Linear accelerator

Medical linear accelerator with telecobalt source head
Include:basic principle,history,working,constructions

1. Linear Accelerator Neha Mannewar Msc 1 st year Medical Phy

2. Inslides Introduction History Components Working principle Construction Application

3. Introduction A linear accelerator is a type of accelerator that greatly increases the velocity of charged subatomic particles or ions by subjecting the charged particles to a series of oscillating electric potentials along a linear beam line.  It delivers high-energy x-rays or electrons to the region of the patient's tumor.

4. History In the late 1940s ,the idea of using linear accelerator in the medical application become interested. Medical Linear Accelerators have been in clinical use since the early 1950s. First one was installed in Hammersmith in 1952 with 8MeV, limited gantry motion, large and bulky machine.

5. A 2 year old boy was the first patient to receive radiation in operation therapy from LINAC at Stanford.

6. Basic Principle Consist of cylindrical shape electrodes, of increasing length arranged as fig. Enclosed in glass vacuum chamber . Alternate cylinders are connected together, odd and even to different terminals. Ions from ion source kept at one end, move along the axis of the tubes and are accelerated on crossing the gaps between the tubes. C:\Users\admin\Desktop\linac images\550px-Linear_accelerator_animation_16frames_1.6sec (1).gif

7. Ions are not affected inside the tubes because the potential is constant there. If ions are positively charged and moving from left to right. If 1 st cylinder is positive and the second is negative, the ions are accelerated and travel to the 2 nd cylinder at a velocity which is constant but greater than the velocity in the 1 st cylinder. The 2 nd cylinder is long enough so that when the ions reach the gap between 2 nd and 3 rd cylinders, the potential are reversed. The 2 nd cylinder is now positive and 3 rd is negative, so the ions are again accelerated in gap between 2 nd and 3 rd . Since the velocity is increased of ions again in gap between 2 nd and 3 rd ,the 3 rd cylinder need to be longer in order for ions to spend another half period between the gaps.

8. Accelerated electron called the “wave guide” in the part of accelerator. These electrons collide with heavy metal target and so high energy photons are produced. These high energy X-ray will be directed towards patient tumor Equation Ln = λ√ (( neV+C )/m), Where, Ln = the length of cylinder ne= nth electrode from the energy equation V=average potential drop from the particle sees in passing gap. C=constant from equation, total length of the accelerator is proportional to the wavelength , λ of the radio frequency signal. The largest linac in the world ,is at Stanford university, is 3.2 km long . It is capable of accelerating electrons to an energy of 50GeV (50 giga electrons volts ).

9. Question Linear Accelerator accelerate electron to produce Neutron Proton X-ray All of the above

10. Linac are usually mounted isocentrically and operational system are distributed over five major and distinct sections of the machine Modulating Cabinet Control Console Gantry Gantry stand and support Patient support assembly Iscocentric arrangement The axis of rotation of the three structures : Gantry Collimator Couch Coincide at a point known as Isocenter

11. Components The main operating components of a medical linac usually grouped into 6 classes. Injection system RF power generating system Accelerating wave guide Auxiliary system Beam transport system Beam collimation and Monitoring system

12. In the simplest and most practical configuration :

13. Block diagram showing the component system of a linear accelerator Power supply and modulator Microwave power generator Cooling system Beam Monitor Electron gun Electron accelerator Treatment head Magnetic focusing and guiding system Vacuum system Gantry support and drive system Patient support system Safety interlocks Control system

14. Working principle : The electron accelerator is a waveguide structure which is energized at microwave frequency, most commonly at 3000MHz . The microwave radiation is supplied in short pulses and this is generated by supplying high voltage pulses of about 50kV from the modulator to the microwave generator . Which is most commonly magnetron valve, in some higher energy accelerators a klystron valve is used as a microwave power source. The electron gun is also pulsed so that high velocity electrons are injected into the accelerating waveguide at the same time as it is energized The electron gun and accelerating waveguide system have to be evacuated to a pressure such that the mean free path of electrons between atomic collision is long compared with the electron path through the system.

