Linac Beam Treatment Head Components of the dose

Linac Beam

Treatment Head Components of the dose in water primary photons scattered photons in the head (photons and Electrons of contamination) P middle scattered photons in the middle

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

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

specification of beam quality: Radioactive source Nature and mass of radionuclide • XR beams of low energy • U(k. V) + 1 st HVL • 1 st HVL + 2 nd HVL • 1 st HVL + (1 st HVL / 2 nd HVL) XR beams of high energy 20 • U(MV) + TPR 10

M 10 10 cm x 10 cm water TPR 20, 10 20 cm 10 cm SAD = 100 cm The specification of a beam of high energy XR is a parameter called TPR 20, 10 (Tissue Phantom Ratio) or I quality index. M 20 = M 10 10 cm x 10 cm M 20 10 cm x 10 cm water at SAD = 100 cm

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

B. Attenuation coefficient µ dx N = N 0 exp (-µ 0 x) µ = s + t + p N 0 x x+dx

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

D. Tissue Air Ratio TAR SAD A Dair TAR (Z, A) = DZ / Dair z A DZ • 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

E. BSF (Back Scatter Factor) DSA A Dair zmax A Dzmax 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 SAD zmax TMR(Z, A) = DZ / Dm A Dm z A DZ 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)

120 100 dose (%) 80 TMR_6 MV 60 TMR_18 MV 40 20 0 0 500 1000 1500 Depth (mm) 2000 2500

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

H. The Collimator opening Factor : Output Factor DSA z. R AR DR(AR) z. R P A DT(A) 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

Telecobalt 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 Me. V ) 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 2 cm , this lead to produce wide penumbra 6 -the components of the machine are technically less complicated 7 -in expensive and breakdowns are less frequent Linear Accelerator 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 ) 4 -the output radiation rate is variable and weekly calibration is required. 5 -focal size is small (5 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