MVCT Image
Guidance and QA
Robert Staton,, PhD DABR
MD Anderson Cancer Center Orlando
ACMP Annual Meeting 2011
Disclosures
• MDACCO has received grant funding from TomoTherapy,
Inc.
Overview
• TomoTherapy MVCT review
• Recommended QA procedures
• Implications for Adaptive Radiotherapy
Megavoltage CT Imaging
6 MV Accelerator (tuned to 3.5 MV for
MVCT)
P i
Primary
Collimator
C lli
t
6mm slice width (1mm for 4.x software)
Binary MLC
85
85cm
(all open during MVCT)
85 cm Gantry Aperture
40 cm CT FOV
Approximately
50cm
Spectral Energy Distribution
MVCT Imaging
• 3 choices of reconstructed slice thickness
• Fine (2mm)
• Normal (4mm)
• Coarse (6mm)
• Gantry Period = 10 sec
• Couch speed:
• Fine = 4 mm/rotation
• Normal = 8 mm/rotation
• Coarse = 12 mm/rotation
• Typical scan times
• 2-4 mins
Imaging Time: Examples
Prostate:
P t t
Fine, 34 slices, 6.8 cm
=> 170 seconds
Head and Neck:
Normal, 42 slices, 16.8 cm
=> 210 seconds
Thorax:
Coarse, 44 slices, 26.4 cm
=> 220 seconds
Prostate
Diagnostic KVCT
TomoTherapy MVCT
Head and Neck
Diagnostic KVCT
TomoTherapy MVCT
Imaging Frequency
• At M. D. Anderson-Orlando, we MVCT for every tx
• What if ….
• we had imaged only 1st day?
• we had imaged only first 5 days?
• we had imaged weekly?
Imaging Frequency
• Zeidan et al., IJROBP, 67(3), pp. 670–677, 2007
• Retrospective analysis of 24 HN patients (802 tx)
• All 802 fx were IGRT
• Replay
R l different
diff
t protocols:
t l
• Use the daily MVCT images
• Calculate positioning errors for different frequencies of IGRT
Imaging Frequency
IG %
- Protocol A: no imaging (system. corr.)
0%
- Protocol B: Image 1,
1 1-3
1 3, 1-5
1 5,1
1-7
7 day
3-21%
- Protocol C: Weekly imaging
20%
- Protocol D: first 5, then weekly
31%
- Protocol E: every other day IG, running average 50%
Imaging Frequency
• Results
• systematic errors are reduced with minimal image workload (i.e. 9%
IGRT)
• Random errors are not reduced for non-IG treatments
Imaging Frequency
% of Residual Error vs. % IGRT
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
% Image guidanced
> 3mm (all-Tx)
> 5mm (all-TX)
> 3mm (non-IG)
> 5mm (non-IG)
60
MVCT Imaging QA
• What do I need to QA?
• How often?
TG-148 Imaging QA
• Geometric
• Image Quality
• Imaging
I
i Dose
D
TG-148 Daily QA
Daily QA Example
-Test imaging, registration, alignment chain
3) Align & test
automatic
couch setup
1) Scan
2)R
Register
i t
& compare to
k
known
offsets
ff t
Tolerance:
Consistency
within 1 mm
TG-148 Monthly
TG-148 Quarterly
Monthly QA Example
• One MVCT scan
•
•
•
•
•
•
Geometric
Noise
Uniformity
Spatial resolution
MVCT HU
Dose
Suggested Test & Tolerances
• Geometric
• Verify dimensions of phantom are consistent
with known values
• Alignment of fiducials
• TG-148:
TG 148: 2mm/1mm (SRS/SBRT)
• Noise
• Select a consistent ROI in a homogeneous
portion of the cheese phantom
• TG-148: Consistency with Baseline
• MDACCO
• Std Dev of Central ROI
• Reference value: ~40 HU
Suggested Test & Tolerances
• Uniformity
• Compare central and peripheral MVCT HU
in uniform phantom
• TG-148
• Compare to Baseline
• Within 25 HU if using MVCT for dose
calculations
Suggested Test & Tolerances
• Noise
• Assess low contrast visibility using
phantom with various density plugs
• TG-148: Compare
p to baseline
• For example, can you visualize the 1.02
density plug?
