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
Components of the dose in water
Treatment Head
P
middle
<0,5 to 8 cm
70 to 95 %
5 to 30 %
< 5%
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
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:
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
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
dx
N0
x x+dx
B. Attenuation coefficient µ
N = N0 exp (µ0 x)
µ = s + t + p
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
Superficial beam
Orthovoltage
beam
SAD
z
A
A
Dair
DZ
D. Tissue Air Ratio TAR
TAR (Z, A) = DZ / Dair
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.
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)
120
100
80
TMR_6MV
dose (%)
60
TMR_18MV
40
20
0
0
500
1000
1500
2000
2500
Depth (mm)
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)
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
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