06 chap 04 clinical radiation generators


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06 chap 04 clinical radiation generators

  1. 1. Chapter 4 Clinical Radiation Generators4.1 Kilovoltage Units Machine Energy Treatment depth (90% depth dose) Grenz-Ray therapy < 20 kV - Contact therapy 40-50 kV 1–2 mm (endocavitary) Superficial therapy 50-150 kV 5 mm Orthovoltage therapy 150-500 kV 2-3 cm (deep therapy) Supervoltage therapy 500-1000 kV Megavoltage therapy > 1 MV 1
  2. 2. 4.1 Kilovoltage Units (cont’d)• Filters (typically made of aluminum or copper) are often usedto ‘harden’ the energy.• Beam quality expressed as the half-value layer (HVL),defined as the thickness of a specified material that wouldreduce the exposure rate by half.• High skin dose and sharp decrease of depth dose distributionsare the major limitations of the low-energy machines.• Conventional transformer is used for up to 300 kV. Resonanttransformer units used for higher energies. 2
  3. 3. Cobalt-60 Orthovoltage HVL=2.0mm Cu Contact therapy Superficial therapy HVL=1.5mm Al HVL=3.0mm Al Grenz raysHVL=0.04mm Al 3
  4. 4. 4.2 Van de Graaff GeneratorAn electrostatic acceleratordesigned to accelerate chargedparticles.In radiotherapy, it accelerateselectrons to produce high-energyx-rays, typically at 2 MV. Fig. 4.4 4
  5. 5. 4.3 Linear Accelerator The linac uses high-frequency electromagnetic waves (3000 MHz microwaves, wavelength about 10 cm) to accelerate electrons to high energies through a linear tube. There are two types of design used in radiotherapy: traveling and stationary (standing) waves.The traveling wave design requires a The stationary wave design providesterminating load at the end of the maximum reflection on both ends,structure to prevent reflected wave. when combined with the forward wave, forms the stationary wave. 5
  6. 6. 4.3 Linear Accelerator (cont’d) 6
  7. 7. 4.3 Linear Accelerator (magnetron & klystron)The magnetron is a device that produces microwave pulses ofseveral microseconds duration at a repetition rate of severalhundreds pulses per second. Typically, magnetron is used forlow-energy linacs (6 MV or less).The klystron is a microwave amplifier that needs to bedriven by a low-power microwave oscillator. It is used forhigher energy linacs. 7
  8. 8. 4.3 Linear Accelerator (X-ray beam) Bremsstrahlung x-rays are produced when the electrons are incident on a target of a high-Z material (tungsten). The energy spectrum is continuous with the maximum energy equal to the incident electron energy. The average energy is about 1/3 of the maximum energy. 8
  9. 9. 4.3 Linear Accelerator (treatment head) 9
  10. 10. 4.3 Linear Accelerator (target and flattening filter) The electrons hitting the target are in the MeV range, as a result, the x- rays produced from the target are forward peaked. In order to make the intensity more uniform, a flattening filter is inserted in the beam before it leaves the treatment head. The flattening filter is usually made of lead, although tungsten, uranium, steel, aluminum have also been used. The geometrical shape of the flattening filter is usually designed to produce a flat field at 10 cm depth in water. 10
  11. 11. 4.3 Linear Accelerator (beam collimation and monitoring) The treatment beam is first collimated by a fixed primary collimator. Below that is the monitor chamber, which monitors the dose rate, integrated dose, and field symmetry. The monitor chamber is sealed, so that its response is NOT affected by the temperature and pressure in the treatment room. After passing through the monitor chamber, the beam is further collimated by movable x-ray collimators (jaws), which provide a range of field sizes from 0×0 up to 40×40 cm2. 11
  12. 12. 4.3 Linear Accelerator (electron beam) The electron beam, as it emerges from the waveguide tube, has a diameter of about 2-3 mm. In the electron mode of linac operation, this beam is made to strike an electron scattering foil in order to spread the beam to wider area as well as to get a more uniform fluence across the treatment field. The scattering foil consists of thin metallic foil (Pb), so that most electrons are scattered. But some bremsstrahlung photons are also produced, called x-ray contamination of the electron beam. In some linacs, the broadening of the electron beam is accomplished by electromagnetic scanning. 12
