Novalis Tx System Research Articles

두개저 수막종 환자에 대한 저분할 방사선 수술의 임상적 효용성

Background : Various modalities of stereotactic radiosurgery (SRS) have been proven to be effective and safe in the treatment of intracranial meningiomas. Depending on the characteristics of each modality, various results can be obtained. To analyze the therapeutic effect of hypofractionated SRS (hSRS) on skull base meningioma, we retrospectively reviewed the results of clinical and follow-up image studies. Materials and Methods : Between January 2011 and December 2020, thirty-two patients (median age, 58 years; 5 males and 27 females) had been treated with hSRS using the Novalis Tx system for skull base meningiomas (mean volume, 7.86 cm3). Tumor was located in most frequently cavernous sinus (n = 7, 21.9%), cerebellopontine angle (n = 7, 21.9%), clinoidal (n = 6, 18.8%) locations. Thirteen patients had previously undergone surgical treatment including radiosurgery. The median cumulative dose was 25 Gy (range, 21.0-27.5 Gy) in five fractions (range, 3-5 fractions). Results : During a median follow-up of 50 months (range, 13-98 months) hSRS achieves 100% tumor control rate in all patients without tumor progression. As a result of volumetric analysis using magnetic resonance imaging showed partial response (volume decrease > 20%) in 26 patients (81.3%), stable in 4 patients (12.5%). The tumor volume decreased by an average of 55.5% when compared with the beginning. Conclusions : This study showed that hSRS is an effective and safe treatment modality with a high tumor control rate and functional outcomes for skull base meningioma.

SU‐E‐J‐48: Imaging Origin‐Radiation Isocenter Coincidence for Linac‐Based SRS with Novalis Tx

PurposeTo implement and evaluate an image‐based Winston‐Lutz (WL) test to measure the displacement between ExacTrac imaging origin and radiation isocenter on a Novalis Tx system using RIT V6.2 software analysis tools. Displacement between imaging and radiation isocenters was tracked over time. The method was applied for cone‐based and MLC‐based WL tests.MethodsThe Brainlab Winston‐Lutz phantom was aligned to room lasers. The ExacTrac imaging system was then used to detect the Winston‐ Lutz phantom and obtain the displacement between the center of the phantom and the imaging origin. EPID images of the phantom were obtained at various gantry and couch angles and analyzed with RIT calculating the phantom center to radiation isocenter displacement. The RIT and Exactrac displacements were combined to calculate the displacement between imaging origin and radiation isocenter. Results were tracked over time.ResultsMean displacements between ExacTrac origin and radiation isocenter were: VRT: −0.1mm ± 0.3mm, LNG: 0.5mm ± 0.2mm, LAT: 0.2mm ± 0.2mm (vector magnitude of 0.7 ± 0.2mm). Radiation isocenter was characterized by the mean of the standard deviations of the WL phantom displacements: σVRT: 0.2mm, σLNG: 0.4mm, σLAT: 0.6mm. The linac couch base was serviced to reduce couch walkout. This reduced σLAT to 0.2mm. These measurements established a new baseline of radiation isocenter‐imaging origin coincidence.ConclusionThe image‐based WL test has ensured submillimeter localization accuracy using the ExacTrac imaging system. Standard deviations of ExacTrac‐radiation isocenter displacements indicate that average agreement within 0.3mm is possible in each axis. This WL test is a departure from the tradiational WL in that imaging origin/radiation isocenter agreement is the end goal not lasers/radiation isocenter.

Feasibility of using Geant4 Monte Carlo simulation for IMRT dose calculations for the Novalis Tx with a HD-120 multi-leaf collimator