15. The accelerating electrons tend to diverge, partly by mutual coulomb repulsion but mainly because the electric field in the waveguide structure has a radial component. They can be focused back on their straight path by use of coaxial magnetic field. The field is supplied by the coils which themselves are coaxial with the accelerating waveguide. There are also additional coils which can be use to steer the electron beam to make sure it emerges from the accelerator structure at the required position and direction, these function are labeled magnetic focusing and guiding system. The energy an electron can acquire from the microwave frequency field in the waveguide clearly depend on the amplitude of the electric field, i.e. it depends on the microwave power. Available microwave valves can only operate at the necessary instantaneous power level in the pulsed mode and therefore the system can only deliver accelerated electrons in pulses. The electron accelerator will deliver pulses of high energy electrons into the treatment head, where the machine is being used as an x-ray generator .The treatment head will contain target, filtering system, the beam monitor detectors and the beam defining system

16. If the electron beam is to be used for treatment , it will emerge from the vacuum system through a thin window into the treatment head, where it again monitored and if necessary, scattered to give the required field coverage. Some machines have dual purpose treatment head for x-ray and electrons, in which case rather elaborate mechanical and electrical arrangements are required to allow the changeover from one mode of treatment to the other. For instance, it will be necessary to have an arrangement for retracting the x-ray target and to have an interlock system to verify that this has been done correctly before an electron treatment can start. The treatment head pointer, or an electron field applicator ,or for any other attachments which may be placed in the useful beam. The temperature of certain components in the system is critical for efficiently operation .In particular the temperature of the accelerating guide structure and the microwave valve has to be controlled because dimensional changes associated with thermal expansion will significantly change their characteristics. The x-ray target also needs to be cooled .The cooling system has to operated using a thermostat system.

17. The major mechanical elements are both the gantry support and drive systems, which serve to position the radiation source with respect to the patient, and the patient support system which will place the patient in the desired position. The control system and safety interlocks clearly relate to the system as a whole and the use of the large box. For the gantry rotation there is no limit to the length of the accelerating guide. The patient support system needs to be able to provide the movements indicated .i.e. vertical, longitudinal and lateral with respect to the long axis of the patient and roational about vertical axis.

18. Injection System Electron Gun Electrons are produced by thermionic emission from a heated cathode. Two basic types of electron gun exist: The diode type Electrostatic fields used to accelerate the electrons in the diode guns are supplied directly from the pulsed modulator in the form of a negative pulse delivered to the cathode The triode type The cathode is held at a static negative potential (typically 20kV) . The grid of triode gun is normally held sufficiently negative with respect to the cathode to cut of the current to the anode

19. The injection of electrons into 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. The electrostatic fields used to accelerate the electrons are supplied directly from the pulsed modulator in the form of negative pulsed delivered to the cathode of the gun.

20. RF power generating system Microwave radiation used in the accelerating waveguide to accelerate electrons to the desired K.E is produced by RF power generation system. RF system consist of : RF power source (Magnetron or Klystron) 2. Pulsed modulator Power supply provides dc power to the modulator, which includes pulse forming network and a switch tube know as Hydrogen thyratron . High voltage pulses from the modular section are flat topped dc pulses of few µs in duration. These pulses are delivered to magnetron and klystron and simultaneously to the electron gun.

21. RF Power source : Either magnetron or klystron both are device used electron acceleration and deceleration in vacuum for production of high power RF fields. Magnetron: It produces microwave required for electron acceleration. Low power operation(3-6MW) Low voltage(45KV) 6MeV for low power ,lifetime comparatively low.(5-8yr) Variable PRF

22. Magnetron Device that produce microwaves. It has cylindrical construction , having central cathode and outer anode. Space between cathode and anode is evacuated. Cathode is heated by an inner filament and electrons are generated by thermionic emission. Static magnetic field is applied perpendicular to the plane of cross section of cavities and pulsed dc electric field is applied between cathode and anode. Electrons emitted by cathode are accelerated towards anode by the action of pulsed dc electric field and by influence of magnetic field electrons move in complex spirals towards resonant cavities, radiating energy in the form of microwaves.

23. RF System- Klystron It acts as a RF power amplifier. 12MeV for high power lifetime is comparatively high. High Voltage 140k. High power (7+MW) Typical 10000 hr life.(6-10 yr) Electromagnet (solenoid). Mainly using in high energy linacs Fixed PRF

24. Klystron It is a microwave amplifier. The electrons produced by the cathode are accelerated by a negative pulse of voltage into first cavity called Buncher Cavity . The microwave set up alternating electric field across the cavity. The velocity of the electron is altered by the action of this electric field to a varying degree velocity modulation. Some electrons are speed up while others are slow down and some are unaffected results and this results in bunching of electrons. As the electrons bunch reaches catcher cavity they induce charges on the ends of the cavity and there by generate a retarding electric field . The electrons suffer deceleration and by the principle of conservation of energy the K.E of electrons is converted into high power microwaves.

