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Linear accelerator vinay

LINEAR ACCELERATOR is a radiation therapy machine which produces x-rays of high energy which are useful in treatment of cancer.

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Linear accelerator vinay

  1. 1. Medical Linear Accelerator (LINAC) Vinay Desai M.Sc Radiation Physics KIDWAI MEMORIAL INSTITUTE OF ONCOLOGY
  2. 2. Medical Linear Accelerator (LINAC) • The linear accelerator is a device that uses high-frequency electromagnetic waves to accelerate charged particles such as electrons to high energies through a linear tube. • Electron trajectories are linear in the accelerator tube hence the name ‘LINEAR ACCELERATOR’. • The high-energy electron beam itself can be used for treating superficial tumors, or it can be made to strike a target to produce x-rays for treating deep-seated tumors.
  3. 3. History of the medical linear accelerator: • 1952: Henry Kaplan and Edward Ginzton begin building a medical linear accelerator. • 1956: The first medical linear accelerator in the Western Hemisphere is installed at Stanford Hospital in San Francisco. • 1959: Stanford medical school and hospital move to the Palo Alto campus, bringing the medical linear accelerator. • 1962: Kaplan and Saul Rosenberg begin trials using the linear accelerator with chemotherapy to treat Hodgkin's disease, an approach that dramatically improves patient survival. • 1994: First use of the CyberKnife, invented at Stanford, which uses sophisticated computerized imaging to aim a narrow X-ray beam precisely. • 1997: Stanford pioneers the use of intensity-modulated radiation therapy, which combines precise imaging with linear accelerators that deliver hundreds of thin beams of radiation from any angle. • 2004: Implementation of four-dimensional radiotherapy, which accounts for the motion of breathing during imaging and radiation delivery. • Medical linear accelerators have become the backbone of radiation therapy for cancer worldwide.
  4. 4. History of LINAC • A 2-year-old boy suffering from a tumor in his eye, was the first to undergo X-ray treatment from a medical linear accelerator that was developed by Henry Kaplan, MD with campus physicists. • The treatment saved the child's sight and he lived the rest of his life with his vision intact. The first patient to receive radiation therapy from the medical linear accelerator at Stanford was a 2- year-old boy. Henry Kaplan (left), and head of radiologic physics Mitchell Weissbluth, the first physicist Kaplan hired, at the working end of the Stanford accelerator.
  5. 5. Generation’s of Linac • The first one was installed in Hammersmith in 1952. • In 1956 ,the first patient was treated at Stanford University in USA. • The LINAC had an 8-MV Xray beam with limited gantry motion. • These LINACs were large and bulky.
  6. 6. • The second generation was isocentric and could rotate 360o around the gantry axis. • They were built between 1962-1982. • They increased the accuracy and precision of Dose delivery. Second generation
  7. 7. Third generation • Better accelerator waveguides and bending magnet systems and more beam modifying accessories. • Wider range of energies , dose rates, field sizes and operating modes. • Higher reliability and computer driven.
  8. 8. Linear accelerator consists of : 1) Electron injection system 2) Microwave system 3) Power supply system 4) Beam transport beam monitoring system 5) Auxiliary system 6) Safety interlock system 7) Computer controlled feedback system 8) Beam collimator/applicator system 9) Cooling system 10) Control console system
  9. 9. TYPE OF SYSTEM COMPONENTS Electron injection system Electron Gun, to provide electrons for acceleration. Microwave system Magnetron or Klystron to provide microwaves. Power supply system Modulator to provide High voltage & short duration pulses in synchronization. Beam transport beam monitoring system Accelerating waveguide ,provides acceleration to electrons. Auxiliary system Vacuum pump, circulating cooling water ,RF frequency tune, Pressurized dielectric gas for RF transmission , RF isolator & Thyratron. Safety interlock system Both Hardware and software interlocking system. Computer controlled feedback system Monitor chamber, Hardware position encoders, limiting micro switches. Beam collimator/applicator system Jaw collimators, multi-leaf collimators(MLC), micro multi leaf collimators (mMLC).
