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Chapter 4 Clinical Radiation Generators

Chapter 4 Clinical Radiation Generators. The physics of Radiation Therapy, pp. 45 - 70. Kilovoltage Units Van de Graaff Generator Linear Accelerator Betatron Microtron Cyclotron Machine Using Radionuclides Heavy Particle Beams. X-ray machines for radiotherapy. The main components of a

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Chapter 4 Clinical Radiation Generators

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  1. Chapter 4 Clinical Radiation Generators The physics of Radiation Therapy, pp. 45 - 70

  2. Kilovoltage Units • Van de Graaff Generator • Linear Accelerator • Betatron • Microtron • Cyclotron • Machine Using Radionuclides • Heavy Particle Beams

  3. X-ray machines for radiotherapy The main components of a radiotherapy x-ray machine are: • X-ray tube • Ceiling or floor mount for the x-ray tube • Target cooling system • Control console • X-ray power generator

  4. The components of a radiotherapy x-ray machine: • X-ray tube • Applicators

  5. The main components of a typical therapy x-ray tube are: • Water or oil cooled target (anode) • Heated filament (cathode)

  6. X-ray machines for radiotherapy With x-ray tubes the patient dose is delivered using a timer and the treatment time must incorporate a shutter correction time. 􀀁 In comparison with diagnostic radiology x-ray tubes, a therapy x-ray tube operates: • At about 10% of instantaneous current. • At about 10 times average energy input. • With significantly larger focal spot and a fixed rather than rotating anode.

  7. Kilovoltage Units • Up to above 1950 • X-rays generated at voltages up to 300 kVps • Still some use in the present era, esp. treatment of superficial skin lesions • Kilovoltage Therapy • Grenz-Ray Therapy • Contact Therapy • Superficial Therapy • Orthovoltage Therapy or Deep Therapy • Supervoltage Therapy

  8. Kilovoltage Units • Grenz-Ray Therapy • Energy : < 20 kV • Very low depth of penetration • No longer used in R/T • Contact Therapy • Energy: 40 – 50 kV • Short SSD (< 2 cm) • Produces a very rapidly decreasing depth dose • Max irradiated tissue : skin surface • Application: Tumor not deeper than 1 – 2 mm

  9. Kilovoltage Units • Superficial Therapy • Energy: 50 – 150 kV • HVLs: 1.0- – 8.0-mm Al • Applicator or cone attached to the diaphragm • SSD: 15 – 20 cm • Tube current: 5 – 8 mA • Application: tumors confined to about 5-mm depth

  10. Kilovoltage Units • Orthovoltage Therapy or Deep Therapy • Energy: 200 – 300 kV • Tube current: 10 – 20 mA • HVLs: 1 – 4 mm Cu • Cones or movable diaphragm (continuous adjustable field size) • SSD: 50 cm • Application: tumor located < 2 –3 cm in depth • Limitation of the treatment: • skin dose • Depth dose distribution • Increase absorbed dose in bone • Increase scattering

  11. Kilovoltage Units • Supervoltage Therapy • Energy: 500 – 1000 kV • Technical problem • Insulating the high-voltage transformer • Conventional transformer systems were not suitable for producing potential > 300 kVp • The problem solved by invention of resonant transformer

  12. Kilovoltage Units • Resonant transformer units • Used to generate x-rays from 300 to 2000 kV • At resonant frequency • Oscillating potential attains very high amplitude • Peak voltage across the x-ray tube becomes very large

  13. Megavoltage Therapy • X-ray beams of energy > 1 MV • Accelerators or γray produced by radionuclides • Examples of clinical megavoltage machines • Van de Graaff generator • Linear accelerator • Betatron • Microtron • Teletherapy γray units (e.g. cobalt-60)

  14. Clinical x-ray beams • In the diagnostic energy range (10 - 150 kVp) most photons are produced at 90 from the direction of electrons striking the target (x-ray tube). • In the megavoltage energy range (1 - 50 MV) most photons are produced in the direction of the electron beam striking the target (linac).

  15. Kilovoltage Units • Van de Graaff Generator • Linear Accelerator • Betatron • Microtron • Cyclotron • Machine Using Radionuclides • Heavy Particle Beams

  16. Van de Graaff Generator • Electrostatic accelerator • Energy of x-rays: 2 MV (typical), up to 10 MV • Limiation: • size • high-voltage insulation • No longer produced commercially • Technically better machine (e.g. Co-60 units & linear accelerators)

  17. Kilovoltage Units • Van de Graaff Generator • Linear Accelerator • Betatron • Microtron • Cyclotron • Machine Using Radionuclides • Heavy Particle Beams

  18. LINACS Medical linacs are cyclic accelerators that accelerate electrons to kinetic energies from 4 to 25 MeV using microwave radiofrequency fields: • 103 MHz : L band • 2856 MHz: S band • 104 MHz: X band 􀀁 In a linac the electrons are accelerated following straight trajectories in special evacuated structures called accelerating waveguides.

