cyberknife: treatment planning, qa, and clinical applications
TRANSCRIPT
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David ShepardSwedish Cancer Institute
Seattle, WA
CyberKnife: Treatment Planning, QA, and Clinical Applications
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• Cristian Cotrutz – Swedish Cancer Institute• Bill Main - Accuray Inc.
Acknowledgements
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1) To provide an overview of the physics of CyberKnife radiosurgery.
2)To review the treatment planning and quality assurance procedures for the CyberKnife.
3)To discuss clinical applications of the CK.
Objectives
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CyberKnife Basics
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CyberKnife - Basics
• A treatment unit designed for both intracranial and extracranial radiosurgery.
• CyberKnife uses a compact linear accelerator mounted on a robotic arm, which has 6-degrees of freedom.
• Pencil beams of radiation are delivered sequentially as the robot moves around patient.
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Image Guided Radiosurgery
• The CyberKnife delivers frameless radiosurgery.• During delivery, the patient position is monitored
and the delivery is modified to correct for patient movement.
• Orthogonal kilvoltage (kV) x-ray sources are mounted to the ceiling and directed at amorphous silicon detectors on both sides of the table.
• kV images are obtained before and during the treatment to monitor the alignment of the patient.
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Linear Accelerator
Manipulator
ImageDetectors
X-ray Sources
IMAGINGSYSTEM
ROBOTICDELIVERYSYSTEM
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CyberKnife – Treatment Delivery
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• 6 MV accelerator• 12 interchangeable circular
collimators• At an SAD of 80cm,
collimators provide a beam diameter from 5 to 60 mm
• SAD can be varied from 65 to 100 cm
CyberKnife – Beam Characteristics
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• Radiation is delivered at a discrete set of linac positions (called nodes).
• A typical treatment plan will use approximately 40-50 nodes (short path) or 60-75 nodes (full path).
• The nodes are distributed approximately uniformly over about one half of a sphere centered on the treatment site.
CyberKnife – Delivery
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Optic Apparatus Meningioma
†Case provided courtesy of Barrow Neurological Institute - Phoenix
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At an SAD of 80 cm, the circular collimators provide a beam diameter that ranges from:
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0% 1. 1 to 10 mm2. 3 to 40 mm3. 3 to 80 mm4. 5 to 60 mm5. 10 to 100 mm
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Answer:
• At an SSD of 80cm, collimators provide a beam diameter from 5 to 60 mm
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CyberKnife Treatment Planning
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1. Identify the treatment site.– Each anatomical location has
a predetermined group of nodes that are considered a good selection for that site.
Treatment Planning Steps
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2. Select single-isocenter, multi-isocenter, or a non-isocentric plan.– Single isocenter used for the delivery of a spherical region
of high dose (Gamma Knife “shot”).– Multi-isocenter used for a ball-packing approach.– Non-isocentric technique is used most commonly due to
greater flexibility.
Treatment Planning Steps
Multi-isocenter Non-isocentric
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• For non-isocentric delivery the planning system defines up to 12 discrete pointing directions (vectors) at each node.
• The combination of nodes and pointing vectors provides a set of 1320 “beams” from which to construct a plan.
Non-isocentric Delivery
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3. Select the size of the collimator(s).• For most cases, a collimator diameter is chosen that
is equal to approximately 75% of the lesion’s smallest projected cross-section.
4. Define the treatment goals run the inverse planning software.
Treatment Planning Steps
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Setting the Planning Goals
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Lung Met to T-Spine
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Meningioma
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Nasopharyngeal Tumor
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Prostate
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• No dose volume constraints.• Hard constraints may lead to an “infeasible”
solution, requiring the user to:1. Increase the range of allowable dose to tumor2. Increase the max. dose allowed to critical structures3. Reduce constraint weights 4. Repeat until a solution is found
CyberKnife’s LP Based Optimization
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• Accuray now provides a scripting approach where the users can prioritize the treatment goals.
• A series of optimization are run in succession with one new goal added at time.
• This approaches always guarantees a feasible solution.
New Sequential Optimization
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Clinical Results: HDR-like Prostate Plan
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What type of optimization algorithm is used in CyberKnife inverse
planning?
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0% 1. Linear programming2. Mixed integer programming3. Nonlinear programming4. Simulated annealing5. Genetic algorithm
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Answer:
• The CyberKnife inverse planning algorithm uses linear programming.
• Linear programming is a procedure for finding the maximum or minimum of a linear function where the arguments are subject to linear constraints. The simplex method is one well known algorithm.