  10. 10. Energies of LINAC Photons beam energy 6 MV 18 MV Electron Beam energy 6MeV 9MeV 12MeV 15MeV 18MeV
  11. 11. The major components of medical Linear Accelerator • Power Supply • Modulator • Magnetron Or Klystron • Electron Gun • Wave Guide system • Accelerator Tube • Bending Magnet • Treatment Head • Treatment table(Couch)
  12. 12. Block diagram of LINAC
  13. 13. Modulator and power supply • This important component of the linear accelerator is usually located in the treatment room In some Units. • The Modulator cabinet contains three major components i. Fan control (cooling the power-distribution system). ii. Auxiliary power distribution system (contains the emergency off button that shuts off the power to the treatment unit ). iii. Primary power-distribution system . • 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 or klystron and simultaneously to the electron gun.
  14. 14. Magnetron • The magnetron is a device that produces microwaves. • It is a high-power oscillator, generates microwave pulses of frequency of about ~3,000 MHz. • Magnetron is cylindrical construction consists of evacuated central cathode and an outer anode with resonant cavities machined out of a solid piece of copper. • The cathode is heated by an inner filament and the electrons are generated by thermionic emission. • Both the electron gun and the Magnetron are fed with High voltage power supply & short duration pulses in synchrony with the Modulated power supply system. • Typical high voltage pulse of about 50kVp is a few micro seconds long and is repeated a few hundred times per second. • Pulse repetition frequency (PRF) OR Pulse per second differs according to manufacturer but pulse width remains constant.(Pulses are of about 4µs duration & are delivered at a PRF of 250Hz.) • PRF or PPS determines the dose rate from a LINAC.
  15. 15. WORKING: • 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. Under the simultaneous influence of the magnetic field. • The electrons move in complex spirals toward the resonant cavities, radiating energy in the form of microwaves. The generated microwave pulses are led to the accelerator structure via the waveguide.
  16. 16. Klystron • The Klystron is a microwave amplifier. It is driven by a low-power microwave oscillator. • The electrons produced by the cathode are accelerated by a negative pulse of voltage into 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.
  17. 17. • Electrons form bunches due to variation in velocity resulting in bunching of electrons as the velocity-modulated beam passes through a field-free space in the drift tube. • As the electron bunches arrive at the catcher cavity, they induce charges on the ends of the cavity and thereby generate a retarding electric field. • The electrons suffer deceleration, and by the principle of conservation of energy, the kinetic energy of electrons is converted into high-power microwaves. Klystron
  18. 18. Electron Gun • It is responsible for producing electrons and injecting them into the accelerator structure . • Tungsten Mesh/coil produces a stray of electrons due to thermionic emission when voltage is applied in terms of “Filament current”. • The electron gun and the source are pulsed so that the high velocity electrons are injected into the accelerating waveguide at the same time as it is energized by the microwaves. • The number of electrons ejected depends upon the temperature of the filament. • 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.
  19. 19. Wave Guide system • Microwave power (produced in the klystron) is transported to the accelerator structure, in which corrugations(wrinkle) are used to slow the waves. • Accelerating electrons tends to diverge, partly by the mutual coulomb repulsion and mainly by the radial component of electric field in waveguide structure. • Electrons are focused back to their path by the use of co-axial magnetic focusing field generated by the coaxial coils which are coaxial with accelerating waveguide.(Also called as steering coils) Accelerator Guide : Also called as the accelerator structure , mounted in the gantry: i) Horizontally (High-energy machines) ii) Vertically (low-energy machines ).
  20. 20. Types of waveguide system 1) Travelling waveguide system 2)Standing wave guide system Travelling waveguide system • Travelling wave guide structure require relatively longer accelerating waveguide. • Functionally, traveling wave structures require a terminating, or " dummy," load to absorb the residual power at the end of the structure, thus preventing a backward reflected wave.
  21. 21. Standing wave guide system • Standing wave guide structure helps in reducing the accelerating length due to option of side coupling cavities. • The standing wave structures 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 as the microwave power is coupled into the structure via side coupling cavities. • Such a design tends to be more efficient than the traveling wave designs since axial, beam transport cavities, and the side cavities can be independently optimized.