The aim of this study was to develop an independent dose verification system by using a Monte Carlo (MC) calculation method for intensity modulated radiation therapy (IMRT) conducted by using a Varian Novalis Tx (Varian Medical Systems, Palo Alto, CA, USA) equipped with a highdefinition multi-leaf collimator (HD-120 MLC). The Geant4 framework was used to implement a dose calculation system that accurately predicted the delivered dose. For this purpose, the Novalis Tx Linac head was modeled according to the specifications acquired from the manufacturer. Subsequently, MC simulations were performed by varying the mean energy, energy spread, and electron spot radius to determine optimum values of irradiation with 6-MV X-ray beams by using the Novalis Tx system. Computed percentage depth dose curves (PDDs) and lateral profiles were compared to the measurements obtained by using an ionization chamber (CC13). To validate the IMRT simulation by using the MC model we developed, we calculated a simple IMRT field and compared the result with the EBT3 film measurements in a water-equivalent solid phantom. Clinical cases, such as prostate cancer treatment plans, were then selected, and MC simulations were performed. The accuracy of the simulation was assessed against the EBT3 film measurements by using a gamma-index criterion. The optimal MC model parameters to specify the beam characteristics were a 6.8-MeV mean energy, a 0.5-MeV energy spread, and a 3-mm electron radius. The accuracy of these parameters was determined by comparison of MC simulations with measurements. The PDDs and the lateral profiles of the MC simulation deviated from the measurements by 1% and 2%, respectively, on average. The computed simple MLC fields agreed with the EBT3 measurements with a 95% passing rate with 3%/3-mm gamma-index criterion. Additionally, in applying our model to clinical IMRT plans, we found that the MC calculations and the EBT3 measurements agreed well with a passing rate of greater than 95% on average with a 3%/3-mm gamma-index criterion. In summary, the Novalis Tx Linac head equipped with a HD-120 MLC was successfully modeled by using a Geant4 platform, and the accuracy of the Geant4 platform was successfully validated by comparisons with measurements. The MC model we have developed can be a useful tool for pretreatment quality assurance of IMRT plans and for commissioning of radiotherapy treatment planning.

SU‐E‐T‐300: Spatial Variations of Multiple Off‐Axial Targets for a Single Isocenter SRS Treatment Plan in ExacTrac 6D Robotic Couch System

Purpose:To evaluate the spatial variations of multiple off‐axial targets for a single isocenter stereotactic radiosurgery (SRS) treatment plan in ExacTrac 6D robotic couch system (BrainLab AG).Methods:Five metallic ball bearing (BB) markers were placed sparsely in 3D off‐axial locations (non‐coplanar) inside a skull phantom as the representatives of multiple targets mimicking multiple brain metastases. The locations of the BB markers were carefully chosen to minimize overlapping of each other in a port imaging detector plane. The skull phantom was immobilized by a frameless mask and CT scanned with a BrainLab Head&Neck Localizer using a GE Optima MDCT scanner. The CT images were exported to iPlan software (BrainLab AG) and a multiple target PTV was drawn by combining all the contours of the BBs. The margin of the MLC opening was selected as 3 mm expansion outward. Two coplanar arc beams were placed to generate a single isocenter SRS plan to treat the PTV. The arc beams were delivered using Novalis Tx system with portal imaging acquisition mode per 10% temporal resolution. The locations of the BBs were visualized and analyzed with respect to the MLC aperture in the treatment plan similar to the Winston‐Lutz test.Results:All the BBs were clearly identified inside the MLC openings. The positional errors for the BBs were overall less than 1 mm along the rotational path of the two arcs.Conclusion:This study verified that the spatial deviations of multiple off‐axial targets for a single isocenter SRS treatment plan is within sub‐millimeter range in ExacTrac 6D robotic couch system. Accompanied with the Winston‐Lutz test, this test will quality‐assure the spatial accuracies of the isocenter as well as the positions of multiple off‐axial targets for the SRS treatment using a single isocenter multiple target treatment plan.

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.

SU-E-T-143: Dosimetric Characteristics of TrueBeam STx System with High Definition Multi-Leaf Collimator: Commissioning of Stereotactic Radiosurgery

