Gamma Knife and CyberKnife:
y
and Quality
Q
y Assurance
Physics
David Shepard
Swedish Cancer Institute
Seattle, WA
A k
Acknowledgements
l d
t
•
•
•
•
•
•
•
•
•
Cristian Cotrutz , Swedish Cancer Institute
Peng Zhang, Hillcrest Medical Center
Steven Goetsch,
Goetsch San Diego GK Center
Paula Petti, Washington Hospital GK Center
Jean Régis,
g Timone University
y Hospital
p
Bill Main, Accuray Inc.
Chad Lee, CK Solutions
M tin Murphy,
Martin
M ph VCU
Michelle Lee, Elekta
Objectives
1)) To p
provide an overview
w of
f the p
physics
y
of
f
Gamma Knife and CyberKnife radiosurgery.
2) To review the quality assurance procedures
f th
for
the Gamma
G
Knife
K if and
d CyberKnife.
C b K if
Gamma Knife - Basics
•
•
A treatment unit designed specifically for intracranial radiosurgery.
radiosurgery
First Gamma Knife built in 1967 under direction of Lars Leksell in
Stockholm, Sweden.
Leksell Gamma Knife®
269 Units installed Worldwide- May 2009
Europe - 38
N th America
North
A
i - 123
Japan - 52
Middle East-7
China - 17
Other Asia - 31
South
America 2
Gamma Knife
f Evolution
1968
Model S
1986
Model U
2 delivered
27 delivered
Sh ff
Sheffield,
,
Buenos Aires
D cr as
Decreased
Improved
mpro
body dose
collimator
design
Computer
dose planning
Courtesy of David Larson
1987
Model B
127 delivered
1999
Model C
2004
Model 4C
2006
Perfexion
152 delivered
49 delivered
36 delivered
S m ro ot c
Semi-robotic
patient
positioning
Improved dose
conformity
St in
Still
n
production
Improved
software
M
Merge/fusion
/f i
capability
Improved
mpro
conformity
Larger cavity
Very low body
d
dose
Rapid
treatment
Full
automation
Expert panel
Recent Advances in Gamma Knife Technology
Model 4C
2006
P f
Perfexion
2004
Model 4C
Gamma Knife
Procedure
Step 1: A stereotactic
head frame is attached
to the patient
patient’ss head
under local anesthesia.
Step 2: The patient is imaged using either MRI
or CT with a fiducial box attached to the
patient’s stereotactic frame.
Step
p 3: A treatment plan
p
is developed.
p
Step 4: The patient’s stereotactic head
f
frame
is affixed
ff d to the
h Gamma Knife’s
f
automatic positioning system.
Step 5: The doors to the treatment unit
open. The patient is advanced into the
shielded
hi ld d treatment vault.
l
• Inside the shielded vault,, the beams from 201 Co-60 sources
are focused so that they intersect at a single location.
• An elliptical region of high dose is produced with a rapid falloff
in dose outside the boundary
y of the ellipse.
p
• Each exposure is referred to as a shot of radiation.
• F
Four focusing
f
h
helmets
l
are available.
l bl
• Each focusing helmet includes 201 collimators that dictate the
size of the shot of radiation (4, 8, 14, or 18 mm).
Creating
g the Treatment Plan (1)
• F
For sm
small
ll sspherical
h i l llesions,
si s th
the planning
l
i is
straightforward.
p , here a single
g 8mm shot
• For example,
covered the target (6mm in diameter).
Creating the Treatment Plan (2)
• For tumors that are large or irregularly
shaped, the planning process becomes more
complex.
• These cases typically require several shots
of radiation.
radiation
• Through an iterative trial-and-error
approach, the user must determine how
many shots
h t to
t use along
l
with
ith their
th i sizes,
i
locations and weights.
M t – 8 shots
Met
h t (18mm)
(18
)
Meningioma – 12 shots
h
(8mm)
(
)
Leksell Gamma Knife® Perfexion™
Gamma Knife Perfexion
• Major redesign of the Gamma Knife.
Knife
• July 2006 – 1st system became operational at
Timone University Hospital of Marseille France.
• August 2006 - FDA approval
Gamma Knife Perfexion
Beam Collimation (1)
• The most critical change in the Perfexion is the
new collimator system.
• The new system replaces the multi-helmet
collimator setup with a single integrated permanent
collimator system that incorporates openings for
4mm, 8mm, and 16mm treatment beams.
Gamma Knife Perfexion
Beam Collimation (2)
• The collimator is partitioned into 8
independently moveable sectors each delivering
24 beams of radiation (192 total sources).
• Beam
B
size
i can b
be changed
h
d dynamically
d
i ll b
by sector.
• Individual sectors can be blocked to provide
further shaping of each shot of radiation.
Gamma Knife Perfexion
Collimator system 16-16-16-16-16-16-16-16
16 16 16 16 16 16 16 16
Collimator system 8-16-16-16-16-16-16-16
8 16 16 16 16 16 16 16
Collimator system 8-16-8-16-16-16-16-16
8 16 8 16 16 16 16 16
Collimator system 8-16-8-16-8-16-16-16
8 16 8 16 8 16 16 16
Collimator system 8-16-8-16-8-16-8-16
8 16 8 16 8 16 8 16
Single Shot Dynamic Dose Shaping
Courtesy of David Larson
Positioning
g System
y
Design
g
• The GK Perfexion positions the patient by moving the couch
rather than moving the patient
patient’ss head within an APS.
