Information about LINACs various components

1. Radiation generating
equipments of LINAC
SUBHRENDU GHOSH
INTERN MEDICAL PHYSICIST
Moderator- Dr. Kanan Jassal

2. Contents
Information about Linac and a brief history
Working
Kilovoltage Units
Evolution to various stages
Components

3. What is a LINAC??
• A linear accelerator is a device that uses high Radio-Frequency
(RF)electromagnetic waves to accelerate charged particles (i.e.
electrons) to high energies in a linear path, inside a tube like
structure called the accelerator waveguide.
• The resonating cavity frequency of the medical linacs is about 3
billion Hertz (cycles/sec) .
• This is the most common device to treat cancer with external beam
radiation.

4. How Does it Work
• The linear accelerator (Linac), uses
microwave technology to accelerate
electrons in a part of the linac called
waveguide , then allows these electrons to
collide with a heavy metal target . As a
result of these collisions , high energy X--
Rays (Photons) are produced from the
target .

5. Working(contd.)
• These high energy photons
will be directed to the
patient’s tumor.
• Radiation can be delivered
to the tumor from any angle
by rotating the gantry and
moving the treatment
couch.

9. Kilovoltage Units
Grenz-ray Therapy
Contact Therapy
Superficial Therapy
Orthovoltage Therapy
Supervoltage Therapy
Megavoltage Therapy

10. Grenz-ray therapy
•Very low energy x-rays are used.
•Below 20kV energy is used for production of x-ray.
•Depth penetration is very low , so not in use

11. Contact Therapy
•Very small SSD (2cm or below).
•Machines operates at 40kV to 50kV.
•Typically at a tube current of 2mA.
•Rapid dose fall off.( Typically within 2cm).
•Quality of radiation is useful for tumors not deeper than
1mm to 2mm.

12. Superficial Therapy
•50kV to 150kV is used for production of x-ray.
•Filtration is used for production of desired beam by using
1mm to 6mm Al.
•Typical SSD ranges between 15cm to 20cm.
•5 to 8mA is used.
•Tumor of 5mm depth can be treated with 90%depth dose.

13. Orthovoltage Therapy
•50kV to 500kV is used for production of x-ray.
•Mostly operated at 200kV to 300kV.
•10mA to 20mA.
•A movable diaphragm produces desired field size.
•SSD usually set at 50cm.
•Good for tumors around 2cm depth.
•Skin dose is very high.

14. Supervoltage Therapy
•500kV to 1000kV is used for production of x-ray.
•Insulation of high voltage transformer is difficult.

15. Megavoltage Therapy
•X-ray beam of energy 1MV or greater.
•γ ray emitting radionuclides are also included in this
category if energy is 1MeV or greater.

17. Evolution
DC Voltage Accelerators
RF-Accelerators
Linear Accelerators (Linacs)

18. Van De Graaff Generator
The tandem Van de Graaff accelerator at Western
Michigan University is used mainly for basic research,
applications and undergraduate instruction.

19. 14

20. Betatron
The main principle is that electron in a changing magnetic
field experience a motion in a circular orbit.
• The betatron was the first machine capable of
producing electron beams at energies higher than
could be achieved with a simple electron gun (6-
40MeV).

21. • The betatron was developed in 1935 by Max
Steenbeck in Germany to accelerate electrons
• In a betatron, the changing magnetic field from the
primary coil accelerates electrons injected into the
vacuum torus, causing them to circle around the
torus in the same manner as current is induced in the
secondary coil of a transformer

22. The stable orbit for the electrons satisfies
ϴ=2𝜋r2H ,
ϴ is the flux of electron.
R is the radius of electron orbit.
H is the magnetic field

24. From DC to RF
• The limit of high-‐voltage equipment is several million volts. The
plants are very complex for higher energy, and higher voltage cause
spark discharges.
• Proposal of the Swedish scientist Ising 1924 to use fast-‐changing
high-‐frequency voltage to accelerate instead of DC.
• The Norwegian scientist Wideröe 1928 successfully tested the first
linear accelerator, which is based on this principle.
• Today almost all accelerators use RF systems for accelerating
particles.
15

27. Early Accelerator
The first one was
installed at
Hammersmith in 1952.
In 1956, the first patient
was treated at Stanford
University in the United
States.
The Linac had an 8 MV
X-‐ray beam with
limited gantry motion.
These linacs were large
and bulky.

28. 2nd Generation Linacs
• The second generation,
were isocentric units,
which can rotate 360
degrees around the gantry
axis.
• They were built between 1962
and 1982.
• They improved in precision
and accuracy of dose delivery.

