Linear Accelerators
- 1. Clinical Radiation Generators Linear Accelerator 1Linear Accelerators By P.Velliangiri M.Sc., MBA., North Bengal Oncology Centre, Rangapani, Near Siliguri Darjeeling Dist, West Bengal Mail ID: p.velliangirii@gmail.com
- 2. Introduction : • Medical linacs are cyclic accelerators that accelerate electrons to kinetic energies from 4 to 25 MeV. • Non-conservative microwave RF fields in the frequency range from, • 103 MHz : L band • 2856 MHz: S band • 104 MHz: X band • Various types of linac are available for clinical use. • Some provide X rays only in the low megavoltage range (4 or 6 MV), while others provide both X rays and electrons at various megavoltage energies. • A typical modern high energy linac will provide two photon energies (6 and 18 MV) and several electron energies (e.g. 6, 9, 12, 16 and 22 MeV). 2Linear Accelerators
- 3. Linear Accelerators 3 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.
- 4. Linac generations • 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. 4Linear Accelerators
- 5. Linear Accelerators 5 Linear Accelerator Basic Linac Machine :
- 6. Components of modern linacs 6Linear Accelerators • Linacs are usually mounted isocentrically and the operational systems are distributed over five major and distinct sections of the machine, the: ● Gantry; ● Gantry stand or support; ● Modulator cabinet; ● Patient support assembly (i.e. treatment table); ● Control console.
- 7. Linear Accelerators 7 The length of the accelerating waveguide depends on the final electron kinetic energy, and ranges from ~30 cm at 4 MeV to ~150 cm at 25 MeV. The main beam forming components of a modern medical linac are usually grouped into six classes: (i) Injection system; (ii) RF power generation system; (iii) Accelerating waveguide; (iv) Auxiliary system; (v) Beam transport system; (vi) Beam collimation and beam monitoring system.
- 9. Injection system • The injection system is the source of electrons; it is essentially a simple electrostatic accelerator called an electron gun. ● Two types of electron gun are in use as sources of electrons in medical linacs: — Diode type; — Triode type. • Electrons are thermionically emitted from the heated cathode, focused into a pencil beam by a curved focusing electrode and accelerated towards the perforated anode through which they drift to enter the accelerating waveguide. Linear Accelerators 9
- 10. Diode type : • The electrostatic fields used to accelerate the electrons in the diode gun are supplied directly from the pulsed modulator in the form of a negative pulse delivered to the cathode of the gun. 10 Triode type : In a triode gun, however, the cathode is held at a static negative potential (typically –20 kV). The grid of the triode gun is normally held sufficiently negative with respect to the cathode to cut off the current to the anode. The injection of electrons into the accelerating waveguide is then controlled by voltage pulses, which are applied to the grid and must be synchronized with the pulses applied to the microwave generator. A removable triode gun of a high energy linac
- 11. Radiofrequency power generation system • The microwave radiation used in the accelerating waveguide to accelerate electrons to the desired kinetic energy is produced by the RF power generation system, which consists of two major components: ● An RF power source; ● A pulsed modulator. The RF power source is either a magnetron or a klystron. A magnetron is a source of high power RF required for electron acceleration, while a klystron is an RF power amplifier that amplifies the low power RF generated by an RF oscillator commonly called the RF driver. Linear Accelerators 11
- 12. Linear Accelerators 12 The Magnetron The magnetron has a cylindrical construction, having a central cathode and an outer anode with resonant cavities machined out of a solid piece of copper The space between the cathode and the anode is evacuated.. The cathode is heated by an inner filament and the electrons are generated by thermionic emission.
- 13. Linear Accelerators 13 The Magnetron • A static magnetic field is applied perpendicular to the plane of the cross section of the cavities and a pulsed DC electric field is applied between the cathode and the anode. • The electrons emitted from the cathode are accelerated toward the anode by the action of the pulsed DC electric field. • The generated microwave pulses are led to the accelerator structure via the waveguide
- 14. Linear Accelerators 14 The Klystron The klystron is not a generator of microwaves but rather a microwave amplifier. It needs to be driven by a low-power microwave oscillator.
- 15. Linear Accelerators 15 The Klystron • The electrons produced by the cathode are accelerated by a negative pulse of voltage into the first cavity, called the buncher cavity, which is energized by low-power microwaves. • The microwaves set up an alternating electric field across the cavity. • The velocity of the electrons is altered by the action of this electric field to a varying degree by a process known as velocity modulation. Some e- are speed up Other are slowed down
- 16. Linear Accelerators 16 The Klystron • This results in bunching of electrons as the velocity- modulated beam passes through a field-free space in the drift tube. • Electrons arrive catcher cavity 1. Generate a retarding E-field 2. Electrons suffer deceleration 3. KE of electrons converted into high-power microwaves
- 17. Linear Accelerators 17 The high voltage (~100 kV), high current (~100 A), short duration (~1 s) pulses required by the RF power source (magnetron or klystron) and the injection system (electron gun) are produced by a pulsed modulator. The circuitry of the pulsed modulator is housed in the modulator cabinet, which, depending on the particular linac installation design, is located in the treatment room, in a special mechanical room next to the treatment room or in the linac control room. Pulsed Modulator :
- 18. Accelerating waveguide • Waveguides are evacuated or gas filled metallic structures of rectangular or circular cross-section used in the transmission of microwaves. • The accelerating waveguide is evacuated to allow free propagation of electrons. The cavities of the accelerating waveguide serve two purposes: —To couple and distribute microwave power between adjacent cavities; —To provide a suitable electric field pattern for the acceleration of electrons. • Two types of waveguide are used in linacs: RF power transmission waveguides Accelerating waveguides. 18
- 19. RF power transmission waveguides • The power transmission waveguides transmit the RF power from the power source to the accelerating waveguide in which the electrons are accelerated. Linear Accelerators 19 Accelerating waveguides : Two types of accelerating waveguide have been developed for the acceleration of electrons: (i) Travelling wave structure; (ii) Standing wave structure.