  22. 22. Treatment head • Treatment head comprises of components that are designed to shape and monitor the treatment beam.  Bending magnet:  Shielding material:  X-ray target:  Primary collimator  Beam flattening filter:  Scattering foil:  Beam monitoring devices:  Secondary collimators:  Field light:
  23. 23. • The electrons exit the waveguide and enter the ‘flay tube’ where electron beam is redirected towards the target, the electrons travel along a ‘Slalom’ path within the flay tube. • Three pairs of magnets on the either side of the Flay tube, cause the electron beam to bend through the turns of the Slalom. • This process not only positions the beam to strike the target, but also focuses the beam to a diameter of 1mm. • The design of the magnets enables them to focus the electrons of slightly different energies on to the same point on the target (Achromatic behavior) Bending magnet
  24. 24. • In the higher-energy linac’s, however, the accelerator structure is too long and, therefore, is placed horizontally or at an angle with respect to the horizontal. • The electrons are then bent through a suitable angle (usually about 90 or 270 degrees) between the accelerator structure and the target. • 90 degree magnets (Achromatic) have the property that any energy spread results in spatial dispersion of the beam. • 270 degree magnets (chromatic) designed to eliminate spatial dispersion. Slalom 900 Bending (Achromatic ) Chromatic 2700 Bending
  25. 25. • Shielding material :The treatment head consists 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 surroundings, patient as well as the radiation workers. • X-ray target: The pencil electron beam strikes on the x-ray target to produce photons. • X-ray target used is transmission type target .It used is mainly made of Tungsten due to its high atomic number(Z = 74) & High melting point 33700C.
  26. 26. • Primary collimator : The treatment beam is first collimated by a fixed primary collimator located immediately beyond the x-ray target. 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. • Flattening filter: Modifies the narrow, non- uniform photon beam at the isocenter into a clinically useful beam through a combination of attenuation of the center of the beam and scatter into the periphery of the beam. • It is made up of lead, steel or copper.
  27. 27. • Scattering foil: In the electron mode of linac operation, narrow pencil electron beam, about 3 mm in diameter., 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. • The scattering foil consists of a thin high-Z metallic foil (e.g., lead, tantalum) . • The thickness of the foil is such that most of the electrons are scattered instead of suffering bremsstrahlung. • Carrousel is a device in treatment head which helps in the movement of ’Flattening filters of different energies as well as Scattering foils’.
  28. 28. • The function of the ion chamber is to monitor dose rate, integrated dose, and field symmetry. • As the chambers are in a high-intensity radiation field and the beam is pulsed, the ion collection efficiency of the chambers should remain unchanged with changes in the dose rate. • 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. • Beam monitoring devices: The flattened x-ray beam or the electron beam is incident on the dose monitoring chambers. • The monitoring system are transmission type ion chambers or a single chamber with multiple plates. cylindrical thimble chambers have also been used in some linac’s.
  29. 29. • Secondary collimators: the beam is further collimated by a continuously movable x-ray collimators. • This collimators consists of two pairs of lead of tungsten blocks (jaws} which provide a rectangular opening (from 0X0 to 40X40 cm2) projected at a standard distance such as 100 cm from the x-ray source. • The collimator blocks are constrained to move so that the block edge is always along a radial line passing through the x-ray source position. • Field light: The field size definition is provided by a light localizing system in the treatment head. • A combination of 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. allows accurate positioning of the radiation field in relationship to skim marks or other reference points.
  30. 30. Field Light and lasers • Field Light : • It is a Field localizing device, Used to display the position of the radiation field on the patient skin. • An high accuracy bulb is placed at 450 angle with the Mercury mirror placed in the path of the beam (Transmission type mirror) . • The light field size is in congruence with the radiation field size.Field size can be varied with the help of this ‘Light field size) • Lasers: • The accuracy of the laser guides in determining Isocenter position. • Isocenter is a virtual point where the central axis of Gantry, Collimator and couch meets. • 2 Side lasers, saggital and Ceiling lasers are mounted on walls of LINAC unit. • Tolerance of laser position is 2 mm
  31. 31. Treatment table(Couch) • Treatment table (Couch) : Treatment table is a mechanically movable motor driven couch . • Patient is positioned over the treatment table according to the desired co-ordinates of planning. • Patient is immobilized using the Immobilization devices. • Treatment table can be moved Horizontal, Vertical as well as Rotational directions. • Hand Pendent: It contains all the control switches which can be used to access the movement of Gantry, Couch, Collimator jaws(Field size),SSD etc.,
  32. 32. Cooling system • Heat dissipation in linear accelerator is an important step in maintenance in large setup and heavy patient load in hospitals. • The x-rays produced are almost the 1 percent of the electron energy which is striking on the target. • Hence 99% of the energy is converted to heat. • This heat is needed to be cooled and that is achieved by the ‘Cooling system’. • Cooling system consists of ‘water chiller’ for cooling the water and water inlets and outlets to various parts of LINAC including X-ray target.
