Brief description of Linear accelerator machine

linear accelerator machine and its components.its implication in radiotherapy for treatment of cancer patients.

1. LINEAR ACCELERATOR BY- PALLAVI JAIN MODERATOR- ANKUR MOURYA SIR

2. HISTORY The first linear accelerator principle was invented by Rolf Wideroe in 1928 to accelerate heavy ions In 1952, the first linear accelerator designed for radiotherapy was installed at Hammersmith hospital, London In 1956, the first patient was treated at Stanford University in USA A model of the 1953 8 MeV linac installation at Hammersmith Hospital.

3. LINAC Device that uses high-frequency electromagnetic waves to accelerate charged particles such as electrons to high energies through a linear tube using accelerator wave guide. It can be made to strike a target to produce x-rays.

4. Major Components of linac Power Supply Modulator Magnetron Electron Gun Wave Guide system Accelerator Tube Treatment Head Treatment Couch Control Console

5. A Block Diagram Of Typical Medical Linear Accelerator

6. TYPE OF SYSTEMS COMPONENTS ELECTRON EJECTION SYSTEMS ELECTRON GUN MICROWAVE SYSTEMS MAGNETRON OR KLYSTRON POWER SUPPLY SYSTEMS MODULATOR CABINET BEAM TRANSPORT SYSTEMS ACCELERATING WAVE GUIDE AUXILLARY SYSTEMS VACCUM PUMP,CIRCULATING COOLING WATER, RF FREQUENCY TUBE, PRESSURIZED DIELECTRIC GAS (for RF transmission), RF ISOLATOR OR THYRATRON SAFETY INTERLOCK SYSTEMS Both HARDWARE and SOFTWARE INTERLOCKING systems COMPUTER CONTROLLED FEEDBACK SYSTEM MONITOR CHAMBER, hardware position encoders , limiting micro-switchers BEAM COLLIMETER/APPLICATOR SYSTEMS JAW COLLIMETERS, MLCS

7. Drive stand It is large rectangular cabinet that is firmly secured to the floor of the room Gantry rotates on horizontal axis in the drive stand. Major components of the drive stand is: Power supply Modulator Cooling system/ Water tank Magnetron or klystron DRIVE STAND

8. Power supply Provides DC current power to the modulator which includes pulse forming network High voltage pulses from modulator section are flat topped DC pulses of few microseconds in duration.

9. Modulator Cabinet Contains three major subcomponents- Fan control ( cooling the power distribution systems) Primary power distribution systems (gives power to machine) Auxiliary power distribution systems. (emergency off button to shut off power to treatment unit) Supplies pulsed power to magnetron and electron gun when triggered by a timing pulse from control console.

10. COOLING SYSTEM/ WATER TANK Locted in drive stand Provide thermal stability to the system Helps in maintaining a constant temperatures so that the components in drive stand and gantry functions properly. Demineralized water is used for cooling. Normal water temperature is 20-30 C SF6 is used which acts as dielectric and thus prevent sparking.

11. MAGNETRON Device that produces microwaves Functions as a high-power oscillator, generating microwave pulses of 3000MHz High voltage short duration pulses in synchrony with electron gun is supplied to inner circular cathode which result in thermo-ionic electron emission to outer anode in presence of perpendicular magnetic field .This produces microwaves. Cross-sectional diagram showing principle of magnetron operation

12. Klystron Klystron is not a generator of microwaves but rather a microwave amplifier  It needs to be driven by a low-power microwave oscillator Cross-sectional drawing of a two-cavity klystron 

13. GANTRY It can rotate by 360 degree It is isocentrically mounted C- Type It consist of : Electron gun Wave guide systems Treatment head

14. The linac injection system is the source of electrons, referred to as the electron gun Produced thermionically Injected onto the central axis of the waveguide. Electron Gun

15. Electron gun It consist of: Cathode – negatively charged ,1 cm3 of barium aluminate, produce electron by thermo-ionic emission Gride / gate- switch like mechanism , consist of copper screen Anode – positively charged , attracts electron, perforated so that some electrons are passed into accelerating wave guide

16. Wave guide system 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 The accelerating waveguide is evacuated to allow free propagation of electrons. Often pressurized with dielectric freon or SF6 to reduce possibility of electric breakdown and increase power handling capacity.

17. Types of accelerating waveguide Travelling wave structure : structures require a terminating, or “dummy,” load to absorb the residual power at the end of the structure, thus preventing a backward reflected wave. Standing wave structure : provide maximum reflection of the waves at both ends of the structure so that the combination of forward and reverse traveling waves will give rise to stationary waves, more efficient than the traveling wave
designs since axial, beam transport cavities, and the side cavities can be independently optimized 


18. treatment Head Bending magnet X-Ray Target – tungsten Primary Collimator Flattening Filter –to produce a “flat” beam Monitor Chambers – ion chamber Asymmetrical jaws- X and Y MLCs Shielding against leakage - tungsten, or lead–tungsten alloy Light localizer system

19. Bending magnet Projects the electron beam onto the target. Produce different beam path for different energies. Present in high energies linacs Two common bending magnets afe : 90 degree- chromatic deflection 270 degree- achromatic deflection 112.5 degree- slalom

20.       X ray target Bremsstrahlung x-rays are produced when the electrons are incident on a target of a high- Z material such as tungsten target is water cooled target is thick enough to absorb the incident electrons the electron energy is converted into a spectrum of x-ray energies  maximum energy equal to the incident electron energy average photon energy of the beam is approximately one-third of the maximum energy 

21. The electron beam It exits the window of accelerator tube , a narrow pencil of about 2mm in diameter This beam instead of striking the target ,is made to strike the on electron scattering foil to spread the beam as well get a uniform electron fluence across the treatment field.

