The document provides information on the physics and operation of medical linear accelerators. It discusses the history and development of linear accelerators from 1st to 5th generation machines. The key components of a modern linac are described, including the electron injection system, RF power generation using klystrons or magnetrons, the accelerating waveguide, electron beam transport using bending magnets, and beam collimation and monitoring systems using components like flattening filters, ionization chambers, and multileaf collimators. Modern linacs can accelerate electrons to energies over 20 MeV and are able to treat deep-seated tumors with high precision using computer-controlled systems.

1. PHYSICS AND OPERATION
OF MEDICAL
LINEAR ACCELERATOR
1
SAILAKSHMI . P
Medical Physicist
Department of Radiation Oncology
Medanta -The Medicity , Gurgaon-122001

2. HISTORY OF MEDICAL APPLICATIONS OF
ACCELERATORS
 1895 W. Conrad Rontgen(1845-1923) discovers the X-
Ray on 8th November.
 1896 On 23rd January Rontgen announced his discovery
and demonstrated the new kind of radiation by
a photograph of the hand of his wife.
 1897 First treatment of tissue with X-Ray is done by
Leopold Freund at University in Vienna.
 1901 W. C. Rontgen got Nobel prize in Physics.
2

3. HISTORY OF MEDICAL APPLICATIONS OF
ACCELERATORS
X Ray Tube
 The Coolidge Tube, is the first produced
X ray tube by William Coolidge in 1913.
 It is the forerunner of all the types of x-
ray tubes we are using today
 It works based on the principle of
thermionic emission.
3

4. HISTORY OF MEDICAL APPLICATIONS OF
ACCELERATORS
4
S.No X Ray therapy in kV Range Voltage range
1 Grenz-ray Therapy < 20kV
2 Contact Therapy
(The Quality of radiation is useful for tumors not deeper than 1 to
2mm)
40 to 50 kV
3 Superficial Therapy
(useful for irradiating tumors confined to about 5mm depth) 50 to 150 kV
4 Ortho voltage or Deep Therapy
(useful for irradiating tumors confined to about 2 to 3cm depth) 200 to 300 kV
5 Super voltage therapy
(Suitable for deep seated tumors )
500 kV to 1MV

5. VAN DE GRAAFF GENERATOR
 The Van de Graff machine is an electrostatic
accelerator designed to accelerate charged
particles up to 10 MV.
 In radiotherapy, the unit accelerates electrons
to produce high energy x rays, typically at
2MV.
 High energy production is limited by its size
and it required high voltage insulation.
5

6. PARTICLE ACCELERATORS
6
CYCLIC ACCELERATORS
LINEAR PARTICLE
ACCELERATORS
Cyclotron Betatron
Drift tube
LINAC
Wave guide
LINAC

7. 7
BETATRON CYCLOTRON

8. WHAT IS LINEAR ACCELERATOR?
A linear particle accelerator is a
device that uses high frequency
electromagnetic waves to
accelerate charged particles such
as electrons to high energies
through linear tube.
8

9. 1ST GENERATION ACCELERATORS
 The first one was installed at
Hammersmith Hospital in London in
1952.
 In 1956, the first patient was treated
at Stanford university in the united
states.
 The linac had low energy photons up
to 8MV with limited gantry motions.
 These linac were large and bulky.
9

10. 2nd GENERATIONACCELERATORS
 The second generation were isocentric
units, which can rotate 360⁰ around the
gantry axis.
 They were built between 1962 and 1982.
 They improved in precision and accuracy
of dose delivery.
10

11. 3rd GENERATIONACCELERATORS
 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.
11

12. 4th and 5th GENERATIONACCELERATORS
 4th generation linear accelerators is having
computer controlled operation , dynamic
wedges, electronic portal imaging and multi
leaf collimator(MLC).
 5th generation linac having photon beam
intensity modulation with MLC , full dynamic
conformal dose delivery and compatible for
Flattening filter free beam.
12

