The document summarizes the key components and evolution of linear accelerators (linacs) used for radiation therapy. It describes the major components of linacs including the electron gun, accelerating waveguide, bending magnets, and treatment head. It outlines the process of electron acceleration and production of x-rays. It also discusses the different generations of linacs and modern capabilities like intensity modulated radiation therapy using multileaf collimators.

1. Linear Accelerator
DR . NEENA JOHN
JUNIOUR RESIDENT
RADIATION ONCOLOGY
MCH KOTTAYAM

2. Discovery of X rays by Roentgen in 1895→ radiotherapy
The technology has first been aimed towards producing ever higher
photon and electron beam energies and intensities
More recently towards computerization and intensity modulated beam
delivery.

3. PARTICLE ACCELERATORS
Numerous types built for basic research in nuclear and high energy
physics
Most of them have been modified for at least some limited use in
radiotherapy
Conditions to be met for particle acceleration
◦ The particle to be accelerated must be charged
◦ An electric field must be provided in the direction of particle acceleration.

4. Linear Accelerator
Device that uses high-frequency electromagnetic waves to accelerate
charged particles eg- electrons to high energies through a linear tube.
Accelerate electrons to kinetic energies from 4 to 25 MeV

5. 5 distinct generations of medical
linacs:
Low energy photons:(4-8MV)
Straight through beam, fixed flattening filter,
external wedges, symmetrical jaws, single
transmission ionization chamber, isocentric
mounting.
Medium energy photons (10-15MV) and electrons
Bent beam, movable target, flattening filter,
scattering foils, dual transmission ionization
chambers, electron cones.

6. High energy photons(18-25MV) and electrons
Dual photon energy, multiple electron energy,
achromatic bending magnet, dual scattering
foils/scanned electron pencil beam, motorised
wedge, assymetric jaws.
High energy photons and electrons: -fourth
Computer controlled operations, dynamic wedges,
electronic portal imaging device (EPID), MLC.
5th generation:
Intensity modulation with MLC.

7. The high-energy electron beam - for treating
superficial tumors
made to strike a target to produce x-rays -
treating deep-seated tumors

8. Major linac components
Gantry
The Gantry stand or support
Modulator cabinet
Treatment couch
Control console

9. Electron gun Accelerating wave guide
modulator
Power supply
Magnetron/klystron
straight beam
Bending magnet
DC power
Treatment head
Radiation beam

10. Beam forming components:
1. Injection system
2. RF power generation system
3. Accelerating wave guide
4. Auxiliary system
5. Beam transport system
6. Beam collimation and beam monitoring

11. Power supply provides direct current (DC) power to the
modulator
Modulator includes the pulse-forming network.
High-voltage DC pulses from the modulator are delivered to the
magnetron or klystron and simultaneously to the electron gun.

12. Pulsed microwaves produced in the magnetron or klystron
are injected into the accelerator tube via a waveguide
system.
At the proper instant electrons, produced by an electron
gun, are also pulse injected into the accelerator structure.

13. Electrons injected into the accelerator structure with an
initial energy of about 50 keV -interact with the
electromagnetic field of the microwaves.
The electrons gain energy from the sinusoidal electric field
by an acceleration process

14. High-energy electrons emerge in the form of a pencil beam
of about 3 mm in diameter.
In low-energy linacs (up to 6 MV) with relatively short
accelerator tubes, the electrons proceed straight and strike
on a target for x-ray production.

15. In higher-energy linacs the accelerator structure is too long and is
placed horizontally or at an angle
The electrons are then bent through a suitable angle (usually
about 90 or 270 degrees) between the accelerator structure and
the target.
The precision bending of the electron beam is by the beam
transport system consisting of bending magnets, focusing coils,
and other components.

