ViewRay System Commissioning

Transcription

1 ViewRay System Commissioning Kyle R. Padgett, PhD, DABR Assistant Professor University of Miami Viewray Team: Matt Studenski, PhD Chet Ford, PhD Yidong Yang, PhD Nesrin Dogan, PhD Medical Physics Residents: Naru Lamichhane, PhD Xing Li, PhD

2 Disclosures None 2

3 Overview Overview of the MRIdian system Commissioning Safety MRI Mechanicals RT System Treatment Planning System End to End Testing

4 Commonly known as Viewray MRIdian Integrated magnetic resonance (MR)-guided radiation therapy (RT) instrument designed to provide simultaneous MR Imaging (MRI) and a range of external beam RT options Consists of three major subsystems MRI System RT System Adaptive RT System Introduction

5 The MRI Gapped horizontal solenoidal superconducting magnet 0.35 T whole body MRI, capable of volumetric and real-time imaging The MRI system consists of two separate magnets abutted with a 28 cm gap. Split gradient coil has an inner diameter of 80 cm. Resolution as low as 0.75 mm Planar real-time imaging at up to 4 frames per second. MR isocenter matched to RT isocenter Body & Surface coils are thin & uniformly attenuating

6 Split Super Conducting Magnet Allows Unobstructed Beam Path Fits in Standard Vaults, Pop-apart design for non-destructive rigging 0.35T Field strength provides 50cm DSV for large FOV imaging Less image distortion and less patient heating Minimal distortion of dose distribution 70cm bore to accommodate large patients Split gradient with a 28cm gap, slew 200mT/m/ms, 18mT/m peak, 30kW heat removal. The MRI

7 Low Field MRI High-field causes a loss of spatial integrity Magnetic susceptibility artifacts due to the patient scales with field strength High field distorts the dose distribution Electron return effects get worse with field strength High field heats the patient -SAR

8 Electron Return Effect The Magnetic Field has an effect upon the secondary electrons The stronger the Magnetic Field the more pronounced the effect Raaijmakers et. al. Phys. Med. Biol. 53 (2008)

9 MRI Sequences Available FISP - Fast Imaging with Steady Precession GE Gradient Echo (Only one small FOV option) FISP is a coherent technique that uses a fully balanced gradient waveform (true FISP) The image contrast is determined by T2*/T1 and mostly depends on the TR The speed and relative motion insensitivity of acquisition help to make the technique reliable. Other MRI sequences may become available on future software releases.

10 Setup Imaging All setup images shown use FISP imaging FOV ranges from 22 54cm Resolution ranges from mm Acquisition time ranges from sec GE setup image has a resolution of 7.5mm, a FOV of ~27cm and a acquisition time of 12min

11 Real Time Imaging / Gating All CINE images shown use FISP imaging FOV ranges from 27cm to 45cm Resolution ranges from 3.5mm to 10mm All are 4 frames per second for a single slice

12 The RT delivery system Equipped with three robotic 60 Co treatment heads mounted with 120 separation on a rotating gantry. Each treatment head operates independently and they can be operated simultaneously. Each head can deliver up to 1.8Gy per minute at the machine isocenter, which is 105 cm from each source, with the nominal source strength of 15kCi. The source is shielded by a depleted-uranium safe in the retracted position

13 The RT delivery system Rotating Gantry Assembly 3 Independent Co60 Headed Design Enabling IMRT, SBRT or 3D- Conformal External Beam Radiation with Asynchronous Delivery Mounted with 120 degree separation 15,000 Ci per source degree Rotation for 2 or 3 Head Operation for increased Reliability. 3 Doubly Focused MLC Systems 180 MLC Leaves. 60 per head 1.05 leaf thickness projected at isocenter

14 Source Operation Beam Indicator Beam Indicator Beam OFF Beam On 14

15 3 Doubly Focused MLC Systems 180 MLC Leaves Per Head 1.05cm leaf thickness projected at isocenter Fully interdigitating Average Leakage less than 0.375% The MLC

16 Adaptive RT System An integrated high-performance radiation planning and delivery software capable of auto contouring, Monte Carlo dose computation, and IMRT or conformal RT planning or both are used to support 3-dimensional conformal RT, IMRT, and on-couch ART. The speed of the TPS (9 field plans with complete optimization, leaf motion calculation, and dose calculations can be accomplished in less than 30 seconds) enables ART treatments based on the volumetric image of the day

17 Commissioning Equipment Water tank with manually driven chamber holder Non-magnetic ion chamber: Exradin Nonmagnetic A28 & standard A12 chambers MRI-compatible IMRT QA device: ArcCheck MR, Sun Nuclear Corporation MRI-compatible beam profiler: ArcCheck IC Profiler 1122-mr, Sun Nuclear Corporation Gafchromic Films and Film Processor 2D Spatial Integrity Phantom Spherical Water Phantoms CIRS Gating Phantom ACR MRI Phantom

