Ch 4. Clinical Radiation Generators

1. Ch 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. 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

5. 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

6. 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

7. 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

9. 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

10. 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

11. 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)

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

13. 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)

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

15. 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

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

17. 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

24. 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)

25. 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

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

27. 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

28. Auxiliary system The linac auxiliary system consists of several services that are not directly involved with electron acceleration, yet make the acceleration possible and the linac viable for clinical operation. ● A vacuum pumping system producing a vacuum pressure of ~10–6 torr in the accelerating guide and the RF generator; ● A water cooling system used for cooling the accelerating guide, target, circulator and RF generator; ● An optional air pressure system for pneumatic movement of the target and other beam shaping components; ● Shielding against leakage radiation.

30. 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)

31. linac treatment head 31

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

33. 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

34. The Effect of Flattening Filter 34

35. Vertical and Horizental Chambers

36. 36

37. Linac generations ● Low energy photons (4–8 MV): straight-through beam; fixed flattening filter; external wedges; symmetric jaws; single transmission ionization chamber; isocentric mounting. ● Medium energy photons (10–15 MV) and electrons: bent beam; movable target and flattening filter; scattering foils; dual transmission ionization chamber; electron cones. ● High energy photons (18–25 MV) and electrons: dual photon energy and multiple electron energies; achromatic bending magnet; dual scattering foils or scanned electron pencil beam; motorized wedge; asymmetric or independent collimator jaws. ● High energy photons and electrons: computer controlled operation; dynamic wedge; electronic portal imaging device (EPID); multileaf collimator (MLC). ● High energy photons and electrons: photon beam intensity modulation with MLC; full dynamic conformal dose delivery with intensity modulated beams produced with an MLC.

38. 38

39. Sections of a Linac The linacs are usually mounted isocentrically and the operational systems are distributed over five major and distinct sections of the machine: (I) gantry (2) gantry stand or support (3) modulator cabinet (4) patient support assembly, i.e., treatment couch (5) control console 39

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

42. Betatron • Electron in a changing magnetic field experiences acceleration in a circular orbit • Energy of x-rays: • 6 – 40 MV • Disadvantage: • low dose rate • Small field size

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

44. Microtron • Electron accelerator which combines the principles of both linear accelerator and the cyclotron • Advantage: • Easy energy selection, small beam energy spread and small size

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

46. Cyclotron • Charged particle accelerator • Mainly used for nuclear physics research • As a source of high-energy protons for proton beam therapy • Have been adopted for generating neutron beams recently

47. Cyclotron • Structures • Short metallic cylinder divided into two section (Ds) • Highly evacuated • Placed between the poles of a direct current magnet • Alternating potential is applied between two Ds

48. Cyclotron Positive charged particles (e.g. protons or deuterons) are injected at the center of the two Ds Under B-field, the particles travel in a circular orbit Accelerated by E-field while passing from one D to the other Received an increment of energy Radius of its orbit increases

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

50. Machines Using Radionuclides • Radionuclides have been used as source of γrays for teletherapy • Radium-226, Cesium-137, Cobalt-60 • 60Co has proved to be most suitable for external beam R/T • Higher possible specific activity • Greater radiation output • Higher average photon energy