TG-104: In-room kV Computed Tomography
for Image-guidance
D.A. Jaffray, Ph.D.
Radiation Medicine Program
Princess Margaret Hospital/Ontario Cancer Institute
Professor
Departments of Radiation Oncology and Medical Biophysics
University of Toronto
AAPM’10
Acknowledgements
• Princess Margaret Hospital
– Douglas Moseley, Michael Sharpe, Elizabeth White, J.P.
Bissonnette, Bern Norrlinger, Jason Smale, D. Letourneau,
Winnie Li
Disclosure
Research agreements funded with the following
commercial entities:
Elekta, Philips, Raysearch
The presenter has financial interest in two
technologies reported here: (i) cone-beam CT
imaging technology, and (ii) a phantom for
daily QA.
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Learning Objectives
• Become aware of guidance documents for
IGRT QA.
• Understand in-room kV computed
tomography IG technologies.
• Familiar with the level of targeting
performance these systems can achieve.
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Volumetric kV CT Systems
Siemens
PRIMATOM™
Elekta Synergy™
Siemens MVision™
Varian OBI™
Siemens Artiste™
kV CT
Approach
kV and MV Cone-beam CT
Approach
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kV Fluoroscopic, Radiographic,
and CT Functionality
Fluoroscopic
Radiographic
Tomographic
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TG-104
The role of kV imaging in
patient setup.
Description.
IGRT Workflow.
Specific Processes.
Manpower.
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IGRT is Quality Assurance
• Provides measurement of patient position in
treatment position.
–
–
–
–
Quantitative, accurate, repetitive
Minimally invasive
Large field-of-view
Markers, bone, soft-tissue, skin-line
• Verify consistency of planned and actual
geometry
– Provides a critical data source for rational
margin design
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PTV Design + IGRT
• IGRT is part of a system-wide QA
activity
– Geometric quality assurance / uncertainty
management
• Treatment localization reduces, but does
not eliminate, geometric uncertainties
• Reduced uncertainties may allow
reduced PTV margins
• PTV margin selection relies on
assumptions about stability of guidance
performance.
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From “Modern
Technology of
Radiation Therapy –
Supplement (Vol. 2)”
Ed: J. van Dyk
IG Systems, Processes,
and QA for CBCT
Systems
Jaffray ,Craig, Bissonnette -2003
Yoo et al. 2006
• Quality Assurance program
specific to Varian On-Board
Imager (OBI)
• Includes specific mechanical
tests associated with robotic
arms.
• General concepts applicable
to all vendors
Med. Phys. 33 (11) November 2006
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* Recommendations for Imaging System QA
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* Recommendations for Imaging System QA
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* Recommendations for Imaging System QA
In-room Conventional CT for
IGRT
Memorial Sloan Kettering Cancer Center
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Siemens CT-on-Rails System
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CT – Linac system with RTRT
Yamaguchi University Hospital, Japan
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In-room
Conventional
CT for IGRT
Kuriyama et al. Int.J.Rad.Onc.Biol.Phys. 55(2) Feb 2003
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In-room
Conventional
CT for IGRT
Positional Accuracy:
0.2 mm (LAT)
0.18 mm (VERT)
0.39 mm (LONG)
Onishi et al. Int.J.Rad.Onc.Biol.Phys. 56(1) May 2003
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Varian CT-on-Rails at MDACC
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Central
guide rail
Magnetic
encoder
strip
Side rail to
provide
balance
Fig. IIA.-3 There are three rails in this moving-gantry CT scanner. The central rail
contains positional sensor and drive mechanism; and the two side rails provide
level and balance during movement.