25. Pulsed modulator The high voltage (approx 100kV),high current (approx 100A),short duration (approx 1s)pulses required by RF power source (Magnetron or Klystron ) and the injection system are produced by 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 a special mechanical room next to the treatment room or in the linac control room.

26. Accelerating Waveguide Accelerating waveguide is obtained from a cylindrical uniform waveguide by adding a series of disks (irises) with circular holes at the centre, placed at equal distances along the tube to form a series of cavities. There are two different type of waveguides used in LINAC RF power transmission waveguide Accelerating waveguide RF power transmission waveguide: The power transmission waveguide transmit the RF power from the power source to the accelerating waveguide in which electrons are accelerated Two types of Accelerating waveguide Standing wave structure Travelling wave structure

27. Travelling Wave Stationary wave Microwaves are aborbed at the end of the waveguide or exit the waveguide to be absorbed in a resistive load or fed back to the input end of waveguide Microwaves reflect from distal surface to produce a standing wave. Each end of the wave guide is terminated with a conducting disk to reflect the microwave power.

28. Accelerating wave structure It consist of a copper tube with its interior divided by copper disc or diaphragms of varying aperture and spacing. This section is evacuated to allow free propagation of electrons. The cavities of accelerating wave guide serve two purpose To couple the distribute microwave power between adjacent cavities To provide a suitable electric field pattern for the acceleration of electrons Electrons interact with the electromagnetic field of microwaves. Electron gain energy from the sinusoidal electric field by an acceleration process

29. Accelerating waveguide for intermediate (8-15MV) and high (15-30MV) energy linacs are located Either in the gantry parallel to the gantry axis of rotation or in the gantry stand In both cases , a beam transport system is used to transport the electron beam from the accelerating waveguide to the x-ray target. The radiofrequency power source in both configuration is mounted in gantry stand.

30. Auxiliary System It consist of four system producing high vacuum in the accelerating waveguide. Vacuum pumping system: Producing high vacuum in the accelerating waveguide.(6-10 torr in accelerating waveguide and RF generator) Water cooling system: For cooling the accelerating waveguide,target circulator and RF generator. Air pressure system: for pneumatic movement of the target and other beam shaping component. Shielding against leakage radiation: Producing by the target , beam transport system and RF generator.

31. Electron Beam Transport It is used in transporting the electron beam from the accelerating waveguide to the X-ray target or to the exit window for electron beam therapy. Steering and focusing coils installed on the accelerating wave guide are usually linked with the electron transport system. Beam transport system consist of : Drift tubes Bending magnets Steering coils Focusing coils Energy slits

32. In low energy linac 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 6MeV, where the accelerating waveguides are too long for straight through mounting. The accelerated 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.

33. Linac Head The important component found in a typical head of a fourth and fifth generation linac include : X-ray target Flattening filters and electrons scattering foils (also called scattering filters). Primary and adjustable secondary chambers. A field defining light. Wedges Optional MLC

34. Beam Collimation and Monitoring In a typical modern linac , the photon beam collimation is achieved with two or three collimator device. Target Primary collimator Flattening filter or Scattering coils Dual ion chamber Secondary collimator Multileaf collimator

35. Target There are 3 types of target: thin, intermediate and thick In linac we use thick target due to some practical reason. Efficiency of photon production in thick target is proportional to the atomic number Z of the target material In medical linac high Z target (eg Pb ) would produce beams with highest efficiency.

36. 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-to monitor dose rate ,integrated dose and field symmetry. Bias voltage in the range of 300 to 1000V are applied across the chamber electrodes depending on the chamber design. The Monitor chamber in the treatment head are usually sealed so that their response is not influenced by temperature and pressure of the outside air. The target to surface distance that is SSD is 10*10cm for thin layer in linac i.e =100cm

37. Primary Collimator It defines the largest available circular field size and is essentially a conical opening projecting into a tungsten shielding block. One end of the conical opening of the collimator projecting onto edges of the target and the other to the flattening filter

38. Flattening filter The photon dose distribution produced by a linac is strongly forward peaked. To make the beam intensity uniform across the field , a flattening filter is inserted in the beam. Made of lead , aluminum, tungsten ,uranium ,steel

39. Dual Ionization Chamber The flattened beam is incident on the dose monitoring chambers. The monitoring system consist of several ion chamber or a single chamber with multiple plates. The chambers are usually transmission type i.e flat parallel plate or cylindrical thimble chamber and sealed is used in some linacs

40. Questions: Ionization chamber used in LINAC is Sealed Unsealed Both a and b Condenser What is the target to surface distance in LINAC 150cm 120cm 80cm 100cm

41. Scattering foil In the electron mode of linac operation ,the beam instead of striking the target ,is made to strike an electron scattering foil to spread the beam as well as get a uniform electron fluence across the treatment field. It consist of thin metallic foil usually of lead.

