LINEAR ACCELERATOR PRINCIPAL AND WORKING

Briefly Explanation of linear accelerator

1. LINEAR ACCELERATOR Krishan Murari Radiotherapy Technologist MEDANTA THE MEDICITY

2. What is an Accelerator ?? Accelerator is a device which used to speed up a beam of charged particles to near the speed of light before they collide with a target or with another beam of particles. The sub-nucleus particles created by the collision as well as the radiation emitted are detected and analyzed. Some of these machines accelerate particles in a straight line , and we call them linear accelerators . Other high-energy machines are based on rotating the accelerated particles in a circular path , such as the cyclotrons and the synchrotrons .

3. Applications of Accelerators Radiation Therapy Producing Medical Radioisotopes Electron Beam Sterilization of Medical Disposables and Food Processing Nondestructive Testing (NDT) Ion Implantation in Semiconductor Chip Fabrication Processing of Polymers Security and Inspection Applications

4. Medical Linear Accelerator In its simplest definition, a linear accelerator (linac) is a device that uses electromagnetic waves , in the microwave range, to accelerate electrons. The highly accelerated electron can also be used to treat tumor near the surface of the patient and when the electron is hit to the high atomic number material ( Tungsten ) it produces X-rays that are used to treat the deep down tumors.

5. History In 1952 , two innately talented physicists, Namely Henry Kaplan and Ed Ginzton began working on the concept of a linear accelerator.  In 1956 , in the Stanford hospital in San Francisco, the first linear accelerator was installed. In 1962 , a series of experiments began to see if radiation works best with chemotherapy. Kaplan and Saul Rosenberg worked together to improve the chances of patients' survival.

6. This patient, a 2-year-old boy suffering from retinoblastoma, was the first to undergo X-ray treatment from a medical linear accelerator that Kaplan developed with campus physicists.

7. In 1994 , A great invention was made at Stanford, Something called cyberknife was introduced which narrowed down x-ray beams in a precise manner.  First ever Cyber Knife

8. In 1997 , A further step was taken to advance the use of linear accelerators by combining with intensity modulation radiation therapy . The result was that many thin beams of radiation could be achieved from any desired angle. In 2004 , four-dimensional radiotherapy was implemented. In 2007 ,Volumetric Modulated Arc Therapy (VMAT) was introduced .

9. MAJOR COMPONENTS OF LINACS ● Power Supply ● Modulator ● RF Generation system ● Electron Gun ● Wave Guide system ● Bending Magnet ● Treatment Head ● Treatment Couch

10. Power Supply A Power supply provides Direct current (DC) power to the modulator,which includes the pulse forming network and a switch tube known as Hydrogen thyratron. High voltage pulses from the modulator section are flat-topped DC pulses of a few microseconds in duration. These pulses are delivered to the magnetron and klystron simultaneously.

11. Modulator This vital component of the Linear accelerator is usually located in the treatment room In some Units. This modulator cabinet contains three major components, 1. Fan control (cooling the power-distribution system) 2.Auxiliary power distribution system (contains the emergency off button that shuts off the power to the treatment unit) 3.Primary power-distribution system.

12. RF Power Generation system The electrons are accelerated in the accelerating wave guide using high power RF fields. Which are set up in accelerating wave guide by microwave radiation. This Radiation is produced by microwave generators that are either Magnetrons or Klystrons

13. Magnetron It produces microwave required for electron Acceleration Functions as a high frequency oscillator. Peak power up to 5MV can be produced by Magnetrons Typical high voltage pulse of about 50kVp is a few microseconds long and is repeated a few hundred times per second. Pulse repetition frequency (PRF) differs according to manufacturer, PRF determins the X-ray Energy from Linac.

14. Klystron It is not a generator of microwaves but acts as a RF power amplifier. Driven by low power microwave oscillator. Peak power on the order of 7MV or higher. Mainly using in high energy linacs. It uses High DC voltage.

15. Injection System Injection system or the source of the electrons is importantly a simple electrostatic accelerator known as an Electron gun . When a high voltage is given to the cathode it gets heated up under vacuum which provides sufficient thermal energy to the electrons to draw itself out from the material and get ejected by thermionic emission . The number of electrons produced depends upon the temperature of the filament which is controlled.

