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ELECTRON BEAM RADIOTHERAPY

ELECTRON BEAM RADIOTHERAPY

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ELECTRON BEAM RADIOTHERAPY

  1. 1. DR.K.C PATRO ELECTRON BEAM THERAPY
  2. 2. DEFINITION It is a type of particulate beam radiation therapy used for the treatment of Superficial tumors
  3. 3. BEAM PHOTON PARTICULATE X ray γ ray α,β,proton,e,He,C
  4. 4. WHY ELECTRON BEAM  No exit dose like X-ray  Delivery of reasonably uniform dose from skin to a specific depth  abrupt dose fall off after 90% to 80% isodose curve to near zero level
  5. 5. Photon BeamPit Falls we want  Exit dose  No 100% Skin sparing  No 100% skin dose  No 100% tumor control  No normal tissue sparing  Side scatter  Low LET  High OER  Low RBE  No exit dose  100% Skin sparing  100% skin dose  100% tumor control  Normal tissue sparing  No Side scatter  High LET  Low OER  High RBE
  6. 6. PHOTON vs. ELECTRON  Exit dose  Near 100% Skin sparing  No 100% skin dose  High penetration  No DOSE uniformity  No exit dose  near 100% skin dose  abrupt dose fall off after 80% isodose curve  High scatter  Dose uniformity
  7. 7. MACHINES  Van De Graff generator.  Betatron  Linear accelerator – a device that uses high frequency EM waves to accelerate charged particles viz. electrons to a high energy through a linear tube
  8. 8. Mechanism of production
  9. 9. X-ray Mode/ electron Mode
  10. 10. Parts of treatment head 1. Primary Collimator 2. Scattering foil 3. Flattening filter 4. Ionization chamber 5. Secondary collimator 6. Tertiary collimator ( trimmer , electron cone, auxiliary collimator )
  11. 11. SCATTERING FOIL 1. The scattering foils are made up of high dense material interposed in electron beam 2. Different scattering foils are used for different e- energy 3. The electron beam edges can be sharply defined only if the collimator extended towards the skin of the pt by attachment of trimmers/applicators 4. The electron beam trimmers are optimally designed to give uniform fluency
  12. 12. ELECTRON BEAM FROM MACHINE TO PATIENT  The energetic electron beam emerging from accelerators is pencil beam.it is not suitable for treatment  Hence the beam has to be spread to a larger area for treatment.  Spread can be done by- electromagnetic scattering device or scattering foil  To further spreading electron applicator is used
  13. 13. Mechanism of interaction of electron with matter  Inelastic collision with atomic electron  Inelastic collision with nuclei  elastic collision with atomic electron  elastic collision with nuclei
  14. 14. Fundamental questions before EBT  Indication of EBT  Goal of EBT  Planned Treatment Volume  Planned Treatment dose  Planned Treatment Technique
  15. 15. Indications  Treating skin cancers.  Chest wall irradiation in Ca breast.  Boosting to neck node after 45 Gy.  Total skin irradiation in mycosis fungoides.  Total limb irradiation  Total scalp irradiation  Craniospinal irradiation  Intracavitary irradiation
  16. 16. Dosimetry  Measured by pencil beam algorithm  Accurate in water at standard and extended SSD  Correctly predicts changes in Penumbra  Predict changes in dose in oblique incidence or irregular surfaces
  17. 17. Isodose Curves
  18. 18. Isodose curve characteristic  Rapid dose falloff below 80% isodose  Bulging towards bottom  Ballooning towards edge
  19. 19. Isodose curve in slopping surface
  20. 20. Determination of absorbed dose-  Calorimeter  TLD  Solid state diode  Ionization chamber
  21. 21. Depth dose characteristic/isodose characteristic  There is a abrupt fall of doses beyond 90% to 80% of isodose curve  80% or 90% isodose curve is taken as standard dose for prescription  Depth (cm)of 90% isodose=E(Mev)/4  Depth (cm)of 80% isodose=E(Mev)/3  Electron beam penetrates a finite depth with clear cut range,the photon beam in other hand proceeds infinite range.
  22. 22. Dose dependence on incident energy
  23. 23. Dose dependence on field size
  24. 24. Depth Dose dependence on SSD
  25. 25. dmax  Maximum dose is not on skin  It is somehow away from skin  It is due to electronic equilibrium  6Mev-10mm  9Mev-15mm  12Mev-19mm Approximation
  26. 26. Range of depth  Roughly E/2  E=energy  E.g –for 6Mev range is 6/2=3cm
  27. 27. Surface dose in 10x10 cm field  For 5mev=74%  10mev=82%  16mev=93%  25mev=96%  It depends upon energy,field size and thickness of scattering foil
  28. 28. Energy range  6-20 Mev 6,9,12,15,18,20 mev
  29. 29. Selection of energy Ep0(MeV) = 3.3 x R90 (cm) [R90 exceeds max depth of PTV] Ep0(MeV) = 2 x Rp [Rp is the practical range of electrons ]
  30. 30. Ideal condition for EBT  Electron beam incidence normal to flat surface  Underlying homogenous soft tissue (provide uniform dose in penumbra from surface to R90 after which there is rapid dose fall off)
  31. 31. USE OF BOLUS  To flatten out irregular surface  Reduce the penetration of electrons in part of the field  Increase the surface dose  To act as a missing tissue compensator  Thickness of bolus required increases as the surface dose increases e.g – 6-10mev=1cm – 10-15mev=0.5cm – >15mev=no bolus
  32. 32. BOLUS MATERIAL  Paraffin wax,polysterene,lucite,superstoff,superflab  Flexible bolus that confirms to surface is desirable  Large air gap between the absorber and the surface would result in scattering of e- outside the field and reduction in dose that may not be easily predictable unless specially measured for this condition
  33. 33. MLC vs. BLOCK  No MLC  Wooden MLC  Wax MLC  Customized block
  34. 34. BLOCKING MATERIAL  Usually Cerroband  Cerroband-Pb+Bi+Sn+Cu  Melting point=  Customized block  Thickness of block required to use as block =E/2 e.g for 6mev=6/2=3cm
  35. 35. Monitor unit CGy/MU
  36. 36. Decelerators  A plate of low atomic wt. material such as lucite and polysterene is sometimes used to reduce the energy of electron beam known as decelerators  It must be placed in close contact with patient surface with bolus
  37. 37. Electron arc therapy
  38. 38. Electron field-photon field combination
  39. 39. Total skin irradiation-
  40. 40. Total scalp irradiation
  41. 41. Craniospinal irradiation
  42. 42. Total limb irradiation

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