• The transition between shot locations is typically under 3 seconds
Courtesy of David Larson
Perfexion – Key Advantages
11. Improved
d patient
i
throughput.
h
h
2. Improved patient comfort.
3 Extended anatomical reach.
3.
reach
Improved Patient Throughput
• No helmet changes.
changes
• The patient no longer needs to moved out of
the unit between shots because the beams can
b moved
be
d to the
h off
ff position.
i i
Courtesy of Jean Régis
Improved Patient Comfort
• Increased space
p
inside collimator
m
body
y leads to
reduced patient anxiety.
• The need for eccentric frame positioning is
eliminated.
eliminated
Courtesy of Jean Régis
Larger Collimator Size
Leksell Gamma Knife® PerfexionTM
Leksell Gamma Knife® C
Extended Anatomical Reach
• Most
M
peripheral
p p
lesions can now
w be treated..
• Elekta is developing a fixation device for cervical
spine lesions.
• Lesions
L i
in
i and
d around
d th
the paranasall sinuses,
i
th
the
orbits, and the cervical spine are now accessible.
Courtesy of Jean Régis
Decreased Body Dose Compared to Previous GK Models
Model B
Model C
Perfexion
Courtesy of David Larson
Decreased Body Dose (x10-100) Compared to Other RS Apparatus
Cyberknife
Corvus
Model C
Novalis
Perfexion
Courtesy of David Larson
• First Perfexion in US installed at Washington Hospital,
Fremont, CA.
• First
Fi t patient
ti t ttreated
t dJ
June 2007
First Perfexion™ Patient at WHHS
3 Mets,, 1 Run
Multi-shot Dose Conformity
Courtesy of David Larson
Multi-target
Multi
target Dose Conformity
(22 mets)
Courtesy of David Larson
Gamma Knife eXtend™
• Elekta will soon offer a toolkit for fractionated
treatments in the head and upper
upper-neck
neck region.
region
• The system will use a stereotactic frame with a
vacuum assisted bite block.
• The system has received its CE Mark and FDA
approval is pending. Expected U.S. release is
Octoberr 2009.
Octo
9.
Gamma Knife eXtend
eXtend™
Step 1: A dental imprint
is taken.
Gamma Knife eXtend
eXtend™
Step 2: Patient is set up
with the vacuum
assisted mouth piece
attached to a carbon
fiber frame.
Gamma Knife eXtend
eXtend™
Step 3: A high resolution probe is used to obtain reference
distances that will later be used to validate the accuracy of
the repositioning at the time of treatment.
treatment CT images are
then obtained with the fiducial box in place.
Gamma Knife eXtend
eXtend™
Step 4: At the time of treatment, a reposition check is performed
with the data automatically transferred to the Gamma Knife
control system. The treatment is then delivered.
Gamma Knife
Routine Q
Quality
y Assurance
Gamma Knife Routine Q
QA Procedures
Model 4C or Earlier
Daily QA
•
•
•
•
•
Warmup
Door interlock
Emergency
g
y off
AV communications
Radiation monitor
Weekly QA
•
•
•
•
Monthly QA
•
•
•
•
•
Radiation output
C mputer output
Computer
utput vs.
vs measured
Emergency rod release
Medical UPS battery check
Ti
Timer
constancy,
st
li
linearity,
it and
d
accuracy
Couch release handle
Helmet microswitches
Helmet trunions
Automatic positioning system
accuracy
Annual QA
•
•
•
Relative helmet factors
Isocenter coincidence
Film measurements
Gamma Knife – APS QA
Q
Gamma Knife – APS QA
Q
B
Basic
QA Tests for
f P
Perfexion
f
1) Coincidence
C i id
of
f the
th mechanical
h i l iisocenter
t of
f th
the
patient--positioning system (PPS) with the
patient
radiation--focal p
radiation
point (RFP)
(
)
2) Agreement of measured beam profiles with Leksell
GammaPlan calculations for all collimator sizes in
the XY,
XY YZ and XZ planes
3) Measurement of the absolute dosedose-rate
calibration for largest
g
collimator
4) Confirmation of the relative output factors
(ROFs) for smaller collimators.
Courtesy of Paula Petti
Testing Coincidence Between PPS and RFP
• The patient is not positioned via an automatic
positioning
iti i system
t
b
butt rather
th th
through
h very
precise couch motions
• Collimator settings (4
(4, 8 and 16) are
independent
• Therefore, the must check the coincidence of
the
h PPS
PP and
d RFP for
f each
h individual
i di id l collimator
lli
Courtesy of Paula Petti
Perfexion - Diode Test Tool
Coincidence of PPS and RFP
• Attaches to patient frame
adapter which then
adapter,
attaches to the PPS
• One central diode
• Tool is used to perform
scans for all 3 major
axes.