13. HOW DOES ITWORK ?
 The linear accelerator uses microwave technology to accelerate electrons in a part of the
accelerator called the "wave guide", then allows these electrons to collide with a heavy
metal target. As a result of the collisions, high-energy photons are produced from the
target.
 The high energy x-rays will be directed to the patient’s tumor and shaped as they exit
the machine to conform to the shape of the patient’s tumor. Radiation can be delivered
to the tumor from any angle by rotating the gantry and moving the treatment couch.
13

14. 14

15. The Main Beam Forming Components Of A Medical
Linac Usually Grouped Into 6 Classes :
1. Injection system.
2. RF power generation
system.
3. Accelerating wave guide
4. Beam transport system.
5. Beam collimation and
Monitoring system.
6. Auxiliary system.
15

16. 1. INJECTION SYSTEM
ELECTRON GUN
The electron beam originates at the cathode of
electron gun, by the process of Thermionic
emission.
THERMIONIC EMISSION
Thermionic emission is a process of emission of
charged particle (known as thermion) from the
surface of a heated metal.
Here the charged particle normally are electrons.
16

17. 1. INJECTION SYSTEM
ELECTRON GUN
There are two basic types of electron guns
exist :
 Diode type Guns
 Triode type Guns
17
A triode gun from Varian linac

18. 1. INJECTION SYSTEM
18

19. 2. RF POWER GENERATION SYSTEM
 The electrons are accelerated in the
accelerating wave guide using high power RF
fields. Which are set up in the accelerating
wave guide by microwave radiation.
 This radiation is produced by microwave
generators that are either Magnetrons or
Klystrons.
19

20. 2. RF POWER GENERATION SYSTEM
MAGNETRON
 It produces microwave required for
electron acceleration.
 Function as a high frequency
oscillator.
 Peak power up to 5 MW can be
produced by magnetrons.
20

21. 2. RF POWER GENERATION SYSTEM
MAGNETRON-working
21
 The electrons are emitted from the
cathode by thermionic emission.
 Produced electrons are accelerated
towards the anode by a pulsed electric
field. Under the simultaneous influences of
electric and magnetic field.
 The electrons moves in a complex spiral
towards the resonant cavities, radiating
energy in the form of microwave.

22. 2. RF POWER GENERATION SYSTEM
KLYSTRON
 It is not a generator of microwaves but acts as
a RF power amplifier .
 Driven by a low power microwave oscillator.
 Peak power on the order of 7MW or higher.
 Mainly using in high energy linacs.
22

23. 2. RF POWER GENERATION SYSTEM
KLYSTRON-working
 The electrons produced from the cathode are
accelerated to the first cavity, which is called as
buncher cavity.
 The cavity is energized by a low power microwave.
 The bunches of accelerated electrons reaches the
catcher cavity due to the electric field variation.
 When the electron bunches arrive at the catcher
cavity , It attains high kinetic energy(K.E) and this
K.E converted into high-power microwave.
23

24. MAGNETRON Vs KLYSTRON
 Smaller in size
 Do not require RF driver
 Utilize lower DC voltage
 Operates at lower peak power(3MW)
 Easier to manufacture
 Can be mounted in the rotating
gantry itself
 Lower price
 Less stable
 Bulkier in size
 Requires RF driver
 Utilize high DC voltage
 Operates at higher peak
power(5MW or more)
 Must be mounted within a tank of
insulating oil
 Complicated structure makes them
to place in the gantry stand
24

25. 25

26. 3. ACCELERATING WAVEGUIDE
 Waveguides are evacuated or gas filled metallic
structures.
 Two types of waveguides are used in linac :
 Radio frequency power transmission wave guide
(usually gas filled).
 Accelerating wave guides (usually evacuated).
 For electron transmission Accelerating waveguides
are used.
26

27. 3. ACCELERATING WAVEGUIDE
BASIC STRUCTURE
 Cylindrical uniform waveguide by
adding series of copper discs(irises)
with circular holes at the center.
 Discs are placed at equal distance along
the tube.
 Discs divide the waveguide into series
of cylindrical cavities.
27

