Elekta Synergy - XVI
Cone-Beam CT for IGRT
Dose Calculation on CBCT
Datasets
Elekta System
Daniel Létourneau, PhD, DABR
Field of View (FOV)
Sup-Inf: 12 and 26 cm
Axial: 27 to 50 cm
Selectable mAs and kV
Scatter correction
in software
Single Bowtie Filter
No scatter rejection grid
Elekta Synergy - XVI
Cone-Beam CT for IGRT
Tolerance on Electron Density
Limiting Relative Dose Error to 2%
Field of View (FOV)
Sup-Inf: 12 and 26 cm
Axial: 27 to 50 cm
15 MV
Based on Effective Depth Inhomogeneity Correction
6 MV
Selectable mAs and kV
Co-60
~ 5%
~ 3%
Scatter correction
in software
Single Bowtie Filter
No scatter rejection grid
Kilby et al, PMB, 47: p.1485-92, 2002
1
Impact of Residual Artifacts
Dose Calculation on CBCT Images
Key Requirements for Accurate Dose Calculation
Result: 99.8% within 2% dose diff. and 3 mm DtoA
Dose distribution on CBCT images
-
2.5
E le c tro n D e n s ity
Gamma map
Image Geometric Integrity
HU to ED conversion
2
Scale
No distortion
Skin-line
1.5
1
Reproducible CBCT numbers
0.5
0
0
1000
2000
3000
CT number
Accurate CBCT numbers
Letourneau et al, IJROBP, 67: p.1229-37, 2007
CBCT Image Artifacts
Three Categories:
-
Panel and tube calibration
Post reconstruction processing
Technique
-
kVp, FOV, Panel Offset, Bowtie filter,
Grid, Pre-reconstruction processing
Patient
-
CBCT # from 4 Systems:
Same phantom and imaging technique
2500
- Inter-system
CBCT# variations
- Uniformity
- Lag/Skin-line
- CBCT# variation
Size, prosthesis, intra-scan organ motion
and truncation
- Uniformity
- Streaks
CBCT numbers
System
InterInter-System CBCT# Variation
2000
ES05
ES07
NS09
NS10
1500
1000
500
0
-1500 -1000
-500
0
500
1000
1500
Helical CT numbers
2
InterInter-System CBCT# Variation
CBCT # from 6 Systems:
Same phantom and imaging technique
CBCT numbers
2500
InterInter-System CBCT# Variation
Potential Solutions:
CBCT-to-ED table per machine
ES05
ES07
ES08
NS09
NS10
WS17
2000
1500
1000
500
Flood field calibration
-
Adjust the amount of water to compensate for tube output
variation
Post-Processing algorithm
0
-1500 -1000
-500
0
500
1000
1500
Helical CT numbers
-
User-defined linear CBCT number conversion
All these solutions require periodical QA!!!!
No Detector Calibration in Air?
InterInter-Technique CBCT# Variation
Variation in FOV, kVp, mAs and filter
InterInter-Technique CBCT# Variation
Variation in FOV, kVp, mAs and filter
Same Phantom, Different Imaging Techniques
Change in kVp
(100 to 120 kVp) :
About 500 HU variation
Ritcher et al, Radiation Oncology, 3:42, 2008
Ritcher et al, Radiation Oncology, 3:42, 2008
3
InterInter-Technique CBCT# Variation
Variation in FOV, kVp, mAs and filter
InterInter-Technique CBCT# Variation
CBCT-to-ED table per technique or per anatomic site
Dose on CT and CBCT for a pelvis patient
Change in Sup-Inf FOV
(12 to 25 cm) :
Mean Difference
About 70 HU variation
Single CBCT Imaging Technique
Ritcher et al, Radiation Oncology, 3:42, 2008
Ritcher et al, Radiation Oncology, 3:42, 2008
InterInter-Technique CBCT# Variation
InterInter-Technique CBCT# Variation
CBCT-to-ED table per technique or per anatomic site
CBCT-to-ED table per technique or per anatomic site
Dose on CT and CBCT for a pelvis patient
Dose on CT and CBCT for a pelvis patient
Mean Difference
Mean Difference
Using a technique-based CBCT-to-ED table
Using an anatomic-site-based CBCT-to-ED table
20 cm diameter phantom for pelvis!
Ritcher et al, Radiation Oncology, 3:42, 2008
Ritcher et al, Radiation Oncology, 3:42, 2008
4
InterInter-Technique CBCT# Variation
Summary:
Some cases:
Dose accuracy within 2% is achievable
-
InterInter-Patient CBCT# Variation
Physiological motion
Prosthesis
Truncation
- Streaking
-Photon starvation
- Streaking
-Shift in CBCT #
- Shift in CBCT #
Multiple fields are more forgiving
CBCT-to-ED table per technique and
anatomic sites
- Require maintenance
- Risk of mismatch
Conclusion
Accurate dose calculation on CBCT is possible
Inter-system CBCT# variation
CBCT-to-ED table per technique and
anatomic sites
Periodical QA required
5