Linac Based Radiosurgery and Stereotactic Radiotherapy
Transcript of Linac Based Radiosurgery and Stereotactic Radiotherapy
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Thomas Rockwell Mackie
Professor
Depts. Of Medical Physics, Human Oncology,
and Engineering Physics
University of Wisconsin
Madison WI 53706
Linac Based Radiosurgery and Stereotactic Radiotherapy
Conflict of Interest Statement: I have financial
interest in TomoTherapy Inc.
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Acknowledgements
Peter Hoban, TomoTherapy Inc.
Steve Goetsch, San Diego Gamma Knife
Center
Fang-Fang Yin, Duke University
Chet Ramsey, Thompson Cancer Survival
Center
Karen Rosser, Royal Marsden
Wolfgang Ullrich, BrainLab Inc.
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Outline
� Definition of SRS and SRT
� Stereo Market
� Indications for SRS/SRT
� History of Linac-Based SRS/SRT
� Variety of Systems
� QA for SRS
� Localization
� Imaging
� Small Field Dosimetry
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Stereotactic Radiosurgery
� Usually single fraction delivery
» One large dose instead of ~30 fractions as in standard radiotherapy
» Usually called SRS
� Also multiple fraction delivery
» Often hypo-fractionated
–Small number of fractions (e.g., 5)
» Often called stereotactic radiotherapy (SRT) or fractionated stereotactic radiosurgery (FSRS)
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US Stereotactic MarketDedicated Machines
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In 2003, 83 sites report
plans to purchase in
next few years
32 units in 2004
85% Linac based
15% Gamma Knife
US Stereotactic MarketDedicated Machines
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Half of all dedicated SRS installations are in last 3 years (to 2003)
US Stereotactic MarketDedicated Machines
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US Stereotactic Market
0
20
40
60
80
100
198719
8919
9119
9319
9519
9719
9920
0120
0320
05
cumulative number
GammaKnife
CyberKnife
Novalis
Web Site Claims
2004
To 2005
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Brain Tumors
� Primary brain tumors
» Tumors that originate in the brain
–Malignant (cancerous) or benign
� Metastatic brain tumors
» Malignant cells have spread from
elsewhere
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Malignant Brain Tumors
Glioblastoma Multiforme
Large and diffuse so not very suitable for SRS
Metastases
Smaller well-defined so suitable for SRS/SRT
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Early SRS Developments
� 1951 – Lars Leksell, Swedish neurosurgeon, introduces
the concept of radiosurgery
� 1967 – First Gamma Knife patient treated at Studsvik
nuclear plant, near Stockholm
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History of Linear Accelerator
Based Radiosurgery
� Early reports of linac-based radiosurgery with stereotactic frames in 1980’s
� Winston and Lutz published their results from Joint Center for Radiation Therapy in Boston in 1986
� Early linac treatments required attachment of circular collimators to standard linacs
� Some relied on inherent precision of the linac, others used high precision floor mounts
� Radionics, Leibinger and Fischer, Philips, others began commercial distribution of add-on accessories in 1990s
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Accuray CyberKnife:
Robotic Arm and 6MV Linac
� Industrial robot
arm with 6MV X-
band linac
� Two orthogonal
ceiling mounted
X-rays with floor
mounted flat
panel detectors
� Infrared
positioning
� Extracranial
capability
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BrainLAB Novalis
� Varian SRS 600 modified by adding mMLC
� First dedicated linac radiosurgery system at UCLA in 1995
� 1200 MU/min
� Shaped beam or dynamic arcs
� 84 systems worldwide plus 400 add-on systems
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Varian Trilogy Linear Accelerator
System
� 6MV linac
� Multileaf collimator
� X-ray head
� Silicon flat panel detector w kVCT
� SRS/SRT capability
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Elekta Synergy: Linac with CT
� Cone beam CT
� Pioneered at Princess Margaret Hospital in Toronto (David Jaffray)
� 4D adaptive IGRT
� SRS/SRT capability
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Tomotherapy Hi-ART: MVCT
Scanner and Linac
� First unit
installed at
University of
Wisconsin
� Now 200
worldwide
� 6MV
treatment unit
and also
MVCT imaging
� Capable of
SRS and SRT
treatments
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Mini/Micro-MLCs
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Traditional Linac Stereotactic
Radiosurgery Equipment
Collimator set
Typically ~5-40mm diameter
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Traditional Stereotactic Localization
Rods appear on CT images
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Traditional Stereotactic Localization
Frame attachment
89
1
2
3
5
4
6
7
(x,y,z)T
(x,y,z)1(x,y,z)2
(x,y,z)3
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CT to stereotactic coordinate transformation
CT coordsStereo coords
Traditional Stereotactic Localization
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Cranial Stereotactic Localizer
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Stereotactic Arc Plans
Localizer rods on 3D image
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Isocenter Alignment on LinacAlign lasers with frame crosshairs
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Head and Neck Localiser
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Body Localiser
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QA Reports and Recommendations
� ASTRO/AANS Consensus Statement on stereotactic
radiosurgery quality improvement, 1993
� RTOG Radiosurgery QA Guidelines, 1993
� AAPM Task Group Report 54, 1995
� European Quality Assurance Program on
Stereotactic Radiosurgery, 1995
� DIN 6875-1 (Germany) Quality Assurance in
Stereotactic Radiosurgery/Radiotherapy
� AAPM Task Group 68 on Intracranial stereotactic
positioning systems, 2005
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Recommendations for New
Radiosurgery Programs:
� Rigorous acceptance testing of new
equipment
� Detailed small field dosimetry by
Qualified Medical Physicist
� Detailed investigation of accuracy and
limitations of all imaging equipment:
CT, MR, angio
� Careful examination of all systematic
errors
� Rigorous training for all staff members
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Human Factors QA/QMP
� Flickinger studied error frequency in setting coordinates (IJROBP 1993). Up to 8%, drops to less than 0.1% w two independent observers.
