Linear Accelerator working and construction

2. INTRODUCTION
 WHAT IS AN ACCELERATOR?
An accelerator is a device used to impart
kinetic energy to charged particles.
AIM- To study artificially induced reactions
by bombardment with high energy charged
particles (ones that do not occur naturally).
 As in 1932 cock croft & walton produced first
such accelerator in which Li was charged by
400 KV
 So accelerators are mechanical appliances
where high energy projectiles are produced.

3. TYPES OF ACCERLATORS
 Linac
 Vande - graff generator
 Betatron
 Cyclotron
 Above all accelerators differ only in the
mechanism of acceleration.
 In cyclotron & Betatron - circular paths.
 In linac - linear path

4. VANDEE – GRAFF
GENERATOR
 IN 1937 R.J.VAN de GRAFF developed
first electrostatic accelerator
PRINCIPLE
 It is based on phenomenon of electric
discharge.
FEATURES
 It operates at 200 cGy/minute &provides a
SSD of 100 cm.
 Bulky in size .
 Also no ODI was provided.

5. BETATRON
 IN 1941 KREST developed Betatron with x-
ray of ENERGY 2 Mv
PRINCIPLE:
Electron in changing Mg. field experiences
acc. in circular orbits.
FEATURES
 Both photon & e- beams 6MV & 40 MeV
respectively
 Used dose rate of 200 cGy / min.
 Bulky &noisy.

6. LINEAR ACCERLATORS
 Linac is a device that uses high frequency
e-m waves to accelerate charged particles
such as e- to high energies through a linear
tube.
 This tube employs microwaves resonant
cavities arranged in a straight line.
 Specialised x –ray machine producing 4-20
MV energy beam.

7. DIFFERENCE B/W MEDICAL
LINAC &PHYSICS LINAC
1. Compact so that can be easily incorporated in
hospitals.
2. Generates selected electron and photons energy
beams.

8. Linear accelerator in
general
~
Charged particle

9. MEDICAL LINACS
 Medical linac are accelerators
which accelerate e- to K.E from
4 MeV to 25 MeV using non
conservative microwave RF
Fields in range 10 3(l-band) to
104 (x-band) with vast majority
Running at 2856 MHz(s-band).

10. BASIC PRINCIPLE
 An e- injected into beam of micro-wave at an
appropriate place & time, they will be acted
upon by force applied by electric field &
carried along by the wave with increased
velocity.
 High frequency e. m. waves (alternating
electric & magnetic fields traveling through
space ) used to accelerate e- are called
microwaves
 MW are ordinary radio waves having
frequency thousands times higher
 RF wave must have enough power to carry e-
along. RF needed for Linac operation is
3000MHz.

11. BLOCK DIAGRAM

12. HISTORY
 The first linear accelerator was developed by
wideroe in 1928 to accelerate heavy ions.
 IN 1948 1- Mv linac was installed at Fermi
institute in Chicago by Henry Kaplan having a
mile long waveguide.
 In June 1952 medical research council,
Atomic energy research establishment &
Metropolitian Vickers electric company
installed Linac in Hammersmith hospital.
 IN 1953 in New castle hospital a linac with 4
Mv photon beam was installed.

13. LINAC Installed at Hammersmith hospital in london in
1953 .
FEATURES
 8 MV x-ray beam
 Limited gantry motion
 Extremely large and bulky.
The first 100 cm SAD fully isocentric linac was
manufactured in united states and was installed in 1961
by varian associates.
ITS CHARACTERSTIC FEATURES
 High energy beams in Mv ranges
 Greater skin sparing effects
 Less penumbra
 Less radiation leakage
 Use of computer technology

14. Generations of linac
 These are generations of linac with new features:
 Low energy photons (4–8 MV): straight-through beam; fixed
flattening filter; external wedges; symmetric jaws; single
transmission ionization chamber; isocentric mounting.
 Medium energy photons (10–15 MV) and electrons: bent
beam; movable target and flattening filter; scattering foils; dual
transmission ionization chamber; electron cones.
 High energy photons (18–25 MV) and electrons: dual photon
energy and multiple electron energies; achromatic bending
magnet; dual scattering foils or scanned electron pencil beam;
motorized wedge; asymmetric or independent collimator jaws

