Formation

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Linac Beam
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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
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Components of the dose in the middle
lt0,5 to 8 cm
70 to 95
5 to 30
lt 5
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specification of beam quality
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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
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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
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B. Attenuation coefficient µ
N N0 exp (-µ0 x) 
µ s t p
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C. The yield on the depth of the beam axis
(percentage 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

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

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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.
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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
8) TMR (Z, A)
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G. Tissue Phantom Ratio TPR
TPR (Z,A) DZ / DZR   If ZR Zmax, so TMR(Z,A)
TPR (Z,A)  
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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
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