18 Radiation shielding survey was performed by the University of Miami radiation safety office. All points outside the treatment vault showed radiation levels in compliance with regulatory limits specified in 10 CFR, part 20 (limit of 2mR/hr) Safety

19 Safety Door Interlocks: Beam was successfully interrupted when door is opened Emergency stop buttons: The 5 emergency stop buttons were enabled and generated safety interlocks on the console Beam on Warning Light: Warning light successfully operated during irradiation Prime Alert Functionality: Prime alert functionality was confirmed with a check source Intercom and AV Monitoring: Confirmed with successful communication between two individuals Backup Timer: Primary timer was disabled and the backup timer successfully terminated the beam Emergency Door operation: Operates when power is off Emergency Couch Retract: Can be initiated manually and when power is off Many other safety interlocks were also tested during ATP

20 Magnetic Field Homogeneity Phased Array Coil Elements Image Homogeneity Spatial Integrity High Contrast Resolution MRI Tests Slice Thickness Slice Position Accuracy Low Contrast Resolution Ghosting Ratio

21 Magnetic Field Homogeneity The homogeneity of the MRI system was measured using a large spherical water phantom A water spectrum was then collected to determine the frequency spread at FWHM Repeated at several different Gantry Angles and with the different RF coils FWHM Gantry 0 Gantry Gantry Gantry Gantry ppm 1.46ppm 1.44ppm 1.60ppm 1.74ppm <= 5ppm

22 Coil Element Tests The signal characteristics for the coil elements for all phased array coils were measured. 12 element Torso Coil 12 element Head and Neck Coil All SNR measurements significantly exceeded specifications Shape of signal profile was also qualitatively evaluated.

23 Homogeneity Homogeneity of the Body, Torso, and H&N Coils were measured and exceeded specifications 60% Body Coil 50% Torso Coil 50% H&N Coil The Signal to Noise of the Body, Torso, and H&N Coils were measured and exceeded specifications SNR > 12 Body Coil SNR > 30 Torso Coil SNR > 30 H&N Coil Uniformity % = 100 * 1 [(ROI Signal max ROI Signal min) / (ROI Signal max + ROI Signal min)] SNR = (ROI signal mean) * 0.66 / (ROI Noise SD)

24 Spatial Integrity 2D spatial integrity was measured with a custom made phantom provided by Viewray All measurements within inner circle must have less than 1mm of distortion All measurements within outer circle must have less than 2mm of distortion This test is also performed in our Monthly QA Pass-Rate Pass-Rate Max error 1mm Ring 2mm Ring Axial 100.0% 100.0% 1.764mm Coronal 99.3% 98.7% 2.134mm Sagittal 100.0% 100.0% 1.298mm centered Sagittal 12.5cm N/A 100.0% 1.387mm Patient Left Sagittal 12.5cm Patient Right N/A 100.0% 1.060mm

25 High Contrast Resolution To assess the scanner s ability to resolve small objects when the contrast to noise ratio is sufficiently high. Test utilizes the MRI ACR phantom Should be able to visualize/resolve several of the rows and columns of four. This test is also performed in our Monthly QA Figure 6: from ACR Phantom Test Guidance for Use of the Small MRI Phantom

26 To assess the accuracy with which a slice of specified thickness is achieved Test utilizes the MRI ACR phantom Two signal ramps with a slope of 10 to 1 are utilized Thickness should be 5mm ± 0.7mm This test is also performed in our Monthly QA Slice Thickness Slice thickness = 0.2 x (top x bottom)/(top + bottom)

27 Slice Position Accuracy To assess the accuracy with which slices can be prescribed at specific locations Test utilizes the MRI ACR phantom Two crossed wedged ramps of 45 degrees are utilized The difference in the two bar length determines the slice position accuracy ½ of the measured length is the positional discrepancy This test is also performed in our Monthly QA

28 Low Contrast Resolution To assess the extent to which objects of low contrast are discernible in the images Test utilizes the MRI ACR phantom The low contrast disks are holes drilled in thin sheets of plastic mounted in the phantom Must find 9 spokes on 4 subsequent slices in both the T1 and T2 acquisitions This test is also performed in our Monthly QA

29 Ghosting Ratio To assess the level of ghosting in the images Test utilizes the MRI ACR phantom ROIs are drawn inside the phantom and in 4 surrounding areas to calculate the ghosting. Ratio must be less than This test is also performed in our Monthly QA Ghosting Ratio = [(top + btm) (left + right)] / (2 x Large ROI)