• Tomo spec: 3-cm
3 cm object of 3% difference
in electron density
Suggested Test & Tolerances
• Spatial Resolution
• TG-148 – 1.6mm high contrast object
• 3rd row of contrast detail plug
Image Artifacts
• Button/Zipper Artifact
• Due to sharp dropoff of detector response in
the center of the detector array
• Possible solutions
• Energy spectral recalibration
• Repositioning of detector
Suggested Test & Tolerances
• MVCT Dose
•
•
•
•
Measure MSAD in phantom with ion chamber in a consistent location
Typical values 1-3 cGy
TG-148:
TG
148: compare to baseline
MDACCO
• Scan of entire cheese phantom (18 cm)
• Normal
• 1.2 – 1.8 cGy
Suggested Test & Tolerances
• MVCT HU calibration
• Important if using dose calculations on
MVCT
• TG-148:
• Near Water – within 30 HU of baseline
• Lung/Bone – within 50 HU of baseline
TG-148 Annual QA
• Verification of imaging/treatment/laser coordinate system
• Design a phantom end-to-end test
• Film or diode array
• TG-148 tolerance: 2mm/1mm (SRS/SBRT)
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Component Replacements
• Imaging QA is recommended after major component
replacements
•
•
•
•
Magnetron
SSM
Target
Linac
Adaptive Radiotherapy
• Allows assessment of the
dosimetric impact of
anatomical
t i l changes
h
during
d i
the course of treatment
Adaptive Protocol at MDACCO
• Daily MVCT scans are collected for H&N patients
• Images are run through a research version of TomoTherapy
Planned Adaptive software
• Automatically
A t
ti ll deforms
d f
contours
t
to
t MVCT images
i
andd
recalculates dosimetry for that fraction
• C
Cumulative and dailyy DVHs
V can be used to tract changes
g in
the dosimetry
• Replans are triggered by physician review
Planned Adaptive – Research Version
Thorax Phantom DVH – No
Deformation
Previous MVCT Study
Thorax Phantom DVH
Previous Calculation
K M Langen et al, “The use of megavoltage CT
(MVCT) images for dose recomputations,” Phys Med
Biol 50,
50 4259
4259-76
76 (2005).
(2005)
Thorax Phantom Density Comparison:
Original IVDT
MVCT Uniformity
• Cup of water used to calibrate
IVDT
27 HU
• On one machine,
machine the HU value
increased near the edge of the FOV
5 HU
• This
Thi is
i illustrated
ill
d by
b the
h bottle
b l off
water shown at the right
15 HU
37 HU
Virtual Water (Cheese) CT
Calibration Phantom
• Water plug shown at
upper-right
• Cheese phantom
configuration shown at
lower-right
Thorax Phantom Density Comparison:
New IVDT
New Thorax Phantom DVH
L
Less
th
than 1% error att D50
Tomo1 Temporal MVCT Number
Variability
Tomo1 Solid Water Variation
120
103 HU
MVCT N
Number
100
80
60
40
20
5 HU
0
-20
Tomo2 Temporal MVCT Number
Variability
Tomo2 Solid Water Variation
120
100 HU
MVC
CT Numberr
100
80
60
40
20
31 HU
0
Energy variation
HI*ART II S/N 002, ENERGY CHECK
0.4350
T
Target
t changes
h
0.4250
0.4200
0.4150
