8/16/2011
Intracranial Stereotactic Radiosurgery
(SRS) and Stereotactic Radiotherapy (SRT)
Kamil M. Yenice, PhD
University of Chicago
Radiosurgery
• The use of radiation as a “surgical”
tool
• Small volumes of tissues within the
brain are treated with large doses
delivered in a single fraction
• Normal tissues are protected by
the rapid dose falloff and by
delivering the treatment with high
precision
*
*
Focused high intensity radiation dose requires crossfiring
of many beams.
Target size determines the dose falloff characteristics
beyond the target boundary
Small target, narrow beams
High dose is focused to where
beams intersect over the target
Large target, broad beams
Increased beam overlap beyond target
boundary
Figures: Jürgen Arndt
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8/16/2011
GammaKnife System Evolution
First Gamma Unit Design for SRS (1967)
Model 4C: computer control and APS (2004)
Model U Gamma Unit (1986)
201 sources
4, 8, 14, 18 mm helmets
Manual positioning
GK Perfexion Unit (2006)
192 sources
No collimator helmets
Only 4, 8, 16 mm collimation
Early Gamma Unit collimator with elliptical beam collimation designed for functional SRS
The Co-60 sources are evenly distributed over the surface of the hemispherical source core so
that each beam is directed at a common focal spot at the center
Original Linac Radiosurgery System at the Joint Center (~1986)
MLC Based Linac Radiosurgery by Varian/BrainLAB: Technology
Evolution
NOVALIS: 6 MV treatment beam + M3 MLC + ExacTrac x-rayimaging
NOVALIS TX: Dual Mode Machine + HDMLC + OBI + ExacTrac x-ray imaging
TrueBeam STX: Refinement of Imaging and treatment delivery (FFF mode for SRS)
Linacs for dedicated radiosurgery are also available from other vendors
Radiation is delivered via small cones in multiple arc geometry with gantry motion
Patient immobilization and setup is achieved with a floor stand
Photo courtesy of Wendell Lutz, PhD
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8/16/2011
CyberKnife: dedicated robotic linac radiosurgery system
Beam Shaping
• Small fields shaped by
tertiary collimating system
• precisely machined
• closer to patient smaller geometric
penumbra
• diverging beam
shaping further
minimizes penumbra
Kilby et al, “The CyberKnife® Robotic Radiosurgery System in
2010,” Technology in Cancer Research and Treatment (2010)
Why need a tertiary collimation system?
Tertiary Collimation minimizes the
geometric penumbra and
positioning error
SRS Treatment Process: Linac and GammaKnife
•
•
•
Upper
Jaw
Lower
Jaw
Tray
Collimator
Tertiary
Collimator
Distance from isocenter (mm)
72
62
35
23
Geometrical penumbra for 2mm
focal spot (mm)
5.1
3.3
1.1
0.6
Positional error due to 0.5mm
displacement of X-ray target (mm)
1.3
0.8
0.3
0.15
Positional error due to 0.5 mm
displacement of collimator (mm)
1.8
1.3
0.8
0.65
Smith et al Radiation Oncology Investigations (1993)
•
•
•
•
Frame placement: rigid immobilization for imaging and treatment
Imaging
– CT, MR, and Angiography/DSA
Treatment Planning
– Stereotactic localization and image registration
– Target and structure delineation
– Beam (shot) selection and placement
– Iterative optimization
– Dose selection and normalization
Treatment Plan Evaluation
– Dose distribution
– DVH analysis for target and critical structures
– Various Conformity measures
Treatment Plan QA
Treatment Machine and Patient QA
Setup and treatment
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Stereotactic Frames
Radiosurgery Target Delineation
– patient immobilization
• rigid fixation of cranial anatomy
• Is it really rigid?
– 0.36±0.2 mm (Li et al Med Phys 2011)
– target localization
• precise identification of target
coordinates in a stereotactic
coordinate frame
– treatment setup
• patient setup must guarantee
accurate placement of target
coordinates to the nominal isocenter
of the linac
?