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CyberKnife Immobilization and Tracking
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• Intracranial lesions:– Immobilization with aquaplast mask– Patient positioning is monitored using bony
landmarks• Extracranial lesions:
– Immobilization with vacuum bag– Patient positioning is monitored using either:
1. implanted fiducial markers2. spinal cord anatomy3. lung tumor silhouette
Frameless Radiosurgery
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Skull Tracking
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Fiducial Tracking
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Xsight Spine Tracking
Spine tracking without fiducials based on bony anatomy
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Live kV image
Image B
Image A
DRR (from CT) Displacement Field
Xsight™ Spine Tracking System
Enables registration of non-rigid skeletal anatomy
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Synchrony™ Respiratory Tracking System
• Tracking for tumors impacted by respiratory motion
• Patient wears a vest with optical markers that serve as a surrogate for tumor position.
• Camera system monitors position of implanted fiducial markers.
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Synchrony™ Respiratory Tracking System
• Before the treatment, a correspondence model between the optical markers and the tumor position is constructed using the camera and multiple orthogonal x-rays.
• Model is updated continuously during treatment by further x-ray imaging.
• During delivery, the tumor position is tracked using the live camera signal and the correspondence model.
• The robot is moved in real-time to maintain alignment with the tumor.
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Xsight Lung Tracking
• Can be used to track lung tumors without implanted fiducials.
• Tracks the silhouette of the lesion that appears on the orthogonal images.
• Works for 20-30% of lung cases.• Tumor must be peripherally
located and > 2.0 cm in diameter.
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Align Global Patient Position (6D) Using Xsight® Spine Tracking System
Track Moving Tumor (3D)Using Direct Tumor Registration
Move Patient from Align Center to Treatment Center Using AutoCouch
Static spine region
Dynamic lung tumor
Xsight® Lung Tracking System
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The tracking techniques available for the CyberKnife include all of the
following except:
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0% 1. Skull tracking2. Electromagnetic beacon tracking3. Fiducial tracking4. Spine tracking base on bony anatomy5. Tracking of lung tumor silhouette
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Answer:
• Electromagnet Beacon Tracking (e.g. Calypso) is currently not an available option for the CyberKnife.
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CyberKnife Quality Assurance
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CyberKnife Routine QA Procedures
• Linac Output• Various voltages and currents• Robot perch position• Safety interlocks• Coincidence of treatment beam
with imaging center (AQA)
Daily QA
• Beam Energy• Flatness/symmetry/penumbra• Robot pointing (b.b. test)• End-to-end test• Laser/radiation coincidence
Monthly QA
• Imaging system alignment• Couch position accuracy
Quarterly QA
• Spot check beam data• Treatment planning system
beam data and calculation checks.
Annual QA
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Daily QA – Linac Output Constancy
• In air measurement using “birdcage” phantom.• CyberKnife’s ion chambers are vented to the atmosphere.
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Monthly QA - End-to-end Test
• QA test designed to measure total accuracy of the system including localization, mechanical targeting, and planning errors.
• Measurements are performed using an anthropomorphic head phantom loaded with a target ball and orthogonal pieces of gafchromic film.
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Anthropomorphic Head Phantom
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Anthropomorphic Head/Neck phantom
2.5” Ball Cube in cranium for fiducial and skull tracking QA
1.25” Ball Cube in neck for Xsight Spine QA
Courtesy of Accuray Inc.
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Ball-Cube Film Cassette
• Allows accuracy measurements using only two films• Contains fiducials for QA for extracranial treatments.
Courtesy of Accuray Inc.
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End-to-end Test
• The head phantom is imaged using CT.• A treatment plan is developed with the goal of
conforming the 70% isodose line to the target ball.• After the delivery, the orthogonal films are scanned
and analyzed using software from Accuray that determines the shift between the centroid of the 70% isodose curve and the center of the film.
• Test is repeated for each tracking technique: skull tracking, fiducial tracking, spine tracking, and synchrony based tracking.
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TPS Images
70% contour aimingat 31.75 mm ball target.
Courtesy of Accuray Inc.
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Digital Centroid Analysis Software
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• The total error should be below 0.9mm for skull tracking, fiducial tracking, and X-sight spine tracking.
• The total error should be less than 1.5mm for tracking using Synchrony.
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Synchrony End-to-end Test
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Ball Phantom and Ball Cube
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New CyberKnife Features
• Sequential Optimization• 800 MU/min accelerator• Monte Carlo Dose Calculation• Iris Variable Aperture Collimator• RoboCouch
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800 MU/min. LINAC
• Provides reduced treatment times relative to existing 600 MU/min design.
