David ShepardSwedish Cancer Institute

Seattle, WA

CyberKnife: Treatment Planning, QA, and Clinical Applications

• Cristian Cotrutz – Swedish Cancer Institute• Bill Main - Accuray Inc.

Acknowledgements

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

CyberKnife Basics

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.

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.

Linear Accelerator

Manipulator

ImageDetectors

X-ray Sources

IMAGINGSYSTEM

ROBOTICDELIVERYSYSTEM

CyberKnife – Treatment Delivery

• 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

• 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

Optic Apparatus Meningioma

†Case provided courtesy of Barrow Neurological Institute - Phoenix

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

CyberKnife Treatment Planning

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

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

• 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

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

Setting the Planning Goals

Lung Met to T-Spine

Meningioma

Nasopharyngeal Tumor

Prostate

• 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

• 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

Clinical Results: HDR-like Prostate Plan

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.

CyberKnife Immobilization and Tracking

• 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

Skull Tracking

Fiducial Tracking

Xsight Spine Tracking

Spine tracking without fiducials based on bony anatomy

Live kV image

Image B

Image A

DRR (from CT) Displacement Field

Xsight™ Spine Tracking System

Enables registration of non-rigid skeletal anatomy

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.

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.

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.

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

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.

CyberKnife Quality Assurance

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

Daily QA – Linac Output Constancy

• In air measurement using “birdcage” phantom.• CyberKnife’s ion chambers are vented to the atmosphere.

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.

Anthropomorphic Head Phantom

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.

Ball-Cube Film Cassette

• Allows accuracy measurements using only two films• Contains fiducials for QA for extracranial treatments.

Courtesy of Accuray Inc.

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.

TPS Images

70% contour aimingat 31.75 mm ball target.

Courtesy of Accuray Inc.

Digital Centroid Analysis Software

• 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.

Synchrony End-to-end Test

Ball Phantom and Ball Cube

New CyberKnife Features

• Sequential Optimization• 800 MU/min accelerator• Monte Carlo Dose Calculation• Iris Variable Aperture Collimator• RoboCouch

800 MU/min. LINAC

• Provides reduced treatment times relative to existing 600 MU/min design.

• More compact

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

• 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

Example 1: Prostate treatment plan, homogeneous dosedistribution; 35Gy delivered in 5 fractions

Fixed collimator600MU/min. LINAC

Iris™ Variable Aperture Collimator800MU/min. LINAC

Example 2: Lung treatment plan, 60Gy delivered in3 fractions

Fixed collimator600MU/min. LINAC

Iris™ Variable Aperture Collimator800MU/min. LINAC

RoboCouch®

RoboCouch®

• 6D robotic couch• Converts between

seated and flat positions

• 500lb weight capacity

• Seated load height approximately 16 inches from the floor

• Table top converts between seat and flat positions

RoboCouch® - Key Features

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.

CyberKnife at the SCI

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

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.

CK Imaging for PBI: 1st patient

CK Imaging for PBI: Fiducials

1st imaging study

CK Imaging for PBI: Sychrony

1st imaging study

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.

• 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.

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

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.

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.

Film is indexed on the edge of the ball cube

Courtesy of Accuray Inc.

Ball-Cube Film Cassette

Stereotactic Targeting Accuracy Measurement

Single axis targeting error

70%

Dose Distribution

Target Sphere

Courtesy of Accuray Inc.

Synchrony®®

camera Linearaccelerator

Manipulator

Imagedetectors

X-ray sourcesTARGETING SYSTEM

ROBOTIC DELIVERY SYSTEM

Treatment Couch

Dose Comparison – MC and Ray-tracing

Courtesy of Charlie Ma

Courtesy of Charlie Ma

Dose Comparison – MC and Ray-tracing

• 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

Xsight® Lung Tracking System

DRR

LIVE

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