CT
T1
Flair
CT is primary imaging modality (except for GK), structural discrimination is based
on relative atomic composition (electron density info), has high spatial fidelity
MR provides improved soft tissue contrast based on nuclear spin properties of
Hydrogen atoms in tissues, imaging is subject to many sources of errors (distortions)
Images Courtesy of Y Cao, Univ. of Michigan
Inter-observer variability for GTV delineation using CT alone
and impact of MRI
C. Weltens et al. / Radiotherapy and Oncology (2001)
Inter-observer variability in delineating target volume and organs at risk in benign
tumor for SRS (analyzed 21 plans made by 11 clinicians in seven CyberKnife centers)
Yamazaki et al. Radiation Oncology 2011
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AVM Localization on DSA
CT and MR Registration
Rectangular Fiducial Markers
AP view
Registration uncertainties are ~1mm (Wang et al JACMP 2009)
GammaKnife Planning
AVM Target Delineation Process
Nidus
Embolized AVM volume
Lateral view
Conventional Angiography: as contrast material is injected through cerebral vasculature,
Orthogonal x-ray transmission images capture cerebral architecture with respect to a
stereotactic coordinate system.
• Cobalt-201 sources uniformly distributed over an angular
segment of160°×60°uses the idea of the 2p geometry
• Single iso plan
– The shot location and size
– Plug pattern
• Multi-iso plan: sphere packing-manual or algorithm
–
–
–
–
–
DSA images are registered to CT/MR through stereotactic localization
the number of shots
The shot sizes
The shot locations
The shot weights
Iterative optimization of above
• See the talk by D. Shepard – AAPM 2009
Angiography helps identification of the nidus position and differentiation from feeding
arteries and draining veins, not easily identifiable on CT or MR images
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8/16/2011
Linac Based Circular Arc Techniques
Standard University of Florida five-arc set
Conventional arcs with circular cones
3050
2700
550
3400
200
Arcs are achieved through couch and
gantry rotations
Most techniques were developed to mimic GK delivery by early investigators
First attempt at “conformal” planning with circular cones and jaws were explored by
the JCRT group
• Multiple isocenter linear accelerator radiosurgery treatment
planning optimization based on optimal sphere packing
arrangement with circular cones.
• Planning reduces to determining positions and sizes of the
multiple spherical high-dose regions that will be used to fill up
the target volume
67%
70%
35%
14%
38%
13%
Clinical Plan (20 isocenters, 68 arcs, PITV=1.05)
Target volume
Test Plan (20 isocenters, 100 arcs, PITV=1.27)
Sphere packing arrangement
Rx isodose (64%) surface superimposed over target volume.
Wagner et al, “A geometrically based automated radiosurgery planning” IJROBP 2000
3D wireframe representation
of target volume and target
volume with sphere packing
arrangement (5, 10, 12, 20mm).
Wagner et al IJROBP (2000)
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Linac Conformal Radiosurgery with micro MLC
Conformal Arcs
Conventional arc
Geometry with
Conformal beam
shaping
Conformal Static
Beams
Utilization of BEV field
Shaping and
simplification of
planning process
Static Conformal Beam Stereotactic Radiosurgery
• Beam Geometry
– Maximize the solid angle
irradiated: 2p or 4p
– Use a reasonable
number of beams
• How many beams are
reasonable?
• The higher the number of
fields the lower the
peripheral dose
– Use unopposed fields
– Diminishing gains
beyond 11 static beams
compared to a single-iso
4 arc plan
Bourland and McCollough IJROBP 1993
Linac Radiosurgery: Isocenter placement
– Usually at the
geometrical center of
the PTV
– collimator size is set to
encompass most of the
target volume
– Multiple non-coplanar
beams (8-12) or 4-5 arcs
used
– Limit no of beams/arcs
in ANT/POST directions
Dose Selection/Prescription
• Dose selection depends on
– lesion volume
– lesion location
– pre-existing neurologic deficit
– proximity to radiosensitive structures
– lesion pathology
– previous treatments
• Dose prescribed to an isodose line (shell) that
conforms to the periphery of the target
– typically 80% line (sharper dose fall-off outside
the target)
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Prescription Isodose Line : 80 % or 90%?
Single isocenter (arcs or static fields): 80% is near the steepest point of dose falloff .
Multi-isocenter : steepest dose falloff region moves near 70% IDL
GammaKnife: 50% is near the steepest dose falloff
Dose fall-off along axial plane
100
90
80
relative dose
70
60
d80-40=2.7 mm
d90-45=3.3 mm
50
d80-40=4.3 mm
d90-45=5.8 mm
40
30
20
10
0
-50
-45
-40
-35
-30
-25
-20
-15
-10
lateral (mm)
A Trigeminal Neuralgia Case
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Does MLC Leaf Size Matter for SRS?
