8/2/2012
SRS using the CyberKnife
Sonja Dieterich, PhD, DABR
Associate Professor
University of California Davis
Disclaimer/Conflict of Interest
Consulting agreements with Broncus Medical and
CyberHeart, Inc.
Scientific Advisory Board, MGS Research
4 Essentials of CK Physics
1.
Dose Delivery Accuracy
2.
Small-field Dosimetry for SRS
3.
Patient Localization
4.
Treatment Planning
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The CyberKnife System
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n
n
n
1-30 Fractions in 15-60 min
1000 Mu/min
Image-guidance during
treatment
Corrects:
– 10 mm in translation,
– 1.5 degree roll/pitch angle
– 3 degree yaw angle
n
<0.95 mm accuracy
as defined by E2E test
Image courtesy of Accuray Inc.
1. Dose Delivery Accuracy
Winston-Lutz Test
•Reference:
W. Lutz, K. R. Winston and N. Maleki, "A system for
stereotactic radiosurgery with a linear accelerator,"
Int J Radiat Oncol Biol Phys 14, 373-381 (1988).
Aimed at frame-based
SRS
• Image-based setup not
included
•
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Modified Winston-Lutz: AQA
• Modified Winston-Lutz:
• 2 beams
• Image-guided setup using fiducials
• Performed daily
• AQA is lacking:
• Dose distribution is overlay of
many beams
• 4 localization modalities
• Real-time tracking
Modified Winston-Lutz: End-to-End (E2E)
Process & Results of E2E Test
• E2E includes complete
process:
• Simulation
• Contouring
• Planning
• Setup
• Localization
• Treatment delivery
• Specification < 0.95 mm
• Actuals 0.3 mm – 0.7 mm
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Delivery Accuracy in Patient
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n
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Near real-time image
guidance
Frequency based on
acceptable PTV margin
Small PTV margin vs.
image dose and Tx time
Murphy, M. J. (2009). "Intrafraction Geometric
Uncertainties in Frameless Image-Guided Radiosurgery."
International Journal of Radiation
Oncology*Biology*Physics 73(5): 1364-1368.
Furweger et al 2010
SAMS Question 1
Which test defines the overall accuracy of a
CyberKnife?
20%
1.
20%
2.
20%
3.
20%
4.
20%
5.
Winston-Lutz test
Daily beam pointing accuracy (AQA)
Robot calibration
Delivery QA of patient plan (DQA)
End-to-end test (E2E)
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C ountdown
SAMS Answer 1
Which test defines the overall accuracy of a CyberKnife?
n
n
n
n
End-to-end (E2E)test
Simulates patient treatment from CT scan through delivery
All localization algorithms in anthropomorphic phantom
0.3 mm – 0.7 mm in clinical practice
References:
Chang, S. D., W. Main, et al. (2003). "An analysis of the accuracy of the CyberKnife: a robotic
frameless stereotactic radiosurgical system." Neurosurgery 52(1): 140-146; discussion 146-147.
Yu, C., W. Main, et al. (2004). "An Anthropomorphic Phantom Study of the Accuracy of CyberKnife
Spinal Radiosurgery." Neurosurgery 55(5): 1138-1149.
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2. Small Field Dosimetry
IAEA Formalism for Reference Dosimetry
Alfonso, R., P. Andreo, et al. (2008). "A
new formalism for reference dosimetry of
small and nonstandard fields." Med Phys
35(11): 5179-5186.
TG-51 for Flattening-Filter Free Beam
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TG51 uses %dd(10)x
Details in:
Kalach & Rogers, Med
Phys 30 (2003) 1546
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TG 51: How do we get kQ ?
•MC simulation of kQ
• 0.3% difference in linac vs. CK
• Use 6MV linac kQ or measure with 60 mm collimator
•Araki, Med Phys (2006), 2955
TG-51: Measuring %dd(10)x
n CK:
60 mm cylindrical collimator at 80 cm SAD
n Measure
at 10 cm depth, 100 cm SSD, 60 mm collimator
n Calculate
equivalent square
n Interpolate
to 80 cm SAD using the BJR data
TG-51: Chamber selection
Kawachi et al, Med Phys
(2008) 4591
• Dose flatness insufficient for Farmer-type chamber
• Cavity length should be 1 cm or shorter
• Option: cross calibrate a short chamber with Farmer-type chamber
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SAMS Question 2
Why does the dosimeter reading of a Farmer-type chamber
need to be corrected for reference dosimetry?
