beam because D, for 6-MV photons are scattered in the decreases rapidly lorward

Question

beam because D, for 6-MV photons are scattered in the decreases rapidly lorward direction and because for advantages of x-ray beams from radiation field is one of the 6-MV x-ray a linear accelerator The depth of maximum sso) curves beam is 13 cm below the surface in Margin 7-14 are lor he The isodose curves have been nattened with a cone shaped filter posit x-ray The of the field are sharp, and the dose decreases rapidly utside the primary beam. The region of dose buildup to a d dose 5 cm, and the D. is 83% at 10-cm depth. With megavoltage x he dose delivered to tissue near the exit surface of the patient often is greater than the at the beam entrance Influence of Field Size Two sets of isodose curves are compared in Figure 7-7. The curves on the left represent x 5 cm Co gamma-ray beam with an ssD 80 The curves on ri describe a similar distribution except that the dimensions of the field have been 10 x 10 cm At larger field because a greater the depth dose s greater fraction of the dose is contributed by scattered radiation. The edge of the radiation beam is slightly less sharp with the 10 x 10 cm beam because rays are scattered from the larger volume ofirmadiated medium. This effect is reduced considerably at higher energies, and is almost negligible in a beam MARGIN FIGURE 7-10 of 18 MV x-rays. A 45 wedge filter for a 6 MV x-ray ctron Depth Dose and lsodose Cu (below) D 10 x 10 cm 6-MV x-ray Percent depth doses the central axis of selected electron beams are compared re 7-8. Although a slight reduction in surface dose is provided by a central axis. The isodose curves near 10-om depth are respect to a line electron beam, the dose is fairly uniform from the surface to the depth of maximum dose. The depth of maximum dose increases gradualy with energy of the incident electrons. Beyond this depth, the dose de abrup and somewhat less rapidly for of higher energy For electrons o a particular energy, the decrease in depth dose is slightly more rapid for beams of very small The eenetration depth (depth for 10% depth dose) may be estimated roughly with the expression Penetration depth lElectron enen (Mevv2 Mevcmj An eloupan beam dose distribution is For example, the depth of penetration is about 3 cm for a 6-Mev electron beam about 5 cm for a 10 MeV beam, and about 10 cm for electrons of 20 MeV Energy loss during interaction lsodose curves for electron beams are frequently constructed from densitometric of primary electrons of exposed x-ray film radiographs exposed "edge to electrons of 4 and 26 MeV are ometrically Because electrons are sganci y as an electron beam pencirates a Directional changes of primary medium, the field size for an electron beam expands rapidly below the surface When electron-beam depth-dose and isodose curves are measured with ioniza- Production of bremsstrahlung must be applied for the change in stopping power with Beam divergence causing electron energy According to the AAPM." the mean incident energ Eoofan electron fall-off of electron fluence

Explanation / Answer

As electrons travel through a medium they interact with atoms by a variety of Coulomb force interactions that may be classified as follows:

(a)inelastic collisions with atomic electrons resulting in ionisation and excitation of atoms and termed collisional or ionisational loss;

(b) inelastic collisions with nuclei resulting in bremsstrahlung production and termed radiative loss;

(c)elastic collisions with atomic electrons; and

(d)elastic collisions with atomic nuclei resulting in elastic scattering whichis characterized by change in di

rection but no energy loss

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