28. 3. ACCELERATING WAVEGUIDE
WORKING
28

29. 3. ACCELERATING WAVEGUIDE
WORKING
29

30. 3. ACCELERATING WAVEGUIDE
 There are two types of accelerating wave
guides have been developed for
acceleration of electron:
 Standing or stationary wave
accelerator.
 Traveling wave accelerator.
30

31. 3. ACCELERATING WAVEGUIDE
TRAVELLING WAVE GUIDE
 Microwaves enters the waveguide on
the gun side
 Propagate at the high energy end of
the waveguide
 At the end of the waveguide either
they are absorbed without any
reflection
 Or fed back to the input end.
31

32. 3. ACCELERATING WAVEGUIDE
STANDING WAVE GUIDE
 If each end of an iris loaded waveguide
is terminated with an conducting disc,
microwave power will reflected at
either end and standing wave can
build up in the system.
32

33. TRAVELLING WAVEGUIDE Vs STANDING WAVEGUIDE
 Waveguide length will be greater.
 At the end of the waveguide
microwaves are absorbed without
any reflection or fed back to the
input.
 It requires low microwave peak
power (eg: 2MW).
 Requires lower mean RF power.
 Side coupling will reduce the
waveguide length.
 At the end of the waveguide
microwaves are reflected back to
the input.
 It requires high microwave peak
power than the travelling
waveguide (eg: 2.5MW).
 Requires higher mean RF
power(25% more)
33
Here, Neither type of the two accelerating structure has a clear advantage over other.

34. 34

35. 4. ELECTRON BEAM TRANSPORT
 It consists of the evacuated drift tubes
and bending magnets, which are used in
transporting the electron beam from the
accelerating waveguide to the x ray target
or to the exit window for electron beam
therapy.
 Steering and focusing coils installed on
the accelerating wave guide are usually
linked with the electron transport system.
35

36. 36

37. 4. ELECTRON BEAM TRANSPORT
STEERING COILS
 The steering coils keep the
accelerated electron pencil beam as
close as possible to the axis of the
cylindrical accelerating waveguide.
 And it will steer the beam toward the
opening which connects the
accelerating waveguide to the target.
37

38. 4. ELECTRON BEAM TRANSPORT
FOCUSING COILS
 Focusing coil is to focus the accelerated
pencil beam in order to minimize the
beam divergence and cross section.
 Divergence results from a small radial
component of the electric field in the
accelerating waveguide and from the
repulsion among electrons in the pencil
beam.
 The focusing solenoid coils are coaxial
with the accelerating waveguide
38

39. BENDING MAGNET
 Changes the direction of electron beam,
downward toward the isocentre.
 Needs for energy greater than 6MeV.
 There are three types of bending magnets;
 90° bending
 270° bending (achromatic)
 112.5° bending (slalom)
39
4. ELECTRON BEAM TRANSPORT

40. Three systems for electron beam bending
90° bending 270° bending 112.5° bending
( achromatic) (slalom)
40
4. ELECTRON BEAM TRANSPORT

41. BENDING MAGNET
90° BENDING
 It is the simplest of the three.
 Higher energy electrons in the electron
beam spectrum are bent less than the lower
energy electrons.
 So the focal spot on the target becomes
elongated in the longitudinal direction
resulting in an elliptical shape.
41

42. BENDING MAGNET
270° BENDING
 Here the electron bending is achromatic.
 It refocuses the electron spectral spread
and directional spread.
 Provides a small focal spot on the target.
 They are bulky
42

43. BENDING MAGNET
112.5° BENDING
 Slalom system offers the advantages of both the
90° and 270° bending systems.
 It is achromatic and requires no more vertical
space than 90° bending.
 We can achieve this with two 45° bending
magnets in opposite directions and final 112.5 °
bending magnet , total bending of 202.5°.
43

44. 44

45. 5. BEAM COLLIMATION AND MONITORING
SYSTEM
Target
Primary collimator
 Flattening filter or scattering foils
Dual Ion chamber
Secondary collimator
Multi leaf collimator
Wedges
45