20%
4.
Difference in kQ for flattening filter vs. 6 MV flattening filter
free beam
To adjust for detector alignment uncertainties
Off-axis beam profile for non-flattened beam changes over
chamber length
High dose rate effect (1000 MU/min) on chamber
20%
5.
To adjust for energy-dependent detector response
20%
20%
20%
1.
2.
3.
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C ountdown
SAMS Answer 2
Why does the dosimeter reading
of a Farmer-type chamber need
to be corrected for reference
dosimetry?
n Off-axis beam profile for nonflattened beam changes over
chamber length
Reference: Kawachi, T., H. Saitoh, et al. (2008).
"Reference dosimetry condition and beam quality
correction factor for CyberKnife beam." Med Phys 35(10):
4591-4598.
3. Patient Localization
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The Principle of 2D-3D registration
Fu, D. and G. Kuduvalli (2008). "A fast, accurate, and automatic 2D-3D image registration for image-guided
cranial radiosurgery." Med Phys 35(5): 2180-2194.
Cranial Localization
XSight Spine Localization
D. Fu, G. Kuduvalli, C. J. Maurer, J. Allision and J. Adler, "3D target localization using 2D local displacements of skeletal structures in
orthogonal x-ray images for image-guided spinal radiosurgery," Int J CARS 1, 198-200 (2006).
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Spine Localization Accuracy
Fürweger, C., C. Drexler, et
al. (2010). "Patient Motion
and Targeting Accuracy in
Robotic Spinal
Radiosurgery: 260 SingleFraction Fiducial-Free
Cases." International
Journal of Radiation
Oncology*Biology*Physics
78(3): 937-945.
SAMS Question 3
Which tracking algorithm would be used for a
C3 spine target?
20%
1.
20%
2.
20%
3.
20%
4.
20%
5.
Cranial tracking
Spine Tracking
Fiducial tracking
LOT tracking
Spine segmentation
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C ountdown
SAMS Answer 3
Which tracking algorithm would be used for a C3 spine
target?
n Spine Tracking
n C3 barely visible in DRR
n Highly flexible cervical spine means cranial tracking not
accurate
n Spine segmentation removes mandible interference, allows
for stable tracking
Reference: Fürweger, C., C. Drexler, et al. (2010). "Patient Motion and Targeting Accuracy in
Robotic Spinal Radiosurgery: 260 Single-Fraction Fiducial-Free Cases." International Journal of
Radiation Oncology*Biology*Physics 78(3): 937-945.
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4. Treatment Planning
Sequential Optimization Example
1.
Initial optimization step,
optimize the minimum dose in
the PTV
2.
Set a constraint on the minimum
dose, optimize the conformality
3.
Set a constraint on conformality,
optimize total beam weight:
19895 MU → 18567 MU
Sequential Optimization
Schlaefer, A. and A. Schweikard (2008).
"Stepwise multi-criteria optimization for
robotic radiosurgery." Med Phys 35(5):
2094-2103.
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Example of Sequential Planning
PDD Calculations for 4 Collimators
with Raytrace and Monte Carlo
• Change in PDD most
pronounced for small
collimators
• SRS near air cavities,
e.g. nasal sinuses
• MC calculation time:
15 - 20 min for 2%
uncertainty at dmax
SAMS Question 4
What is the treatment planning algorithm used to for
inhomogeneity corrections?
20%
1.
20%
2.
20%
3.
20%
4.
20%
5.
Convolution-Superposition
Equivalent tissue-air-ratio
Monte Carlo
Linear Boltzman transport equation
Simulated annealing
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SAMS Question 4
What is the treatment planning algorithm used to for
inhomogeneity corrections?
n Monte Carlo
n Use might be necessary near air cavities or large metal
implants
n AVMs: CT artefact vs. accuracy of dose calculation?
References:
Wilcox, E. E. and G. M. Daskalov (2008). "Accuracy of dose measurements and calculations
within and beyond heterogeneous tissues for 6 MV photon fields smaller than 4 cm produced by
Cyberknife." Med Phys 35(6): 2259-2266.
Summary
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CK dose delivery accuracy in E2E meets SRS
standards
Well understood small field reference dosimetry
Integrated image guidance system for frameless
& fiducial-less localization and tracking
MC dose calculation & Sequential optimization
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