IntroductionDosimetric characteristics of one TrueBeam STx treatment unit with a 120‐leaf high definition multileaf collimator (HD120MLC) are systematically measured for commissioning of stereotactic radiosurgery (SRS). Due to limited data available, it is important to make the dosimetric data available for reference.MethodsSRS commissioning data were acquired for two photon beam calculation algorithms of the treatment planning system iPlan (BrainLAB iPlan4.5): Clarkson method and Pencil Beam dose calculation (PBC) method. Major beam data required include percent depth dose (PDD), beam profile, and relative scatter factors for various field sizes. Generally, the PBC is used for HD120MLC based treatment planning, while Clarkson method is for cone‐based planning. ResultsPDD, beam profile and relative scatter data for the circular cones with diameters of 4, 6.0, 7.5, 10, 12.5, 15 mm were measured for 6X flattening‐filter‐free (FFF) and 10X FFF. The relative scatter factors range from 1.12 to 1.45 for 6X FFF and 0.81 to 1.23 for 10XFFF. Beam penumbra of 6XFFF and 10XFFF of each the circular cone were less than 2.5 mm for the 20% to 80% criterion. PDD and relative scatter data for HD120MLC were measured for various MLC fields with corresponding jaw settings for various energy modes: 6X, 6X FFF, 10X, 10X FFF and 15X, with an MLC filed down to 5mmx5mm. The transverse and diagonal beam profiles for HD120MLC were measured at different depths to generate source function correction models in iPlan. HD120MLC leakage for open jaw and close jaw are ∼1% and 0.0% for all the energy modes. The measured HD120MLC dynamic leaf shifts are 0.15, 0.11, 0.19, 0.18, 0.21 mm for 6X, 6XFFF, 10X, 10XFFF and 15X, less than 1/2 of those of the Novalis Tx system previous published.ConclusionThe commissioning data provided us valuable insights and might be useful for future reference.

Implementation and evaluation of modified dynamic conformal arc (MDCA) technique for lung SBRT patients following RTOG protocols

To implement modified dynamic conformal arc (MDCA) technique and Radiation Therapy Oncology Group (RTOG) protocols in our clinic for stereotactic body radiation therapy (SBRT) treatment of patients with Stage I/II non–small cell lung cancer. Five patients with non–small cell lung cancer have been treated with SBRT. All the patients were immobilized using CIVCO Body Pro-Lok system and scanned using GE 4-slice computed tomography. The MDCA technique that was previously published was adopted as our planning technique, and RTOG protocols for the lung SBRT were followed. The patients were treated on Novalis Tx system with cone-beam computed tomography imaging guidance. All the patient plans passed the RTOG criteria. The conformal index ranges from 0.99 to 1.12 for the planning target volume, and the biological equivalent dose for the planning target volume is overall 100Gy. Critical structures (lung, spinal cord, brachial plexus, skin, and chest wall) also meet RTOG protocols or published data. A 6-month follow-up of one of the patients shows good local disease control. We have successfully implemented the MDCA technique into our clinic for the lung SBRT program. It shows that the MDCA is useful and efficient for the lung SBRT planning, with the plan quality meeting the RTOG protocols.

Dosimetric comparison of 2.5 mm vs. 3.0 mm leaf width micro-multileaf collimator-based treatment systems for intracranial stereotactic radiosurgery using dynamic conformal arcs: implications for treatment planning

The objective of our study was to explore any significant dosimetric differences between different leaf width (3.0 mm vs. 2.5 mm) micro-multileaf collimator (mMLC)-based treatment systems for intracranial stereotactic radiosurgery using dynamic conformal arcs (DCAs). The systems included a 3 mm leaf width mMLC (m3) mounted on a nondedicated linac, and the Novalis Tx system with an integrated 2.5 mm width mMLC (HD120). Thirty plans for brain metastases were replanned for both systems using a uniform method for target definition and treatment planning for baseline comparison. The target coverage values for the 80% isodose surface (IDS) and the D95 values in the HD120 plans were significantly lower than those for the m3 plans. The ratios of lower isodose volumes to the target for the HD120 were smaller than those for the m3. When a 1 mm leaf margin was added to the HD120 plans, these differences were reversed, but statistically significant differences were still observed. Significant dosimetric differences were observed between these systems. Different planning methods are required for the two systems to attain similar target coverage values with selected IDS, which can be achieved by adjusting the leaf margin with 0.1 mm increments or isocenter dose settings.

