Comparison of surface dose
measurements and treatment
planning system modeling for a
carbon fiber couch top
D. Jacqmin, N. Koch, D. McDonald, J. Peng, M.
Ashenafi, A. Ellis, M. Fugal, and K. Vanek
Alliance Oncology
Medical University of South Carolina
Charleston, SC
Disclosures
• None
Outline
1.
2.
3.
4.
Motivation
Couch Surface Dose Methods
Results
Discussion and Conclusions
Motivation: AAPM TG-176
“Dosimetric effects caused by couch tops and
immobilization devices: Report of AAPM Task
Group 176”
• Couch tops and immobilization devices
cause beam attenuation and affect surface
dose and the dose distribution
• These effects should be measured,
modeled and mitigated
Overview of AAPM TG-176
• Dosimetric effects of external devices
• Inclusion of couch tops by treatment planning
systems
• Measurement methods for attenuation and
surface dose from external devices
• Avoidance of external devices during
treatment planning
• Recommendations to TPS and couch top
vendors and physicists
Effect of Couch Tops on Skin Dose
• New IGRT-friendly carbon-fiber couch tops
are much different than their predecessors
in terms of attenuation and surface dose
AAPM TG-176
Surface Dose Measurements
• TG-176 discusses the use of plane parallel
ionization chambers for surface dose
measurements
– Convenient alternative to extrapolation
chambers
– Perturbation corrections required
– Strong polarity effects
Corrections Factors
• Plane parallel measurements must be
corrected in the build-up region due to lack
of charged particle equilibrium
Correction Factors
• Velkley et al., MP 2(1), 1975
– Identified electron fluence perturbation
through chamber side wall as the issue;
proposed corrections
Correction Factors
• Gerbi and Khan, MP 17(1), 1990
– Refined corrections by including the plate
separation and the size of the guard ring
Correction Factors
• Rawlinson et al., MP 19(3), 1992
– Further refinement of the correction based on
wall diameter and flange height
Correction Factors
• Correction factors depend on:
– TPR 20/10: Correction decreases with
increasing energy
– Collector and Guard Ring: Correction
decreases with increasing collector and guard
ring size
– Electrode separation: Correction increases
with increasing separation
Methods
1.
2.
3.
4.
Beam and Phantom Geometry
Measurement Technique
Correction Factors
Treatment Planning System Modeling
Beam and Phantom Geometry
• Varian TrueBeam
• Varian Exact IGRT Couch Top®
• Energies:
– 6x
– 6x FFF
– 10x FFF
Varian Medical Systems
Beam and Phantom Geometry
• Beam Geometry
– AP beam (no couch)
– PA beam through
“thin” couch
– PA beam through
“thick” couch
• Solid Water phantom
Measurement Technique
• Detector
– Classic Markus plane parallel ionization
chamber
• Percent depth dose measurements
– 0, 2, 3, 4, 6 mm …
– Depth of maximum dose
– 100 mm
Measurement Technique
• Correction factors:
– Gerbi and Khan, MP 17(1), 1990
• Data in paper better suited for estimating
corrections for FFF beams
– 2 mm electrode separation
– 0.1 mm collector edge-wall distance
– TPR 20/10 from 0.631 to 0.692
• Polarity correction factors applied
Treatment Planning System
Modeling
• Eclipse v11 AcurosXB algorithm
• “Thin” and “Thick” models
• Default couch structures in TPS
– Couch surface HU = -300
– Couch interior HU = -1000
Results:
Correction Factors for 6 MV
AP (Corrected)
PA Thin (Corrected)
PA Thick (Correctied)
AP (No Correction)
PA Thin (No Correction)
PA Thick (No Correction)
1
Depth
(mm)
0
2
3
4
6
8
10
12
14
% of Max.
Ionization
10.83
4.94
3.33
2.25
1.03
0.47
0.21
0.10
0.04
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2
4
6
8
10
Depth (mm)
12
14
16
18
20
Results: 6 MV
AP
PA (Thin)
PA (Thick)
Eclipse AP
Eclipse PA (Thin)
Eclipse PA (Thick)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2
4
6
8
No Couch
Depth (cm) Measured (%)
Eclipse (%)
0
14.9
29.1
0.2
61.9
66.1
10
66.1
66.5
dmax
1.4
1.5
Difference (%)
14.2
4.2
0.4
0.1
Thin Couch
Depth (cm) Measured (%)
Eclipse (%)
0
86.1
49.2
0.2
93.6
94.7
10
64.4
64.6
dmax
1.0
0.9
Difference (%)
-36.9
1.1
0.3
-0.1
Thick Couch
14
Depth (cm) Measured (%)
Eclipse (%)
Depth
(mm)92.9
0
52.5
0.2
97.3
95.0
10
63.7
64.7
dmax
0.8
0.9
Difference (%)
-40.4
-2.2
1.0
0.1
10
12
16
18
20
Results: 6 MV FFF
AP
PA (Thin)
PA (Thick)
Eclipse AP
Eclipse PA (Thin)
Eclipse PA (Thick)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2
4
6
8
No Couch
Depth (cm) Measured (%) Eclipse (%)
0
19.2
36.1
0.2
68.4
75.1
10
62.9
63.3
dmax
1.2
1.3
Difference (%)
16.9
6.7
0.4
0.1
Thin Couch
Depth (cm) Measured (%) Eclipse (%)
0
89.2
50.6
0.2
95.4
96.4
10
61.2
61.5
dmax
0.8
0.7
Difference (%)
-38.6
1.0
0.3
-0.1
Thick Couch
10
12
14
Depth (cm) Measured (%) Eclipse (%)
0
53.5
Depth
(mm)94.7
0.2
98.0
96.3
10
60.6
61.7
dmax
0.6
0.8
Difference (%)
-41.2
-1.7
1.1
0.2
16
18
20
Results: 10 MV FFF
AP
PA (Thin)
PA (Thick)
Eclipse AP
Eclipse PA (Thin)
Eclipse PA (Thick)
1
0.9
0.8
0.7
0.6
Depth (cm)
0
0.2
10
dmax
No Couch
Measured
Eclipse
12.3
23.7
50.4
56.8
70.5
71.0
2.2
2.2
Difference (%)
11.4
6.3
0.5
0.0
Depth (cm)
0
0.2
10
dmax
Thin Couch
Measured
Eclipse
72.8
42.1
82.3
84.6
68.9
69.1
1.6
1.7
Difference (%)
-30.7
2.3
0.2
0.1
Thin Couch
15
20
Depth (cm) Measured
Eclipse
Depth
(mm)
0
81.5
45.4
0.2
88.1
85.9
10
68.2
69.1
dmax
1.6
1.6
Difference (%)
-36.0
-2.2
0.9
0.0
0.5
0.4
0.3
0.2
0.1
0
5
10
25
30
Discussion
AP Markus
• Beam model:
– Pure CC04 data
imported into TPS
– Would a hybrid
Markus-CC04 PDD
improve the results
of the Eclipse
automodeling?
AP CC04 Scanned
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
5
10
Depth (mm)
15
20
Discussion
• Couch model:
– Default models for both thick and thin couch
are similar in build-up region
– Could a different combination of surface and
interior density overrides improve the model?
Surface
Interior
Conclusions
• The Varian Exact IGRT couch significantly
impacts skin sparing for MV photon
beams.
• The beam attenuation effects of the couch
top on PA beams are modeled well at
depth
• At 2 mm depth and beyond, the effect of
the couch top in the build-up region is
modeled accurately to within 3%