- 20. Travelling wave structure : • In the travelling wave structure the microwaves enter the accelerating waveguide on the gun side and propagate towards the high energy end of the waveguide, where they either are absorbed without any reflection or exit the waveguide to be absorbed in a resistive load or to be fed back to the input end of the accelerating waveguide. • In this configuration only one in four cavities is at any given moment suitable for electron acceleration, providing an electric field in the direction of propagation. Linear Accelerators 20
- 21. Standing wave structure • In the standing wave structure each end of the accelerating waveguide is terminated with a conducting disc to reflect the microwave power, resulting in a buildup of standing waves in the waveguide. In this configuration, at all times, every second cavity carries no electric field and thus produces no energy gain for the electrons. • These cavities therefore serve only as coupling cavities and can be moved out to the side of the waveguide structure, effectively shortening the accelerating waveguide by 50%. Linear Accelerators 21
- 22. Linear Accelerators 22 Cutaway view of a standing wave accelerating waveguide for a 6 MV linac. The cavities are clearly visible: the accelerating cavities are on the central axis; the coupling cavities are off-side. The electron gun is on the left, the target on the right, both permanently embedded.
- 23. 23 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. The linac auxiliary system comprises four systems: ● 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.
- 24. Electron beam transport : • In low energy linacs the target is embedded in the accelerating waveguide and no beam transport between the accelerating waveguide and target is required. • Bending magnets are used in linacs operating at energies above 6 MeV, where the accelerating waveguides are too long for straight-through mounting. • The accelerating waveguide is usually mounted parallel to the gantry rotation axis and the electron beam must be bent to make it strike the X ray target or be able to exit through the beam exit window. Linear Accelerators 24
- 25. Linear Accelerators 25 Three systems for electron bending have been developed: ● 90º bending; ● 270º bending (achromatic); ● 112.5º (slalom) bending. The system consists of evacuated drift tubes and bending magnets. In addition, steering coils and focusing coils, used for steering and focusing of the accelerated electron beam, also form components of the beam transport system.
- 26. Linear Accelerators 26 90º bending :
- 27. Linear Accelerators 27 270º bending (achromatic) :
- 28. Linear Accelerators 28 112.5º (slalom) bending :
- 30. 30 Linac treatment head : The treatment head consists of a thick shell of high density shielding material such as lead, tungsten or lead-tungsten alloy. It contains- X-RAY TARGET SCATTERING FOIL FLATTENING FILTER ION CHAMBER FIXED AND MOVABLE COLLIMATOR LIGHT LOCALIZER SYSTEM The head provides sufficient shielding against leakage radiation in accordance with radiation protection guidelines
- 31. Linear Accelerators 31 Bremsstrahlung x-rays are produced when electrons are incident on a target of a high-z material such as tungsten. The target is water cooled system and is thick enough to absorb most of the incident electrons. As a result of bremsstrhlung type interactions,the electron energy is converted into a spectrum of x-ray energies with maximum energy equal to the incident electron energy. The average photon energy of the beam is approximately one third of the maximum energy. X-Ray Beam :
- 32. Linear Accelerators 32 Target and Flattening Filter • Linear accelerators produce electrons in the megavoltage range, the x-ray intensity is peaked in the forward direction. • To make the beam intensity uniform across the field, a flattening filter is inserted in the beam. • This filter is usually made of lead, although tungsten, uranium, steel, aluminum, or a combination has also been used or suggested.
- 33. Linear Accelerators 33 Clinical Radiation Generators Lead or tungsten Opening from 0 x 0 to 40 x 40 cm at SSD 100 cm
- 34. Linear Accelerators 34 The electron beam, as it exits the window of the accelerator tube is a narrow pencil beam of about 3mm in diameter. In the electron mode of LINAC operation, instead of striking the target the beam is made to strike an electron scattering foil to spread the beam as well as to get the uniform electron fluence across the treatment field. Electron Beam :
- 35. Linear Accelerators 35 Clinical Radiation Generators 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
- 36. Linear Accelerators 36 Beam Collimation and Monitoring • The treatment beam is first collimated by a fixed primary collimator located immediately beyond the x-ray target. • The flattened x-ray beam or the electron beam is incident on the dose monitoring chambers. • The function of the ion chamber is to monitor dose rate, integrated dose, and field symmetry. • Bias voltages in the range of 300 to 1,000 V are applied across the chamber electrodes, depending on the chamber design. • The monitor chambers in the treatment head are usually sealed so that their response is not influenced by temperature and pressure of the outside air
- 37. Linear Accelerators 37 Fixed and Movable Collimator • After passing through the ion chambers, the beam is further collimated by a continuously movable x-ray collimator. This collimator consists of two pairs of lead or tungsten blocks (jaws) that provide a rectangular opening from 0 × 0 to the maximum field size (40 × 40 cm or a little less) projected at a standard distance such as 100 cm from the x-ray source (focal spot on the target).
- 38. Linear Accelerators 38 Light Localizer System : • A combination of a mirror and a light source located in the space between the chambers and the jaws projects a light beam as if emitting from the x-ray focal spot. Thus, the light field is congruent with the radiation field.
- 39. Linear Accelerators 39Thank You…