In-Phantom Measurements of the Bremsstrahlung Dose for...

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In-Phantom Measurements of the Bremsstrahlung Dose for Megavoltage Electron Beams Incident on Metals for Use in an electron Multileaf Collimator Chester S. Reft Department of Radiation Oncology University of Chicago Medical Center

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In-Phantom Measurements of the Bremsstrahlung Dose for Megavoltage

Electron Beams Incident on Metals for Use in an electron Multileaf Collimatoran electron Multileaf Collimator

Chester S. ReftDepartment of Radiation Oncology

University of Chicago Medical Center

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Sources of bremsstrahlung radiation in electron beams

accelerator head (sc foil & coll)

cerrobend blocks

patient

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dmax = 0.145 E0

µ(E0) = 0.084 E0-0.218

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Materials

Varian Cl 2100C – 6, 9, 12, 16 and 20 MeV electron beams

Materials attached to end of 6x6 cm 2 conebrasscerrobendtungstenpolyethylenepolyethylene

Solid Water Phantom – RMI 451

DetectorsTLDs – bremsstrahlung doseEDR film – CA PDD and beam profilesAttix parallel-plate chamber – CA PDD

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Some Physical properties of the materials

material Thickness(cm)

ρ(g/cm3)meas (ρZ/A)rel wat

polyethylene 15.5 0.94 0.968

brass 3.0 8.4 6.79brass 3.0 8.4 6.79

cerrobend 1.7 9.4 6.79

tungsten 2.0 18 12.8

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Brass

Solid Water

Film

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Some Electron Beam Parameters

E(MeV) R50(cm) Rp(cm)

6 2.4 2.8

9 2.7 4.59 2.7 4.5

12 6.1 6.3

16 6.8 8.4

20 8.5 10.6

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Figure Summary of dose measurements at Rp+2 for the metals normalized to the dose at

dmax for an open field

2.5

3

3.5

4

4.5

5

%[D

(Rp+

2,Z

)/D

(dm

ax,o

pen)

] 2 cm brass1.7 cm cerrobend2 cm tungstenopen

0

0.5

1

1.5

2

2.5

0 5 10 15 20 25

Energy (MeV)

%[D

(Rp+

2,Z

)/D

(dm

ax,o

pen)

]

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% dose at 1 cm depth for bremsstrahlung radiation produced by 20 MeV electrons incident on various

materials relative to the dose for an open field at dmax

for 20 MeV electrons

% [Dbrem(1cm) / D20E(dmax)]

Open# Poly15.5 cm

Brass3 cm

Cerrob1.7 cm

W2 cm

4.9 4.7 6.1 8.5 3.5

# measured at depth of Rp + 2 cm = 12.6 cm

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E(MeV)Mat

6 9 12 16 20

Tungsten 0.20 0.30 0.60 1.4 2.22.2

% Doseblock (d=3cm) / Dose open (dmax,E)6x6 cm 2 Cone

Tungsten 0.20 0.30 0.60 1.4 2.22.2

Brass 0.24 0.50 1.1 2.8 5.35.3

Cerrobend 0.40 ------ 3.7 3.7 ------

OpenRp+2cm

0.50 0.90 1.5 3.2 4.54.5

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OD vs dose EDR film

0.8

1

1.2

1.4

1.6

1.8

OD

- F

og(.

28)

CA calib

OAR calib

Poly. (CA calib)

OD=-0.04769+0.006266D+0.000004148D^2

0

0.2

0.4

0.6

0 50 100 150 200 250

Dose (cGy)

OD

- F

og(.

28)

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Profiles at 1 cm depth in SW of the bremsstrahlung radiation produced by 20

MeV electrons

80

100

120

% d

ose

rel C

A

0

20

40

60

80

-4 -3 -2 -1 0 1 2 3 4

distance (cm)

% d

ose

rel C

A

W 2 cm

Brass 3 cm

Poly 15.5 cm

Open Rp+2 cm

Cerrobend 1.7 cm

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CA PDD of the bremsstrahlung produced by 20 MeV electrons incident on the materials. Note that polyethylene produces the most

penetrating beam.

0.8

1

1.2

PD

D (

SW

)

W 2 cmCerrobend 1.7 cmBrass 3 cmPolyethylene 15.5 cm

0

0.2

0.4

0.6

0 5 10 15 20 25depth (cm)

PD

D (

SW

)

Open at Rp +2 cm

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Poly15.5 cm

Brass3 cm

Cerrob1.7 cm

W2 cm

d PDD d PDD d PDD d PDD

Summary of PDD in SW for the bremsstrahlung radiation Produced by 20 MeV electrons incident on

various materials. Notice that polyethylene produces the most penetrating radiation

cm cm cm cm

10 68 6.2 50 8.2 50 9.5 50

16.5 50 10 38 10 45 10 49

20 44 20 23 20 26 20 28

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Summary of PDD in SW for the bremsstrahlung radiation Produced by 6 MeV electrons incident on polyethylene and W. The beam qualities are very

similar.

