Linac Beam

primary photons scattered photons in the head

(photons and Electrons of contamination)

scattered photons in the middleP

Tre

atm

ent

Hea

dm

idd

le

Components of the dose in waterComponents of the dose in water

Components of the dose in the middleComponents of the dose in the middle

70 to 95 %

5 to 30 % < 5%

<0,5 to 8 cm

primary photons + scattered photons + e- contamination

P

Collimator

Accessoire

XR Tube

inhérentFiltre additionnel

Filtre

X

e-

P

60Co

e-

Source

e-

P

Accelarator

Monitor

e-

FlqtteningFiltre

Tqrget

e-

e-

ph Kerma

Dose

specification of beam quality:

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

M20 water20 c

m

SA

D =

100

cm

M10 water

10 c

m

10 cm x 10 cm 10 cm x 10 cm

TPR20,10 = M20

M10

10 cm x 10 cm

at SAD = 100 cm

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

Parameters used to characterize the beam

Source: geometric center of the target or face the source outputBeam axis: axis through the source and the geometric center of the collimatorSSD : Source Skin DistanceSAD : Source Axe Distance SCD : Source Collimateur Distance (SCD)Field: intersection of the beam with a plane perpendicular to the axis at a given distance

A. Geometrical C haracteristics of Linac

Accélérateure-

Champd’irradiation

Accelaratore-

Collimator

Axe of rotation

SCD

SSD SAD

Field

B. Attenuation coefficient µ

µ = + +

N = N0 exp (-µ0 x) 

x x+dx

dx

N0

C. The yield on the depth of the beam axis (percentage depth dosepercentage depth dose PDD)

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 considers the attenuation and inverse square distance

• The source detector distance is not fixed

DmDZ

zmax z

SSD = cte

source source

A A

Ionisation Chambre

water water

Photon percentage depth dose comparison for Photon percentage depth dose comparison for photon beamsphoton beams

Superficial beam

Orthovoltagebeam

ADair DZ

SAD

zA

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

ADair Dzmax

DSA

zmax

A

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.

zmaxA

Dm

zA

DZ

SAD

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)

0

20

40

60

80

100

120

0 500 1000 1500 2000 2500

Depth (mm)

do

se (

%) TMR_6MV

TMR_18MV

zRA

DZR

zA

DZ

DSA

G. Tissue Phantom Ratio TPR

TPR (Z,A) = DZ / DZR

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

 

zRAR

DR(AR)

A

DT(A)

zR

DSA

P

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

Linear Accelerator Telecobalt

1-generates a spectrum of differ x -rays

energies

2-dose provide differ of electron beam

3-we can control the x -ray energy that

produced in the range of 4 to more than

5MV )

4-the output radiation rate is variable and

weekly calibration is required .

5-focal size is small (5mm ) hence the

penumbra is narrow with defined field

borders .

6-the electric , mechanical component of

the machine is complicated

7-expemsive and breakdowns are more

frequent

1-produces monenergetic ?-rays

2-dose not provide electron beam

3-through a natural phenomenon (the ?-

rays energy cannot be changed or

controlled by external factors , two ?-rays

are produced 1.17,1.34 MeV )

4-radiatio rate changes very slowly T 1/2

of cobalt -60 is 5.26 Yr , calibration every

1 to 3 months is required

5-cobalt - 60 source has 2cm , this lead to

produce wide penumbra

6-the components of the machine are

technically less complicated

7-in expensive and breakdowns are less

frequent