  33. 33. Radiation Safety/interlock system • Similar to treatment from radiation the Safety from radiation also plays an important role in Radiotherapy. • Various Interlocks are present in LINAC to avoid the mis-happens or wrong treatment to the patient. • Interlocks indicates the problem in particular device in the LINAC assembly and interlocking system helps in solving the particularly and easily. • Safety Interlocks include: 1) Last Man Out Switch(LMO) 2) Door interlock 3) Beam ON/OFF Key etc., • Emergency switches are provided at all the systems of an LINAC unit to completely turn Off the entire Unit with only single switch during emergency situations.
  34. 34. Working: Photon Beam Therapy • Photons of energies 6MV & 18MV can be produced for treatment in Medical linear accelerator. • Modulator produces the necessary potential for the whole accelerator mainly the Magnetron/Klystron and the electron gun. • Electrons are produced by the electron gun and proceeded to the wave guide system. • An 3mm electron ‘pencil beam’ is produced due to acceleration and the focusing coil’s fixed all around the waveguide. • Focused beam is bent by the bending magnets and also the beam is further focused due to reflective force of the bending magnets. • The electron beam is made to strike on the X-ray producing target. • X-rays/Photon energy produced is not uniform in flatness & is asymmetrical in nature, which is made uniform by the flattening filter. • The uniform flattened beam is then passes by the monitoring chambers and the dose rate as well as the symmetry is verified. • The beam is then ready to treat the tumor/Patient.
  35. 35. Working: Electron Beam Therapy • Electron of energies 6MeV, 9MeV, 12MeV, 15MeV, 18MeV & 20MeV can be produced for treatment in Medical linear accelerator. Modulator produces the necessary potential for the whole accelerator mainly the Magnetron/Klystron and the electron gun. • Electrons are produced by the electron gun and proceeded to the wave guide system. • An 3mm electron ‘pencil beam’ is produced due to acceleration and the focusing coil’s fixed all around the waveguide. • Focused beam is bent by the bending magnets and also the beam is further focused due to reflective force of the bending magnets. • The electron beam is made to strike on the Scattering foil to make the electrons to distribute in forward direction. • Electron beam produced is uniform in flatness & is asymmetrical in nature, which can be directly used for treatment. • The uniform flattened beam is then passes by the monitoring chambers and the dose rate as well as the symmetry is verified. • The beam is then ready to treat the tumor/Patient.
  36. 36. Accessories used with LINAC: • Wedges • Dynamic wedge • Blocks • Multileaf Collimator (MLC) • Electronic Portal Imaging (EPID)
  37. 37. Wedges • Physical wedges are graduated pieces of lead that have a thick end and a thin end. • The thin end causes less attenuation than the thick end; this causes a shift in the isodose curves within the treated volume. • The wedge is denoted by the angle it tilts the isodose curves • eg. a 30o wedge would cause a 30o tilt in the isodose curves. • Physical wedges are not in common use due to the ability of the independent jaws to perform dynamic wedging.
  38. 38. Custom Blocks • Significant irradiation of the normal tissue outside this volume must be avoided as much as possible. • These restrictions can give rise to complex field shapes, which require intricate blocking. • Custom blocking system uses a low melting point alloy, Lipowit metal ( Cerrobend) , which has a density of 9.4 g/cm3 at 20°C (83% of the lead density) • This material consists of, 1) 50.0% bismuth 2) 26.7 % lead, 3) 13.3 % tin, and 4) 10% cadmium • The main advantage of Cerrobend over lead is that it melts at about 70°C and can be easily cast into any shape. • At room temperature, it is harder than lead.