15.  High energy photons and electrons: computer controlled
operation; dynamic wedge; electronic portal imaging device
(EPID); Multi leaf collimator (MLC).
 High energy photons and electrons: photon beam
intensity modulation with MLC; full dynamic conformal dose
delivery with intensity modulated beams produced with an
MLC.
 High Energy Photons and electrons with attached KV
CBCT/MV CBCT or both; photon beam intensity modulated
beams MLC; full dynamic conformal IMRT .

16. COMPONENTS OF LINAC
 GANTRY( c- arm / drum types)
 Gantry stand (or support)
 Modulator cabinet
 Patient support assembly
or treatment couch.
 Control console.

17. DRIVE STAND
 Large rectangular cabinet.
It Consist of :
 Magnetron / klystron :a microwave
generator & amplifier.
 Wave guide: hollow tube delivers
microwaves to accelerator guide.
 Circulator : prevents reflected microwaves
returning to klystron.
 Water cooling system : to maintain thermal
stability.
 Oil tank :for heat dissipation

18. Configuration of modern
linacs

19. MODULATOR
 Modulator is a unit that contains components
which distribute and control primary electrical
power to the parts of machine through utility
connections.
 It consist of :
 3 phase full wave rectifier ,diodes & capacitors.
 Pulse forming network.
 Switch tube hydrogen thyratron.
 Its main functions :
1. Supply voltage pulses to microwave generator.
2. Peak power of pulses depends upon
requirements of mw generator.

20. Contd…
 High-voltage pulses from the
modulator section are flat-topped
D C.
 These pulses are delivered to the
magnetron or klystron and
simultaneously to the electron gun.
 Pulsed microwaves produced in
the magnetron or klystron are
injected into the accelerator tube
or structure via a waveguide
system.
V
time
5 µ sec

21. MW CAVITIES

22. MAGNETRON
 Magnetron is a device that
produces microwave and also
works as a high power oscillator.
 It generates microwave pulses
of several microsecond duration
with a repetition rate of hundred
Pulses per second.
 Each pulse is associated with a frequency of 3000
MHZ (3 GHZ).

23. STRUCTURE
 It consist of a hot cathode with a negative potential
placed at the centre in a evacuated circular chamber.
 Space around rim is covered with cylindrical cavities
directly connected to anode (+ve potential).
 Pulsed D.C voltage is applied b/w anode and
cathode.
 Electrons emitted by cathode are made to move in
spiral paths by the action of magnetic field.
 These spiral e- causes resonance in cavities and high
frequency mw power is delivered to accelerator
waveguide.
 For 10 MEV e - energies a magnetron operating at
peak power of 2.5 Mw is required.

24. KLYSTRON
 Klystron is a device which is a microwave amplifier
driven by a power low power microwave
oscillator.
ITs structure consist of a :
1. Buncher cavity (energised by low power
microwaves).
2. Catcher cavity (retarding E.F de-accerlates e’s &
by conservation of energy K.E of e- converted into
high power microwaves).

25. Contd….
 Klystron is not a microwave generator but a MW amplifier.
 Electrons produced by hot cathode are acc. by –ve pulse of
voltage in buncher cavity & are mixed with low power
microwave.
 MW set E.F across the cavity & velocity modulation takes
place.
 This velocity modulation causes acc. & de- acc .i.e. few e -
are speeded up, some slows down while few remain un
affected. & hence bunching of e - takes place.
 In catcher cavity e- induces charge & generates retarding
E.F & e- suffer de- acc. Hence by conservation of energy e -
energy is converted to high power microwaves.
 It is placed in a thin metal container.

26. Klystron

27. Buncher
•The purpose of the buncher is to
accelerate the pulsing electrons as
they come out of the electron gun
and pack them into bunches.
•To do this the buncher receives
powerful microwave radiation from
the magnetron or klystron.
•The microwaves accelerate the
electrons in somewhat the same
way that ocean waves accelerate
surfers on surfboards.

