Brief discussion about the parts of LINAC and Other Generators.

1. CLINICAL GENERATORS
Dr.Avilash Banerjee
PGY1,Department of Radiation Oncology
Yashoda Superspeciality Hospital

2. CLINICAL GENERATORS
GENERATORS
LOW ENERGY GENERATORS( < 1MV) HIGH ENERGY GENERATORS(>1MV)
1.Van De Graff generator
2.Betatron
3.Cyclotron
4.Microtron
5.Radionucleotide
Machines(Eg.Co 60
Teletherapy Machine)
6.Linear Accelerator
NAME. ENERGY SSD USE.
GRENZ RAY: <20 kV VERY LESS
Treatment of psoriasis, eczema,
neurodermatitis
Contact
therapy:
40-50 kV 2 cm or less
Useful for tumours not deeper than 1
to 2 mm,Eg, superficial rectal cancers
Superficial
therapy:
50-150 kV 15 to 20 cm Tumours confined to about 5 mm
depth
Orthovoltage
therapy:
150 to 500 kV 50 cm For Tumours located upto 2 cm
depth
Supervoltage
therapy:
500 to 1000 kV 60 cm Technical problem of insulating the
high voltage transformers

3. LOW ENERGY MACHINES
GRENZ RAY THERAPY ORTHOVOLTAGE THERAPY SUPERFICIAL THERAPY

4. DEPTH DOSE CURVES OF VARIOUS MACHINES
• Depth-dose curves in water or soft tissues for various quality beams.
• Line a: Grenz rays, HVL = 0.04 mm Al, field diameter = 33 cm, SSD = 10 cm.
• Line b: Contact therapy, HVL = 1.5 mm Al, field diameter = 2.0 cm, SSD = 2
cm.
• Line c: Superficial therapy, HVL = 3.0 mm Al, field diameter = 3.6 cm, SSD =
20 cm.
• Line d: Orthovoltage, HVL = 2.0 mm Cu, field size = 10 × 10 cm, SSD = 50 cm.
• Line e: Cobalt-60 γrays, field size = 10 × 10 cm, SSD = 80 cm
HVL:A material's half-value layer (HVL), or half-value thickness, is the thickness of the material at which the
intensity of radiation entering it is reduced by one half.[1] HVL can also be expressed in terms of air kerma
rate (AKR), rather than intensity: the half-value layer is the thickness of specified material that, "attenuates
the beam of radiation to an extent such that the AKR is reduced to one-half of its original value. In this
definition the contribution of all scattered radiation, other than any [...] present initially in the beam
concerned, is deemed to be excluded."[2] Rather than AKR, measurements of air kerma, exposure, or
exposure rate can be used to determine half value layer, as long as it is given in the description.
kerma is an acronym for "kinetic energy released per unit mass",

5. MORDERN MEGAVOLTAGE MACHINES
• FOUNDATION OF MODERN RADIOTHERAPY.
• PHOTON BEAMS ARE PRODUCED.
• MAIN DIFFERENCE FROM ORTHOVOLTAGE MACHINES IN THE SKIN SPARING
EFFECT.
• MAXIMUM DOSE AT SUB EPIDERMAL LAYER.
• SKIN SPARING EFFECT IS DIRECTLY PROPORTIONAL TO ENERGY OF PHOTONS.

6. VAN DE GRAFF GENERATOR
• PRINCIPLE:
1.WHEN A CHARGED CONDUCTOR IS
BROUGHT IN INTERNAL CONTACT OF A
HOLLOW CONDUCTOR THE CHARGED
CONDUCTOR TRANSMITS ITS CHARGES
TO THE SURFACE OF THE HOLLOW
CONDUCTOOR.
2.THERE IS NO CHARGE INSIDE THE
HOLLOW CONDUCTOR

7. VANDE GRAFF GENERATOR
-20-40 kV applied across a moving belt of
insulating materialDischarge, e- sprayed onto
belt.
- e- are carried to top, removed by a collector
connected to a spherical dome. As negative
charges collect on sphere high potential
develops between sphere and ground.
- This potential is applied across x-ray tube
consisting of filament, series of metal rings
and target. Rings are connected to resistors to
provide a uniform potential drop
from bottom to top.
-Can reach up to 10 MV, limited by size and HV
insulation.

