# Linac Beam

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06-Jan-2016Category

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Linac Beam

primary photons scattered photons in the head (photons and Electrons of contamination)

scattered photons in the middle

PTreatment HeadmiddleComponents of the dose in water

Components of the dose in the middle

70 to 95 %5 to 30 %< 5%

specification of beam quality:

M20water20 cmSAD = 100 cmM10water10 cm10 cm x 10 cm10 cm x 10 cmTPR20,10 = M20M1010 cm x 10 cmat SAD = 100 cmThe 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 beamSource: 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

B. Attenuation coefficient = s + t + pN = N0 exp (-0 x)

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

Photon percentage depth dose comparison for photon beamsSuperficial beamOrthovoltagebeam

D. Tissue Air Ratio TAR TAR (Z, A) = DZ / DairThe RTA depends on the depth Z, the field size but does not depend on the distance source detectorThe source detector distance is fixed

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.

F. Tissue Maximum Ratio TMR TMR(Z,A) = DZ / DmThe 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)

G. Tissue Phantom Ratio TPR TPR (Z,A) = DZ / DZRIf ZR = Zmax, so TMR(Z,A) = TPR (Z,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