• More compact
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Iris™ Variable Aperture Collimator
• Description– 2 stacked banks of 6 tungsten
segments creates a 12-sided variable aperture
– Variable aperture automatically replicates sizes of the existing 12 fixed collimators (5 to 60 mm)
– All segments are driven by a single motor
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• Benefits– Can use up to 12 different aperture sizes in a
single treatment path– Reduces treatment time by consolidating multiple-
path sets and multiple-collimators into a single path set
– Better plan quality can be achieved by using multiple collimators
– Automatically changes the size of the variable aperture without having to re-enter the treatment suite
Iris™ Variable Aperture Collimator
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Example 1: Prostate treatment plan, homogeneous dosedistribution; 35Gy delivered in 5 fractions
Fixed collimator600MU/min. LINAC
Iris™ Variable Aperture Collimator800MU/min. LINAC
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Example 2: Lung treatment plan, 60Gy delivered in3 fractions
Fixed collimator600MU/min. LINAC
Iris™ Variable Aperture Collimator800MU/min. LINAC
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RoboCouch®
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RoboCouch®
• 6D robotic couch• Converts between
seated and flat positions
• 500lb weight capacity
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• Seated load height approximately 16 inches from the floor
• Table top converts between seat and flat positions
RoboCouch® - Key Features
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Monte Carlo Dose Calculation
• Accuray is now offering a Monte Carlo dose engine.
• This provides a significant improvement in dose accuracy relative to their current ray-tracing algorithm.
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CyberKnife at the SCI
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CK Prostate Protocol
• 15 institution trial• 5 fractions – 36.25 Gy to PTV and 40 Gy
to prostate• 4 or more gold fiducials are placed for CK
tracking
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CK for PBI – A feasibility study
• 4 to 6 gold fiducials placed at time of surgery• 2 separate fiducial tracking sessions• 5 to 10 initial x-rays image sets are obtained to
see if an initial alignment can be achieved.• 5 to 10 additional live images are then obtained
to develop a 3D model of the lupectomy site movement during respiration.
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CK Imaging for PBI: 1st patient
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CK Imaging for PBI: Fiducials
1st imaging study
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CK Imaging for PBI: Sychrony
1st imaging study
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CyberKnife - Summary
• Radiosurgery delivered using an x-band linear accelerator mounted on a robotic arm.
• Uses a frameless approach and is capable of intracranial and extracranial radiosurgery.
• Real time image-guidance is accomplished using 2 kilovoltage imagers.
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• Advantages of the CyberKnife– Frameless
• Fractionated delivery– Can be used for both intracranial and
extracranial stereotactic delivery.• Disadvantages of the CyberKnife
– The use of a pencil beam based delivery is inefficient and can lead to treatment times that can be up to several hours.
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U.S.U.S.56 Installed CK66 Pending Install CK
EuropeEurope7 Installed CK8 Pending Install CK
AsiaAsia22 Installed CK22 Pending Install CK
85 CyberKnife Systems Installed85 CyberKnife Systems Installed97 CyberKnife Systems Pending Install97 CyberKnife Systems Pending Install
Americas (outside U.S.)Americas (outside U.S.)1 Pending Install CK
*October 2006
85 CyberKnife Units Installed Worldwide
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CyberKnife - Basics
• A treatment unit designed for both intracranial and extracranial radiosurgery.
• CyberKnife uses a compact linear accelerator mounted on a robotic arm, which has 6-degrees of freedom.
• Pencil beams of radiation are delivered sequentially as the robot moves around patient.
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CyberKnife - Summary
• Radiosurgery delivered using an x-band linear accelerator mounted on a robotic arm.
• Uses a frameless approach and is capable of intracranial and extracranial radiosurgery.
• Real time image-guidance is accomplished using 2 kilovoltage imagers.
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Film is indexed on the edge of the ball cube
Courtesy of Accuray Inc.
Ball-Cube Film Cassette
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Stereotactic Targeting Accuracy Measurement
Single axis targeting error
70%
Dose Distribution
Target Sphere
Courtesy of Accuray Inc.
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Synchrony®®
camera Linearaccelerator
Manipulator
Imagedetectors
X-ray sourcesTARGETING SYSTEM
ROBOTIC DELIVERY SYSTEM
Treatment Couch
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Dose Comparison – MC and Ray-tracing
Courtesy of Charlie Ma
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Courtesy of Charlie Ma
Dose Comparison – MC and Ray-tracing
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• 2D Tumor contour generated in each DRR projection in MultiPlan® Treatment Planning System
• Window created containing tumor contour plus surrounding contrasted background
• Window utilized as the matching block in tumor registration
contour
window
DRR A DRR B
Xsight® Lung Tracking System
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Xsight® Lung Tracking System
DRR
LIVE
Maximum similarityMatching window
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