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

Kilovoltage Radiotherapy

Treats superficial cancers

Melanoma. Healthline

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

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.

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

Backscatter Factors

Water Air

Published BSFs

Chica et al. Phys. Med. Biol. 53 (2008) AAPM TG-61 Protocol. Ma et al. Med. Phys. (2001)

Geant4 and TOPAS

Geant4 simulation. SLAC Perl et al. Med. Phys. (2012)

Radiochromic Film

Gafchromic film

Radiochromic.com

Optically Stimulated Luminescence

Thermally and optically stimulated luminescence: a simulation approach.

TOPAS Calculations

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

Benchmarking TOPAS Calculating HVLs

Calculating depth dose and compare to EGSnrc and experimental values

Monte Carlo Backscatter Factors In-air dose

Dose at the surface of a phantom

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)

Experimental Backscatter Factors

Filter

Kilovoltage unit

Applicator

Solid water >10 cm

Detector, sits on a layer of cling film

Empty container

Detector

Results

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

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

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

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)

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

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

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

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

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