16. Bending magnets:

17. Bending magnets:

18. Injection system:
Source of electrons:
simple electrostatic accelerator called electron gun.
Heated filament cathode and perforated grounded anode.
Electrons are thermionically emitted from the heated cathode
Removable electron triode gun

19. RF power generation system:
Produces the microwave radiation used to accelerate electrons to
the desired kinetic energy
Two major components:
● An RF power source;
● A pulsed modulator.
Magnetron. Klystron.
Source:

20. Pulsed modulator
Produces the high voltage (~100 kV), high current
(~100 A), short duration (~1 s) pulses required by
the RF power source (magnetron or klystron) and
the injection system (electron gun)
The circuitry of the pulsed modulator is housed in
the modulator cabinet

21. The magnetron
Device that produces microwaves.
Functions as a high-power oscillator
Generates microwave pulses of several microseconds' duration and with a
repetition rate of several hundred pulses per second.
The frequency of the microwaves within each pulse is about 3,000 MHz.
Peak power output:
◦ 2 MW (for low-energy linacs, 6MV or less)
◦ 5 MW (for higher-energy linacs)

22. The magnetron has a cylindrical construction
central cathode and an outer anode with resonant cavities
The space between the cathode and the anode is evacuated.

24. The klystron
The klystron is not a generator of microwaves
It is a microwave amplifier.
It needs to be driven by a low-power microwave oscillator.
2 cavity klystron

26. Accelerating wave guide
Evacuated or gas filled metallic structures usually
Cu of rectangular or circular crossection used in
transmission of microwaves.
Electrons are injected into the accelerator structure
with an initial energy of abt 50 KeV.
They interact with E field of microwaves and gain
energy inside the wave guide

27. Difference lies in the design of the
accelerator structure
Two types of accelerating waveguide
developed for the acceleration of electrons:
(i) Travelling wave structure
(ii) Standing wave structure.

28. Travelling wave guide:
Standing wave guide

29. 1. Travelling EM wave
◦ Require a terminating (“dummy”) load to
absorb the residual power at the end of
the structure
◦ Prevent backward reflection wave

30. Standing EM wave
◦ Maximum reflection of the waves at both ends of the
structure
◦ Combination of forward and reverse traveling waves give
rise to stationary waves

31.  Standing EM wave
◦ More efficiency
◦ More expensive as requires installation of
◦ a circulator (or insulator) between the power
source
◦ the structure to prevent reflections from
reaching the power source

33. Auxiliary system:
•Vaccum pumping
•Water cooling system -for cooling the accelerating guide,
target, circulator and RF generator
•Optional air pressure system for pneumatic movement of
target and other beam shaping components,
•Shielding against leakage radiation.

34. The Linac X-ray Beam
Production of x-rays
◦ Electrons are incident on a target of a high-Z material (e.g.
tungsten)
◦ Target – need water cooled & thick enough to absorb
most of the incident electrons
◦ Bremsstrahlung interactions
◦ Electrons energy is converted into a spectrum of x-rays energies
◦ Max energy of x-rays = energy of incident energy of electrons
◦ Average photon energy = 1/3 of max energy of x-rays

35. Designation of energy of electron beam and x-rays
◦ Electron beam - MeV (million electron volts,
monoenergetic)
◦ X-ray beam – MV (megavolts, voltage across an x-ray tube,
hetergeneous in energy)
The Varian Clinac 18 unit produces electron beams of energy 6, 9, 12,
15, and 18 MeV and x-rays of energy 10 MV

36. The Electron Beam
The electron beam is a narrow pencil about 3 mm in diameter.
In the electron mode of linac operation ,is made to strike an electron scattering foil
Spread the beam as well as get a uniform electron fluence
The scattering foil - a thin metallic foil, usually of lead.

37. Most of the electrons are scattered instead of suffering bremsstrahlung.
A small fraction of the total energy is still converted into bremsstrahlung
and appears as x-ray contamination

38. Treatment Head
A thick shell of high-density shielding material such
as lead, tungsten, or lead-tungsten alloy.
Contains an x-ray target, scattering foil, flattening
filter, ion chamber, fixed and movable collimator,
and light localizer system.
Provides sufficient shielding against leakage
radiation

39. •X-ray targets (5mm tungsten
embedded in cu.electron
window made of nickel)
•Flattening filters/Scattering
foils
•Dual transmission ionization
chamber
•Retractable wedges (optional)
•Field defining light and range
finder
•Jaws/MLC(optional)
Treatment head:

41. Target and Flattening Filter
To make the beam intensity uniform across the field, a
flattening filter is inserted in the beam
This filter is usually made of lead, although tungsten,
uranium, steel, aluminum, or a combination has also been
used or suggested.