29. WAVEGUIDE
 These are rectangular or circular in shape.
ITS FUNCTION
Microwave power from the
Klystron/ magnetron is supplied
to the accelerator structure by
a system of hollow pipes called
Waveguide.
 Also known as feed
Waveguide.
 Pressurized with freon or sulphur hexa
 flouride to reduce electrical breakdown.

30. Beam forming components
1. Injection system
2. RF power generation system
3. Accelerating waveguide;
4. Auxiliary system;
5. Beam transport system; and
6. Beam monitoring system and beam
collimation.

31. INJECTION SYSTEM
 It consist of a electron gun.
 Electron gun is a source of electrons i.e. a simple
electrostatic generator.
Types of e - guns
1. Diode type
2. Triode type
 Both have a heated cathode maintained at –ve
potential of order 25 kv.
 Also a perforated grounded anode.
 In addition triode gun has a grid placed b/w cathode &
anode maintained at –ve pot. to cut off e- to anode.
 Cathode in diode type is a flat spiral of tungsten.
 While in triode oxide coated cathode . Electrons
emitted thermionically by cathode are focussed into
pencil beam & are accelerated towards anode then
they drift into accelerating waveguide.

32. RF POWER
GENERATION
 Produces high power MW used for e-
acceleration
 It consist of :
 RF power source
 Magnetron/klystron with low
power oscillator.

33. ACCERLATING
WAVE GUIDE
 Accelerating waveguides are special type of metal
tube in which electrons are accelerated.
 The electrons are accelerated in the accelerating
waveguide by means of an energy transfer from
the high power, which are set up in the waveguide
and are produced by the RF power generators
 Electrons experiences a zero voltage & + Ve to
right i.e. Charged particle is pulled to right &
accelerated.

34.  The simplest kind of accelerating waveguide is a
cylindrical uniform waveguide obtained by adding
a series of cu discs (irises) with circular holes at
the centre, placed at equal distances along the
tube.
 These discs divide the waveguide into a series of
cylindrical cavities that forms the basic structure
of the accelerating waveguide.
 Its length varies from 30 cm for 4 MV to 1 m for
higher energies.
 The whole system is vaccum tight .

35.  In the initial bunching section some e- get captured
& grouped together by e-m waves i.e e -are
captured if arrive while a region of positive charge
exists ahead of them &if fall too far behind will loss
to acc. process.
 Two types of waveguides are:
1. Travelling waveguide
2. Standing waveguide
Travelling wave guide – surf rider
Standing waveguide – strings of violin.

36. TRAVELLING STANDING
WAVE WAVE
 Microwaves enter on
gun side.
 Only one incident wave.
 In this irises are not equally
spaced
 Mw is absorbed in a resistive
load.
 No cavity can be moved out as
they provide E field in dir n of
propagation i.e.no side coupling.
 Less efficient & expensive.
 Microwaves can enter from
anywhere along accerlator
structure.
 Two waves incident & reflected
wave.
 Equal spacing & dimensions of
cavities.
 Acc. Structure terminated with
conducting disc for reflection.
 Cavity with no E field can moved
out to side i.e. side coupling.
 More efficient & expensive

37. Cutaway view of a standing wave accelerating waveguide for a 6
MV linac. The accelerating cavities are on the central axis; the
coupling cavities are off-side. The electron gun is on the left, the
target on the right, both permanently embedded.
electron
source

38. AUXILLARY SYSTEM
IT consist of :
 Pressure system= waveguide is filled
with sulphur hexaflouride gas
To prevent electrical breakdown from MW.
 WATER COOLING SYSTEM
 VACCUM SYSTEM
 AFC (AUTOMATIC FREQUENCY
CONTROL) SYSTEM

39. WATER COOLING
SYSTEM
 It cools various components that dissipate
heat energy and establishes a stable
temperature suitable for proper functioning
 Components cooled by
circulating water include
Accelerator structures ,klystron
Circulator, target.
 IN a 4 mv linac water flowing through
pipe around periphery of target.

