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Linear Accelerators

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1948: the British Ministry of Health brought together the three main groups in England who were working on the linac. – PowerPoint PPT presentation

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Title: Linear Accelerators


1
Linear Accelerators
  • Chapter 7 W/L
  • Chapter 9 S/S

2
Radiation Therapy Equipment
  • Low-energy machines Uses x-rays generated at
    voltages up to 300kVp
  • Grenz rays
  • 10-15kVp
  • Treatment of inflammatory disorders (Langerhans
    cells), Bowens disease, patchystage mycosis
    fungoides, herpes simplex
  • Contact therapy
  • Superficial skin lesions
  • Endocavitary treatments for curative intent
    (rectal)
  • hemangiomas
  • Superficial equipment
  • 50-150kVp
  • Skin cancer and tumors no deeper than 0.5 cm
  • Orthovoltage machines
  • 150-500kVp
  • Skin, mouth, and cervical carcinoma
  • Experience limitation in the treatment of lesions
    deeper than 2 to 3 cm.

3
Limitations of Low Energy Machines
  • Can not reach deep-seated tumors with an adequate
    dosage of radiation.
  • Do not spare skin and normal tissues.

4
Central Axis Depth Dose
  • Central axis depth dose and physical penumbra are
    related to beam quality.
  • The central axis depth dose distribution for a
    specific beam depends on the energy.
  • Isodose curve a line representing various points
    of similar value in a beam along the central axis
    and elsewhere
  • The depth of an isodose curve increases with beam
    quality
  • The absorbed dose in the medium outside the
    primary beam is greater for low-energy beams than
    for those of a higher energy.
  • Limited scatter outside the field for megavoltage
    beams occurs because of predominantly forward
    scattering of the beam.

5
Linear Accelerator
  • Treatment machine that uses high-frequency
    electromagnetic waves to accelerate charged
    particles such as electrons to high energies via
    a linear tube.
  • Charged particles travel in straight lines as
    they gain energy from an alternating
    electromagnetic field.
  • Higher energy beams can be generated with greater
    skin sparing, field edges are more sharply
    defined with less penumbra and computer
    technology shapes the treatment beam and
    personnel receive less exposure to radiation
    leakage.

6
History
  • 1948 A working 1MV linear accelerator was
    installed at the Fermi Institute in Chicago.
  • Mile long waveguide ran under University
    Boulevard at the University of Chicago.
  • 1948 the British Ministry of Health brought
    together the three main groups in England who
    were working on the linac.
  • Medical Research Council
  • Atomic Energy Research Establishment
  • Metropolitan Vickers Electric Company
  • 1952 linac installed at Hammersmith Hospital in
    London, first treatment in 1953 with an 8MV beam.
  • 1956 first clinically used in the US at the
    Stanford University Hospital.
  • 1961 The first 100 cm SAD fully isocentric
    linear accelerator was manufactured and installed
    in the US.

7
Accelerator Generations
  • Early Accelerators (1953-1961)
  • Extremely large and bulky
  • Limited gantry motion
  • Second Generations (1962-1982)
  • 360 degree rotational
  • Allow treatment to a patent from any gantry angle
  • Improvement in accuracy and dose delivery
  • Third generation accelerators
  • Improved accelerator guide
  • Magnet systems
  • Beam-modifying systems to provide wide ranges of
    beam energy, dose rate, field size
  • Operating modes with improved beam
    characteristics
  • Highly reliable
  • Compact design
  • May include dual photon energies, multileaf
    collimation, several electron energies
    electronic portal verification systems

8
Components
  • Modulator cabinet
  • Console
  • Drive Stand
  • Klystron
  • Waveguide
  • Circulator
  • Water-cooling system
  • Gantry
  • Electron gun
  • Accelerator structure
  • Treatment head
  • Bending magnet
  • Flattening filter
  • Scattering foil
  • Treatment couch
  • Other accessories

9
Modulator Cabinet
  • Modulator cabinet contains components that
    distribute and monitor primary electrical power
    and high-voltage pulses to the magnetron or
    klystron
  • Located in the treatment room
  • Three major components
  • The fan control automatically turns the fans off
    and on as the need arises for cooling the power
    distribution
  • Auxiliary power-distribution system contains the
    emergency off button that shuts off the power to
    the treatment unit.
  • Primary power-distribution system