46. 5. BEAM COLLIMATION AND
MONITORING SYSTEM
TARGET
 When the accelerated electrons strike the
target it will undergo three types of
interactions:
 Collisional losses
 Radiative (bremsstrahlung) losses
 Scattering
46

47. 5. BEAM COLLIMATION AND
MONITORING SYSTEM
 There are 3 types of target :
 Thin
 Intermediate and
 Thick.
47
TARGET

48. 5. BEAM COLLIMATION AND
MONITORING SYSTEM
 The target thickness is related to the practical
range of a electron beam in the target material.
 In linac we use thick targets .
 Efficiency for photon production in thick target
is proportional to the atomic number (Z) of the
target material.
 In linacs high Z targets (eg :Tungsten) would
produce beams with highest efficiency.
48
TARGET

49. 5. BEAM COLLIMATION AND
MONITORING SYSTEM
 It defines the largest available circular field size
and is essentially a conical opening projecting
into a tungsten shielding block.
 One end of the conical opening of the collimator
projecting on to edges of the target and the
other to the flattening filter.
 It is designed to attenuate the primary x-ray
beam intensity to less than 0.1% of the initial
value
49
PRIMARY COLLIMATOR

50. 5. BEAM COLLIMATION AND
MONITORING SYSTEM
 The photon dose distribution produced by a linac is
strongly forward peaked.
 To make the beam intensity uniform across the field, a
flattening filter is inserted in the beam.
 Which attenuate the central portion of the beams to level
equal to those on the periphery some 20cm off axis.
 The filter is usually made of Pb, although tungsten,
uranium, steel , aluminum or a combination has also
suggested.
50
FLATTENING FILTER

51. 5. BEAM COLLIMATION AND
MONITORING SYSTEM
 In the electron mode of linac operation , the
beam 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.
 It consist of a thin metallic foil, usually of lead.
51
SCATTERING FOILS

52. 5. BEAM COLLIMATION AND
MONITORING SYSTEM
 The dose delivered to the patient is measured and
controlled by the monitoring system i.e., ionization
chambers .
 The chambers are usually transmission type. We are
using flat parallel plate type ionization chambers .
 Here the first chamber is the Primary dosimeter, it
measure and stops the radiation when the required
dose is delivered.
 The other chamber is the backup one it stops the
radiation when primary chambers fails.
52
DUAL IONIZATION CHAMBER

53. 5. BEAM COLLIMATION AND
MONITORING SYSTEM
 It consist of four blocks, two forming the upper
and two forming the lower.
 Provide rectangular or square field at the linac
isocenter.
 This collimators are able to rotate about their axis
and this degree of freedom is referred to as
collimator rotation.
 Usually made of lead or tungsten.
53
SECONDARY COLLIMATOR

54. 5. BEAM COLLIMATION AND
MONITORING SYSTEM
54
SECONDARY COLLIMATOR

55. 5. BEAM COLLIMATION AND
MONITORING SYSTEM
 Using fine tungsten leaves we can conform the
treatment fields to the tumor volume.
 The number of leaves in the commercial MLC’s are
steadily increasing.
 leaves: 82, 120,160 & width: 1cm and 1.5 to 6mm
is currently available.
 Each leaves is controlled by computer controlled
motors.
55
MULTILEAF COLLIMATOR

56. 5. BEAM COLLIMATION AND
MONITORING SYSTEM
56
MULTILEAF COLLIMATOR
VarianSiemensElekta

57. COLLIMATOR PARAMETERS OF OUR MACHINES
57
SYNERGY INFINITY TOMOTHERAPY
Leaf width (cm) 0.4cm 1cm 0.625cm
Number of leaf 40pairs 40pairs 64 (binary leaf)
Maximum field size
(cm x cm)
16x21 40x40 5x40
Central Axis
overtravel
Complete -12.5cm NA

58. 5. BEAM COLLIMATION AND
MONITORING SYSTEM
58
WEDGES
These are the beam modifying devices.
It causes a progressive decrease in the intensity
across the beam, resulting in a tilt of the isodose
curves from their normal positions.
Usually made of dense material such as lead or steel.
Mainly there are 3 types of wedges;
Physical wedges
Motorized wedges
Dynamic wedges