16. The electron gun and waveguide system are evacuated to a low pressure to make the mean free path of electrons between atomic collisions long compared to path in the system. Electron GUN

17. WAVEGUIDE The waveguide is a channel directing the microwave power of the klystron/magnetron to the Accelerating wave guide. Waveguides are evacuated or gas filled (Sulfur Hexaflouride)metallic structures. Two types of waveguides are used in linac : 1. Radiofrequency power transmission wave guide (usually gas filled) 2 . Accelerating wave guides (usually evacuated) For electron transmission Accelerating waveguides are used.

18. Structure of Accelerating waveguide Accelerating waveguide

19. Accelerating Waveguide Electrons are accelerated through the holes in the discs using an oscillating electric field supplied by the radio-frequency generator (magnetron or klystron) and modulator. Accelerating waveguides may be either of the traveling-wave or standing-wave based on the time dependent variation of their electric field

20. Accelerating Waveguide Travelling Waveguide Waveguide length will be greater At the end of the waveguide microwaves are absorbed without any reflection or fed back to the input It requires low microwave peak power Requires lower mean RF power Standing Waveguide Side coupling will reduce waveguide length At the end of the waveguide microwaves are reflected back to the input It requires high microwave peak power than travelling wave guide Requires higher mean RF power (25% more)

21. Electron Beam Transport It consist of the evacuated drift tubes and bending magnets ,which are used in transporting the electron beam from the accelerating waveguide to the X-ray target or the exit window for electron beam therapy. Steering and focusing coils installed on the accelerating waveguide are usually linked with electron transport system Bending magnet Electron Gun Electron Beam

22. Steering Coils : The steering coils keep the accelerated electron pencil beam as close as possible to the axis of the cylindrical accelerating waveguide. It will steer the beam towards the opening which connects the accelerating waveguide to the target.

23. Focusing Coils : Focusing coil is to focus the accelerated pencil beam in order to minimize the beam divergence and cross section. Divergence results from a small radial component of the electric field in the accelerating waveguide and from the repulsion among electrons in the pensil beam. The focusing solenoid coils are coaxial with the accelerating waveguide.

24. Treatment Head Bending magnet. Shielding material. X-ray target. Primary collimator. Beam flattening filter and Scattering foil. Beam monitoring devices. Secondary collimators. MLC and Wedge. Field light

25. 1. BENDING MAGNET Changes the direction of electron beam downward towards the isocentre. Slalom (Achromatic) 270° bending (Chromatic) Bending magnet not only directs the beam to strike the target,but also focuses the beam to diameter of 1mm. The design of magnets enables them to focus the electron of slightly different energies on the same point on target (Achromatic behavior)

26. 2. SHIELDING MATERIAL The treatment head consist of a thick shell of high-density shielding material such as lead, tungsten, or lead-tungsten alloy. Shielding material is used to avoid the unnecessary irradiation to the surrounding, patient and the radiation workers as well.

27. 3. X-RAY TARGET The pencil electron beams strikes on the X-ray target to produce photons. X-ray target used is transmission type target. It is made of Tungsten due to its high atomic number (Z=74) and High melting point 3370°C.

28. 4. PRIMARY COLLIMATOR The treatment beam is first collimated by a fixed primary collimator located immediately beyond the X-ray target. Conical opening projecting into a tungsten shielding block. It attenuates the primary x-ray beam intensity to less than 0.1% of the initial value In the case of X-rays, the collimated beam then passes through the flattening filter . In the electron mode, the filter is moved out of the way.