SU‐E‐T‐123: Evaluation of Integrated Gating Systems on a Novalis Tx System Using Quantitative Motion Tracking Analyses and Efficient Time‐Delay Measurements

Purpose: To investigate accuracy of motion tracking and radiation delivery controlling of integrated gating systems on a Novalis Tx system. Methods: The study was performed on a Novalis Tx system, which is equipped with Varian RPM and BrainLAB ExacTrac gating systems. In this study, the two systems were assessed on accuracy of both motion tracking and radiation delivery controlling. To evaluate motion tracking, one artificial sinusoidal profile and three patientsˈ respiratory profiles were used. The motion trajectories acquired by the two gating systems were compared against the references. To assess radiation delivery controlling, time‐delays were measured using a single‐exposure method. More specifically, radiation is delivered with a 4‐mm diameter cone within 10%–45% phases for BrainLAB ExacTrac system and within 0%–25% phases for Varian RPM system during expiration, each with three times. Radiochromic films were used to record the radiation exposures and to calculate the time‐delays. In the work, the discrepancies were quantified using mean and standard deviation (SD). Pearsonˈs product moment correlational analysis was used to test correlation, which is quantified using a parameter of r. Results: The trajectory profiles acquired by the gating systems show good agreement with those references. A quantitative analysis shows that the average mean discrepancy between BrainLAB ExacTrac system and known references is 1.68 mm for artificial and patient profiles, with the maximum motion amplitude of 28.0 mm. As for Varian RPM system, the corresponding average mean discrepancy is 1.54 mm for artificial and patient profiles. With the single‐exposure method, the time‐delays are found to be 0.20±0.03 seconds and 0.09±0.01 seconds for BrainLAB ExacTrac and Varian RPM systems, respectivelyConclusions: The results indicate the gating systems on the Novalis Tx system can track motion and control radiation delivery with reasonable accuracy. The single‐exposure method has been demonstrated feasible in measuring time‐delays efficiently.

Evaluation of integrated respiratory gating systems on a Novalis Tx system

The purpose of this study was to investigate the accuracy of motion tracking and radiation delivery control of integrated gating systems on a Novalis Tx system. The study was performed on a Novalis Tx system, which is equipped with Varian Real‐time Position Management (RPM) system, and BrainLAB ExacTrac gating systems. In this study, the two systems were assessed on accuracy of both motion tracking and radiation delivery control. To evaluate motion tracking, two artificial motion profiles and five patients' respiratory profiles were used. The motion trajectories acquired by the two gating systems were compared against the references. To assess radiation delivery control, time delays were measured using a single‐exposure method. More specifically, radiation is delivered with a 4 mm diameter cone within the phase range of 10%–45% for the BrainLAB ExacTrac system, and within the phase range of 0%–25% for the Varian RPM system during expiration, each for three times. Radiochromic films were used to record the radiation exposures and to calculate the time delays. In the work, the discrepancies were quantified using the parameters of mean and standard deviation (SD). Pearson's product‐moment correlational analysis was used to test correlation of the data, which is quantified using a parameter of r. The trajectory profiles acquired by the gating systems show good agreement with those reference profiles. A quantitative analysis shows that the average mean discrepancies between BrainLAB ExacTrac system and known references are 1.5 mm and 1.9 mm for artificial and patient profiles, with the maximum motion amplitude of 28.0 mm. As for the Varian RPM system, the corresponding average mean discrepancies are 1.1 mm and 1.7 mm for artificial and patient profiles. With the proposed single‐exposure method, the time delays are found to be 0.20±0.03 seconds and 0.09±0.01 seconds for BrainLAB ExacTrac and Varian RPM systems, respectively. The results indicate the systems can track motion and control radiation delivery with reasonable accuracy. The proposed single‐exposure method has been demonstrated to be feasible in measuring time delay efficiently.PACS numbers: 87.56.bd, 87.56.‐v, 87.55.‐x

Toward acquiring comprehensive radiosurgery field commissioning data using the PRESAGE®/ optical-CT 3D dosimetry system

