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An Analysis of OBI Automatic Match Results for non-SBRT Lung Cancer Patients
Michael Grams, Ph.D. Clinical Medical Physics Fellow
Mayo Clinic, Rochester
1
Background
• Daily CBCTs for non-SBRT lung patient localization
• CBCT localization has the potential to allow for tighter margins because we have the ability to directly localize the target prior to treatment
• For lung tumors, the tumor volume can change over time
• Increases the uncertainty associated with localizing the target
• Potential for dosimetric impact due to this uncertainty
• Automatic matching is faster than manual matching and removes inter-observer variability
• It is important to understand the behavior of the IGRT tools in different clinical situations
2
Automatic Match Process
• The automatic match is a 3-step process
• First step is a coarse match between CT pixels within the ROI and the CBCT
• Second step is a repeat but uses increased resolution
• Third step is a match to a structure VOI only (e.g., the PTV) and is a refinement to the coarse match with the highest resolution
Ref CT CBCT
Steps 1 and 2
Step 3
Planning CT
CBCT, Fraction 29 of 30
7
Automatic match to ITV
8
Automatic match to ITV, slightly
smaller ROI
Methods
• Varian Linac with OBI system
• Retrospectively auto-match the daily CBCTs using the ITV and PTV as the structure VOI for 3 different ROI sizes
• Compare the auto-matches to a “gold standard” manual match performed by a physician
9
“Small ROI” - extends ~ 2 cm
beyond the longest dimension of
the PTV in each direction
“Medium ROI” - arbitrary extension
of the small ROI of ~ 3 – 4 cm in
each direction
“Large” ROI - encompasses the
entire CBCT
72.2
77.8
88.9
20.8
15.3
6.9 6.9 6.9 4.2
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
100.0
Vert Long Lat
Fre
qu
en
cy (
%)
MD vs Automatic (ITV) Small ROI
0 -2 mm
3 - 4 mm
5 mm and Greater
N = 72 fractions
Results
72.2 77.8
88.9
20.8 15.3
6.9 6.9 6.9 4.2
0.0
20.0
40.0
60.0
80.0
100.0
Vert Long Lat
Fre
qu
en
cy (
%)
MD vs Automatic (ITV) Small ROI
0 -2 mm
3 - 4 mm
5 mm and Greater
N = 72 fractions
77.8 75.0 83.3
15.3 22.2
12.5 6.9
2.8 4.2
0.0
20.0
40.0
60.0
80.0
100.0
Vert Long Lat
Fre
qu
en
cy (
%)
MD vs Automatic (ITV) Medium ROI
0 -2 mm
3 - 4 mm
5 mm and Greater
N = 72 fractions
59.7 65.3
80.6
27.8 27.8
15.3 12.5
6.9 4.2
0.0
20.0
40.0
60.0
80.0
100.0
Vert Long Lat
Fre
qu
en
cy (
%)
MD vs Automatic (ITV) Large ROI
0 - 2 mm
3 - 4 mm
5 mm or Greater
N = 72 fractions
95.8 88.9
94.4
2.8 11.1
5.6 1.4
0.0 0.0 0.0
20.0
40.0
60.0
80.0
100.0
Vert Long Lat
Fre
qu
en
cy (
%)
MD vs Automatic (PTV) Small ROI
0 - 2 mm
3 - 4 mm
5 mm and Greater
N = 72 fractions
94.4 90.3 93.1
5.6 9.7
5.6
0.0 0.0 1.4
0.0
20.0
40.0
60.0
80.0
100.0
Vert Long Lat
Fre
qu
en
cy (
%)
MD vs Automatic (PTV) Medium ROI
0 -2 mm
3 - 4 mm
5 mm or Greater
N = 72 fractions
94.4 87.5 88.9
4.2 9.7 8.3
1.4 2.8 2.8
0.0
20.0
40.0
60.0
80.0
100.0
Vert Long Lat
Fre
qu
en
cy (
%)
MD vs Automatic (PTV) Large ROI
0 -2 mm
3 - 4 mm
5 mm or Greater
N = 72 fractions
72.2 77.8
88.9
20.8 15.3
6.9 6.9 6.9 4.2
0.0
20.0
40.0
60.0
80.0
100.0
Vert Long Lat
Fre
qu
en
cy (
%)
MD vs Automatic (ITV) Small ROI
0 -2 mm
3 - 4 mm
5 mm and Greater
N = 72 fractions
95.8 88.9
94.4
2.8 11.1
5.6 1.4
0.0 0.0 0.0
20.0
40.0
60.0
80.0
100.0
Vert Long Lat
Fre
qu
en
cy (
%)
MD vs Automatic (PTV) Small ROI
0 - 2 mm
3 - 4 mm
5 mm and Greater
N = 72 fractions
77.8 75.0 83.3
15.3 22.2
12.5 6.9
2.8 4.2
0.0
20.0
40.0
60.0
80.0
100.0
Vert Long Lat
Fre
qu
en
cy (
%)
MD vs Automatic (ITV) Medium ROI
0 -2 mm
3 - 4 mm
5 mm and Greater
N = 72 fractions
94.4 90.3 93.1
5.6 9.7
5.6
0.0 0.0 1.4
0.0
20.0
40.0
60.0
80.0
100.0
Vert Long Lat
Fre
qu
en
cy (
%)
MD vs Automatic (PTV) Medium ROI
0 -2 mm
3 - 4 mm
5 mm or Greater
N = 72 fractions
59.7 65.3
80.6
27.8 27.8
15.3 12.5
6.9 4.2
0.0
20.0
40.0
60.0
80.0
100.0
Vert Long Lat
Fre
qu
en
cy (
%)
MD vs Automatic (ITV) Large ROI
0 - 2 mm
3 - 4 mm
5 mm or Greater
N = 72 fractions
94.4 87.5 88.9
4.2 9.7 8.3
1.4 2.8 2.8
0.0
20.0
40.0
60.0
80.0
100.0
Vert Long Lat
Fre
qu
en
cy (
%)
MD vs Automatic (PTV) Large ROI
0 -2 mm
3 - 4 mm
5 mm or Greater
N = 72 fractions
16
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
0 5 10 15 20 25 30
Dis
pla
cem
en
t (c
m)
Fraction
Displacement Between Automatic Match and MD Match vs Fraction
ITV
PTV
Small ROI
21% Reduction
36% Reduction
47% Reduction
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
0 5 10 15 20 25 30
Dis
pla
cem
en
t (c
m)
Fraction
Displacement Between Automatic Match and MD Match vs Fraction
ITV
PTV
Medium ROI
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
0 5 10 15 20 25 30
Dis
pla
cem
en
t (c
m)
Fraction
Displacement Between Automatic Match and MD Match vs Fraction
ITV
PTV
Large ROI
Conclusions
• The results of the automatic match will depend on the size of the ROI
• The results of the automatic match will depend on the structure VOI chosen for the last step
• Our preliminary results indicate that automatic matching to the PTV rather than the ITV will provide a result closer to the physician’s manual match (“gold standard”) even when the tumor changes
17
Questions?