29. 5. FLATTENING FILTER The photon dose distribution produced by a Linac is strongly forward peaked. Modifies the narrow, non-uniform photon beam at the isocenter into a clinically useful beam through a combination of attenuation at the center of the beam and scatter into the periphery beam

30. It is made up of Pb, Tungsten. Carroesel is a device in the treatment head which helps in the movement of flattening filters of different energies and Scattering foil as well. Moves flattening filter away from the beam in FFF (Flattening filter free) mode either. Structure of Flattening filter Structure of Carroesel

31. 6.SCATTERING FOILS 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 a thin metallic foil, usually made of lead

32. 7 . BEAM MONITORING DEVICE The flattened X-ray beam or the electron beam is incident on the Beam monitoring devices. The dose delivered to the patient, Dose rate and field symmetry are measured and controlled by the monitoring system. The chambers are usually transmission type .It consist of two flat parallel plate type ionization chambers .

33. Here the first chamber is the Primary dosimeter , it measure and stops the radiation when the required dose is delivered. The other chamber is the backup one it stops the radiation when primary chambers fails. 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.

34. 8 .SECONDARY COLLIMATOR It consist of four blocks, two forming the upper and two forming the lower. Provide rectangular or square field (up to 40x40cm ) at the linac isocenter. This collimators are able to rotate about their axis and this degree of freedom is referred to as collimator rotation . Usually made of lead or tungsten. Set-up A with collimator Rotation Set-up B with shielding blocks

35. 9 .WEDGES These are the beam modifying devices. It causes a progressive decrease in the intensity across the beam, resulting in a tilt of the isodose curves from their normal positions. Degree of tilting depends on slope of wedge. Wedge angles used are: 15,30,45 & 60 Usually made of dense material such as lead or steel.

36. Mainly there are 3 types of wedges; 1.Physical wedges 2.Motorized wedges 3.Dynamic wedges

37. 10. MULTILEAF COLLIMATOR (MLC) Group of collimating leaves that can be driven automatically, independent to each other to generate a field of any shape. Made up of tungsten alloy. The number of leaves in commercial MLC’s are steadily increasing Perpetual modulation of MLC plays vital role in VMAT/Rapid Arc and IMRT by allowing complex shaped fields can be generated in sequence.

38. At Medanta we have, Agility MLC on Elekta’s Linacs 160 Leaves 5mm width 65mm/s leaf speed 15cm maximum overtravel. Binary MLC on Accuray’s Tomotherapy 64 leaves 6.25 mm width 20 msec open –close time.

39. 11. FIELD LIGHT It is a field localizing device, used to display the position of the radiation field on the patient skin. A high accuracy bulb is placed at 45° angle with the Mercury mirror placed in the space between the chambers and the jaw projects a light beam as if emitting from X-ray focal spot. Thus the light field is congruent with the radiation field.

40. Lasers The accuracy of the laser guides in determining isocenter position. Isocenter is a virtual point where the central axis of the Gantry,Collimator and Couch meets. 2 side lasers,sagittal and ceiling lasers are mounted on walls of LINAC Unit. Tolerence of Laser position is 2mm.

41. Treatment Couch Treatment table is a mechanically movable motor driven couch. It can move the patient Lateral (X) , longitudinal (Y) , vertical (Z) Provides Isocentric rotation. Hexapod can be mounted on Treatment couch. Hand pendent conatins switches can be used to access movement of table, Gantry , Couch,Collimator jaws (field size),SSD etc..

42. Electron Applicators In electron-beam mode, a well defined electron beam passing through the thin window of the accelerator vacuum system is transported through a scattering foil system in order to broaden the beam. Electrons undergo significant scattering in the air between the scattering foils and the patient. It is therefore necessary to collimate the beam with an applicator attached to the treatment head.

43. The electron applicator currently in use constructed of light-weight fiberglass and aluminum.

44. Accessories and Systems used with LINAC Electronic Portal Imaging device (EPID) Cone Beam CT (CBCT) / On Board Imaging. HEXAPOD Active Breathing Co-ordinator Treatment and Verification system Treatment Planning system

45. Accessories and Systems used with LINAC Electronic Portal Imaging device (EPID) Cone Beam CT (CBCT) / On Board Imaging. HEXAPOD Active Breathing Co-ordinator Treatment and Verification system Treatment Planning system

46. Accessories and Systems used with LINAC Electronic Portal Imaging device (EPID) Cone Beam CT (CBCT) / On Board Imaging. HEXAPOD Active Breathing Co-ordinator Treatment and Verification system Treatment Planning system

