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CALCULATION OF BACKSCATTER FACTORS FOR LOW ENERGY X …
Transcript of CALCULATION OF BACKSCATTER FACTORS FOR LOW ENERGY X …
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Emily Hewson1
Martin Butson1,2
Robin Hill1,2
1 Institute of Medical Physics, School of Physics, University of Sydney
2 Chris O’Brien Lifehouse, Sydney
CALCULATION OF BACKSCATTER FACTORS FOR LOW ENERGY X-RAYS USING THE TOPAS MONTE CARLO CODE
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Kilovoltage Radiotherapy
Treats superficial cancers
Melanoma. Healthline
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Applications of kV beams • Therapeutic x-ray beams in radiotherapy
• Various skin cancers
• Keloids
• Other clinical conditiosn
• Intraoperative units low energy x-rays (breast cancers) • Small field sizes, lower energies
• Dosimetry of OBI units
• Biological irradiators • Used for animal studies in cancer treatments
• Issue of very small field sizes – 1 mm diameter
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Why is kilovoltage x-ray beam dosimetry important? 1. There are now more kilovoltage x-ray units installed in
Australia/NZ.
2. More treatment units released on the market.
3. The international dosimetry protocols based on air kerma standards or dose to water standards
4. Uncertainties in terms of dosimeters to use for dose measurements: – Depth dose calculations – Output factors – Backscatter factors
5. BJR Report 25 – Some consider to be the gold standard for dosimetry but there are a number of issues.
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Photon Interactions
Photoelectric Effect Compton Scattering
Photoelectron Incoming photon
Incoming photon
Scattered photon
Recoil electron
Kilovoltage energy beams → more backscatter
→ larger dose at surface
Practical Radiotherapy Physics and Equipment
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Backscatter Factors
Water Air
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Published BSFs
Chica et al. Phys. Med. Biol. 53 (2008) AAPM TG-61 Protocol. Ma et al. Med. Phys. (2001)
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Geant4 and TOPAS
Geant4 simulation. SLAC Perl et al. Med. Phys. (2012)
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Radiochromic Film
Gafchromic film
Radiochromic.com
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Optically Stimulated Luminescence
Thermally and optically stimulated luminescence: a simulation approach.
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TOPAS Calculations
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Measuring Beam Quality
Filter Peak Potential
(kVp)
1 50
2 75
3 100
4 125
5 180
8 280
Kilovoltage unit
Attenuator
Rig
Film
Filter
Applicator
Thimble ionisation chamber
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Benchmarking TOPAS Calculating HVLs
Calculating depth dose and compare to EGSnrc and experimental values
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Monte Carlo Backscatter Factors In-air dose
Dose at the surface of a phantom
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0.00E+00
1.00E-02
2.00E-02
3.00E-02
4.00E-02
5.00E-02
0 10 20 30 40 50 60
50 kVp
0.00E+00
5.00E-03
1.00E-02
1.50E-02
2.00E-02
2.50E-02
3.00E-02
3.50E-02
0 20 40 60 80 100
75 kVp
0
0.01
0.02
0.03
0.04
0.05
0 20 40 60 80 100 120
100 kVp
0
0.02
0.04
0.06
0.08
0.1
0 50 100 150 200
180 kVp
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0 20 40 60 80 100 120 140
125 kVp
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0 50 100 150 200 250 300
280 kVp
Rel
ativ
e W
eigh
t
Energy (keV) SpekCalc. Poludniowski et al. Med.Phy. (2007)
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Experimental Backscatter Factors
Filter
Kilovoltage unit
Applicator
Solid water >10 cm
Detector, sits on a layer of cling film
Empty container
Detector
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Results
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HVLs
Filter Peak Potential
(kVp) HVL
1 50 1.5 mm Al
2 75 2.5 mm Al
3 100 3.8 mm Al
4 125 6.4 mm Al
5 180 0.8 mm Cu
8 280 3.3 mm Cu
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
No
rmal
ised
Rea
din
g
Al Thickness (mm)
Filter 3 100 kVp
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TOPAS HVL Calculations – using SpekCalc
Filter HVL 𝑰
𝑰𝟎
1 1.5 mm Al 0.47 ± 0.2
2 2.5 mm Al 0.48 ± 0.2
3 3.8 mm Al 0.48 ± 0.2
4 6.4 mm Al 0.49 ± 0.2
5 0.8 mm Cu 0.52 ± 0.2
8 3.3 mm Cu 0.48 ± 0.2
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0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10
Re
lati
ve D
ose
Depth (cm)
TOPAS and EGSnrc
TOPAS 10 keV
EGSnrc 10 keV
TOPAS 50 keV
EGSnrc 50 keV
TOPAS 100 keV
EGSnrc 100 keV
TOPAS 300 keV
EGSnrc 300 keV
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0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10
Rel
ativ
e D
ose
2 cm Field Size
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10
6 cm Field Size
TOPAS 50 kVp
Advanced MarkusChamber 50kVp
TOPAS 100 kVp
Advanced MarkusChamber 100 kVp
TOPAS 280 kVp
Advanced MarkusChamber 280 kVp
Depth (cm)
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1.10
1.12
1.14
1.16
1.18
1.20
1.22
1.24
3 cm 6 cm 12 cm
BSF
Field Size
50 kVp 1.5 mm Al
1.10
1.15
1.20
1.25
1.30
1.35
3 cm 6 cm 12 cm
BSF
Field Size
75 kVp 2.5 mm Al
1.00
1.10
1.20
1.30
1.40
3 cm 6 cm 12 cm
BSF
Field Size
100 kVp 3.8 mm Al
AAPM TG61
TOPAS
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1.10
1.15
1.20
1.25
1.30
1.35
1.40
1.45
3 cm 6 cm 12 cm
BSF
Field Size
125 kVp 6.4 mm Al
1.10
1.15
1.20
1.25
1.30
1.35
1.40
1.45
3 cm 6 cm 12 cm
BSF
Field Size
180 kVp 0.8 mm Cu
1.00
1.05
1.10
1.15
1.20
1.25
1.30
3 cm 6 cm 12 cm
BSF
Field Size
280 kVp 3.3 mm Cu
AAPM TG61
TOPAS
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1.00
1.05
1.10
1.15
1.20
1.25
1.30
3 cm 6 cm 12 cm
BSF
Field Size
50 kVp 1.5 mm Al
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
1.40
3 cm 6 cm 12 cm
BSF
Field Size
75 kVp 2.5 mm Al
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
1.40
1.45
3 cm 6 cm 12 cm
BSF
Field Size
100 kVp 3.8 mm Al
TOPAS
Film
OSL
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1.00
1.10
1.20
1.30
1.40
1.50
1.60
3 cm 6 cm 12 cm
BSF
Field Size
125 kVp 6.4 mm Al
1.00
1.10
1.20
1.30
1.40
1.50
1.60
1.70
3 cm 6 cm 12 cm
BSF
Field Size
180 kVp 0.8 mm Cu
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
1.40
1.45
3 cm 6 cm 12 cm
BSF
Field Size
280 kVp 3.3 mm Cu
TOPAS
Film
OSL
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Conclusions
TOPAS successfully calculated low energy x-ray dose calculations – measurements and EGSnrc
TOPAS calculated the BSFs with good agreement to published values (AAPM TG61)
Film was also successful for BSF measurements, but OSLs are not recommended for larger field sizes
Future work should investigate how BSF calculations vary with dose scoring thickness
Find a standard for measuring clinical machine’s BSFs