59. The Main Operating Components Of A Medical Linac
Usually Grouped Into 4 Classes :
1. Gantry
2. Modulator Cabinet
3. Patient support assembly
4. Control console
59

60. MODULATOR CABINET
 This is the noisiest of the linac system
components and is located inside the
treatment room.
 The primary function of the modulator is to
supply high voltage pulses to the cathode of
the microwave generator valve and electron
gun.
 And it contains the emergency off button that
shuts the power to the linac.
60

61. THE GANTRY
 The Drum mounting: In this gantry arm is carried
on a cylindrical or drum shaped structure.
 The Pendulum mounting: Here the weight is
carried on a vertical stand which is fixed to a
frame which is bolted down onto earth.
The arm is carried on a main support which is
free to swing as a pendulum.
And this pendulum is mounted on this vertical
stand by a slewing ring
61

62. TREATMENT COUCH
62
 The treatment couch is the area on which patient’s
are positioned to receive their radiation treatment,
and it has the ability to move .
 up+/- down (Z)
 Right+ /-left (X)
 In+/-out (Y)
 Hexapod ( 6 degrees of motions are available)
 The linac couch 6D displacements
are vertical, longitudinal, lateral, yaw, roll and pitch
Pitch- X Roll - Y, Yaw – Z,

63. 63

64. 6. AUXILLARY SYSTEMS
The linac auxiliary system comprises four systems:
 Vacuum pumping system
 Water cooling system
 High Pressure gas system
 Air pressure system(optional)
64

65. 6. AUXILLARY SYSTEMS
VACUUM PUMPING SYSTEM
 Vacuum condition is one of the major requirement of the linac.
 It is determined by the fact that electrons being accelerated should not be
deflected by collisions with the gas atoms.
 So the vacuum will make a free path for the acceleration of the electron.
 Producing a vacuum pressure of ~10-6 tor in the accelerating guide and in the
RF generator using ion pumps.
65

66. 6. AUXILLARY SYSTEMS
WATER COOLING SYSTEM
 Used for cooling the accelerating wave guide, beam
transport system ,target and RF generator.
 For these components cooling is necessary in order
to maintain precise temperature control for
stability of operation.
 Cooling is provided with water of fixed flow rate
and temperature(~1°C).
66

67. 6.AUXILLARY SYSTEMS
HIGH PRESSURE GAS SYSTEM (ISOLATER)
 The transition section, between magnetron and transmission wave guide
needs to be gas filled (Nitrogen, freon or SF6)to provide gas cooling and
operated at high pressure to prevent sparking.
 The external microwave load at the higher energy end of the accelerating
waveguide also need to be gas filled.
 Both the gas filled section of microwave system are separated from the
evacuated sections.
67

68. OTHER MODERN LINACS
CYBERKNIFE TOMOTHERAPY
68

69. 69
Stanford linear accelerator to the modern miniature form of linacs

70. MRI LINAC
70

71. BRACHYTHERAPY WITH MINIATURE
ELECTRONIC X-RAY SOURCE
 Electronic brachytherapy(eBx) applies
interstitial irradiation without
radionuclides.
 Here, one miniature x ray tube inserted
into a flexible cooling catheter
71
X-Ray Tube size
X-Ray sourceIn Vivo Target

72. SUMMARY
 Linear accelerators are technically advanced and sophisticated equipment.
 Optimal and proper use of the such equipment is highly essential.
 As linear accelerators are being used for the healthcare purposes, optimal and safe use of
linear accelerator is even more crucial.
 Hence, in order to ensure safe use of linear accelerator, one need to perform all adequate and
necessary quality assurance tests.
 Daily, weekly monthly and yearly QA and radiation survey is mandatory for the safe practice
of linear accelerator in radiation oncology.
 At our center, international protocols (TG-40, TG-142 and TRS-398) are being followed to
ensure Mechanical, dosimetrical and geometric accuracy of the linear accelerator.
72

73. 73