (BrainLAB) with a 4‐mm diameter cone using 79 arcs with differential‐weighting for Novalis‐TX 6MV‐SRS(1000MU/min) beam and optimized to minimize brainstem dose. Winston‐Lutz test was performed before each treatment delivery with sub‐millimeter isocenter accuracy. Quality assurance of frame placement was maintained by helmet‐bobble‐measurement before simulation‐CT and before patient setup at treatment couch. OBI‐CBCT scan was performed for patient setup verification without applying shifts. On clinical follow up, treatment response was assessed using Barrow Neurological Institute Pain Intensity Score(BNI‐score:I–V).Results:26/28 TNR‐patients (16‐males/10‐females) who were treated with following single‐fraction point‐dose to isocenter: 80Gy(n=22),75Gy(n=1),70Gy(n=2) and 60Gy(n=1, re‐treatment) were followed up. Median follow‐up interval was 8.5‐months (ranged:1–48.5months). Median age was 70‐yr (ranged:43–93‐yr). Right/left TNR ratio was 15/11. Delivered total # of average MUs was 19034±1204. Average beam‐on‐time: 19.0±1.3min. Brainstem max‐dose and dose to 0.5cc were 13.3±2.4Gy (ranged:8.1–16.5Gy) and 3.6±0.4Gy (ranged:3.0–4.9Gy). On average, max‐dose to optic‐apparatus was ≤1.2Gy. Mean value of max‐dose to eyes/lens was 0.26Gy/0.11Gy. Overall, 20‐patients (77%) responded to treatment: 5(19%) achieved complete pain relief without medication (BNI score: I); 5(19%) had no‐pain, decreased medication (BNI‐score:II); 2(7.7%) had no‐pain, but, continued medication (BNI‐score:IIIA), and 8(30.8%) had pain that was well controlled by medication (BNI‐score: IIIB). Six‐patients (23.0%) did not respond to treatment (BNI‐score:IV–V). Neither cranial nerve deficit nor radio‐necrosis of temporal lobe was clinically observed.Conclusion:Linac‐based SRS for medically/surgically refractory TNR provided an effective treatment option for pain resolution/control with very minimal if any normal tissue toxicity. Longer follow up of these patients is anticipated/needed to confirm our observations.

SU‐F‐T‐529: Dosimetric Investigation of a Rotating Gamma Ray System for ImagedGuided Modulated Arc Radiotherapy

Purpose:Because of their effectiveness and efficiency, rotational arc radiotherapy (MAT) techniques have been developed on both specialty machines such as Tomotherapy and conventional linear accelerators. This work investigates a new rotating Gamma therapy system for image guided MAT and SBRT of intra/extracranial tumors.Methods:The CyberMAT system (Cyber Medical Corp., China) consists of a ring gantry with a gamma source (effective source size 1cm and 1.7cm respectively), a 120‐leaf MLC, a kV CBCT and an EPID. The treatment couch provides 6‐degrees‐of‐freedom motion compensation and the kV CBCT system has a spatial resolution of 0.4mm for target localization. The maximum dose rate is &gt;4.0 Gy/min and the maximum field size is 40cm × 40cm. Monte Carlo simulations were used to compute dose distributions and compare with measurements. A retrospective study of 125 previously treated SBRT patients was performed to evaluate the dosimetric characteristics of CyberMAT in comparison with existing VMAT systems.Results:Monte Carlo results confirmed the CyberMAT design parameters including output factors and 3D dose distributions. Its beam penumbra is 5mm to 10mm for field sizes 1cm to 10cm, respectively and its isocenter accuracy is &lt;0.5mm. Compared to the 6 MV photons of Tomotherapy and conventional linacs, Cobalt beams produce lower‐energy secondary electrons that exhibit better dose properties in low‐density lung tissues. Cobalt beams are ideal for peripheral lung tumors with half‐arc arrangements to spare the opposite lung and other critical structures. Superior dose distributions have been obtained for brain, head and neck, breast, spine and lung tumors with half/full arc arrangements.Conclusion:Because of the unique dosimetric properties of Cobalt sources and its accurate stereotaxy/dose delivery CyberMAT is ideally suited for image guided MAT and SBRT. Full‐arc arrangements are superior for brain and H&amp;N treatments while half‐arc arrangements produce best dose distributions for thoracic tumors.