Achieving accurate small field dosimetry is challenging. This study investigates the utility of a radiochromic plastic PRESAGE® read with optical-CT for the acquisition of radiosurgery field commissioning data from a Novalis Tx system with a high-definition multileaf collimator (HDMLC). Total scatter factors (Sc, p), beam profiles, and penumbrae were measured for five different radiosurgery fields (5, 10, 20, 30 and 40 mm) using a commercially available optical-CT scanner (OCTOPUS, MGS Research). The percent depth dose (PDD), beam profile and penumbra of the 10 mm field were also measured using a higher resolution in-house prototype CCD-based scanner. Gafchromic EBT® film was used for independent verification. Measurements of Sc, p made with PRESAGE® and film agreed with mini-ion chamber commissioning data to within 4% for every field (range 0.2–3.6% for PRESAGE®, and 1.6–3.6% for EBT). PDD, beam profile and penumbra measurements made with the two PRESAGE®/optical-CT systems and film showed good agreement with the high-resolution diode commissioning measurements with a competitive resolution (0.5 mm pixels). The in-house prototype optical-CT scanner allowed much finer resolution compared with previous applications of PRESAGE®. The advantages of the PRESAGE® system for small field dosimetry include 3D measurements, negligible volume averaging, directional insensitivity, an absence of beam perturbations, energy and dose rate independence.

SU‐FF‐T‐309: Quality Assurance for Imaging Guided Stereotactic RadioSurgery with Novalis Tx™ System

Purpose: To present QA measurements over extended periods at our institute and recommend a QA program for image‐guided radiosurgery on Novalis Tx system. Method and Materials: Novalis Tx system is developed based on a Trilogy linear accelerator featuring BrainLAB 6D‐ExacTrac, HD120‐MLC, MV‐EPID and KV‐OBI&CBCT. Utilization of any new technology and equipment necessitates a comprehensive QA program to maintain and monitor system performance characteristics. In this study, we carried out a systematic QA program, including linear accelerator QA, treatment planning QA, planning MRI and CT QA, treatment imaging QA, patient‐specific QA, and system setup procedures for image‐guided radiosurgery. Methodology, frequencies and criteria of QA tests were based on AAPM TG40, TG42, TG51, TG66, and ACR MRI QC. Results: Comprehensive QA was performed from March to December 2008. 1) Daily Winston‐Lutz tests of 199 measurements showed that isocenter of gantry rotation was 0.44±0.21mm; isocenter of couch rotation was 0.34±0.20mm, within 1mm tolerance. Other linear accelerator QA results were within TG40 criteria. 2) Treatment planning QA was conducted with spot‐checks and a RPC SRS head phantom test. In spot‐checks, dosimetric discrepancies were within 3% (3.5% for the 4mm cone). RPC radiosurgery test was passed with less than 1% dosimetric deviation, and 100% geometric match. 3) Daily MRI QA showed that average geometric distortion was −0.2±0.4mm, −0.3±0.4mm, and −0.9±0.1mm along vertical, lateral, and longitudinal directions, within 1mm on phantom, 50% of ACR specifications. 4) OBI QA showed that imaging isocenter displacement was 0.46±0.30mm at AP and 0.76±0.19mm at RLAT. 5) Over ten months, 128 patients with intracranial lesions were treated with radiosurgery. All second MU checks were within 3%, and there were none treatment errors associated with the SRS treatments. Conclusion: Comprehensive QA program for image‐guided radiosurgery has been developed with proper methodology, frequency and criteria on Novalis Tx system, demonstrated with extensive measurements.

SU‐EE‐A3‐02: Imaging Guided Frameless Stereotactic RadioSurgery Using CBCT 6D Image Registration and 6D Couch On Novalis Tx™ System