40. The vacuum system
 The minimum vacuum condition required for the
for the operation of the system is determined by
the fact that the electrons being accelerated
should not be deflected by collisions with gas
atoms.
 The mean free path b/w collisions with gas atoms
needs to be long compared to the length of the
acc. Waveguide.
 This length is typically 1-3 m.
 The maximum gas pressure to meet this condition
is 10-5 torr ( where 1 torr = 1mm Hg).
 Nowadays ion pumps are used

41. CIRCULATOR
 Circulator prevents MW power reflected from
standing wave accelerator structure from
reaching Klystron where it can lead to instability
& damage.
 It is made up of a ferrite material i.e ceramic
made of iron.
 It allows the waveguide to pass through it in
only one direction which can be either the
direction of microwave

42. TREATMENT HEAD
.
1. x-ray target.
2. Flattening filter & scattering foil.
3. Collimators(primary and secondary).
4. Dual transmission ion chamber.
5. Field defining light & SSD.
6. Slots for wedges, blocks & e- cones.

43. Energy
Switch
e- gun
Waveguide
Bending Magnet
Focal Spot
Carrousel
Ion
Chamber
Jaws
MLCs

44. BENDING MAGNETS
 Bending magnets are either permanent or
electromagnets & are of mainly three types :
1. Achromatic bending
2. Chromatic bending
3. Slalom (112.50)
 Linac with horizontally mounted acc. Structure
requires bending magnets which forces e- into arcing
path towards target or scattering foil
 Degree of arc depends upon design of acc. & particular
requirements

45.  90 0 bending not suitable for
high energies e - to bend & is a
energy differentiator.
 As Lower energy travel arc of
smaller radius than high
energy & achromatic
magnets (2700) are used to
bring all e- to focus at required
target pt.
Advantages
1. Stable focal spot
2. Maximum output
Disadvantage
Bulky &require large head
 Slalom are achromatic but not
bulky

46. TARGET
 It is a high z material where e-s strike
resulting in the production of x-rays.
 Spatial dist of x-rays represent that they
are produced in same direction in which e
are bombarded.(transmission type)
 To maximise x- ray intensity transmission
targets are used.
Charactersics
 Few mm thick .
 Made of high z material as tungsten, gold
or platinum.
 Cooled by water system.

47. PRODUCTION OF X-RAYS
When e- interact with matter:
 Movement of atomic electron to optical
orbit.
 Ionization of atom.
 Characteristic x-ray emission.
 Continous spectrum(bremsstrahlung).
The last two processes results in
production of x-rays.

48. Bremsstrahlung:
 When a free e- with sufficient
K.E. rapidly de accelerate while
passing near the vicinity of nucleus,
t hen electron losses some of its
energy in the form of x-rays.
 The acc. of the free e- is inversely
proportional to the distance(impact
parameter) it is from the nucleus.
The energy of emitted photons
depends upon this acceleration.
 While the prob. of a brem.
Interaction increases linearly with
this distance.

49. FLATTENING FILTER
 These are conical metal
filters of low atomic no. as
Al & cu .
 Thick at centre &thin
at periphery.
 Different filters for different
energies.
 Used for flattening the
beam( for uniform dose distribution)
modified in spatial dist n by thick targets.

50. ELECTRON
SCATTERING FOIL
 Thin metal sheets made of steel, brass,
AL.
 Used for broadening electron beams
(scatter).
 Multiple foils are there for different beam
energies present on a round structure
called carousell.
 Improves the beam flatness for clinical
use.

51. SYMMETRY & FLATNESS
SYMMETRY REFERS TO the measure of the
intensity difference b/w the two opposite sides of
beam from its central axis.
Acceptable difference is 4% i.e. +- 2% on either side
of the centre.
 Symmetry is of two types TRN & RDL
 Due to assymetric beams non uniform fluence
results in hot & cold spots.
 FLATTNESS
is the constancy of intensity across the beam &is
measured in percent at aparticular depth in
phantom i.e. at 10 cm.
 Acceptable limit in most beam +- 2% with a
clinical useful limit of +- 3%.