30 Couch Motions Couch Level Couch Sag Couch orthogonal to imaging plane Laser / MRI Coincidence Mechanicals Radiation / MRI Coincidence Radiation Isocentricity Gantry Angle Accuracy

31 Couch Couch Motions Couch Level: A level was placed on the couch oriented in the transverse and longitudinal directions and recorded the values Couch Sag: Solid Water was placed on Head Side and then Foot Side and displacements measured with MRI 0.3mm Head 0.39 Foot Couch motion vs MRI coordinates: Spatial Integrity Phantom was imaged at two known locations then fused to demonstrate proper couch motion Couch orthogonal to MRI coordinates: Spatial Integrity Phantom was imaged in different orientations to confirm orthogonality

32 Radiation/MRI versus Laser Isocenter The radiation versus laser isocenter was determined by using the IC profiler and confirmed with starshots Laser and MRI coincidence was determined with daily QA phantom and MRI visible fiducials. The offset was determined by measuring the fiducials location versus the isocenter location. Sagittal Laser Offset Coronal Laser Offset Axial Laser Offset -0.7 mm 0.0 mm 0.8 mm

33 Gantry Radiation Isocenter 4 Starshots were collected at various gantry angles determine the size of the isocenter Laser / RT coincidence Due to the design of the system films cannot be placed at isocenter manually The Daily QA phantom has a circular film holder for this purpose but requires laser cut film

34 Gantry Angle Accuracy Radiation beams were delivered to the Arccheck device utilizing all three heads at various gantry angles and analyzed in the Arccheck software to determine gantry angle accuracy Gantry angles were also confirmed utilizing starshots collected on the Daily QA phantom Head Set Angle VR Readout AC Measured H H H H H H H

35 MLC Leakage MLC leakage films were collected on all three heads at different gantry angles to ensure leakage is within tolerance and isn t dependent on gantry rotation.

36 Employs a wire jig with 4cm separation between wires Measures MLC positioning accuracy Align the wire jig to the sagittal laser Treatment plan was created for a series of MLC segments centered at 0,- 12,-8,-4,4,8,12cm The MLC positioning accuracy is 0.2cm In house software written to process data, see us at AAPM in Washington DC. MLC Accuracy

37 Field Size Accuracy Square fields were measured with the IC Profiler with a 1 cm depth (the intrinsic buildup of the IC Profiler device) Field Size Field Size Axis Head Gantry Flatness Symmetry Neg_Penumb Pos_Penumb Beam Center Field Size Difference (mm) 4.2x4.2 x-measured x-measured x4.2 Y-measured Y-measured x10.5 X-measured X-measured x10.5 Y-measured Y-measured x21 X-measured X-measured x21 Y-measured Y-measured x27.3 X-measured X-measured x27.3 Y-measured Y-measured x4.2 x-measured x-measured x-measured x4.2 Y-measured Y-measured Y-measured x10.5 X-measured X-measured X-measured x10.5 Y-measured Y-measured Y-measured x21 X-measured X-measured X-measured x21 Y-measured Y-measured Y-measured x27.3 X-measured X-measured X-measured x27.3 Y-measured Y-measured Y-measured

38 Output Calibration A28 MRI compatible chambers employed Calibration conditions: 5 cm depth in solid-water 105 cm SAD, 10.5 x 10.5 cm field. Beams delivered from 90 degrees in solid water phantom For verification, heads 1 and 3 were measured in liquid water at 0 degrees (AP beam), and these measurements matched the measurements in solid water to less than a 0.5% difference.

39 RPC OSLDs Report

40 Percent Depth Doses A28 TPS-measured % Measured vs TPS depth (mm) 4.2x x x % % 0.90% 2.94% 80.00% % 0.00% 0.00% 70.00% % -0.11% -0.12% Percent Depth Dose 60.00% 50.00% 40.00% 30.00% 4.2 x 4.2 TPS 10.5 x 10.5 TPS 27.3 x 27.3 TPS 4.2 x 4.2 Measured % -0.44% -0.02% % -0.46% -0.04% % -0.66% 0.05% 20.00% 10.5 x 10.5 Measured % 0.14% 0.12% 10.00% 27.3 x 27.3 Measured % -0.17% 0.07% 0.00% % -0.67% 0.35% Depth (mm) % -0.87% 0.14%