� U.S. NRC reported on 15 gamma stereotactic radiosurgery misadministrations over a 10 year period in the United States.
� Goetsch analyzed these errors: 14 would have been prevented with the modern Gamma Knife with Automatic Positioning System (APS) and a record and verify capability (IJROBP 2002).
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Failures of Quality Assurance
� U.S. NRC issued NUREG “Medical Misadministrations Caused by Human Errors in Gamma Stereotactic Radiosurgery” in 1993
� Listed 15 known misadministration reports: wrong helmet, wrong coordinates, wrong side, wrong patient, shots repeated or skipped, failure to enter intended radiation dose (treated w nominal dose)
� Gamma Knife Center reported to CDRH accidental administration up to 52% overdose to 77 patients in 2004-2005 due to miscalibration
� Linac radiosurgery center in Florida mistakenly loaded wrong factor in initial calibration, accidentally over-dosed 10 patients
� Death reported to FDA in Sep 2006 from linac radiosurgery: failure to attach accessory cone mount (field too big)
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Correction Actions after
Reported Incidents
� Regulatory authorities mandate that coordinates must be manually set by one person, then checked by two more staff members (“double check”)
� Record and Verify systems more common now
� Elekta software now demands that a dose be entered (nominal dose of 10Gy at 100% no longer valid)
� Elekta now has password protected calibration file
� Extramural dose checks should be performed before first treatment with new system
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Detailed Physics QA
Recommendations
� Multiple films of test object taken from selection of couch/gantry angles each time equipment used
� Convergence should be within designated limits
� Gamma Knife claim 0.5mm, linacs attempted comparable accuracy
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Localization
� The problem with radiotherapy is…
� We are treating something we can’t see with something we can’t see!
� Need to know where the target is relative to something we can see
� Stereotactic techniques utilize a visible reference frame that is fixed relative to the target
» Traditionally a frame screwed to the head
» Can also be an image whose location is known relative to the treatment machine
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Traditional Patient Setup
beamspatient image
skin marks
Via skin marks
Locate tumor in room…
Done indirectly via…
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Traditional Patient Setup
beamspatient image
skin marks
Via skin marks
Locate tumor in room…
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Stereotactic Patient Setup
beamspatient image
Via a stereotactic frame
stereotactic frame
Locate tumor in room…
at least for the head…
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Image Guided Patient Setup
beamspatient image
Via pre-treatment images
Locate tumor in room…
Tomo Image = beams
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MRI Image Distortion Problems
� Image distortions of up to 9mm reported
� Some sequences notorious: particularly coronal or axial acquisitions
� MRI image problems extremely scanner dependent
� Some scanners exhibit severe metal artifacts
� Vital to work with MRI expert and manufacturers engineer
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Initial Acceptance of Imaging
Systems
� ACR now offers accreditation of CT programs, MRI programs; also Radiation Oncology departments
� CT, MRI and Angio devices should be accepted by a Qualified Medical Physicist
� Ongoing QA should be maintained in accordance with ACR standards
� Each device must be qualified for use in a radiosurgery program
� DICOM compatibility and rapid transmission of images is vital
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CT/MR Fusion
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CT/MR Fusion
MR with no frameCT scan with frame attached
Image registration
MR in stereotactic coordinates
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Example of MRI distortion
Artifact Caused by Metallic
Dental Work
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Flat Panel Imager
20.5 x 20.5 cm2
Yin et al., Henry Ford Hospital, Detroit, MI
Ceiling-
Mounted
X-Ray TubesCalibration
Phantom
Referenced to
Isocenter
Iso-center
reproducibility
based on the
imaging system
is within 1mm.