  19. Linac generations During the past 40 years medical linacs have gone through five distinct generations, each one increasingly more sophisticated: (1) Low energy x rays (4-6 MV) (2) Medium energy x rays (10-15 MV) and electrons (3) High energy x rays (18-25 MV) and electrons (4) Computer controlled dual energy linac with electrons (5) Computer controlled dual energy linac with electrons combined with intensity modulation

  20. Linear Accelerator • Use high frequency electromagnetic waves to acelerate charged particles (e.g. electrons) to high energies through a linear tube • High-energy electron beam – treating superficial tumors • X-rays – treating deep-seated tumors

  21. Linear Accelerator • Types of EM wave 1. Traveling EM wave • Required a terminating (“dummy”) load to absorb the residual power at the end of the structure • Prevent backward reflection wave 2. Standing EM wave • Combination of forward and reverse traveling waves • More efficiency • Axial beam transport cavities and the side cavities can be independently optimized • More expensive • Requires installation of a circulator (or insulator) between the power source • the structure prevent reflections from reaching the power source

  22. Linear Accelerator Fig 4.5. A block diagram of typical medical linear accelerator

  23. Accelerating waveguide In the standing wave accelerating structure each end of the accelerating waveguide is terminated with a conducting disk to reflect the microwave power producing a standing wave in the waveguide. Every second cavity carries no electric field and thus produces no energy gain for the electron (coupling cavities In the travelling wave accelerating structure the microwaves enter on the gun side and propagate toward the high energy end of the waveguide. Only one in four cavities is at any given moment suitable for acceleration

  24. Microwave power transmission 􀀁 The microwave power produced by the RF generator is carried to the accelerating waveguide through rectangular uniform waveguides usually pressurized with a dielectric gas (freon or sulphur hexafluoride SF6). 􀀁 Between the RF generator and the accelerating waveguide is a circulator (isolator) which transmits the RF power from the RF generator to the accelerating waveguide but does not transmit microwaves in the opposite direction.

  25. The Magnetron • A device that produces microwaves • Functions as a high-power oscillator • Generating microwave pulses of several microseconds with repetition rate of several hundred pulses per second • Frequency of microwave within each pulse is about 3000 MHz • Peak power output: • 2 MW (for low-energy linacs, 6MV or less) • 5 MW (for higher-energy linacs, mostly use klystrons)

  26. The Magnetron The cathode is heated by an inner filament Electrons are generated by thermionic emission Pulse E-field between cathode & anode Electron accelerated toward the anode Static B-field perpendicular to the plane of cavities Electron move in complex spirals toward the resonant cavities Radiating energy in form of microwave

  27. The Klystron • Not a generator of microwaves • Microwave amplifier • Needs to be driven by a low-power microwave oscillator

  28. The Klystron Electrons produced by the cathode Passed in the drift tube (field-free space) Electrons are accelerated by –ve pulse into buncher cavity Lower level microwave set up an alternating E field across the buncher cavity • Electrons arrive catcher cavity • Generate a retarding E-field • Electrons suffer deceleration • KE of electrons converted into high-power microwaves • Velocity of e- is altered by the action of E-field (velocity modulation) • Some e- are speed up • Other are slowed down

  29. Schematic diagram of a modern fifth generation linac

  30. Electron beam transport Three systems for electron beam bending have been developed: • 90o bending • 270o bending • 112.5o (slalom) bending

  31. The Linac X-Ray Beam • Production of x-rays • Electrons are incident on a target of a high-Z material (e.g. tungsten) • Target – need water cooled & thick enough to absorb most of the incident electrons • Bremsstrahlung interactions • Electrons energy is converted into a spectrum of x-rays energies • Max energy of x-rays = energy of incident energy of electrons • Average photon energy = 1/3 of max energy of x-rays • Designation of energy of electron beam and x-rays • Electron beam - MeV (million electron volts, monoenergetic) • X-ray beam – MV (megavolts, voltage across an x-ray tube, hetergeneous in energy)

  32. Linac treatment head 􀀁 Components of a modern linac treatment head: • Several retractable x-ray targets (one for each x-ray beam energy). • Flattening filters (one for each x-ray beam energy). • Scattering foils for production of clinical electron beams. • Primary collimator. • Adjustable secondary collimator with independent jaw motion. • Dual transmission ionization chamber. • Field defining light and range finder. • Retractable wedges. • Multileaf collimator (MLC).

  33. Physical Wedge Beamline

  34. Virtual Wedge Beamline Dose Rate Control MU/min Jaw Speed Constant mm/sec

  35. Virtual Wedge Beamline Dose Rate Control MU/min Jaw Speed Constant mm/sec

  36. Virtual Wedge Beamline Dose Rate Control MU/min Jaw Speed Constant mm/sec

  37. Multi Leaf Collimator (MLC)

  38. Siemens Elekta Varian Source Source Source MLC Y Y Y Jaw Jaw Y Y X 2 X 1 39.2 cm MLC X X 55.0 cm 57.6 cm Jaw X X X X MLC Accessory Accessory Holder Holder Holder Accessory 29.2 cm 32 cm 43 cm 100 cm 1.0 cm 1.0 cm 1.0 cm Isocenter Resolution Resolution Resolution

  39. Lead or tungsten Opening from 0 x 0 to 40 x 40 cm at SSD 100 cm

  40. Production of clinical x-ray beams Typical electron pulses arriving on the x-ray target of a linac. Typical values: Pulse height: 50 mA Pulse duration: 2 μs Repetition rate: 100 pps Period: 104 μs

  41. Collimation System In modern linacs the x-ray beam collimation is achieved with three collimation devices: • Primary collimator. • Secondary adjustable beam defining collimator (independent jaws). • Multileaf collimator (MLC). 􀀁 The electron beam collimation is achieved with: • Primary collimator. • Secondary collimator. • Electron applicator (cone). • Multileaf collimator (under development).

  42. Production of clinical electron beam 􀀁 To activate the electron mode the x-ray target and flattening filter are removed from the electron pencil beam. 􀀁 Two techniques for producing clinical electron beams from the pencil electron beam: • Pencil beam scattering with a scattering foil (thin foil of lead). • Pencil beam scanning with two computer controlled magnets

  43. Narrow pencil about 3 mm in diameter Uniform electron fluence across the treatment field e.g. lead Electron scatter readily in air Beam collimator must be achieved close to the skin surface

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