Fiducial Transfer Method
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kV Cone-Beam CT for IGRT
Robust 2D
Detector
Elekta Synergy (XVI)
Varian iX (OBI)
Feasible Reconstruction Method
kV IG Systems: Front of Mind
• Baseline Radiographic/Fluoroscopic Performance
• Stability of Geometric Calibration/MV-kV
Correspondence
• Dosimetry of Imaging Techniques
• Baseline Performance of Image Quality
• Image-guidance Performance (Phantom
Orientation, Residuals)
• Automated Couch Adjustment Performance
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kV IG System Commission/Test
X-ray Tube
kVp
Linearity
Timer accuracy
mR/mAs
HVL
CBCT Image Quality
Uniformity
Spatial Resolution
CNR, Low-Contrast
CT # (linearity)
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Automatic Couch
Accuracy
Reproducibility
Flat-Panel
Linearity
[ADU/mR]
Defects
Dark noise
Lag
MV/kV Iso-centre
Coincidence
Stability
kV/MV Calibration Concept
Winston-Lutz et al, IJROBP 14 1988
y
MV Mechanical Iso-centre
kV
MV Radiation Iso-centre
x
z
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Calibrated Iso-centre
Reconstruction Iso-centre
kV/MV Calibration Concept – Elekta
Flex Maps: Synergy (XVI) Units
FOV: 26 cm
Rot. Dir: CW
XVI1
XVI2
XVI3
XVI4
XVI5
26 cm
CCW
40 cm
CW
40 cm
CCW
50cm
CW
50 cm
CCW
All Flex Maps: Synergy (XVI) Units
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OBI1 Approach – “Residual”
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Accept if within specified tolerance.
Sensitivity of Cone-beam CT Performance
to Geometric Miscalibration
• Shift panel 5 mm in “u”
• E.g. Nasopharynx
v
u
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Sensitivity of 2D kV Radiographic
Localization Performance to Miscalibration
• Image quality is not
sensitive to
detector/source shifts
• No physical
reference marks
– i.e. cross-hair or
field-edge
• Electronic field
size/isocentre
* Vendor Notices
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Clinical Assessment of Remote Couch for
IGRT
Background
• The XVI system is linked to the Remote
Figure 2: Frequency Histograms for Translational x, y, z & Rotational x, y, z
Automatic Table Movement (RATM).
• The patient is shifted per the imageguidance system via the remote couch
interface.
Sarcoma
9%
Lymphoma
3%
• The stability of the system and residual error
is measured through verification scans
acquired following a table shift.
Upper GI
21%
Head and Neck
26%
Lung
41%
Methodology
Figure 1: Patient Population by Tumor Site
• Collection
Period Oct 19th ‘06 – Nov 17th ‘06
• Patients with repeat (verification) scans
were measured and matched using an
Automatic Algorithm
• 34 patients
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with 135 scans W Li et al. J Appl Clin Med Phys. 2009 Oct 7;10(4):3056
IG Performance:
Connectivity,Orientation/Scale Checks
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Image Source: GE and Philips CT
Planning: Philips Pinnacle v7.4
IG Performance:
Connectivity,Orientation/Scale Checks
Residual Error
•
•
•
•
•
•
Anthropomorphic phantom
4 Orientations
Target bony anatomy
Arbitrary initial shifts
Plot residual error
5 XVI units
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IG Performance:
Connectivity,Orientation/Scale Checks
Residual Absolute Error [cm]
0.18
0.16
0.14
0.12
L/R
S/I
A/P
0.10
0.08
0.06
0.04
0.02
0.00
1
All XVI’s
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Daily Geometry QA
• Align phantom with lasers
• Acquire portal images (AP
& Lat) & assess central
axis
• Acquire CBCT
• Difference between
predicted couch
displacements (MV & kV)
should be < 2 mm
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Daily Geometry QA
• Align phantom with lasers
• Acquire portal images (AP
& Lat) & assess central
axis
• Acquire CBCT
• Difference between
predicted couch
displacements (MV & kV)
should be < 2 mm
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Full kV/MV Calibration - Monthly
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Compare Portal Image & DRR
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Iso_ODI = Q/A Phantom = Lasers/ODI
E2 (CBCT)
E1 (EPID’s)
Iso_kV
MV Mechanical
Isocenter
E3
MV Nomimal
Isocenter
z
y
x
Iso_MV
MV Radiation
Isocenter
Current* Action Level on E 3: 2 mm (in any one direction)
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* Due to large observer variability in MV alignment
2006 XVI Daily QA Results (E3 Error)
XVI1
XVI2
[mm]
M
XVI3
L/R
XVI4
S/I A/P
(-0.04, -0.72, 0.52)
Ave(σ) (0.96, 0.82, 1.04)
~ 6 Months
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of Daily QA Testing
XVI5
2007 OBI Daily QA Results (E3 Error)
OBI1
[mm]
OBI2
L/R
OBI3
S/I
OBI4
A/P
M = (-0.08, -0.19, 0.27)
Ave(σ) = (0.51, 1.02, 0.71)
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TG-104: QA Summary
• Guidance documents are available to assist in
the establishment of QA programs for IGRT
technologies.
