### Formation

```Linac Beam
P
middle
Components of the dose in water
 primary photons
 scattered photons in the head
(photons and Electrons of
contamination)
 scattered photons in the middle
Components of the dose in the middle
<0,5 to 8 cm
70 to 95 %
5 to 30 %
< 5%
XR Tube
inhérent
Filtre
Accelarator
60Co
Tqrget
Flqttening
Filtre
Monitor
Filtre
Collimator
e-
Source
g
X
e-
e-
Accessoire
e-
e-
eP
P
P
primary photons + scattered photons + e- contamination
Yph  Kerma
Dose
specification of beam quality:
•
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
The specification of a beam of high energy XR is a parameter
called TPR20, 10 (Tissue Phantom Ratio) or I quality index.
M1
0
10 cm x 10 cm
water
20 cm
10 cm
TPR20,10
M20
=
M10
10 cm x 10 cm
M2
0
10 cm x 10 cm
water
Parameters used to characterize the beam
A. Geometrical C haracteristics of Linac
e-
Accélérateur
Accelarator
SCD
Collimator
SSD
Axe of rotation
Champ
Field
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
SCD : Source Collimateur Distance (SCD)
Field: intersection of the beam with a plane perpendicular to the axis at a given
distance
B. Attenuation coefficient µ
dx
N = N0 exp (-µ0 x)
µ=s+t+p
N0
x x+dx
C. The
yield on the depth of the beam axis
(percentage depth
dose PDD)
source
source
SSD = cte
zmax
Ionisation
Chambre
A
A
water
Dm
z
water
DZ
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
Photon percentage depth dose comparison for
photon beams
Superficial beam
Orthovoltage
beam
D. Tissue
Air Ratio TAR
A
Dair
z
A
DZ
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
E. BSF
(Back Scatter Factor)
DSA
A
Dair
zmax
A
Dzmax
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.
F. Tissue
Maximum Ratio TMR
zmax
A
Dm
z
A
DZ
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)
120
100
dose (%)
80
TMR_6MV
60
TMR_18MV
40
20
0
0
500
1000
1500
Depth (mm)
2000
2500
G. Tissue
Phantom Ratio TPR
DSA
zR
A
z
DZR
TPR (Z,A) = DZ / DZR
If ZR = Zmax, so TMR(Z,A) = TPR (Z,A)
A
DZ
H. The
Collimator opening Factor : Output Factor
DSA
zR
AR
DR(AR)
zR
P A
DT(A)
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
Telecobalt
1-produces monenergetic
Linear Accelerator
?-rays
2-dose not provide electron beam
1-generates a spectrum of differ x -rays
energies
2-dose provide differ of 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 )
3-we can control the x -ray energy that
produced in the range of
4 to more than
5MV )
4-radiatio rate changes very slowly T
1/2
of cobalt -60 is 5.26 Yr , calibration every
1 to 3 months is required
4-the output radiation rate is variable and
weekly calibration is required .
5-cobalt - 60 source has 2cm , this lead to
produce wide penumbra
5-focal size is small (5mm ) hence the
penumbra is narrow with defined field
borders .
6-the components of the machine are
technically less complicated
6-the electric , mechanical component of
the machine is complicated
7-in expensive and breakdowns are less
frequent
7-expemsive and breakdowns are more
frequent
```