10
Console
  • Console electronic cabinet provides a central
    location for monitoring and controlling the linac
  • Take the form of a digital display, push button
    panel or video display terminal (VDT)
  • All interlocks must be satisfied for the machine
    to allow the beam to be started
  • Provides a digital display for prescribe dose
    (monitor units), mechanical beam parameters such
    as collimator setting or gantry angle

11
Drive Stand
  • Drive Stand a stand containing the apparatus
    that drives the linear accelerator
  • Open on both sides with swinging doors for easy
    access to gauges, valves, tanks, and buttons
  • Klystron/Magnetron power source used to generate
    electromagnetic waves for the accelerator guides
  • Waveguide hollow tube-like structure that guide
    the electromagnetic waves from the magnetron to
    the accelerating guide where electrons are
    accelerated
  • Circulator directs the RF energy into the
    waveguide and prevents any reflected microwaves
    from returning to the klystron
  • Water-cooling system allows many components in
    the gantry and drive stand to operate at a
    constant temperature

12
Klystron
  • Klystron A linear beam microwave amplifier
    requiring an external oscillator or
    radiofrequency (RF) source driver
  • A form of radiowave amplifier, multiplies the
    amount of introduced radiowaves greatly.
  • Electron tube that is used to provide microwave
    power to accelerate electrons
  • Microwave frequencies needed for linear
    accelerator operation are about three billion
    cycles per second

13
Magnetron
  • Magnetron device that provides high-frequency
    microwave power that is used to accelerate the
    electrons in the accelerating waveguide.
  • Electrons are emitted from the cathode and spiral
    in the perpendicular magnetic field. The
    interaction of the spiraling electrons with the
    cavities in the anode creates the high-frequency
    EM waves.
  • oscillator and amplifier used in low-energy

14
Gantry
  • Gantry responsible for directing the photon
    (x-ray) energy or electron beam at a patients
    tumor.
  • Electron gun produce electrons and injects them
    into the accelerator structure
  • Accelerator structure a special type of wave
    guide in which electrons are accelerated.
  • Treatment head components designed to shape and
    monitor the treatment beam

15
Accelerator Structure
  • Microwave power (produced in the klyston) is
    transported to the accelerator stricture in which
    corrugations are used to slow up the waves
    synchronous with the flowing electrons. After
    the flowing electrons leave the accelerator
    structure, they are directed toward the target
    (for photon production) or scattering foil (for
    electron production) located in the treatment
    head.
  • Amplification that occurs in the accelerator
    structure is in the closed ended, precision
    crafted copper cavities where the electrical
    power provides momentum to the low-level electron
    stream mixed with the microwaves. Alternating
    positive and negative electric charge accelerates
    the electrons toward the treatment head, the
    negative voltage repels electrons while the
    positive voltage attracts then, thereby pushing
    and pulling the electrons along.
  • Charged particles experience the equivalent of a
    small voltage multiple times, ending up with a
    large amount of kinetic energy

16
Accelerator Structure
  • Length varies depending on the beam energy of the
    linac, as more cavities are used, higher energy
    is derived
  • Traveling wave an electromagnetic wave travels
    to the right along with the electron, the
    electron is continuously accelerated as it moves
  • Limitation the electron and the electric field
    must move at the same velocity
  • Irises washer shaped metal discs that provide
    resistance to the travel of the electromagnetic
    waves.
  • As the electron increases in energy and velocity,
    the need for irises is reduced, so irises are
    increasingly far apart and have increasingly
    wider openings.
  • Standing wave microwave power is joined into the
    structure by side-coupling cavities, rather than
    through the beam aperture, provides a shorter
    accelerating tube
  • Makes use of the concept if interference
  • More efficient, more costly

17
Treatment head
  • Treatment head components designed to shape and
    monitor the treatment beam
  • Bending magnet direct the electrons vertically
    toward the patient
  • X-ray target
  • Primary collimator designed to limit the maximum
    field size
  • Beam flattening filter shaped the x-ray beam in
    its cross sectional dimension
  • Ion chamber monitors the beam for its symmetry
    in the right-left and inferior-superior direction
  • Secondary collimators upper and lower collimator
    jaws
  • Field light outlines the dimensions of the
    radiation field as it appears on the patient,
    allows accurate positioning of the radiation
    field in relationship to skim marks or other
    reference points

18
Bending Magnet
  • Bending magnet bends the electron beam through a
    right angle, so it ends up pointed at the patient
  • 90 degree magnets (chromatic) have the property
    that any energy spread results in spatial
    dispersion of the beam.
  • Electrons are bent in proportion with their
    energy, the lower energy electrons are bent more,
    the higher energy electrons less
  • Results in a beam that is spread from side to
    side according to energy
  • Energy sensitive, act as energy differentiators
  • 270 degree magnets (achromatic) designed to
    eliminate spatial dispersion
  • So not significantly disperse the different
    electron energies in the beam.

