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%

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

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

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

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

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

(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 . 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 beams Superficial beam Orthovoltage beam

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

. 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.

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)

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

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)

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

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