Purpose: To investigate localization accuracy for image‐guided frameless stereotactic‐radiosurgery (IG‐SRS) using CBCT 6D image registration and 6D‐couch on Novalis Tx system. Method and Materials: Novalis Tx system is a powerful radiosurgery system featuring BrainLAB 6D‐couch and Varian CBCT. In this work, we performed phantom and patient studies to investigate IG‐SRS with CBCT 6D guidance. In phantom studies, a head phantom was positioned on couch, scanned with CBCT, and matched with planning CT using 6D image registration. Matching results were used to test accuracy of CBCT 6D/6D match for correcting translational and rotational setup errors. In patient studies, 10 intracranial SRS cases were randomly selected. Patients were initially positioned with BrainLAB mask system with target‐positioning‐overlays (TAPO). Patients were then scanned with CBCT, and positions were adjusted based on CBCT. Discrepancies between CBCT 3D/3D match and 6D/6D match were quantitatively analyzed. Results: Phantom experiments showed that translational discrepancies in root‐mean‐square (RMS) were 0.39, 0.23, and 0.36mm along lateral, longitudinal, and vertical directions. Rotational discrepancies in RMS were 0.38°, 0.19°, and 0.22° in pitch, roll, yaw, respectively. In retrospective patient studies, setup displacements measured in RMS between BrainLAB TAPO and CBCT 6D/6D match were upto 2.5mm translationally and 1.0° rotationally. If positioning was corrected using CBCT 3D/3D match, translational discrepancies in RMS were <0.5mm except 0.71mm in vertical, and rotational displacements in RMS were 0.67°, 0.88° and 0.22° in pitch, roll, yaw, as compared with CBCT 6D/6D match. CBCT guided 6D correction was performed in two cases to achieve necessary positioning accuracy. After 6D adjustments, translational and rotational displacements of upto 4mm and 0.9° were measured in CBCT 6D/6D match were fully corrected using 6D‐couch. Conclusion: CBCT 6D/6D match offers unprecedented view to verify patient positioning in both translations and rotations. Combined with 6D‐couch, translational and rotational setup errors can be effectively minimized.

TH-D-BRB-03: The Application of a Novel Three Dimensional Dosimetry System to the Acquisition of Commissioning Data of Small Photon Fields

Purpose: To investigate the application of a radiochromic plastic 3D dosimeter to the measurement of commissioning data in radiosurgery fields. Method and Materials: In order to measure total scatter factors, a single cylinder of a radiochromic plastic (PRESAGE) was irradiated with five different radiosurgery fields uniformly spaced in a circular geometry. These square fields were delivered with a Novalis Tx system equipped with a high definition multileaf collimator (HDMLC) and ranged from 40 mm to 5 mm. The radiation‐induced change in optical density within the dosimeter was digitized using a first generation laser based optical CT system. Additionally, a cylinder of PRESAGE was irradiated in a water bath with one 10 mm beam to measure percent depth dose. This dosimeter was digitized using a novel in‐house high‐resolution CCD based optical CT system. Gafchromic EBT film was used to validate the assumption of negligible cross‐field scatter associated with the combined field geometry. It also served as an independent check of all PRESAGE measurements. Results: The total scatter factor measurements made with PRESAGE agreed with radiosurgery mini‐ion chamber commissioning measurements with a percent error of less than 3.2% for 30, 20, and 5 mm fields. The percent error for the 10 mm field was 5.5%. The scatter measurements made with radiochromic film confirmed the validity of the combined field geometry and agreed with the commissioning data within 2%. PDD measurements showed good agreement with the high‐resolution diode commissioning measurements with a competitive level of resolution. Conclusion: The PRESAGE and optical CT dosimetrysystem presents a new approach for the commissioning of radiosurgery fields, with many advantages. These include high‐resolution 3D beam data measured with energy, dose rate, and directional independence, as well as a reduction in placement and volume averaging errors.

Dosimetric characteristics of Novalis Tx system with high definition multileaf collimator

A new Novalis Tx system equipped with a high definition multileaf collimator (HDMLC) recently became available to perform both image-guided radiosurgery and conventional radiotherapy. It is capable of delivering a highly conformal radiation dose with three energy modes: 6 MV photon energy, 15 MV photon energy, and 6 MV photon energy in a stereotactic radiosurgery mode with 1000 MU/min dose rate. Dosimetric characteristics of the new Novalis Tx treatment unit with the HDMLC are systematically measured for commissioning. A high resolution diode detector and miniion-chamber detector are used to measure dosimetric data for a range of field sizes from 4 x 4 mm to 400 x 400 mm. The commissioned Novalis Tx system has passed the RPC stereotactic radiosurgery head phantom irradiation test. The Novalis Tx system not only expands its capabilities with three energy modes, but also achieves better beam conformity and sharer beam penumbra with HDMLC. Since there is little beam data information available for the new Novalis Tx system, we present in this work the dosimetric data of the new modality for reference and comparison.