47. Accessories and Systems used with LINAC Electronic Portal Imaging device (EPID) Cone Beam CT (CBCT) / On Board Imaging . HEXAPOD Active Breathing Co-ordinator Treatment and Verification system Treatment Planning system

48. Care and Usage The way a linear accelerator is cared for, and how often its used, can also determine its useful life. Proper maintenance, like adhering to scheduled Preventative Maintenance Inspections (PMI) , can uncover small problems before they become bigger.  Another chief consideration is the linear accelerators usage, often measured in beam hours or filament hours. Newer technologies like VMAT (volumetric arc therapy) require much more use of “beam on” time and can reduce the lifespan of a linear accelerator

49. The X-ray produced are almost 1 percent of the electron energy which is striking on a target, rest of the energy (99%) is converted into heat. This heat needs to be cooled and is achieved by Cooling system consist of water chiller. Air-conditioning and humidity control in the treatment room with a certain number of air volume replacements per hour is required for proper functioning

50. Safety The facility should be prepared and equipped to deal with any emergency situation such as unwanted Radiation exposure, Fire and other accidents. To minimise hazards due to high voltage, caution sign boards with danger signs and visual indication should be put up near such locations Audio visual communication between the treatment room and control room.

51. LINACS AT HOUSE

52. Elekta’s Synergy and Infinity YEAR OF INSTALLATION SYNERGY – 2009 INFINITY - 2010 ENERGY (X-RAY) 4MV, 6MV and 15 MV ENERGY (ELECTRON) 4-22Mev DOSE RATE (Maximum) 1400 MU/m (FFF), 600 MU/m (FF) IMAGING XVI (CBCT) , iViewGT (EPID) MLC 160 LEAVES (AGILITY MLC) MLC SPEED 65mm/s MLC THICKNESS 5mm FIELD SIZE (max) 40cm*40cm MICROWAVE SOURCE MAGNETRON ELEKTA SYNERGY is equipped with HEXAPOD Evo RT system

53. Accuray’s Tomotherapy-H A fusion of Radiotherapy Linac with Computered Tomography. It is armed to provide Helical treatments which is boon for large sized PTVs. Flat couch provided allows automatic translation during treatment, is calliberated by IEC 6MV FFF Integrated MVCT system (3.5 MV)

54. Accuray’s Tomotherapy-H A fusion of Radiotherapy Linac with Computered Tomography. It is armed to provide Helical treatments which is boon for large sized PTVs. Flat couch provided allows automatic translation during treatment, is calliberated by IEC 6MV FFF Integrated MVCT system (3.5 MV)

55. YEAR OF INSTALLATION 2015 ENERGY 6MV FFF DOSE RATE 865 MU/minute IMAGING C-TRUE (3.5 MV) MLC 64 BINARY LEAVES MLC THICKNESS 0.625 MM MLC SPEED ( OPEN TO CLOSE) 20 ms FIELD SIZE (MAX) 5CM*40CM GANTRY ROTATION (MAX) 6 rpm Tomotherapy Linear Accelerator

56. Accuray’s Cyberknife VSI Sub-milimetre accuracy treatment anywhere in the body, with continual image guidance. 6D Robotic Arm-Mounted Linac. Enables the delivery of a large number of non-isocentric , non-coplanar beams individually directed at unique points within the intented target. 6MV FFF 1000 MU/Minute

57. Accuray’s Cyberknife VSI Sub-milimetre accuracy treatment anywhere in the body, with continual image guidance. 6D Robotic Arm-Mounted Linac. Enables the delivery of a large number of non-isocentric , non-coplanar beams individually directed at unique points within the intented target. 6MV FFF 1000 MU/Minute

58. CK’ S Linear Accelerator

59. THE FUTURE OF LINAC

60. Treat cancer with a FLASH PHASER (Pluridirectional high-energy agile scanning electronic radiotherapy) Rapid ( <1s )freeze-motion dose delivery – Highest precision Dragon LINAC 300x higher dose rate 50Gy/Second FLASH RT biological advantage

61. Thank You For Your Attention