29. 3rd Generation LA
• Better accelerator
waveguides and
bending magnet
systems and more
beam modifying
accessories.
• Wider range of beam
energies, dose rates,
field sizes and operating
modes.
• Higher reliability and
computer driven.

30. Components

31. Drive stand
• The gantry rotates on horizontal axis bearings located inside the
Drive Stand, a large rectangular cabinet that is firmly secured to
the treatment room floor.
• Major components located in the Drive Stand:
• 1. Klystron or Magnetron
• 2. RF Waveguide
• 3. Circulator (connects item 1 and 2 above)
• 4. Cooling water system.

32. ANALOGY
• Envisage that electrons as
being surfers carried along by
a sea wave.
• It is hard for the surfers to
catch the right wave at the
right time.
• To gain speed they must
remain on top.

33. Magnetron
• Electron tube that provides
microwave power to
accelerate the electrons.
• Preferred for lower electron
energies, 4 MeV to 6 MeV
linacs.
• For higher energies the Klystron
is a better choice.

34. Physics behind
• It is basically a high power oscillator.
• Generates microwave pulses of several microseconds
with a repetition rate of several 100 pulses per
second.
• The cathode is heated by inner filament and electrons
are emitted by thermionic emission.

35. • A static magnetic field and DC electric field is applied
between cathode and anode.
• The electrons starts moving towards anode by action
of DC electric field.
• Due to magnetic field electrons move in a complex
spiral towards resonant cavities, radiating energy in
the form of microwave.

37. The Klystron
• Provides the
source of
microwave
power to
accelerate the
electrons.
• This is done by
amplifying
introduced Radio
Frequency (RF)
electromagnetic
waves.

38. Physics behind
• It is a microwave amplifier
• It needs to be driven by a low power microwave
oscillator.

39. Accelerator Guide
• This is sometimes called the accelerator structure or accelerator
wave guide.
• It can be mounted in the gantry horizontally for high energy single
or dual energy machines with klystrons.
• Can be mounted vertically for low energy machines with
magnetrons.
• The microwave power produced in the klystron or magnetron is
transported to the accelerator structure to accelerate the pulsed
electron bunches.

40. Gantry
• Helps direct the X-‐ray (photons) or electron beams to the
patient’s tumor.
• It rotates 360 degrees around a line/point, called the
Isocenter.
• The gantry has the following compontents:
• 1. Electron Gun
• 2. Accelerator Structure.
• 3. Treatment Head.

41. Pulse Forming Network
• This network sores electrical energy to provide flat
topped DC pulses to the Thyratron.
• The Thyratron uses these pulses as a high-‐tech switch to deliver
these pulses to the electron gun.

42. Treatment Head
• Contains components
designed to shape and
monitor the treatment
beams.
• For photon therapy, they
consist of the bending
magnet, target, primary
collimator, beam flattening
filter, ion chambers,
secondary collimators and
one or more slots for trays,
wedges, blocks and
compensators.

43. Modulator Cabinet
• This is the noisiest of the
linac system components
and is located inside the
treatment room.
• Contains 3 subcomponents:
• Fan Control, cooling the
power distribution system.
• The auxiliary power
distribution system, contains
the emergency off button
that shuts the power to the
linac.
• Primary power distribution
system.

44. Bending Magnet
• Changes the direction of
the electron beam,
downwards toward the
patient.
• Bends the pulsed electron
beam towards the target for
X-‐rays or toward the
scattering foil for electron
treatments.

45. X-‐Ray Target
• The collision of the electrons with the high density transmission
target creates the X-Rays (photons), forming a forward peaking
shaped X-ray beam in the direction of the patient’s tumor.
• The X-ray target is located at the focus of the Bending Magnet.
• 94% of the electrons energy goes into heat.

46. Spatial Distribution

47. The Water Cooling System
Located in the Drive Stand and
Gantry
• Provides thermal stability
to the system.
• Allows many components
in the Drive Stand and
Gantry to operate at a
constant temperature.

48. Beam Flatening Filter
• It is a conical shaped metal absorber, that absorbs more forward
peaking photons than the ones in the periphery.
• It is required to create a flattened beam of sufficient area,
uniformity and symmetry.
• It is usually made of Tungsten, Steel, Lead, Uranium and
Aluminum.
• In dual energy photon linacs, two flattening filters are required for
the low and the higher photon energies.

52. References
• F.M.Khan-The physics of radiation therapy
• Attix-Introduction to radiological physics and radiation
dosimetry
• Images are googled.
• “Medical linear accelerator” by Victor M. Tello, Florida
Hospital cancer Institute.

53. THANK YOU