Courtesy of Paula Petti
GK QA Notes
• Task Group No. 178 - Gamma
Stereotactic Radiosurgery Dosimetry
and Quality Assurance
p g a calibration
• TG-178 is developing
protocol for GK units based on ADCL
calibrated ionization chambers
Courtesy of Paula Petti
Additional GK Information at AAPM
• TU-E-213A-1 – 4-5:30 PM – “Quality Assurance
f th
for
the Leksell
L k ll Gamma
G
Knife
K if Perfexion”
P f i ” - Paula
P l
Petti - Washington Hospital, Fremont, CA
• SU-FF-T-532
FF
5
– “Immobilization
mm
zat n Accuracy
ccuracy of
fa
Novel Re-Locatable Head Frame Investigated
with a Real-Time Optical Tracking System” Nazanin Nayebi
Nayebi, Princess Margaret Hospital
Hospital,
Toronto
Gamma Knife - Summary
• Radiosurgery delivery technique using beams
from Co
Co-60
60 sources to deliver highly conformal
dose distributions.
• Well established technology used for treating
patients since 1967.
• More than 270 units installed worldwide (36
Perfexion systems) with over 500,000
500 000 patients
treated.
CyberKnife
CyberKnife - Basics
• A treatment unit designed
for both intracranial and
extracranial radiosurgery.
radiosurgery
• CyberKnife uses a compact
linear accelerator mounted
on
n a robotic
b ti arm,
m which
hi h h
hass
6-degrees of freedom.
• Pencil beams of radiation are
delivered sequentially as the
robot moves around patient.
Image Guidance
• 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
x-ray
ray sources are
mounted to the ceiling and directed at amorphous
silicon detectors on either side of the table.
• kV images are obtained before and during the
treatment to monitor the alignment of the patient.
TARGETING SYSTEM
X-ray sources
Manipulator
Synchrony®
camera
Treatment
Couch
Linear
accelerator
ROBOTIC DELIVERY
SYSTEM
Image
d t t
detectors
CyberKnife
y
– Treatment Delivery
y
F
Frameless
l
Radiosurgery
R di
• 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 markers
2)) spine
p
tracking
g (Xsight
(X g spine)
p )
3) synchrony lung tracking
4) soft tissue lung tracking (Xsight lung)
Skull tracking
g window:
Fiducial tracking window:
CyberKnife
y
– Beam Characteristics
• 6 MV accelerator
• 12 interchangeable circular
c llim t s
collimators
• At an SSD of 80cm,
collimators provide a beam
diameter from 5 to 60 mm
• SSD can be varied from 65 to
100 cm
CyberKnife – Delivery
• Radiation is delivered at a
discrete set of linac positions
( ll d nodes).
(called
d )
• A typical treatment plan will
use 110 nodes distributed
approximately uniformly over
about one half of a sphere
centered on the treatment
site.
Meningioma
Nasopharyngeal Tumor
Prostate
™ Respiratory
Synchrony
S
h
R s i t
T
Tracking
ki System
S st m
• Patient wears a vest with optical
p
markers that serve as a
surrogate for tumor position.
• Camera system monitors position of markers.
Synchrony™ Respiratory Tracking System
• Before the treatment, a correspondence model between
the markers and the tumor position
pos t on iss constructed using
us ng
the camera and multiple orthogonal x-rays.
• Model is updated continuously during treatment by
further x-ray
x ray imaging.
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.
Recent 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.
design
• More compact
Monte Carlo Dose Calculation
• This provides a significant improvement in
dose accuracy relative to their current
ray-tracing
y
g algorithm.
g
m.
Dose Comparison – MC and Ray
Ray-tracing
tracing
Courtesy of Charlie Ma
Dose Comparison – MC and Ray
Ray-tracing
tracing
Courtesy of Charlie Ma
Iris™ Variable Aperture Collimator
Iris
• Description
– 2 stacked banks of 6 tungsten
s
segments
ts creates
t s a 12-sided
12 sid d
variable aperture
Variable
able aperture automatically
automat cally
– Var
replicates sizes of the existing 12
fixed collimators (5 to 60 mm)
– All segments
t are d
driven
i
b
by a
single motor
I i ™V
Iris™
Variable
i bl A
Aperture
t
C
Collimator
lli t
• Benefits
B
fi
– Reduces treatment time by consolidating
multiple path sets and multiple-collimators
multiple-path
multiple collimators into
a single path set
– Improved plan quality
– Automatically changes the size of the variable
aperture
p
without having
g to re-enter the
treatment suite
CK - G4 with 8.0
8 0 Delivery Software
• Hillcrest Medical Center (Tulsa,
(Tulsa OK) became 1st center to
treat with Iris Collimator on 7/10/2008.
Comparison of Iris™ Variable Aperture
with Fixed Collimator
• Chad Lee from CK Solutions has performed
plans
l s comparisons
is s between
b t
fixed
fi d collimator
lli t
plans (created in MP 2.1) and IRIS plans
(created in
n MP
M 3.0).
3. ).
• The goal was to achieve similar plan quality and
compare the plans based on # of beams, # of
MUs, and delivery time.