  39. 39. Multi Leaf Collimator • A multileaf collimator (MLC) for photon beams consists of a large number of collimating blocks or leaves that can be driven automatically, independent of each other, to generate a field of any shape. • Typically the MLC systems consists 60 to 80 pairs, which are independently driven. • The individual leaf has a width of 1 cm or less as projected at the isocenter. • The leaves are made of tungsten alloy (p=17.0 to 18.5g/cm3 ) and have thickness along the beam direction ranging from 6cm to 7.5cm, depending on the type of accelerator. • The leaf thickness is sufficient to provide primary x-ray transmission through the leaves of less than 2%. • The primary beam transmission may be further minimized by combining jaws with the MLC in shielding areas outside the MLC field opening.
  40. 40. Electronic Portal Imaging (EPID) • Electronic portal imaging overcomes two problems by making it possible to view the portal images instantaneously 1) real time images can be displayed on computer screen before initiating a treatment. 2) Portal images can also be stored on computer for later viewing or archiving. • EPIDs use flat panel arrays of solid state detectors based on amorphous silicon (a-Si) technology .Flat panel arrays are compact, & easier to mount on a retractable arm for positioning in or out of the field. • A scintillator converts the radiation beam into visible photons. The light is detected by an array of photodiodes implanted on an amorphous silicon panel.
  41. 41. On Board Imaging (OBI) • CT scans acquired with detectors imbedded in a flat panel instead of a circular ring is known as Onboard Imaging . • CT scanning that uses this type of geometry is known as cone- beam computed tomography (CB CT) . • In cone-beam CT, planar projection images are obtained from multiple directions a s the source with the opposing detector panel rotates around the patient through 1800 degrees or more. • These multidirectional images provide sufficient information to reconstruct patient anatomy in 3D , including cross-sectional, sagittal, and coronal planes. • A filtered back-projection algorithm is used to reconstruct the volumetric images. • They are mounted on the accelerator gantry and can be used to acquire volumetric image data under actual treatment conditions. • They enable the localization of planned target volume and critical structures before each treatment. • The system can be implemented either by using a kilovoltage x- ray source or the mega voltage therapeutic source.
  42. 42. • Kilovoltage CBCT: • Kilovoltage x-rays for a kilovoltage CBCT ( kVCBCT) system are generated by a conventional x-ray tube that is mounted on a retractable arm at 900 to the therapy beam direction. • A flat panel of x-ray detectors is mounted opposite the x-ray tube. • This imaging system is versatile and is capable of cone-beam CT as well 2-D radiography and fluoroscopy. • Advantages: 1. Produce volumetric CT images with good contrast an sub millimeter spatial resolution. 2. acquire images in therapy room coordinates, and 3. Use 2-D radiographic and fluoroscopic modes to verify portal accuracy, management of patient motion, and making positional and dosimetric adjustments before and during treatment.
  43. 43. • Megavoltage CBCT • MVCBCT uses the megavoltage x-ray beam of the linear accelerator and its EPID mounted opposite the source. • EPIDs with the a-Si flat panel detectors are sensitive enough to allow rapid acquisition of multiple, low-dose images as the gantry is rotated through 1800 or more. • Multidirectional 2-D images, volumetric CT images are reconstructed from these. • MVCBCT system has good image quality for the bony anatomy and, in even for soft tissue targets. • MVCBCT is a great tool for; a) On-line or pretreatment verification of patient positioning, b) Anatomic matching of planning CT, c) Pretreatment CT, avoidance of critical structures such as spinal cord, and d) identification of implanted metal markers if used for patient setup.
  44. 44. • Advantages of MVCBCT over kVCBCT: a) 1 Less susceptibility to artifacts due to high-Z objects such as metallic markers in the target, metallic hip implants, and dental fillings b) 2. No need for extrapolating attenuation coefficients from kV to megavoltage photon energies for dosimetric corrections • From images we can say that MV-CBCT image of 2.5cGy is sufficient for Bony anatomy verification during patient positioning.
  45. 45. E-mail:- vinaydesaimsc@gmail.com Thank you. Vinay Desai M.Sc Radiation Physics Radiation Physics Department KIDWAI MEMORIAL INSTITUTE OF ONCOLOGY Bengaluru

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