• Published literature demonstrate that these
systems can be accurate, precise, and reliable.
– Compare your results to others.
• Maintenance of IGRT performance is central
to confidence in appropriate PTV margin.
• An integrated daily check for IG system
consistency has been implemented into routine
clinical use with a 15 minute time penalty.
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kV Conebeam CT
Dose
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Patient Dose Estimation: kV-CBCT
Dose depends on
• Beam Quality: HVL (kVp, filtration)
• Tube output: Reference (mR/ mAs)
• Scanning Geometry: SAD, FOV, No. of
projections
• Technique settings: mAs
• Patient Size (Body , Head…)
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Scanning Geometry and Technique
Offset Detector
Asymmetric Col.
26 cm
X1=6.0 cm,
X2=20 cm
41 cm x 41 cm
155 cm
Elekta
Precise: XVI
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Bench top CBCT System
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CBCT: Radial Dose
Phantom: 16 cm dia.
2.4
2.2
Imaging Technique:
2.0
100 kVp
20 ms
330 Projection
Depth
660 mAs
Dose (cGy)
100 mA
1.8
1.6
1.4
FOV: 5 cm x 26cm
FOV: 10 cm x 26cm
FOV: 15 cm x 26cm
FOV: 26 cm x 26cm
1.2
1.0
0.8
0.6
0
2
4
Depth (cm)
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6
8
CBCT: Radial Dose
2.4
FOV: 5 cm x 26cm
FOV: 10 cm x 26cm
FOV: 15 cm x 26cm
FOV: 26 cm x 26cm
2.2
2.0
1.8
Dose (cGy)
Phantom: 30 cm dia.
Imaging Technique:
120 kVp
100 mA
20 ms
330 Projection
660 mAs
Depth
1.6
1.4
1.2
1.0
0.8
0.6
0
2
4
6
8
Depth (cm)
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10
12
14
Dose Comparison: Dose at Isocenter
Dose in cGy/ 640 mAs; FOVx= 26 cm
Phantom
FOVz
Size: dia.
30 cm
16 cm
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100 kVp
120 kVp
10 cm
0.58
1.07
26 cm
0.80
1.5
10 cm
1.52
2.68
26 cm
1.9
3.2
CBCT Imaging Dose
Experimental Setup
Dose vs. Field Size
Total Dose (cGy)
2
D2cm
1.5
1
Dcenter
0.5
0
0
10
20
30
Field Size (z) (cm)
Nuclear Enterprises Free-Air Chamber (0.6 cc)
32 cm “Body” Phantom
330 projections at 2 mAs / proj
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Offset geometry (FOV 40 cm)
Islam et al., Med. Phys. 2006
Deciding Upon the Necessary Image
Quality for the Application
Variation of image quality with lens dose (cGy)
mAs/ Projection
2
1.0
2.0
0.5
1.0
2.0
0.25
0.5
1.0
4.0
1
0.5
80
160
320
Number of Projections
16x
Reduction
Low Dose (1.5 mGy) Pediatric Imaging
for Routine On-line IGRT
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