Case 1: Pancreas – IRIS Collimator
Delivery time = 80 minutes/fraction
Case 1: Pancreas – Fixed Collimator
Delivery time = 102 minutes/fraction
Case 1: Pancreas
• Rx: 11 Gy x 3
• MP 2.1
– 3 even paths: 7
7.5,
5 10
10, 15mm fixed
– 238 beams
minutes/fraction
f
– 102 m
• MP 3.0
– 1 full p
path,, IRIS (7.5,
( , 10,, 12.5,, 15,, 20mm))
– 164 beams
– 80 minutes/fraction
Courtesy of Chad Lee
Case 2: Axilla – IRIS Collimator
Delivery time = 85 minutes/fraction
Case 2: Axilla – Fixed Collimator
Delivery time = 107 minutes/fraction
Case 2: Axilla
• Rx: 7 Gy x 5
• MP 2.1
– 2 sh
shortt p
paths:
ths: 20,
20 40mm fixed
fix d
– 283 beams
– 85 min/fraction
• MP 3.0
– 1 full path
path, IRIS (20
(20, 25
25, 30
30, 35
35, 40
40, 50mm)
– 162 beams
– 107 min/fraction
Courtesy of Chad Lee
CyberKnife Routine QA Procedures
Daily QA
•
•
•
•
•
Linac Output
Various voltages and currents
Robot perch position
Safety interlocks
Test coincidence of treatment
beam with imaging center (AQA)
Monthly QA
•
•
•
•
Annual QA
Quarterly QA
•
•
Laser/radiation coincidence
Imaging system alignment
Beam Energy
Flatness/symmetry/penumbra
Robot p
pointing
g
End-to-end test
•
•
Spot
p check beam data
Treatment planning system
beam data and calculation
checks.
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,
localization
mechanical targeting, and planning errors.
• Measurements are performed using an
anthropomorphic head phantom loaded with a
target ball and orthogonal pieces of
gafchromic
gafchrom
cf
film.
m.
Anthropomorphic Head Phantom
A th
Anthropomorphic
hi H
Head/Neck
d/N k phantom
h t
2.5” Ball Cube in cranium for
g QA
Q
fiducial and skull tracking
1.25” Ball Cube in neck
for Xsight
g Spine
p
QA
Q
Courtesy of Accuray Inc.
Ball-Cube Film Cassette
• Allows accuracy measurements using only two films
• Contains fiducials for QA for extracranial treatments.
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.
ball
• After the delivery, the orthogonal films are scanned
and analyzed using software from Accuray that
d t
determines
i
th
the shift
hift between
b t
th
the centroid
t id of
f th
the 70%
isodose curve and the center of the film.
p
for each tracking
g technique:
q
skull
• Test is repeated
tracking, fiducial tracking, spine tracking, and
synchrony based tracking.
TPS
Images
70% contour aiming
at 31
31.75
75 mm ball target.
target
Courtesy of Accuray Inc.
Digital Centroid Analysis Software
Additional CK Information
• TU-E-213A-3 – Rm 213A – 4-5:30 PM –
“E
“Everything
thi Y
You N
Need
d tto K
Know Ab
Aboutt th
the
Cyberknife, But Were Afraid to Ask” – Mary Ellen
Masterson-McGary,
y, Cyberknife
y
Center of Tampa
p
Bay, Tampa, FL
CyberKnife
y
f - Summary
y
• 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.
radiosurgery
• Real time image-guidance is accomplished using 2
kilovoltage imagers.
• The total error should be below 0.9mm for skull
tracking fiducial tracking,
tracking,
tracking and X
X-sight
sight spine tracking.
tracking
• The total error should be less than 1.5mm for
tracking using Synchrony.
Synchrony End-to-end Test
Ball Phantom and Ball Cube
Perfexion vs. 4C – Prospective Study
• At Timone University Hospital 59 patients were
enrolled in a prospective study comparing the Perfexion
and the Gamma Knife 4C.
4C
• With Perfexion the median total treatment time was
reduced from 65 minutes to 44.5 minutes.
• With
Wi h Perfexion
P f i there
h
were no collision
lli i issues
i
while
hil with
i h
the 4C 21% patients treated in trunion mode.
• The Perfexion unit on average reduced dose to the
gonads by a factor of 15.
Courtesy of Jean Régis
RoboCouch®
RoboCouch®
• 6D robotic couch
• Converts between
seated and flat
positions
• 500lb weight
capacity
251 Units Installed Worldwide – June 2007
North America = 118
Europe = 33
China = 17
Middle East = 4
Japan = 51
Other Asia = 26
South America = 2
©ELEKTA INSTRUMENT AB - Sales &
Marketing (www.elekta.com) - LGK –
June 2007
116 Gamma Knife® units in the U.S.
F b
February
2008
AK
H
I
19 Leksell Gamma Knife® Perfexion™ units
J
June
2008
AK
H
I
111 Leksell Gamma Knife® units installed U.S.
November 2006
AK
HI
%Dosse
Collimator Dose Profiles
x (mm)
M t – 11 shots
Met
h t (18mm)
(18
)
Perfexion vs. 4C – Prospective Study
• At Timone University Hospital 59 patients were
enrolled in a prospective study comparing the Perfexion
and the Gamma Knife 4C.
4C
• With Perfexion the median total treatment time was
reduced from 65 minutes to 44.5 minutes.
• With
Wi h Perfexion
P f i there
h
were no collision
lli i issues
i
while
hil with
i h
the 4C 20.7% patients treated in trunion mode.
