Electron Beam Radiotherapy

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A resident's presentation on Electron Beam Radiotherapy.

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Electron Beam Radiotherapy

  1. 1. ELECTRON BEAM THERAPYAdhikari Abish. MD Resident, Radiation Oncology,NAMS/BPKMCH, Nepalabishadh @ gmail.com 1
  2. 2. Introduction● Photon radiation travels through the whole patient exposing the distal normal tissue.● Electron therapy is suitable for tumors within 5 cm of the surface.● H &N , Skin, Breast.● Fletcher: “there is no alternative treatment to electron-beam therapy”. 2
  3. 3. Electron Production & Interactions Bending Energy Spectrum chamber Relative Accelerator Intensity Exit Window Scatter system Collimators Applicator Energy (MeV) Lead cut-out Largely monoenergetic beam at Patient exit window. Mixture of lower energies after scattering – broader spectrum. Energy spectrum continues to degrade through collimator system. 3
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  5. 5. Interactions with Matter ● Inelastic collisions: can produce ionization, bremsstrahlung or a secondary electron. ● Elastic collisions: No significant loss of energy, but the trajectory is deflected. Could be either with the nucleus or with the outer electron.Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged 5particle, typically an electron
  6. 6. Energy Specifications● Electron beam at exit is a narrow pencil beam (2-3mm) and almost monoenergetic.● Passes through, exit window, scattering foil, monitor chamber and air, suffering energy degradation and scattering.● So, when it reaches the surface, there are different levels of electron energy.● Clinically, energy is specified by the most probable energy at the surface, kE posessed by most of the electrons. 6
  7. 7. Percentage Depth Dose - Electrons High surface dose Almost constant dose to depth justRelative dose [%] beyond dmax Sharp fall off with increasing depth Finite range – sparing of underlying tissues Range varies with electron energy Depth [cm] A: Build-up region B: Dose fall-off region Typical clinical energies: C: depth-dose tail 5, 7, 9, 12, 15, 18, 20 MeV 7
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  10. 10. Clinical Electron Characteristics● Major attractions: Shape of depth dose curve. (mostly 6 -15 MeV). Rapid drop-off.● Modest skin sparing effect.● Percent surface dose increases with energy. (6MeV:70-80% | 25MeV:~95%)● Dmax depends on the energy, but the relation is not linear. =0.46E0.67● Depth of clinical interest are given by: d90=E/3.2, d80=E/2.8 10
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  12. 12. Xray Contamination● By bremsstrahlung interactions of electrons with scattering foil*, chambers, collimator jaws, applicator, air and body tissues.● Contamination increases with energy.● ~0.5% for 6-12 MeV. ~3% for 20MeV. 12
  13. 13. Choice of Energy / Field Size● “The electron energy should be selected so that the maximum of the depth curve is located at the center of PTV.” - ICRU 71.● The choice of field size should be based on adequacy of isodose coverage of PTV.● Ensure that minimum dose to PTV should be adequate to sterilize the tumor and maximum dose doesnt exceed the tolerance of normal tissue. 13
  14. 14. Effects of Oblique surface● The curved contour alters the depth dose distribution.● Ideal situation would be a flat surface.● The more oblique, the more is the surface dose. More xray contamination.● Alteration in the presence of inhomogenous tissues, bone, lung, air cavities. 14
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  17. 17. Bolus● Flattens out an irregular surface.● Reduces the penetration of the electrons.● Build up the surface dose.● Paraffin wax, polysterene, solid sater, superstuff, superflab. 17
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  19. 19. Field Shaping/ Shielding ● Lead or Cerrobend cutouts are used. ● Lead is placed on the treatment surface, cerrobend on the distal end of applicator. ● Thickness required : ~ 1mm / 2MeV ● Eye shields are made of tungsten and plastic and inserted inside the eyelids. 19Cerrobend: An alloy of bismuth, lead, tin, and cadmium having a low melting point.
  20. 20. Total Skin Electron Therapy● For superficial lesions covering large areas like mycosis fungoides.● Different methods are possible to expose the whole body. Scatter place is closer to body.● Traditionally, patient on a stretcher. Modified as standing or rotating.● Modified Stanford technique. 20
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  22. 22. Thank you 22

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