Polyethylene 5.8 cm

W0.5 cm

d PDD d PDDdcm

PDD dcm

PDD

8.2 50 8.0 50

10 43 10 43

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CONCLUSIONS1. These results show that 2 cm thick W stops 20 MeV electrons

while reducing the x-ray dose to a level lower than that produced in the open field – 1% compared to 4.5%.

2. The most penetrating bremsstrahlung radiation was measured for the electrons on polyethylene, the lowest Z material. This is consistent with the work of Podgorsak et al3 which showed that at these beam energies the more penetrating radiations is that at these beam energies the more penetrating radiations is produced in the lower Z material.

3 These results provide measurements to compare with Monte Carlo calculations The % central-axis dose (electron and photon) for 20 MeV electrons at 3 cm depth for open, brass and W are 4.5, 5.3 and 2.3% respectively, in reasonable agreement with the published Monte Carlo calculations3 of 5,5 and 3.1% for 21 MeV electrons.

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OD vs dose EDR film

0.8

1

1.2

1.4

1.6

1.8

OD

- F

og(.

28)

CA calib

OAR calib

Poly. (CA calib)

OD=-0.04769+0.006266D+0.000004148D^2

0

0.2

0.4

0.6

0 50 100 150 200 250

Dose (cGy)

OD

- F

og(.

28)

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Profiles at 1 cm depth in SW of the bremsstrahlung radiation produced by 20 MeV electrons incident on the materials

60

80

100

120

% d

ose

rel C

A

0

20

40

-4 -3 -2 -1 0 1 2 3 4

distance (cm)

% d

ose

rel C

A

W 2 cm

Brass 3 cm

Poly 15.5 cm

Open Rp+2 cm

Cerrobend 1.7 cm

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PDD of the bremsstrahlung produced by 20 MeV electrons incident on materials

0.6

0.8

1

1.2

PD

D(S

W)

Polyethylene 15.5 cmOpen at Rp + 2 cmW 2 cmCerrobend 1.7 cmBrass 3 cm

0

0.2

0.4

0.6

0 5 10 15 20 25

depth (cm)

PD

D(S

W)

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Open# Poly15.5 cm

Brass3 cm

Cerrob1.7 cm

W2 cm

dcm

PDD dcm

PDD dcm

PDD dcm

PDD dcm

PDD

Summary of PDD in SW for the bremsstrahlung radiation Produced by 20 MeV electrons incident on

various materials

cm cm cm cm cm

10 60 10 68 6.2 50 8.2 50 9.5 50

14 50 16.5 50 10 38 10 45 10 49

20 37 20 44 20 23 20 26 20 28

# Rp + 2 cm

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Summary of dose measurements at Rp+2 for the bremsstrahlung produced in metals

relatiave to the dose at dmax for an open field

3

3.5

4

4.5

5

%[D

(Rp+

2,Z

)/D

(dm

ax,o

pen)

] 2 cm brass1.7 cm cerrobend2 cm tungstenopen

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20 25

Energy (MeV)

%[D

(Rp+

2,Z

)/D

(dm

ax,o

pen)

]

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CA PDD of the bremsstrahlung produced by 20 MeV electrons incident on the materials. Note that polyethylene produces the most

penetrating beam.

1.2W 2 cm

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25

depth (cm)

PD

D (

SW

)

W 2 cmCerrobend 1.7 cmBrass 3 cmPolyethylene 15.5 cmOpen at Rp +2 cm

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Abstract

Bremsstrahlung radiation in electron beams is produced in the accelerator head (scattering-foil and collimators), the cerrobend or field defining devices and within the patient. The dominate contribution comes from the head1. However, in areas shielded by electron multileaf collimators the bremsstrahlung produced in the collimating leaves will be bremsstrahlung produced in the collimating leaves will be significant. Here the total bremsstrahlung dose is measured for megavoltage electron beams incident on three metals used to simulate the collimating leaves in a multileaf collimator. Beam profiles at 1 cm depth are obtained as well as percentage depth dose measurements to estimate the beam quality of the radiation distal to the materials. Some results show good agreement with published Monte Carlo calculations2

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Materials and Methods

A Varian Cl 2100C is used to provide 6, 9, 12, 16 and 20 MeV electron beams. At the end of the standard 6x6cm2 cone are positioned materials of appropriate thickness to simulate the multileaves. The materials studied are brass, cerrobend and tungsten. Although polyethylene is not practical for use as multileaves, it is included because it is similar to the bremsstrahlung for an open field and also to compare with the bremsstrahlung for an open field and also to compare with the radiation quality from the higher z metals. Thermoluminescent dosimeters (0.089 cm thick by 0.32x0.32 cm2), EDR film and an Attix parallel-plate ionization chamber are used to measure the central axis x-ray dose in a solid water (RMI 451) phantom at a source-to-surface distance of 100 cm. Film oriented parallel to the beam is used to obtain beam profiles at 1 cm depth and also the central-axis percentage depth dose to estimate the beam quality of the bremstrahlung spectra.