• The Perfexion unit on average reduced dose to the
gonads by a factor of 15.
Courtesy of Jean Régis
Courtesy of Jean Régis
Target
g
1 Shot
2 Shots
3 Shots
4 Shots
5 Shots
Defining the Prescription
• When multiple shots of radiation are used, the
target dose will be highly non-uniform due to the
overlap between the spherical dose distributions.
• Target is covered by typically 50% of the
maximum dose.
• Advantages of the CyberKnife
– Frameless
• Fractionated delivery
– C
Can be
b used
d for
f both
b th intracranial
i t
i l and
d
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.
hours
• 192 sources
• 8 sectors
t
• 72 Collimators per sector
• 3 shots ssize
ze – 4,, 8, 16
6 mm
Ball-Cube Film Cassette
Film is indexed on the edge of the ball cube
Courtesy of Accuray Inc.
Stereotactic Targeting Accuracy Measurement
Single axis targeting error
70%
Target Sphere
Dose
Distribution
Courtesy of Accuray Inc.
Gamma Knife eXtend™
• Carbon fiber frame with a vacuum assisted mouth piece and a
vacuum pillow.
Reposition Check Tool (RCT)
(
)
•
•
•
•
Electro mechanical QA
High resolution Mitutoyo probe
Reference values
Reposition measurements
MRI
Position
reference
Dental
imprint
CT imaging
Patient setup
Coregistratio
n
Position
Treatment
verificatio
For each
n
fraction
Treatment
planning
Dental imprint
Patient setup – at Perfexion
PCU connected to control
system
Treatment – First session
• S
Support
pport in GUI for
fractionated
treatments
• Reference values
from CT entered
• At least two
reference
f
values
l
per
RCT plate (R,L,F,T)
are needed
Treatment
Evaluation of accuracy
y of
ƒ Non-clinical tests
Extend™ Frame
System
performed
- ongoing at Princess
Margeret
- Six volunteers,
Hospital, Toronto
10 sessions each
ƒ Clinical use at
Synergy ongoing
- Comparison
with CBCT
ƒ Clinical use at
Perfexion™
in the Fall of 2009
G
Gamma
Knife
K if eXtend™
Xt d™
G
Gamma
Knife
K if eXtend™
Xt d™
PCU standalone – at CT
• Patient docked with
vacuum
• Vacuum surveillance
active
• Probe readings on
PCU displa
display
• Value locked when
stable
Position reference – at CT
Treatment – First session
• Patient is docked
using the patient
specific parts
• Vacuum
surveillance
handled by control
system; can not
continue without
activating
surveillance
Treatment – First session
• Position to be
measured is
displayed together
g
with corresponding
reference value
• Probe is locked
when
h value
l iis
stable, result is
p y
displayed
• List of
measurement
result needs to be
confirmed to be
able to start
G
Gamma
Knife
K if eXtend™
Xt d™
G
Gamma
Knife
K if eXtend™
Xt d™
Gamma Knife – Spherical
p
Phantom
Gamma Knife – Spherical
p
Phantom
Leksell Gamma Knife® Perfexion™
•
•
•
•
•
•
The patient is positioned via precise couch motions
A frame adapter attaches the Leksell coordinate frame (affixed to the
patient’s skull) to the treatment couch
Collimation system is built into the unit
3 Collimator sizes: 16
16-mm,
mm 8
8-mm
mm and 4
4-mm
mm
192 60Co sources
60Co sources not fixed in space: they reside on 8 moveable sectors
Collimator
Patient Frame
Adapter
Patient Couch: 3-axis
positioning system
Courtesy of Paula Petti
Sector Positions :
S
Sources
i movable
in
bl
5 different sector positions
(listed from back to front):
1) Home: Sources are here
when machine is off
2) 8-mm
8
3) Sector Blocked
4) 4 mm
5) 16 mm
sectors
Back of
unit
Sources shielded when patient is not in planned position, even when
shielding doors are open
ƒ At the beginning of treatment, the sources move from “Home” position to
“Sector Blocked” position
ƒ Sources move to “Sector Blocked” position while patient coordinates are
Courtesy of Paula Petti
changed
Perfexion™
Perfexion
™ Sector Design
8 independent,
identical sectors
Sectors slide back and forth
on outside of collimator
24 sources per
sector
Sector Drives
Sources are arranged in 5 rings
The sources in each sector can be aligned with a different
collimator size or blocked completely
Courtesy of Paula Petti
Example of PinPin-Point Films to Check RFP PPS Coincidence for 1616-mm Collimator
New-Style Film Holder
Old Style Film Holder
Old-Style
Y
X
Z
Y
Courtesy of Paula Petti
Test Specifications and Frequency
• Elekta’s specification
p
for the 4-mm collimator is that Δx,,
Δy and Δz are all ≤ 0.3mm and that
Δr = Δx 2 + Δy 2 + Δz 2 ≤ 0.4mm
Frequency of tests:
1) Master Diode test is done bi-annually by Elekta service
engineers
i
as partt preventative
t ti maintenance
i t
2) Films are usually irradiated annually by the on-site GK
physicist
