commissioning and calibrating a linear accelerator – state ... · fourth, although acceptable...
TRANSCRIPT
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ID/AAPM/10
Indra J. Das, PhD, FACRDepartment of Radiation OncologyIndiana University of School of Medicine & Midwest Proton Radiation Therapy Institute (MPRI), Indiana
Commissioning and Calibrating a Linear Accelerator – State-of-the-Art in 2010
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Preface
Med. Phys. 35(9), 4186-4214, 2008
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ID/AAPM/10
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2008
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Planning for Commissioning Data
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Time= [(PDD + 5 profiles)/beam energy]x ( open + 4 wedges) x 60 points/scanx [(1 s/pts + (1s/movement and delay)]x (15 fields x 2 energies)
∼105 s∼ 30 h
Planning for Commissioning Time
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Rational For Commissioning Beam DataFirst, it is not evident that manufacturing procedures for all linear accelerators have produced a level of reproducibility acceptable for clinical use. For example, variations in beam parameters have been noted between beams with the same nominal energies.
Second, on-site changes made during installation and acceptance of the user’s accelerator e.g., changes in beam energy and/or profiles from beam steering will not be modeled in the golden data.
Third, the beam characteristics of the soft wedges are made by moving jaws that depend on the speed parameters of the jaws and a deviation at site could affect the beam profile of the soft wedge.
Fourth, although acceptable agreement with the golden data set may be found in individual checks, it may be that some clinical setups will have multiple errors, which combine to produce unacceptable results.
TG-106
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Hrbacek, et al Med. Phys. 34, 2917–2927, 2007.
Rational For Not Using Golden Data
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Hrbacek, et al “Quantitative evaluation of a beam-matching procedure using one dimensional gamma analysis,” Med. Phys. 34, 2917–2927, 2007.
Rational For Not Using Golden Data
|xi − x¯ | <∆, ∀ xi
∆ =? (0.5, 1.0 0r 2%)
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Time required for commissioning adual energy linear accelerator with photon and electron beam is
Answer: 4Reference: Das et al, TG-106, Med. Phys. 35(9), 4186-4214, 2008
Question
1. 1 day2. 3 days3. 1 week4. 4-6 weeks5. 2 months
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Standard chamber 10−1 cm3—The active volume for a standard Farmer-type ionization chamber is on average 0.6 cm3.
Minichamber 10−2 cm3—The active volume for a mini-ionization chamber is on average 0.05 cm3.
Microchamber 10−3 cm3—The active volume for a microionization chamber is on average 0.007 cm3
and ideally suited for small field dosimetry such as radiosurgery,gamma knife, CyberKnife, and IMRT
Definition of Detectors
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Water
Air
1 2 3 4 5
Setting Water Tank & Detector
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Know Your Connectors
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Understand Detector, Connector & Cable
See Poster:Srivastava et al, SU-GG-T-270, 2010
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Quality of Cables
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When setting ion chamber in water tank, the correct position of the chamber as viewed in water tank (as seen in figure) is:
Water
Air
1 2 3 4 5
Answer: 3Reference : Das et al, TG-106, Med. Phys. 35(9), 4186-4214, 2008
Question
1. Position # 12. Position # 23. Position # 34. Position # 45. Position # 5
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Setup and Possible Errors
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Electrometer Null Setting Cable subtraction Proper bias >300 V for ion chamber 100 V for diamond 0 v for all diodes
Proper gain Proper mode
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Choose Consistent & Correct Polarity
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Chambers & Gain
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Selection of detector for beam data
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XYZ
XY
Z
Scan Direction
Chamber Orientations
Radiation beam
Choice of Detector Orientation
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Detector Orientation
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0102030405060708090
100110
0 0.5 1 1.5 2
Rel
ativ
e Dos
e
Distance Off Axis (cm)
6MV, 2x2 cm2 field, Illustration of chamber volume effects
Diode, dmax
PTW Pinpoint, dmax
RK chamber, dmax
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The possible setup error that causes the photon beam dose profile in figure is due to:
0
10
20
30
40
50
60
70
80
90
100
-4 -3 -2 -1 0 1 2 3 4Distance (cm)
Rel
ativ
e D
ose 1.5 cm
10 cm20 cm
Answer: 3Reference: Das et al, TG-106, Med. Phys. 35(9), 4186-4214, 2008
Question
1. Gantry tilt
2. Collimator rotation
3. Tank arm tilt
4. Gantry and arm tilt5. Gantry tilt, collimator rotation and
tank arm tilt
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0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0
Depth (mm)
10
20
30
40
50
60
70
80
90
100
0
Dos
e (%