42. Secondary collimator After passing through the ion chamber , the beam is further collimated by a continuously movable x-ray collimator . This collimator consist of four blocks ,two forming upper jaws and two forming lower jaws. Made up of lead or tungsten blocks(jaws) that provide a rectangular opening from zero to maximum field size (40*40cm or a little less). It is projected at a standard distance such as 100 cm from the x-ray source (focal spot on the target)

43. Multileaf Collimator Using MLC’s we can conform the treatment fields to the tumor volume. The number of leaves in the commercial MLC’s are steadily increasing. Leaves:82,120,160 and width:1cm and 1.5cm to 6mm is currently available. Each leaves is controlled by computer controlled motors.

44. Light localizer System A combination of a mirror and a light source located in the space between the chambers. The jaws project a light beam as if emitting from the x-ray focal spot. Thus,the light field is congruent with the radiation field.

45. Treatment Couch The treatment couch is the area on which patient’s are positioned to receive their radiation treatment, and it has the ability to move Up/down Right/left In/out Hexapod (currently 6 degree of motion are available) The Linac couch 6D displacements are vertical,longitudinal,lateral,yaw,roll and pitch.

46. Questions: The angle of bending magnet used in LINAC accelerator is 90 270 Both a and b None of the above

47. Telecobalt To overcome the limitation of kilovotage machines. Radionuclides such as Radium-226,Cesium -137 and cobalt-60 have been used as a source of gamma rays.

48. Source head of telecobalt The source head has the source , shutter and collimator 1. Source The cobalt-60 is a source is a cylinder of 1-2cm diameter and it is positioned with its circular end facing the patient. The source is form of a solid cylinder, disc or pallets, placed in a stainless steel capsule and shield welding. The source is surrounded by sufficient thickness of lead for shielding purpose. The shielding must ensure that the leakage radiation at 1meter from the center of the source must be less than 2mR/hr. The materials commonly used for shielding should have high atomic number and density ,i.e. lead ,tungsten. When the co-60 source decay to Ni-60 with the emission of β particles and two photons of energies 1.17 MeV and 1.33MeV. The β particles are absorbed by the source capsule ,resulting emission of low energy X-ray.

49. Shutter Shutter is a device ,which brings the source in front of an opening (ON position) from which the useful beams comes out. The shutter is used to shutting OFF the radiation beam when it is not needed. The shutter effectively interposes a massive amount of shielding material between the source and the pateint . The opening and closing of the shutter is done by means of an electric motor or hydraulic force.

50. 3.Collimator The collimator is used to control the size and shape of the beam and direct the useful beam to the patient. The collimator consist of two pairs of heavy metal blocks of tungsten or uranium. Each pair can be moved independently towards and away from the axis of the beam to obtain the square of rectangular field. The collimator is also provided with two optical device namely visual field localizer(VFL) and optical distance indicator(ODI). The VFL comprises a lamp and mirror, which can show the size and position of the field of the patient skin The ODI represent the exact SSD on the patient skin surface. It is used to set SSD .

51. Advantages Particles are able to reach very high energies without the need for extremely high voltages. Linear accelerators attack the affected area with higher doses of radiation than other machines.

52. Disadvantages LINAC can cost anywhere between one million to three million dollars. Operating the machine costs about 900,000 dollars annually. The particle accelerator travel in a straight line ,each accelerating segment is used only once .The segment run in short pulses ,limiting average current output and forcing the experimental detectors to handle data coming in short burst ,thus increase the maintenance expense.

53. Techniques used with Medical Linac Variety of treatment techniques are as follows 3D conformal radiotherapy Intensity Modulated radiotherapy Stereotactic Radiotherapy (SRT) Stereotactic Radio surgery (SRS) Dynamic Adoptive Radiotherapy (DART) Image Guided Radiotherapy (IGRT)