Courtesy of Paula Petti
Example of Results using Different Tools to
Check PPS and RFP Coincidence for
WHHS Perfexion™
Perfexion™ GK
Δr = Δx 2 + Δy 2 + Δz 2
(mm)
Collimator =
4-mm
8-mm
16-mm
Master Diode Tool
0.098
0.16
0.16
New-Style
Film Holder
0.09
0.15
0.30
Old-Style
y
Film Holder
0.11
0.20
0.38
Courtesy of Paula Petti
Beam Profiles (film): Measurement Tools
New-style Spherical Phantom
Old-style Spherical Phantom
1) Attaches to patient frame adapter
1) Attaches to dosimetry adapter
2) Film positioned between 2 rods
2) Film positioned in central insert
3) Can irradiate a 3D stack of films
3) Can irradiate only one film at a time
4) Composed of certified Therapy Grade Solid
Water®
4) Presumably composed of polystyrene
5) 3 adapters provided for ion chambers or other
detectors
5) Additional inserts are supplied for ion
chambers and other detectors
Courtesy of Paula Petti
Specifications and Frequency for
B
Beam
Profile
P fil measurements
Specification:
According
A
di tto El
Elekta:
kt M
Measured
d and
d LGP values
l
ffor
FWHM should be within ± 1 mm of each other
Frequency:
Beam profiles should be measured upon acceptance of
the GK unit and annually thereafter
Courtesy of Paula Petti
Results from last Annual QA:
• All FWHM were between ± 0.1
0 1 mm and ±
0.4 mm of Leksell GammaPlan
Courtesy of Paula Petti
16--mm Dose Rate Measurement
16
Performed in spherical phantom
Calibration Protocols:
• TG-21: Ion chamber
calibrated 60Co in-air
– Can be used for various
phantom materials
– Can be used for various
geometrical setups
• TG
TG-51:
51 IIon chamber
h b
calibrated for 60Co in
water.
– Designed to facilitate
linear accelerator
calibration and QA
– phantom must be water,
– 10 cm × 10Courtesy
cm field
size
of Paula Petti
LGK Dose Rate Measurement:
Some Recent Publications
• R Drzymala
y
R Wood,, J Levy:
y Calibration of
the Gamma Knife using a new phantom
following AAPM TG51 and TG21 protocols,
Med. Phys
Med
Phys. 35:514
35:514-521;2008
521;2008
– Compared the 2 protocols in the Elekta old-style
spherical phantom and in a newly designed water
phantom
h t
– TG-51 in water phantom results were 1.4% lower
polystyrene
y y
p
phantom
than TG-21 in p
Courtesy of Paula Petti
LGK Dose Rate Measurement:
Some Recent Publications
• S Griffin Meltsner and LA DeWerd: Air
Kerma based dosimetry calibration for the
Leksell Gamma Knife, Med Phys 36:339350;2009
– Proposes an air-kerma-based dosimetry protocol
using either an in-air or in-acrylic phantom
measurementt
– Modified version of TG-21 specific to LGK
calibration geometry
g
y
– With new protocol, measured dose rates were
between 1.5% and 2.9% higher than those used
clinically by at 7 LGK sites (Models B and C)
Courtesy of Paula Petti
16-mm Dose Rate Measurement: Practical
16I
Issues:
Choice
Ch i off new or oldold
ld-style
t l phantom
h t
Old-style
y p
phantom requires
q
dosimetry adapter
Bhatnagar, et al
Bhatnagar
al. (Med Phys
36:1208-1211;2009):
The dosimetry adapter
attenuates some beams in
the lateral (3 and 7) sectors
off the
th Perfexion™
P f i ™ unit,
it
causing the overall 16-mm
dose rate to be
underestimated by
approximately 1%.
Courtesy of Paula Petti
Choice of phantom
New spherical phantom
• Does not require
dosimetry adapter
• Attaches to patient
adapter
Patient
P
ti t Frame
F
Adapter
– More precise
– Provides better
check of entire
system
Courtesy of Paula Petti
16--mm Dose Rate Measurement:
16
Practical Issues
• Checking measured dose rate: compare
results to
– TLD: either in-house or outside service (e.g.,
RPC SRS phantom)
– EBT GafChromic
Courtesy of Paula Petti
Practical Method for Checking GK
Dose Rate Calibration using EBT Film
Note that EBT film exhibits very little
energy dependence
4.0 Gy
5.0 Gy
6 MV Mini
6-MV
Mini-Calibration
Calibration
Net Cou
unts in Red
Ch
hannel
(Backgroun
nd subtracted)
Irradiate films in 6-MV linear
accelerator beam at 3 dose
levels, e.g. 4, 5 and 6 Gy to
obtain a mini calibration curve
140
130
120
110
100
6-MV Mini-Calibration
90
80
3
4
5
6
7
Dose (Gy)
6.0 Gy
Red channel
extracted,
Background
subtracted,
film intensity
inverted
Courtesy of Paula Petti
Practical Method for Checking Dose
Rate Calibration
6 MV Mini
6-MV
Mini-Calibration
Calibration
Net Cou
unts in Red
Ch
hannel
(Backgroun
nd subtracted)
IIrradiate
di t film
fil with
ith 16
16-mm
collimator, 5.0 Gy @ maximum
140
130
120
110
100
6-MV Mini-Calibration
90
80
3
4
5
6
7
Dose (Gy)
Determine dose at center of
peak region from minicalibration
lib i curve
Compare to expected value
(e g 4
(e.g.