22. Components of treatment head. A: X-ray therapy mode. B: Electron therapy mode

23. Collimation Systems Limiting the radiation beam Beam modifying device Primary/fixed collimation Secondary/movable collimator Tertiary collimator

24. Primary/fixed collimation defines a maximum circular field one end of conical opening of the collimator projecting onto edges of the target and the other end to the flattening filter Depleted uranium/Tungsten Transmission <0.2%

25. Flattening Filter Conical metallic filter Situated between target and ion chamber Produce a uniform intensity distribution Usually made up of lead , steel or copper or tungsten ,aluminum or uranium combinations Modifies the narrow non uniform photon beam at the isocenter into clinically useful flattened beam with uniformity and symmetry.

26. Ionization Chamber   Transmission ionization chambers consist of two separately sealed ionization chambers with completely independent biasing power supplies and readout electrometers for increased patient safety monitor dose rate, integrated dose, and field symmetry

27. Scattering foil – Thin , high Z foil(tungsten or lead or aluminium ) In electron mode of linac Electron beam of 3 mm diameter instead of striking the target ,strike this foil to produce uniform electron fluence across the treatment field Carrousel- Helps in movement of flattening filters of different energies as well as scattering foils

28. Secondary/movable collimator After passing through the ion chambers, the beam is further collimated by a continuously movable x-ray collimator consists of two pairs of lead or tungsten blocks (jaws) mounted on either side of the central axis provide a rectangular opening from 0 × 0 to the maximum field size (40 × 40 cm2 or a little less) projected at a standard distance such as 100 cm from the x-ray source Transmission <0.5%

29. MULTILEAF COLLIMATORS consists of a large number of collimating blocks or leaves driven automatically, independent of each other, to generate a field of any
shape consist of 60 to 80 pairs, independently driven leaves are made of tungsten alloy (density = 17.0 to 18.5 g/cm3) thickness of leaves along the beam direction - 6 cm to 7.5 cm primary x-ray transmission through the leaves - < 2% interleaf (between sides) transmission - > 3% thickness on inner leaves is 0.5 cm - 40 pairs, while its 1cm for the outer 20 pairs

30. Types of MLC Siemens - retain upper but replaces lower Jaw Elekta - retain lower jaw + backup collimators, replaces upper jaw Varian - retain all jaws + MLC

31. Beam Localization The field size definition is provided by light localizing system in treatment head Consists of mirror & light source Located in space between chambers & jaws Provides an intense light field, duplicating in size & shape the radiation field Facilitates positioning of patient’s for treatment

32. Auxiliary Service Vacuum pumping system producing high vacuum in the accelerating waveguide Water cooling system for cooling the accelerating waveguide, target, circulator and RF generator Shielding against leakage radiation produced by target, beam transport system and RF generator

33. Couch/ treatment table It is provided to support the patient to be treated It is isocentrically mounted as the Gantry & Collimator Made up of  carbon fiber

34. Dynamic Floor In Room Display Lasers   Green laser- Silicon diode 2 sagittal , 1 coronal Tolerance of laser position-2mm

35. Couch Movements Can move horizontal, vertical, lateral and rotational directions.

36. Hand pendants Contains all the control switches which can used to access the movement of gantry , couch, collimeters , jaw, etc

37. CONTROL CONSOLE 4D integral treatment console Treatment console/service monitor : Provides status information on treatment modality, accessories in use, prescribed dose & dose delivered, interlock status, emergency off ARIA information system : appointment scheduling  ARIA is a comprehensive information system that addresses the clinical, administrative, and imaging needs of the oncology department. It provides a single user interface that integrates record and verifies the multileaf collimator, and PortalVision Service Monitor ARIA Information System 4D ITC

38. Electronic portal imaging device( epid ) On board imaging MV Xray Real time imaging IGRT Flat panel amorphous silicon Detectors mounted on retractable arm 1024 * 768 resolutions

39. RADIATION SAFETY /INTERLOCK SYSTEMS Last man out(LMOS) switch Door interlock Beam on-off switch Emergency switches Safety lights( red, yellow, green)

40. Emergency buttons Total 7 in number Completely turn off the entire unit with single switch during emergency situations.

41. Audio Visual Monitors

42. Treatment Planning System

43. After completion of the installation, a formal radiation protection survey is carried out –  measurement of head leakage area survey tests of interlocks warning lights emergency switches There are daily ,monthly and annual quality assessment carried out for proper functioning of the LINAC.  

44. THANK YOU

45. Dose monitoring system   standards for the type of radiation detectors, display of monitor units (MUs), termination of radiation and monitoring of beam flatness and dose rate  1 MU corresponds to a dose of 1 cGy delivered in a water phantom at the depth of dose maximum on the central beam axis when irradiated with a 10 × 10 cm2 field at a source to surface distance (SSD) of 100 cm  In addition to monitoring the primary dose in MUs, the dose monitoring system also monitors other operating parameters such as the beam energy, flatness and symmetry