  13. 13. 4.3 Linear Accelerator (electron beam) For electron beams, applicators (cones) are used which are close to the treatment surface to minimize the effects of in-air scattering. Note: Jaw setting are fixed for each particular energy and applicator size. 13
  14. 14. 4.3 Linear Accelerator (gantry)Modern-day linacs are constructed so that the source ofradiation can rotate about a horizontal axis. The intersectionof this axis and the collimator axis (beam central axis) is theisocenter.The source-to-isocenter distance for most machines is 100cm. 14
  15. 15. 4.4 Betatron (medical use byJS Laughlin 1950s)An electron in a changingmagnetic field experiencesacceleration in a circular orbit.Betatron can produce a widerange of energies from lessthan 6 to more than 40 MeV.But compared to linacs,betatron has lower dose rateand smaller field size. 15
  16. 16. 4.5 MicrotronThe microtron is acombination of a linac anda cyclotron. The electronsattain higher energies byrepeated passing throughthe linac component.The final treatment beamcan be extracted atdifferent orbit positions fordifferent desired energies. 16
  17. 17. 4.6 Cyclotron (developed by E.O.Lawrencein the 1930s at UC Berkeley)Alternating potential isapplied to the 2 Ds.Charged particles areaccelerated as they leaveone D and enter another D.The particles travel incircular orbits due to themagnetic field. Withincreased energy, the radiusof the orbit also increases.Cyclotrons are used forproton and neutronmachines 17
  18. 18. 4.7 Machines Using Radionuclidesradionuclide half-life γ-ray energy Γ Value Specific (years) (MeV) (Rm2/Ci-h) activity (Ci/g) Ra-226 1622 0.83 0.825 ~0.98(0.5 mm Pt) Cs-137 30 0.66 0.326 ~50 Co-60 5.26 1.25 1.30 ~200 (1.17, 1.33) 18
  19. 19. 4.7 Machines Using Radionuclides (Co-60 unit) 60 27 Co (5.26 y) - β (Emax=0.32 MeV), 99+% - 2.50 β (Emax=1.48 MeV), (1.17 MeV) 0.1% 1.33 (1.33 MeV)The Co-60 is produced in a 60nuclear reactor from Co-59 by 28 Ni59 Co(n, γ)60Co 19
  20. 20. 4.7 Machines Using Radionuclides (Co-60 unit, cont’d)A typical 60Co source is a cylindrical disc of diameterranging from 1.0 to 2.0 cm, which gives rise to geometricpenumbra.Due to interaction of the primary γ–ray and the sourceitself and other surrounding materials in the treatmenthead, there are low-energy contaminants which contributeto about 10% of the total intensity.In addition, contaminant electrons are also produced due tothese interactions. 20
  21. 21. 4.7 Machines Using Radionuclides (Co-60 unit, cont’d)The source is containedinside a stainless-steelcapsule and sealed bywelding. This capsule itselfis again contained in anotherstainless-steel capsule sealedby welding, for radiationsafety reasons. 21
  22. 22. 4.7 Machines Using Radionuclides (Co-60 unit, cont’d)The source head consistsof a steel shell filled withlead and a device(typically a pneumaticdevice) for bringing thesource in front of anopening when the beam is‘on’. 22
  23. 23. 4.7 Machines Using Radionuclides (Co-60 unit, cont’d) sgeometric penumbra: SDD s ( SSD + d − SDD) SSDPd = SDDTo reduce the geometric dpenumbra, trimmers canbe used to increase SDD. Pd 23
  24. 24. 4.8 Heavy Particle BeamsThese particles include neutrons, protons, deuterons,α- particles, negative pions, and heavy ions.They have the advantages in terms of dose localizationand therapeutic gains (greater dose on tumor than onnormal tissues). 24
  25. 25. 4.8 Heavy Particle Beams (neutrons) High energy neutron beams may be produced by D-T generators, cyclotrons, linacs, and nuclear reactors. D-T generator: 2 1 H + H → He + n + 17.6 MeV 3 1 4 2 1 0 cyclotron: 2 1 H + 4 Be → 10B + 01n 9 5 25
  26. 26. 4.8 Heavy Particle Beams (protons, 150-250 MeV)The major advantage of high energy protons and other heavycharged particles is their characteristic ‘Bragg peak’ in thedepth dose distribution.Particles with the same MeV/ 100A have approximately thesame velocities and range. dose 50For example, 150 MeVprotons, 300 MeV deuteronsand 600 MeV helium ions 0 0 4 8 12 16 20have approx. the same rangein water 16 cm. Depth in water (cm) 26
  27. 27. 4.8 Heavy Particle Beams (negative pions)Negative pions (π−1) of energy close to 100 MeV have beenused in radiotherapy, providing a range of 24 cm in water.The Bragg peak exhibited by pions is more pronounced thanother heavy charged particles because of the additionalnuclear disintegration by pion capture, called star formation. 27