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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