52. MONITORING
CHAMBERS
 For dose monitoring ionisation chambers are used.
 Placed b/w flattening filter or scattering foil & secondary
collimators in treatment head.
 Radiation beam leaving target passes through ionisation
chambers &produces ionising currents proportional to beam
intensity.
 Two I.C are used with independent power supplies
for serving check on another as I.C measures mu if mu1 fails
mu 2 terminates the irradiation.
 chamber is adjusted as 1 MU = 1cGy
It also monitors energy of beam, symmetry& flattness
FIELD DEFINING SYSTEM CONSIST OF
 MIRROR
 LIGHT SOURCE

53. ELECTRON BEAM OBTAINED
FROM LINAC
 Acc. e- beam from exit window is a narrow
pencil beam of about 3mm diameter.
 Falls on scattering foil to give uniform
fluence of e- beam.
 Also x- ray contamination from scattering
foil & collimators occurs
PHOTON BEAM OBTAINED
Bremsstrahlung x- rays are produced whose
avg. energy is 1/3 of max. & is forward
peaked.

55. Non bending tube
Straight-through beam design; the electron gun and target are permanently
embedded into the accelerating waveguide; machine produces only x rays
with energies of 4–6 MV

56. Beam bending Linac - l
Accelerating waveguide is in the gantry parallel to the iso center axis;
electrons are brought to the movable target through a beam transport
system; the RF power generator is located in the gantry stand; machine
can produce megavoltage x rays as well as electrons.

57. Beam bending Linacs - II
Accelerating waveguide and RF -power generator are located in the gantry stand;
electrons are brought to the movable target through a beam transport system;
machine can produce megavoltage x rays as well as electrons

58. COLLIMATION SYSTEM
Collimation refers to limit the area of exposre to a
exposure to a specified value
 Primary collimators
 Secondary collimators
 MLC
 Fixed primary collimators located
beyond the target .
After ion chamber movable secondary x- ray
collimators
 Has two pairs made up of lead or
tungsten blocks (jaws)
 Rectangular field size 0 *0 to 40*40 cm at
ssd=100
 Assymetric jaws

59. MLC
 Multil eaf collimators system consist of two
coplanar sets of blades, each capable of mo
-ving independently by motorized system.
 Each set consist of 40 pairs & each blade presents
1 cm shadow at normal treating distance.
 MLC offer square or rectangular also any shape of
field is acheivable.
 Microprocessor controlled system.
 Useful in IMRT .i.e conformal treatment
method possible.
 Used in conjuction with jaws also.

60. ELECTRONIC PORTAL
IMAGING DEVICE
 Used for verification of iso - centre,
beam position & beam shape
 A flat panel si detector opposited to
gantry head converts x-ray information
to digital image on computer
MECHANISM
 X-RAY→VISIBLE LIGHT →e - (activate
pixels in si layer →DRR

62. TREATMENT
COUCH
 It supports the patient during treatment hence also
called pt. support assembly .
 It is controlled by a hand pendent and can be rotated
around iso- center.
 Table top provided with mobility.
 Couch can be moved up-down, in-out, left-right for
positioning of pt. during treatment .
 Also three lasers are there around it to specify the
iso-centre. i.e. two side & one mid- saggittal laser.

63. CONTROL CONSOLE
 Control &console is located outside
treatment room &consist of digital
display, push button panel, visual
display terminal i.e a electronic cabinet.
 For monitoring & controlling of:
1. Patient treatment information
2. All interlocks
 Emergency off button.

64. ADVANTAGES LINAC OVER
CO-60
 High dose rate (less time).
 Less penumbra.
 Varied photon & electron energies.
 No cost of source replacement
 More Build up depth.
 MLC & IMRT provision.

65. DRAWBACKS
 High cost
 Less reliable
 Output variation is there.
 Less life & maintaience cost is also high.
 Very complex circuitory .
But still Linacs are much suitable with new
technologies evolving.

66. THANKS