19
Flattening Filter
  • Flattening filter (lead, steel, copper etc.)
  • 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
  • Measured in percent at a particular depth in a
    phantom (10 cm)
  • Must be carefully positioned in the beam or the
    beam hitting the patient will be non-uniform,
    resulting in hot and cold spots
  • Flatness a wide beam that is nearly uniform in
    intensity from one side to the other (/- 6)
  • Symmetry the measure of intensity difference
    between its opposite sides (/- 4)
  • Causes include the use of a wedge, misalignment
    of the flattening filter, and misdirection of the
    electron beam before hitting the target.

20
Scattering foil
  • Scattering foil thin metal sheets provide
    electrons with which they can scatter, expanding
    the useful size of the beam
  • Other accessories

21
Monitor Chambers
  • Electron and photon beams must first be measured
    or monitored in order to allow delivery of the
    prescribed amount of radiation.

22
Treatment Couch
  • Treatment couch mounted on a rotational axis
    around the isocenter
  • Also called patient support assembly (PSA)
  • Move mechanically in a horizontal and lengthwise
    direction- must be smooth and accurate allowing
    for precise and exact positioning of the
    isocenter during treatment positioning
  • Support up to 450 lbs
  • Range in width from 45-50 cm
  • Racket-like frame should be periodically
    tightened to provide more patient support and
    reduce the amount of sag during treatment
    positioning.

23
Multimodality Treatment
  • Multimodality treatment units offer several
    advantages
  • Dual photon energies- they can provide backup for
    other treatment units that may experience
    down-time
  • Patients can be treated with multiple beam
    energies on the same treatment unit

24
New Technologies
  • Three-dimensional conformal therapy (3D-CRT) the
    field shape and beam angle change as the gantry
    moves around the patient
  • Images from CT scanners are transferred to
    treatment planning computers, where normal
    tissues and tumor volumes are defined
  • forward planning process beam arrangement are
    tested by trial and error
  • Intensity Modulated radiation therapy (IMRT)
    beneficial in escalating the dose to the tumor
    volume and reducing the dose to normal tissue
  • inverse treatment planning the radiation
    oncologist selects dose parameters for normal
    tissues and the target volume and the computer
    back calculates the desired dose distribution and
    beam arrangements
  • Adjusts the intensity of radiation beam across
    the field with the aid of MLC moving in and out
    of the beam portal under precise computer
    guidance.

25
Additional Advances
  • Independent collimators (dual asymmetrical jaws)
    provide increased flexibility in treatment
    planning
  • MLCs allow an increased number of treatment
    fields without the use of heavy Cerrobend
    blocking
  • Dynamic wedge computerized shaping of the
    treatment field
  • Electronic portal imaging provides feedback on
    single-event setup accuracy or observation of
    treatment in near real-time

26
Additional Advances
  • Verification and record devices
  • Allow incorrect setup parameters to be corrected
    before the machine is turned on
  • Provide data in computer assisted setup
  • Recording of patient data
  • Allowing for data transfer from the simulator or
    treatment planning computer
  • Assisting with quality control
  • Stereotactic radiation therapy involves the
    aiming and delivery of a well defined narrow beam
    to extremely hard to reach places

27
Interlocks
  • Designed to protect the patient, staff members
    and equipment from hazards
  • Patient protection interlocks, including beam
    energy, beam symmetry, dose, and dose-rate
    monitoring, prevent radiation and mechanical
    hazards to the patient
  • Emergency off buttons terminate irradiation and
    machine functions require a complete warm-up
    procedure before the treatment machine can
    produce an electron or photon beam

28
High-energy Machines
  • High-energy machines
  • Van de Graaff generator
  • Betatron particles travel in a circular pattern
  • Cyclotron the particles travel in a spiral
    pattern
  • Linear accelerator
  • Cobalt unit
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