3 mm MLC
Tumor volume of 1.7 cc
13 non-coplanar beams
at 4 couch rotations
1 mm uniform plan margin
V10Gy Ratio= V10Gy (5 mm)
V10Gy (3 mm)
100
5 mm MLC
Normal Brain Dose (5mm vs 3mm MLC)
V** Ratio (5mm/3mm)
Representative Acoustic Neuroma
5 mm MLC
1.40
1.35
1.30
1.25
1.20
1.15
1.10
1.05
1.00
• V Ratio of 5mm to 3mm
decreases with increasing
volume for 10, 5 and 2 Gy
80
<2 cc
2-4 cc
4-6 cc
6-8 cc
>8 cc
V10Gy
V5Gy
V2Gy
Single Lesion (47 cases)
40
• V Ratio of 5mm to 3mm
was higher for high isodose
lines and lower for lower
dose levels
• Normal tissue dose
difference is significant only
for lesions less than 2cc
20
0
0
3 mm MLC
5
10
15
Dose (Gy)
20
25
Surucu and Yenice, AAPM 2010
<2 cc
2-4 cc
4-6 cc
6-8 cc
>8 cc
1.40
V** Ratio (5mm/3mm)
Volume (%)
1.45
PTV 5mm
PTV 3mm
Brainstem 5mm
Brainstem 3mm
R Cochlea 5mm
R Cochlea 3mm
60
1.35
1.30
1.25
1.20
1.15
1.10
1.05
1.00
V10Gy
V5Gy
V2Gy
Multiple Lesions (19 cases)
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8/16/2011
Stereotactic Radiotherapy (SRT)
• Tumors > 4cm
• Tumors involved with a
critical structure
(<4mm), or benign
tumors (acoustic
neuromas,
meningiomas, pituitary
adenomas)
• Fractionation –
Conventional or hypofractionation
• Radiobiology
• Immobilization – GTC
frame, mask or
frameless approach
with IGRT
• More labor intensive!
Static Conformal vs Intensity Modulated Stereotactic
Radiosurgery
• Static Conformal
• Intensity Modulated
Field shape conforms
to the outline of PTV,
uniform intensity
across the field
Field intensity varies
across the field to
achieve optimum dose
distribution
•
•
•
•
Example: Glioblastoma Multiform
Fractionated SRT
PTV =55.35 cc
Previous radiation tx
Brainstem: Dmax= 60 Gy
Organ at risk: brainstem
Beam arrangement:
14 non-coplanar fields at 5
planes:
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100
• Intensity Modulated
PTV
Relative Volume (%)
• Static
Relative Volume (%)
100
Plan Comparison
80
60
40
20
Static
IM
0
0
20
Brainstem
80
60
40
Static
IM
20
0
0
40 60 80 100 120
Relative Dose (%)
10
PTV
Things that have not changed
significantly for the last 25 years
20
30
Relative Dose (%)
B-stem
Normal Tissue
Plan
CI
V80
V90
UI
D02
V24Gy
V12Gy
Static
1.45
100.0%
96.3%
1.22
3.0 Gy
25.0 cc
106.8 cc
IM
1.38
100.0%
99.7%
1.05
3.0 Gy
21.1 cc
105.1 cc
IM90
1.21
-
-
-
2.7 Gy
11.58 cc
83.9 cc
So you think you can hit the target?
PASS RATES FOR PARTICIPATING INSTITUTIONS
Target = 1.9 cm diameter nylon sphere
Parameters
Linac (509)
GammaKnife
(125)
Dose to
Target
93%
91%
Treated
Volume
90%
98%
Meas. Tx
Vol/Tx Vol
92%
88%
Min Dose
80%
49%
All four
54%
39%
Failure to plan adequate target coverage and/or deliver adequate target coverage
contributed to the low percentages for minimum dose to target compliance.
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8/16/2011
Team work around the clock (1968)
We have come a long way!
Gamma Knife Dose planning on the light table (~1968)
Photo Courtesy of Kristiina Hautanen
Dr E-O Backlund,Prof. L Leksell,Dr Åström and engineer Bengt Jernberg
Slide Courtesy of Kristiina Hautanen
“Water tank” used for early
radiosurgery dosimetry at
the Joint Center
Courtesy of Wendell Lutz, PhD
You only get one chance with radiosurgery!
and never forget
FOOLS WITH TOOLS ARE STILL FOOLS
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