0.4100
0.4050
DATE
08
1J
14-Nov-07
28-Apr-07
10-Oct-06
24-Mar-06
5-Sep-05
17-Feb-05
1-Aug-04
14-Jan-04
28-Jun-03
0.4000
10-Dec-02
DEPTH DOSE AT 155 cm
0.4300
Effect of Component Changes
Tomo1 Solid Water Variation for 2009
110
Target
100
Ion Chamber
Magnetron/Circulator
90
MVC
CT Number
80
Magnetron
70
60
50
40
30
20
Magnetron/Gun board
Magnetron/Target
SSM/Gun board
IVDT Temporal Variation
1000
IVDT Curve
Comparison
800
Maximum
600
• IVDT curve appears to maintain
shape over time
400
Minimum
• The slope
p of the curve appears
pp
to
increase over time (with target
wear)
MVCT Numb
ber
200
0
0
0.5
1
‐200
‐400
• There is greater variation in
MVCT number for higher density
materials
•Therefore dosimetric errors
•Therefore,
will likely be greater in regions
containing higher proportions
of these materials
‐600
‐800
‐1000
‐1200
Density (g/cc)
1.5
2
Dosimetric Error Due to Temporal
MVCT Number Variability
PTV1 D50 (Gy)
Error
103
2.02
‐1.7%
86
2.03
‐1.2%
Solid Water MVCT Number
80
69
60
49
2.03
2.04
2.05
2.05
‐1.0%
‐0.6%
‐0.3%
0.0%
Rt Parotid D50 (Gy)
E
Error
0.84
‐0.7%
0 7%
0.84
‐0.5%
0 5%
0.84
‐0.5%
0 5%
0.84
‐0.2%
0 2%
0.84
‐0.1%
0 1%
0.84
0 0%
0.0%
0.85
0 2%
0.2%
0.85
0 4%
0.4%
0.85
0 5%
0.5%
0.85
0 8%
0.8%
Lt Parotid D50 (Gy)
Error
0.53
‐0.8%
0.53
‐0.6%
0.53
‐0.5%
0.53
‐0.5%
0.53
‐0.1%
0.53
0.0%
0.53
0.1%
0.53
0.2%
0.53
0.2%
0.53
0.4%
41
2.06
0.2%
29
2.07
0.5%
20
2.07
0.8%
5
2.08
1.2%
•
All values shown are given for dose recalculations performed on a single MVCT of a head & neck patient
•
Total D50 variation in the primary target was ~3%
•
•
•
If the IVDT or MVCT images were never recalibrated, one could expect to limit the dosimetric error due to temporal
HU variations to 3%
•
•
Parotid D50 variation was ~1.5%
This is consistent with estimates based on simple physics models
If recalibrations were performed to maintain the solid water MVCT number within 30 HU of the commissioned IVDT value,
dosimetric errors should be within ±1%
These errors should increase as depth increases
Dosimetric Error Due to Temporal
MVCT Number Variability
PTV1 D50 (Gy)
Error
103
2.02
‐1.7%
86
2.03
‐1.2%
Solid Water MVCT Number
80
69
60
49
2.03
2.04
2.05
2.05
‐1.0%
‐0.6%
‐0.3%
0.0%
Rt Parotid D50 (Gy)
E
Error
0.84
‐0.7%
0 7%
0.84
‐0.5%
0 5%
0.84
‐0.5%
0 5%
0.84
‐0.2%
0 2%
0.84
‐0.1%
0 1%
0.84
0 0%
0.0%
0.85
0 2%
0.2%
0.85
0 4%
0.4%
0.85
0 5%
0.5%
0.85
0 8%
0.8%
Lt Parotid D50 (Gy)
Error
0.53
‐0.8%
0.53
‐0.6%
0.53
‐0.5%
0.53
‐0.5%
0.53
‐0.1%
0.53
0.0%
0.53
0.1%
0.53
0.2%
0.53
0.2%
0.53
0.4%
41
2.06
0.2%
29
2.07
0.5%
20
2.07
0.8%
5
2.08
1.2%
•
All values shown are given for dose recalculations performed on a single MVCT of a head & neck patient
•
Total D50 variation in the primary target was ~3%
•
•
•
If the IVDT or MVCT images were never recalibrated, one could expect to limit the dosimetric error due to temporal
