1. ELECTRON BEAM THERAPYAdhikari Abish. MD Resident, Radiation Oncology,NAMS/BPKMCH, Nepalabishadh @ gmail.com 1

2. Introduction Photon radiation travels through the wholepatient exposing the distal normal tissue. Electron therapy is suitable for tumorswithin 5 cm of the surface. H &N , Skin, Breast. Fletcher: there is no alternative treatmentto electron-beam therapy. 2 3. Electron Production & InteractionsBendingEnergy SpectrumchamberRelative AcceleratorIntensityExit Window Scatter system Collimators ApplicatorEnergy (MeV) Lead cut-outLargely monoenergetic beam atPatient exit window.Mixture of lower energies afterscattering broader spectrum.Energy spectrum continues todegrade through collimator system. 3 4. 4 5. Interactions with Matter Inelastic collisions: can produce ionization,bremsstrahlung or a secondary electron. Elastic collisions: No significant loss ofenergy, but the trajectory is deflected.Could be either with the nucleus or withthe outer electron.Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged 5particle, typically an electron 6. Energy Specifications Electron beam at exit is a narrow pencilbeam (2-3mm) and almostmonoenergetic. Passes through, exit window, scatteringfoil, monitor chamber and air, sufferingenergy degradation and scattering. So, when it reaches the surface, there aredifferent levels of electron energy. Clinically, energy is specified by the mostprobable energy at the surface, kEposessed by most of the electrons.6 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 energyDepth [cm]A: Build-up regionB: Dose fall-off region Typical clinical energies:C: depth-dose tail5, 7, 9, 12, 15, 18, 20 MeV7 8. 8 9. 9 10. Clinical Electron Characteristics Major attractions: Shape of depth dosecurve. (mostly 6 -15 MeV). Rapid drop-off. Modest skin sparing effect. Percent surface dose increases withenergy. (6MeV:70-80% | 25MeV:~95%) Dmax depends on the energy, but therelation is not linear. =0.46E0.67 Depth of clinical interest are given by:d90=E/3.2, d80=E/2.8 10 11. 11 12. Xray Contamination By bremsstrahlung interactions ofelectrons with scattering foil*, chambers,collimator jaws, applicator, air and bodytissues. Contamination increases with energy. ~0.5% for 6-12 MeV. ~3% for 20MeV. 12 13. Choice of Energy / Field Size The electron energy should be selectedso that the maximum of the depth curve islocated at the center of PTV. - ICRU 71. The choice of field size should be basedon adequacy of isodose coverage of PTV. Ensure that minimum dose to PTV shouldbe adequate to sterilize the tumor andmaximum dose doesnt exceed thetolerance of normal tissue.13 14. Effects of Oblique surface The curved contour alters the depth dosedistribution. Ideal situation would be a flat surface. The more oblique, the more is the surfacedose. More xray contamination. Alteration in the presence ofinhomogenous tissues, bone, lung, aircavities.14 15. 15 16. 16 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 18. 18 19. Field Shaping/ ShieldingLead or Cerrobend cutouts are used.Lead is placed on the treatment surface, cerrobend on the distal end of applicator.Thickness required : ~ 1mm / 2MeVEye 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. Total Skin Electron Therapy For superficial lesions covering largeareas like mycosis fungoides. Different methods are possible to exposethe whole body. Scatter place is closer tobody. Traditionally, patient on a stretcher.Modified as standing or rotating. Modified Stanford technique. 20 21. 21 22. Thank you22