BrainLAB ExacTrac/Novalis
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Pos. 2Pos. 1
Live
X-Rays
DRRs
• X-Ray acquisition on treatment couch.
• Computerized generation of DRRs.• Automatic comparison of live X-ray
images with DRRs.
Image-Guided Extracranial Target Localization
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TomoTherapy Targeting
CT isocenter
Tumor
Tomo isocenter
Objective:
Reproduce planning CT geometry on Hi-Art
Tumor stays at same location relative to isocenter
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Image Guided Stereotactic
TomoTherapy
Images Courtesy of Chet Ramsey, Ph.D. TCSC, Knoxville, TN
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QA of Imaging Devices;
Phantoms
� Numerous phantoms described over the
years for QA of CT, MRI and angio
� Largest uncertainty of target
determination comes from imaging
� Many devices: Coffey: RSVP (1993),
Ramani: LUCY (1995), Walton (1996),
Goetsch: CIRS (2000)
� Initial validation of imaging chain,
followed by regular QA measurement
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CIRS Radiosurgery Head
Phantom
� Epoxy skull with MRI gel,
matrix of interstitial rods with
2 cm spacing
� Now has dosimetry inserts
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Small Field Dosimetry and
Protocols
� Challenging physics measurement for
small circular fields
� Physicist’s primary calibration tool is the
ionization chamber: very difficult to use
for fields less than 1cm diameter
� Extrapolations from larger fields, direct
measurements with diodes, film and TLD
were employed
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Early Papers on Small Field
Dosimetry
� Houdek, Med Phys (1983), Miami: used 0.02 and 0.1cm3 ion chambers
� Rice, PMB (1987), Boston: ion chambers and film
� Kubsad, Mackie, IJROBP (2000), Wisconsin: Monte Carlo and conv./super. dosimetry
� Beddar, Med Phys (1994), Toronto: diode
� Rustgi, Med Phys (1995), Cleveland: diamond detector
� Mack, Med Phys (2002), Munich: alanine and TLD microcubes
� Perks, Med Phys (2005), UC Davis: glass rods
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0.5 1.0 1.5 2.0 2.5 3.00.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Output Factor
Side of Square Field (cm)
Dose to Water for Small Fields
PTW 31014 (Pinpoint)
PTW 31010 (Semiflex)
PTW 30006 (Farmer)
Monte Carlo
Diamond
Diode
LAC+film
Film
From Roberto Capote, IAEA
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Output Factors Measured with
Different Detectors.
output factors relative to 4 cm x 4 cm
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
0 1 2 3 4 5 6 7 8 9 10
equivalent field lenght (cm)
output factor rel to 4x4
standard dataphoton diodediamondpinpoint0.125 cc chamberelectron diode
From Karen Rosser, Royal Marsden
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High Uncertainty in Output Factors
• Example: Statistics of 45 Output Factors for 6 mm and 18 mm square fields (Novalis, SSD = 1000 mm, depth = 50 mm, various detectors)
From Wolfgang Ullrich, BrainLab
Factor of Two in
Beam Calibration!
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Gamma Knife 4mm Output
Factor
� Elekta originally suggested output factor of 4mm helmet of 0.80 (relative to 18mm helmet) in 1987
� Based on single beam dosimetry, Monte Carlo
� Later revised (1997) to 0.87 (9% increase) due to liquid ionization chamber measurements and revised Monte Carlo results (J. Arndt, AAPM Summer Meeting, 1999)
� Very controversial, now well accepted
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Novel Geometries for
Radiosurgery
� Both accepted AAPM external beam calibration protocols: TG 21 (1983) and TG51(1999) are written for diverging point sources with 10 by 10cm2 field size at 100cm SSD, with measurement depths of 5 or 10cm for photons in plastic or liquid water
� Many dedicated radiosurgery devices cannot achieve these SSDs and/or field sizes
� These protocols have been adopted by regulatory agencies in the United States
� Individual physicists may interpolate and extrapolate at their own risk
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Examples of Dedicated
SRS/SRT Systems
� Gamma Knife has maximum 1.8cm diameter field size at 40cm SAD, calibrated in a spherical phantom at 8cm depth
� Cyberknife has maximum 6cm diameter field size at 80cm SAD
� Tomotherapy has maximum 2.5 by 40cm field size at 85cm SAD
� CLEARLY: a new dedicated radiosurgery calibration protocol is urgently needed
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Summary
� SRS/SRT has had more than its share of accidents.
� There are a wide variety of methods and techniques for linac-based SRS/SRT.
� There is no do-overs for stereotactic radiosurgery so QA is very important.
� Small field dosimetry is critical.
� SRS/SRT dosimetry protocol is required.