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Linac Beam. Components of the dose in water. primary photons scattered photons in the head (photons and Electrons of contamination) scattered photons in the middle. Treatment Head. P. middle. Components of the dose in the middle. <0,5 to 8 cm. 70 to 95 %. 5 to 30 %. < 5%.

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Linac Beam

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Linac Beam


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Components of the dose in water

  • primary photons

  • scattered photons in the head (photons and Electrons of contamination)

  • scattered photons in the middle

Treatment Head

P

middle


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Components of the dose in the middle

<0,5 to 8 cm

70 to 95 %

5 to 30 %

< 5%


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Accelarator

XRTube

60Co

e-

Source

Tqrget

inhérent

Filtre

Flqttening

Filtre

additionnel

Filtre

Monitor

Collimator

g

X

e-

e-

Accessoire

e-

e-

e-

P

P

P

primary photons + scattered photons + e- contamination

Yph  Kerma

Dose


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Radioactive source

• Nature and mass of radionuclide

XR beams of low energy

• U(kV) + 1st HVL

• 1st HVL + 2nd HVL

• 1st HVL + (1st HVL / 2nd HVL)

XR beams of high energy

20

• U(MV) + TPR

10

specification of beam quality:


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The specification of a beam of high energy XR is a parameter called TPR20, 10 (Tissue Phantom Ratio) or I quality index.

M20

SAD = 100 cm

TPR20,10 =

M10

10 cm x 10 cm

20 cm

M20

water

M10

water

10 cm

at SAD = 100 cm

10 cm x 10 cm

10 cm x 10 cm


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e

e

-

-

Accélérateur

Accelarator

SCD

Collimator

SSD

SAD

Axe of rotation

Champ

Field

d’irradiation

Parameters used to characterize the beam

A. Geometrical C haracteristics of Linac

Source: geometric center of the target or face the source output

Beam axis: axis through the source and the geometric center of the collimator

SSD : Source Skin Distance

SAD : Source Axe Distance

SCD : Source Collimateur Distance (SCD)

Field: intersection of the beam with a plane perpendicular to the axis at a given distance


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dx

N0

x x+dx

B. Attenuation coefficient µ

N = N0 exp (-µ0 x) 

µ = s + t + p


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C. The yield on the depth of the beam axis

(percentage depth dose PDD)

source

source

SSD = cte

A

A

zmax

water

water

z

DZ

Ionisation

Chambre

Dm

  • PDD (Z, A, SSD) = DZ . 100 / Dm

  • The yield (PDD) depends on the beam quality (Energy), depth Z, the field size A and the SSD.

  • The PDD considersthe attenuation and inverse square distance

  • The source detector distance is not fixed


Photon percentage depth dose comparison for photon beams l.jpg

Photon percentage depth dose comparison for photon beams

Superficial beam

Orthovoltage

beam


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SAD

z

A

A

Dair

DZ

D. Tissue Air Ratio TAR

TAR (Z, A) = DZ / Dair

  • The RTA depends on the depth Z, the field size but does not depend on the distance source detector

  • The source detector distance is fixed


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DSA

zmax

A

A

Dair

Dzmax

E. BSF (Back Scatter Factor)

BSF (A)= Dzmax / Dair

TAR (Zmax, A) = DZmax / Dair = BSF (A)

The back scatter factor is important at low energies decreases ↓rapidly when the energy increases ↑

. BSF increases ↑ when energy decreases ↓ to a given field size.


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SAD

z

zmax

A

A

Dm

DZ

F. Tissue Maximum Ratio TMR

TMR(Z,A) = DZ / Dm

The TMR depends on the beam quality, depth Z, the field size but is independent on the source detector distance.It helps determine the quality index.The TMR considers only the attenuation of the beam.If SSD is infinite, then PDD (Z, A, DSP ∞) ≈ TMR (Z, A)


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120

100

80

TMR_6MV

dose (%)

60

TMR_18MV

40

20

0

0

500

1000

1500

2000

2500

Depth (mm)


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DSA

z

zR

A

A

DZR

DZ

G. Tissue Phantom Ratio TPR

TPR (Z,A) = DZ / DZR

If ZR = Zmax, so TMR(Z,A) = TPR (Z,A)


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DSA

zR

zR

AR

A

P

DR(AR)

DT(A)

H. The Collimator opening Factor : Output Factor

Output ( A ) = DT ( A ) / DR ( AR )

ZR, AR and DR are respectively the reference depth, the reference field size and the reference dose rate

In linear accelerators, Rate variation = fct (open Collimator)  :    

1. Flatness filter 2. Collimator 3. ionization chamber 4. middle


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Telecobalt

Linear Accelerator

1

-

produces monenergetic

?

-

rays

1

-

generates a

spectrum of differ x

-

rays

energies

2

-

dose not provide electron beam

2

-

dose provide differ of electron beam

3

-

th

rough a natural phenomenon

(

the

?

-

3

-

we can control the x

-

ray energy that

rays energy cannot be changed or

produced in the range of

4

to more than

controlled by external factors

,

two

?

-

rays

5

MV

)

are produced

1.17,1.34

MeV

)

4

-

radiatio rate changes very slowly T

4

-

the output radiation rate is variable and

1

/

2

of cobalt

-

60

is

5.26

Yr

,

calibration every

weekly calibration is required

.

1

to

3

months is required

5

-

cobalt

-

60

source has

2

cm

,

this lead to

5

-

focal size is

small

(

5

mm

)

hence the

produce wide penumbra

penumbra is narrow with defined field

borders

.

6

-

the components of the machine are

6

-

the electric

,

mechanical component of

technically less complicated

the machine is complicated

7

-

in expensive and breakdowns are less

7

-

expemsive and breakdowns are more

frequent

frequent


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