HU variations to 3%
•
•
Parotid D50 variation was ~1.5%
This is consistent with estimates based on simple physics models
If recalibrations were performed to maintain the solid water MVCT number within 30 HU of the commissioned IVDT value,
dosimetric errors should be within ±1%
These errors should increase as depth increases
Dosimetric Error Due to Temporal
MVCT Number Variability
PTV1 D50 (Gy)
Error
103
2.02
‐1.7%
86
2.03
‐1.2%
Solid Water MVCT Number
80
69
60
49
2.03
2.04
2.05
2.05
‐1.0%
‐0.6%
‐0.3%
0.0%
Rt Parotid D50 (Gy)
E
Error
0.84
‐0.7%
0 7%
0.84
‐0.5%
0 5%
0.84
‐0.5%
0 5%
0.84
‐0.2%
0 2%
0.84
‐0.1%
0 1%
0.84
0 0%
0.0%
0.85
0 2%
0.2%
0.85
0 4%
0.4%
0.85
0 5%
0.5%
0.85
0 8%
0.8%
Lt Parotid D50 (Gy)
Error
0.53
‐0.8%
0.53
‐0.6%
0.53
‐0.5%
0.53
‐0.5%
0.53
‐0.1%
0.53
0.0%
0.53
0.1%
0.53
0.2%
0.53
0.2%
0.53
0.4%
41
2.06
0.2%
29
2.07
0.5%
20
2.07
0.8%
5
2.08
1.2%
•
All values shown are given for dose recalculations performed on a single MVCT of a head & neck patient
•
Total D50 variation in the primary target was ~3%
•
•
•
If the IVDT or MVCT images were never recalibrated, one could expect to limit the dosimetric error due to temporal
HU variations to 3%
•
•
Parotid D50 variation was ~1.5%
This is consistent with estimates based on simple physics models
If recalibrations were performed to maintain the solid water MVCT number within 30 HU of the commissioned IVDT value,
dosimetric errors should be within ±1%
These errors should increase as depth increases
Dosimetric Error Due to Temporal
MVCT Number Variability
PTV1 D50 (Gy)
Error
103
2.02
‐1.7%
86
2.03
‐1.2%
Solid Water MVCT Number
80
69
60
49
2.03
2.04
2.05
2.05
‐1.0%
‐0.6%
‐0.3%
0.0%
Rt Parotid D50 (Gy)
E
Error
0.84
‐0.7%
0 7%
0.84
‐0.5%
0 5%
0.84
‐0.5%
0 5%
0.84
‐0.2%
0 2%
0.84
‐0.1%
0 1%
0.84
0 0%
0.0%
0.85
0 2%
0.2%
0.85
0 4%
0.4%
0.85
0 5%
0.5%
0.85
0 8%
0.8%
Lt Parotid D50 (Gy)
Error
0.53
‐0.8%
0.53
‐0.6%
0.53
‐0.5%
0.53
‐0.5%
0.53
‐0.1%
0.53
0.0%
0.53
0.1%
0.53
0.2%
0.53
0.2%
0.53
0.4%
41
2.06
0.2%
29
2.07
0.5%
20
2.07
0.8%
5
2.08
1.2%
•
All values shown are given for dose recalculations performed on a single MVCT of a head & neck patient
•
Total D50 variation in the primary target was ~3%
•
•
•
If the IVDT or MVCT images were never recalibrated, one could expect to limit the dosimetric error due to temporal
HU variations to 3%
•
•
Parotid D50 variation was ~1.5%
This is consistent with estimates based on simple physics models
If recalibrations were performed to maintain the solid water MVCT number within 30 HU of the commissioned IVDT value,
dosimetric errors should be within ±1%
These errors should increase as depth increases
Dosimetric Error Due to Temporal
MVCT Number Variability
PTV1 D50 (Gy)
Error
103
2.02
‐1.7%
86
2.03
‐1.2%
Solid Water MVCT Number
80
69
60
49
2.03
2.04
2.05
2.05
‐1.0%