42. Beam collimation
In a typical modern medical linac, the photon beam collimation is
achieved with two or three collimator devices:
A primary fixed collimator;
Secondary movable beam defining collimators;
An MLC (optional).

43. Beam CollimationBeam is first collimated by the fixed primary collimator
Located immediately beyond the x-ray target.
X-rays - the collimated beam passes through the flattening filter.
Electron mode - the filter is moved out of the way – electron beam is made to
strike scattering foil
Incident on the dose monitoring chambers

44. After ion chambers, further collimation -
continuously movable x-ray collimator
Two pairs of lead or tungsten blocks (jaws) – upper
& lower
Provide a rectangular opening from 0 × 0 to the
maximum field size (40 × 40 cm or a little less)
Projected at a standard distance such as 100 cm
from the x-ray source (focal spot on the target).

45. Modern linacs incorporate independent
(asymmetric) jaws that can provide
asymmetric fields

46. MLC:
2 coplanar set of blades,each
blade capable of moving
parallel to but independent of
the other set.
40 pairs of leaf
Width of 1cm or less at
isocentre

47. MLC- multi leaf collimator
•95% alloy of
tungsten

48. Models with 120 leaves (60 pairs) covering fields up to 40 ×
40 cm2 and requiring 120 individually computer controlled
motors and control circuits are currently available.
MLCs are becoming invaluable in supplying intensity
modulated fields in conformal radiotherapy

49. Micro MLCs
Miniature versions of MLCs projecting 1.5–6 mm leaf widths
and up to 10 × 10 cm2 fields at the linac isocentre
May be used in radiosurgery as well as for head and neck
treatments.

50. Electron collimation
◦ As electrons scatter readily in air - beam collimation
must be achieved close to the skin surface
Considerable scattering of electrons from the collimator
surfaces including the movable jaws.

51. Dose rate can change by a factor of two or three as
the collimator jaws are opened to maximum
So the x-ray collimator is kept wide open and an
auxiliary collimator for electrons is attached
◦ Form of trimmers extended down to the skin
surface.
◦ As a set of attachable cones of various sizes.

52. Electron collimation system:
•Attachment of auxiliary
collimators for electrons.
• Done at the end of the applicator
using customized cut-outs

53. Light localizing system and range finder: field light illuminates an area that
coincides with the radiation treatment field on patients skin.
Range finder is used to place the patient at the correct treatment distance by
projecting a cm scale whose image on the pt indicates the dist.
Important to ensure that the light field is congruent with the radiation
field with frequent checks.

54. Monitoring
Treatment beam is incident on the dose monitoring chambers
Monitoring system - several ion chambers or a single chamber with
multiple plates
Are usually transmission type - flat parallel plate chambers to cover the
entire beam
Ion chamber - monitor dose rate, integrated dose, and field symmetry

55. Ion chamber
◦ Ion collection efficiency should remain unchanged with
changes in the dose rate
◦ Are sealed - response is not influenced by temperature
and pressure of the outside air
◦ Periodically checked for leaks

56. The primary ionization chamber measures Mus- monitor
units.
Typically, chamber electrometer circuitry is adjusted in such
that 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.

57. Once the operator preset number of MUs has been reached, the
primary ionization chamber circuitry shuts the linac down
Terminates the dose delivery to the patient.
Before a new irradiation can be initiated, it is necessary to reset the MU
displays to zero.
Furthermore, irradiation is not possible until a new selection of MUs
has been made

58. Portal Imaging
Portal Vision helps ensure treatment plan verification, accurate
patient setup, effective treatment delivery, and more successful
patient outcomes

59. Gantry
Linear accelerators currently constructed - the source of
radiation can rotate about a horizontal axis – isocentric
mounting
As the gantry rotates, the collimator axis moves in a
vertical plane.
Isocenter - point of intersection of the collimator axis
and the axis of rotation of the gantry

60. Isocentric treatment technique - beams are directed from different directions
but intersect at the same point, the isocenter, placed inside the patient.
Nonisocentric units are swivel mounted
◦ treatment head can be swiveled or rotated in any
direction
◦ gantry can move only upward or downward.
◦ not as flexible
◦ mechanically simpler, more reliable and less expensive
than the isocentric models.

61. Thank you

65. Why Linear Accelerator ?
Demand for high penetrating beams