1. Linac Beam

2. 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

3. Components of the dose in the middle
<0,5 to 8 cm
70 to 95 %
5 to 30 %
< 5%

4. primary photons + scattered photons + e- contamination XR Tube
Collimator
Accessoire
XR Tube
inhérent
Filtre
additionnel X e-
60Co g
Source
Accelarator
Monitor
Flqttening
Tqrget
Yph  Kerma
Dose

5. 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

6. 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
M10
M20
water
10 cm
water
at SAD = 100 cm
10 cm x 10 cm
10 cm x 10 cm

7. Parameters used to characterize the beam
A. Geometrical C haracteristics of Linac
Accélérateur e -
Champ
d’irradiation
Accelarator
Collimator
Axe of rotation
SCD
SSD
SAD
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
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

8. µ = s + t + p B. Attenuation coefficient µ N = N0 exp (-µ0 x) dx
x x+dx dx N0
µ = s + t + p

9. (percentage depth dose PDD)
C. The yield on the depth of the beam axis
(percentage depth dose PDD) Dm DZ
zmax z
SSD = cte
source A
Ionisation
Chambre
water
PDD (Z, A, SSD) = DZ / 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

10. Photon percentage depth dose comparison for photon beams
Superficial beam
Orthovoltage
beam

12. The source detector distance is fixed
D. Tissue Air Ratio TAR A
Dair DZ
SAD z
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

13. . BSF increases ↑ when energy decreases ↓ to a given field size.
E. BSF (Back Scatter Factor) A
Dair
Dzmax
DSA
zmax
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.

14. F. Tissue Maximum Ratio TMR
zmax A Dm z DZ
SAD
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)

15. 20 40 60 80
100
120
500
1000
1500
2000
2500
Depth (mm)
dose (%)
TMR_6MV
TMR_18MV

16. zR A DZR z DZ DSA TPR (Z,A) = DZ / DZR G. Tissue Phantom Ratio TPR
If ZR = Zmax, so TMR(Z,A) = TPR (Z,A)

17. zR AR DR(AR) A DT(A) P Output ( A ) = DT ( A ) / DR ( AR ) DSA
H. The Collimator opening Factor : Output Factor zR AR
DR(AR) A
DT(A)
DSA P
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

18. 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 5 MV )
the output radiation rate is variable and
weekly calibration is required .
focal size is
small ( mm
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
produces monenergetic ?
dose not provide electron beam th
rough a natural phenomenon
the
rays energy cannot be changed or
controlled by external factors
two
are produced
1.17,1.34
MeV
radiatio rate changes very slowly T /
of cobalt 60 is
5.26 Yr
calibration every to
months is required
cobalt
source has cm
this lead to
produce wide penumbra
the components of the machine are
technically less complicated
in expensive and breakdowns are less