‐0.6%
‐0.3%
0.0%
Rt Parotid D50 (Gy)
E
Error
0.84
‐0.7%
0 7%
0.84
‐0.5%
0 5%
0.84
‐0.5%
0 5%
0.84
‐0.2%
0 2%
0.84
‐0.1%
0 1%
0.84
0 0%
0.0%
0.85
0 2%
0.2%
0.85
0 4%
0.4%
0.85
0 5%
0.5%
0.85
0 8%
0.8%
Lt Parotid D50 (Gy)
Error
0.53
‐0.8%
0.53
‐0.6%
0.53
‐0.5%
0.53
‐0.5%
0.53
‐0.1%
0.53
0.0%
0.53
0.1%
0.53
0.2%
0.53
0.2%
0.53
0.4%
41
2.06
0.2%
29
2.07
0.5%
20
2.07
0.8%
5
2.08
1.2%
•
All values shown are given for dose recalculations performed on a single MVCT of a head & neck patient
•
Total D50 variation in the primary target was ~3%
•
•
•
If the IVDT or MVCT images were never recalibrated, one could expect to limit the dosimetric error due to temporal
HU variations to 3%
•
•
Parotid D50 variation was ~1.5%
This is consistent with estimates based on simple physics models
If recalibrations were performed to maintain the solid water MVCT number within 30 HU of the commissioned IVDT value,
dosimetric errors should be within ±1%
These errors should increase as depth increases
Patient Example: Linac Change
Linac Change
Solid water:
ater 36 HU to 8 HU
•1.2% increase in dose to PTV1
Baseline MVCT Recalculation Error
D95 (Gy)
MVCT
1.99
Water
kVCT
2.00
D50 (Gy)
2.01
2.01
‐0.4%
2.01
2.04
‐1.3%
2.05
2.03
0.7%
D05 (Gy)
2.03
2.04
‐0.4%
2.04
2.07
‐1.1%
2.09
2.08
0.4%
E
Error
‐0.7%
MVCT
1.97
Head
kVCT
2.00
E
Error
‐1.3%
MVCT
1.99
Thorax
kVCT
2.00
E
Error
‐0.4%
• MVCT images were obtained for three phantoms
• The cheese pphantom was scanned at the same time to obtain an
accurate IVDT and eliminate any temporal effects
• The ggreatest baseline error occurred for the head pphantom at -1.3%
Recommendations
•
MVCT recalibration frequency depends on the dosimetric error that one is willing to
accept
p
•
Based on this data, if the MVCT were never recalibrated, a maximum dosimetric error
of ~4% would be possible
•
•
We recommend recalibrating the MVCT to maintain the MVCT number of water
within ±30 HU of the commissioned value
•
•
•
This would result in an overall dosimetric error of not greater than ±2.5% for the typical head &
neck patient
This is consistent with TG-148 recommendations
We recommend checking the IVDT after major component changes affecting the
beamline
•
•
•
Including baseline error and temporal variations
There appears to be a consistent reduction in MVCT number after linac or target changes
Magnetron changes are less consistent, but some large variations were observed
Apart from major component changes, the IVDT should be checked on a monthly basis
Conclusions
• Imaging QA is important for maintaining an IGRT program
• More frequent QA is needed when using the images for dose
calculations
• TG-148
TG 148 is
i a goodd source off information
i f
ti for
f TomoTherapy
T
Th
users
Acknowledgements
• Katja Langen, PhD – MDACCO, chair of TG-148
• Jason Pukala, MS – UF graduate student