41 Beam Profiles Profiles with different depths of solid water Three depths (1,5, and 10cm) with detector plane maintained at Iso (105cm SAD) 4.2x4.2 cm, 10.5x10.5 cm, 21.0x21.0 cm, and 27.3x27.3 cm field sizes were measured Same profiles were exported from the TPS and the profiler software was used to calculate the flatness and symmetry All profiles matched within specifications Depth = 10 cm Field Size Axis Head Gantry Flatness %Diff Symmetry %Diff 4.2x4.2 X-measured X-measured X-measured X-calc x4.2 Y-measured Y-measured Y-measured Y-calc x10.5 X-measured X-measured X-measured X-calc x10.5 Y-measured Y-measured Y-measured Y-calc x21.0 X-measured X-measured X-measured X-calc x21.0 Y-measured Y-measured Y-measured Y-calc x27.3 X-measured X-measured X-measured X-calc x27.3 Y-measured Y-measured Y-measured Y-calc x27.3 X-measured X-measured X-calc x27.3 Y-measured Y-measured Y-calc x27.3 X-measured X-measured X-calc x27.3 Y-measured Y-measured Y-calc 6 0.1

42 Shutter Timer Error A28 chamber was irradiated several times for two different durations (30s and 240s)

43 Shutter Dose Compensation 1ss nnnn 200ss nnnn/200 DDDDDDDD CCCCCCCCCCCCCCCCCCCCCCCC = 200ss nnnn/200 An A28 chamber was irradiated several times for two different lengths of time (200seconds and 1second) and the compensation time is calculated using the equation shown above Compensation values from the configuration file was recorded and compared to the calculated value

44 Timer Accuracy A 30-second exposure was delivered for each beam. Exposure time was verified with a stopwatch with NISTtraceable calibration. Head Set Time Measured Time Percent Error % % %

45 Timer Linearity Exposures of 5, 15, 30 and 60 seconds were measured with an ion chamber at 1.5 cm depth in solid water. The measured exposures were fit to a line and the line fit was compared to the measured value for each time. H Rdg Fit % Difference Meas-Fit Time (s) 5 s 15 s 30 s 60 s s 15 s 30 s 60 s Shutter Timer Error (sec) H H H

46 CIRS motion phantom Sensor is placed a known distance inferior to the most superior extent of motion Beam should trigger off when motion phantom reaches the level of this optical sensor Latency is discrepancy between actual beam off and optical sensor triggering. Measured with a four channel oscilloscope. Gating Latency

47 Gating Dosimetry

48 Couch Attenuation Relative Attenuation Gantry Angle

49 TPS Statistics Viewray utilizes a Monte Carlo based treatment planning system The number of particle histories used in the calculation increases the accuracy, but Using a high # of histories in adaptive planning can significantly increase the amount of time the patient is on the table. Max Dose Min Dose Mean Dose Standard Deviation 1-Million Histories Million Histories

50 End to End Tests IMRT Delivery IMRT Two Head Delivery RPC Head and Neck Phantom RPC Lung Phantom TG-119 Tests Head and Neck Prostate Multi Target C-Shape

51 IMRT Delivery Delivery Mode Gamma (3%, 3mm) Full Delivery Measured to Calculated Dose 99.4 % Interrupted Delivery: Measured to Calculated 100 % Interrupted Delivery: Measured to Measured 99.9 %

52 IMRT Two-Head Mode Delivery Mode Gamma (3%, 3mm) Full Delivery Measured to Calculated Dose 99.4 % Two Head Mode: (Two Head Mode) Measured to (Three Head Mode) Calculated (Two Head Mode) Measured to (Three Head Mode) Measured 99% 100%

53 RPC Head and Neck Primary PTV Rx Secondary PTV Rx OAR Max Normal Tissue 6.6Gy 5.4Gy 4.5Gy 7.26Gy

54 RPC Head and Neck Constraints Primary PTV Rx Secondary PTV Rx OAR Max Normal Tissue 6.6Gy 5.4Gy 4.5Gy 7.26Gy

55 RPC Head and Neck

56 RPC Lung Primary PTV Rx Cord Max Heart <33% Heart <66% Heart <100% Both Lungs <37% 6.0Gy 5.0Gy 6.0Gy 4.5Gy 4.0Gy 2.0Gy

57 RPC Lung Constraints Primary PTV Rx Cord Max Heart <33% Heart <66% Heart <100% Both Lungs <37% 6.0Gy 5.0Gy 6.0Gy 4.5Gy 4.0Gy 2.0Gy

58 RPC Lung

59 TG-119 Summary Test Gamma (2.5%/2.5mm) Head and Neck 95% Prostate 97.4% Multitarget 98.2% C-Shape 96.3%

60 Acknowledgements Alan Pollack, MD PhD Radka Stoyanova, PhD Viewray Inc: David Holloway, Maria Bellon, Rebecca Sandbrook, James Victoria, Michael Saracen, John Ryan UCLA: Minsong Cao and James Lamb WashU: Olga Green