)
Surface & Buildup Dose, 6 MV
0.6 cc0.3 cc0.125 ccMarkusA16IC-4PinPointDiamondPFDSFD
Reference: Das et al, TG-106, Med. Phys. 35(9), 4186-4214, 2008
Answer: 2
Question
The accurate measurement of the buildup and surface dose could be achieved by a:
1. Diode detector2. Parallel plate chamber3. Cylindrical chamber4. Diamond detector5. PinPoint chamber
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0
50
100
150
200
250
-20 -15 -10 -5 0 5 10 15 20
Rel
ativ
e D
ose
Distance Off Axis(cm)
6 MV 60 Deg Wedge, 10 cm depth: water vs Profiler
Diode Array Profile
Water Profile
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0
20
40
60
80
100
120
140
160
180
200
-20 -15 -10 -5 0 5 10 15 20
Rel
ativ
e D
ose
Distance Off Axis(cm)
18 MV 60 Degree Wedge, 10 cm depth, 100 cm SAD: Water vs Profiler
Diode Array profile
Water Profile
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Scanning Speed
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Scanning Speed
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0
20
40
60
80
100
0 5 10 15 20 25 30 35
Perc
ent D
epth
Dos
e
Depth(cm)
Arithmetic Mean (AM) smoothing: 60 Degree Wedge PDD
UnsmoothedAM x 1AM x 2AM x 3AM x 5
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Electron beam depth doses shown in figure represents the problem of
0
0.2
0.4
0.6
0.8
1.0
1.2
0 2 4 6 8 10 12 14 16 18 20
Depth, z (cm)
Dep
th D
ose
(%)
6 MeV9 MeV
12 MeV
15 MeV
18 MeV
21 MeV
Answer: 4Reference: Das et al, TG-106, Med. Phys. 35(9), 4186-4214, 2008
Question
1. Noise in the cable2. Electrometer gain3. Bias on the electrometer4. Speed of scanning5. Tuning of accelerator
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Future of Beam Data Commissioning
Standardization of linear acceleratorsMonte Carlo based commissioning Newer Radiation Detectors & Cables Newer Scanning Systems Smart algorithms
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Aubin et al., Med Phys, 37(5), 2279-2288, 2010
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Monte Carlo Codes
Aubin et al., Med Phys, 37(5), 2279-2288, 2010
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Simulation of intensity at target
Aubin et al., Med Phys, 37(5), 2279-2288, 2010
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Simulated Profiles
Aubin et al., Med Phys, 37(5), 2279-2288, 2010
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Profiles for different fields
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Depth Dose Simulation
Aubin et al., Med Phys, 37(5), 2279-2288, 2010
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Detector Manufacturer Type volumeSFD Scanditronix Photon diode 1.7x10-5cm3
PFD Scanditronix Photon diode 1.9x10-4cm3
Exradin A-16 Standard Imaging Ion chamber 0.007cm3
Wellhofer-IC4 Scanditronix Ion chamber 0.40 cm3
Pinpoint PTW Ion chamber 0.015cm3
0.125cc PTW Ion chamber 0.125cm3
0.3cc PTW Ion chamber 0.3 cm3
0.6cc PTW Ion chamber 0.6 cm3
Diamond PTW Diamond 0.003cm3
Markus PTW Parallel plate 0.055cm3
Edge Detector Sun Nuclear Diode 10-5cm3
Other
Detectors
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Sensitivity vs Volume of Detectors
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00
Volume (cm3)
Rela
tive s
ensit
ivity
PFD
SFD
A-16 PinPointMarkus
IC4
0.125cc
0.3cc
0.6 cc
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Beam Profile
0
10
20
30
40
50
60
70
80
90
100
110
0 10 20 30 40 50 60 70 80Distance (mm)
Dos
e (%
)
0.6 cc0.3 cc0.125 ccMarkusA16IC-4PinPointDiamondPFDSFD
Answer: 5Reference: Das et al, TG-106, Med. Phys. 35(9), 4186-4214, 2008
Question
Photon beam dose profiles taken with various detectors as shown in figure is possibly due to:
1. Speed of scanning2. Beam asymmetry3. Pb piece in the beam4. Hysteresis of scanning system5. Orientation of scanning detector
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Comparison of Large Tank and Small SRS Cylinder Tank for SRS, TMR & Profiles
ARM Inc., Port Saint Lucie, FL 34983
Moving Tank System for TMR
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No SSD to SAD calculations required, No cubic spline fit of a limited number of fixed data points needed
0
0.2
0.4
0.6
0.8
1
1.2
0
0.2
0.4
0.6
0.8
1
1.2
0 50 100 150 200 250 300 350
Calculated TPR
Measured TPR
Calculated TPR ~2% less at depth Cubic spline fit of 12 data points
Nikesch et al, CyberKnife Center , Palm Beach, FL
Direct TMR Data Acquisition
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Scanner Orientation Advantage
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Sun Nuclear 3D Scanner
2
1
1
32
1. Ring drive maintains consistent scanning direction
2. Diameter drive has maximum scanning range of 640mm
3. Vertical drive has maximum travel of 400mm
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ID/AAPM/10
Conclusions Golden Data should be taken as a
reference only Understand time and amount of data
to be taken View each parameters properly,
double check by another individual Use proper detector for each type of
data collection Set optimum speed for scanning, do
not rush
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ID/AAPM/10
-Conclusions
Understand the limits and measuring condition
Question every unusual data set Do not smooth data too much Write report for future reference Future technology & resources
could help commissioning simpler
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ID/AAPM/10
Thanks