4.96
96 Gy)
Expect ± 2% Courtesy
to 3% agreement
of Paula Petti
Relative Output Factors (ROF) for
the
h 8
8-- and
d4
4--mm Collimators
C lli
• Relative output factor
for Gamma Knife is
defined as:
dDC / dt
dD16 / dt
(100 ,100 ,100 )
i.e., ROF = dose rate of
collimator C relative to 16mm collimator, where both
are measured at isocenter =
(100,100,100) in spherical,
80 mm radius phantom
80-mm
Courtesy of Paula Petti
Unique Features of Perfexion
Perfexion™
™ Geometry
1 2
3 4
5
Co-60 sources
distributed in 5
rings for each
identical sector
Ring
Number
Number of
Sources
1
2
3
4
5
48
32
40
32
40
Courtesy of Paula Petti
ROF: Unique Feature of Perfexion
Perfexion™
™ Geometry
Each collimator within each ring
h a diff
has
differentt beam
b
geometry
t
Courtesy of Paula Petti
Beam Geometry:
There are, therefore, 15 distinct beam geometries: 5 rings
multiplied by 3 beam
beam-on
on positions per ring
ring.
This is in contrast to previous LGK designs for which all of
the beam channels were identical
identical, and there was only one
type of beam.
Each of the 15 beam types
yp has a different:
1) Virtual source-to-isocenter distance (SAD)
2) Output factor
Elekta determined these values by fitting a beam model to
Monte Carlo generated data
data.
Courtesy of Paula Petti
Fitted Values for Relative Output Factor (ROF)
and Virtual SourceSource-to
to--focus distance (Rvsf)
Collimator
Size
Ring
ROF
Rvsf
(mm)
Collimator
Size
Ring
ROF
Rvsf
(mm)
4
1
0.799
521
8
4
0.808
480
4
2
0 815
0.815
546
8
5
0 730
0.730
522
4
3
0.792
533
16
1
0.961
481
4
4
0.725
595
16
2
1.000
459
4
5
0.663
607
16
3
0.986
455
8
1
0.957
431
16
4
0.920
488
8
2
0.946
437
16
5
0.851
519
8
3
0.901
468
Courtesy of Paula Petti
Relative Output Factor for Each Collimator Size
• We cannot measure the 15 ROFs individually
• The ROF for the 88 and 4
4-mm
mm collimators relative to the
16-mm collimator is determined from the equation:
5
ROF (c ) =
∑ n × OF (c )
i =1
i
i
5
∑ n × OF (c = 16mm)
i =1
i
,ni = ( 48,32,40,32,40)
i
Where the sum is taken over all 5 rings
ni represents the number of sources in each ring
ROF(8mm) = 0.924
ROF(4mm) = 0.805
Courtesy of Paula Petti
Two Ways to Determine ROF
Measure dose, Dc, delivered by each
collimator at ((100,, 100,, 100)) for:
• A given treatment time:
ROFc = Dc/D16
• The same prescription dose:
Dc / Tc
ROFc =
D16 / T16
Dc
ROFc ≈
× ROFcno min al
D16
Tc is the irradiation time for
each collimator
The approximately
equal sign is replaced
by an equality if the
dose is prescribed to
(100,100,100) instead
of the point of
Courtesy
of Paula Petti
maximum
dose
Relative Output Factors:
Measurement Techniques
•
•
•
•
Pin-point ion chamber
G fCh
GafChromic
i Fil
Film
TLDs (rods and LiF microcubes)
Glass Rods (Perks, et al.)
Some References:
•
•
•
Mack et al., Precision dosimetry for narrow photon beams
used in radiosurgery - determination of Gamma Knife® output
factors, Med. Phys. 29: 2080-9; 2002
Perks et al
al., Glass rod detectors for small field,
field stereotactic
radiosurgery dosimetric audit, Med. Phys. 32:726-32; 2005
Novotny et al. Measurement of relative output factors for the 8
and 4 mm collimators of the Leksell Gamma Knife Perfexion
by film dosimetry, Med Phys. 36:1768-1774;2009
Courtesy of Paula Petti
ROF Meas: EBT Film and Fixed Dose
¾ Cut and mark Films
¾ Scan un
un-irradiated
irradiated films to obtain background correction
¾ Irradiate 2 films at the 3 dose levels, e.g., 4.5, 5.0 and 5.5 Gy for 16 collimator to
obtain mini-calibration curve (choose either axial, coronal or sagittal plane). It is
reasonable to assume that the calibration curve is piece-wise linear between
measured points (6 films)
¾ Irradiate 2 films the 4- and 8-mm collimators to a dose in the middle of the minicalibration range in the same plane as calibration films (4 films)
¾ “Process” films with ImageJ: extract red channel, invert intensity values, subtract
background
CourtesyROF
of Paula Petti
¾ Sample intensity values in center of films, convert to dose, calculate
My Results: Average of 7 sets of ROF
Measurements
8-mm Collimator: 0.888 ± 0.012 (∼3.9% lower than Elekta)
4-mm Collimator: 0.792 ± 0.007 (∼2.5% lower than Elekta)
Used constant time for 3 sets of measurements, constant
dose for 4 sets
Used different measurement planes (axial, coronal or
sagittal)
Did not change values in LGP
Courtesy of Paula Petti
Results Reported in Literature: Average
of 5 sets of ROF Measurements
Collimator
EDR 2 Film EBT Film
MD-V2-55
MD
V2 55
Film
8 mm
0.904
0
904 ± 0.012
0 012
(-2.1%)
0.917
0
917 ± 0.014
0 014
(-0.8%)
0.906
0
906 ± 0.018
0 018
(-2.0%)
4 mm
0.769 ± 0.010
(-4.5%)
0.810 ± 0.007
(+0.6%)
0.819 ± 0.009
(+1.7%)
Novotny et al
al., Med
Med. Phys
Phys.2009
2009
Courtesy of Paula Petti
Error levels in ROF Measurement
• Neglecting
g
g transit dose:
• Neglecting the
difference between
maximum dose and
dose at (100,100,100)
• Standard deviation in
film pixel values around
point of measurement
0.03Gyy
= 0.6%
5Gy
< 1%
Between 0.5
and 1 count
Courtesy of Paula Petti
What order of magnitude error in film
reading causes a 4% error in dose?
Film Reading Calibration Curve
Read
ding minus Background (inv
verted)
160
140
120
100
Slope in vicinity of 5 Gy ~ 7 counts/Gy
80
Y-intercept ~ 85 counts
60
Calibration Curve from
16mm Data
40
20
0
0
1
2
3
4
5
6
7
8
9
Dose (Gy) at (100,
(100 100
100, 100)
Courtesy of Paula Petti
What order of magnitude error in film
reading causes a 4% error in dose?
From calibration curve on previous slide:
5 Gy ⇒ a reading of 120 counts
48G
4.8
Gy ⇒ a reading
di off 118.6
118 6 counts
t
∴ A difference of only 1.4 counts results in a
p
y
4% dose discrepancy
Courtesy of Paula Petti
Alignment of PatientPatient-Positioning System
(PPS) with Radiation Focal Point (RFP)
Master
Diode Tool:
Tool:
Old-Style
OldFilm
Holder:
Holder
Service
instrument used
during bi-annual
preventative
maintenance
Field
instrument:
used for
annual QA
and
acceptance
t ti
testing
Diode
Tool: Field
New Film
Holder:
Holder
Service and Field
instrument used
for annual QA and
acceptance testing
Courtesy of
instrument
used for
routine
checks (at
least monthly,
but is usually
done more
often)Petti
Paula
Master Diode Tool
Used byy Elekta during
g
Commissioning to allign the
RFP to the PPS
Attaches directly to the PPS
Accomodates up to 5 diodes
There are programmed
scannning sequences for all
three collimators
Courtesy of Paula Petti
Film Holders
New Model
Old Model
Dosimetry
adapter
More precise than old
old-style
style film holder because:
1) Uses the same frame adapter that is used for
patient treatments (not shown above) which
is machined to very high tolerance
2) Has larger film compartment – easier to
interpret 16-mm films
1)) Requires
q
special
p
dosimetry
y adapter,
p , which
is not machined to the same high tolerances
as the patient frame adapter
2) Provides QA for RFP reproducibility, but
d
does
nott provide
id QA ffor patient
ti t setup,
t
since
i
patient adapter is not employed
Courtesy of Paula Petti
QA Reports
R
t and
dR
Recommendations
d ti
1 ASTRO/AANS Consensus
1.
C
St
Statement
t
t on stereotactic
t
t ti
radiosurgery quality improvement, 1993
g y QA
Q Guidelines,, 1993
2. RTOG Radiosurgery
3. AAPM Task Group Report 54, 1995
4. European Quality Assurance Program on Stereotactic
R di
Radiosurgery,
1995
5. DIN 6875-1 (Germany) Quality Assurance in
Stereotactic Radiosurgery/Radiotherapy
u g y/
py
6. AAPM Task Group 68 on Intracranial stereotactic
positioning systems, 2005
Courtesy of Steven Goetsch
16--mm Dose Rate Measurement
16
• Currently no official calibration protocol
specific to the LGK
• Charges of AAPM Task Group 178:
– Suggest a protocol for calibration with
ionization chambers calibrated at an ADCL
that can be used with all Gamma
Stereotactic Radiosurgery (GSR) devices
– Work
W k with
i h the
h working
ki group on d
dosimetry
i
calibration protocol for beams that are not
compliant with TG-51
TG 51
Courtesy of Paula Petti
134 CyberKnife Units Installed Worldwide
Asia
35 Installed CK
U.S.
87 Installed CK
Europe
12 Installed CK
*June 2008
Coincidence of PPS and RFP for Perfexion™
Perfexion™
• No helmets or microswitches to check
• Collimator
ll
settings (4, 8 and 16) are
independent
• Therefore,
Therefore the must check the coincidence
of the PPS and RFP for each individual
collimator
Courtesy of Paula Petti
Iris™ Variable Aperture Collimator
Iris
• Benefits
f
– Can use up to 12 different aperture sizes in a
single treatment path
– Reduces treatment time by consolidating multiplepath 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