8. GENERAL PRINCIPLES OF PARTICLE ACCLERETORS
• A CHANGING MAGNETIC FIELD CAN ACCELARETE A CAHARGED PARTICLE
• A ELECTRICAL FIELD CAN ALSO ACCELARETE A CAHARGED PARTICLE
• AND IF THE CHARGED PARTICLE IS ELECTRON, A HIGH VELOCITY ELECTRON
CAN BE MADE TO HIT A TARGET ,IONISING RADIATION IS PRODUCED.
IF TARGET IS HIGH ATOMIC NUMBER ELEMENT X RAY IS PRODUCED

9. BETATRON
• PRINCIPLE:
A CAHRGED PARTICLE IS MADE TO
ACCELARATE IN A CHANGING
PERPENDICULAR MAGNETIIC
FILED IN A CIRCULAR TUBE OF
CONSTANT RADIUS.
OUT SIDE VACCUM CHAMBER IS
THERE.

10. BETATRON
• 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 (Faraday's law).
• Accelerating tube shaped like hollow doughnut, placed btn poles of magnet.
• e- pulse injected into doughnut and spin w/ increasing velocity inside tube.
• e- achieve max energy and are deflected out of orbit, striking target and
producing x-rays.
• Provides e- beams from 6 to >40 MeV, but low field size capabilities and x-ray
dose rates compared to linacs.

11. MICROTRON
• A type of particle accelerator
concept originating from the
cyclotron in which the
accelerating field is not applied
through large D-shaped
electrodes, but through a linear
accelerator structure.
• Electron acclerator that
combines the principles of both
linear acclerator and cyclotron
Animated Video of
Microtron

12. MICROTRON
• e- are accelerated by oscillating electric field of single
microwave cavity.
• Magnetic field constrains e- to circular motion, and
return to cavity.
• Repeated passes through cavity mean higher e- energy.
• Cavity voltage, frequency and magnetic field are
adjusted so e- are in phase.
• e- are deflected and extracted, deflection tube moves
to correct orbit for desired e- energy.

13. RACE TRACK MICROTRON
• Racetrack microtron is a larger-
scale microtron which uses two
electromagnets instead of one.
Both electromagnets supply a
homogeneous magnetic field in
a half-circle formed region, and
the particles path between both
magnets is thus straight.

14. Cyclotron
• Principle
Consists of two metallic Ds.
1.Small electric field present
between two Ds.
2.Large Perpendicular magnetic
field exists in both the metal Ds.

15. CYCLOTRON
• Dees are highly evacuated, and
placed between poles of
magnet, producing constant
magnetic field.
• Alternating potential is applied
between the Dees.
• Positive particles injected at
center of Ds, travel in circular
orbit.
• Can achieve E~30 MeV
Animated Video of
Cyclotron

16. Limitation of Cyclotron
• can not acclarate uncharged
particles eg.neutron.
• can not accelarate electrons.

17. Linear Accelerators (LINAC)
• BASIC PRINCIPLE
High frequency electromagnetic
field accelaretes the charged
particle(eg.electron) in linear
fashion.
Trajectory of the particle is linear
instead of a cyclical fashion(like
cyclotron) in LINACS.

18. Principle of Linear Particle Accelertaor
When a charged particle is placed in an electromagnetic field it
experiences a force given by the Lorentz force law:
q is charge on the particle,v is velocity,B is magnetic field,E is Electrical field.
Most accelerators use some form of radiofrequency (RF) acceleration. In RF acceleration, the
particle traverses a series of accelerating regions, driven by a source of voltage in such a way
that the particle sees an accelerating field as it crosses each region

19. Principles of LINAC
• They accelerate the electrons by using either
traveling or stationary electromagnetic waves
of frequency in the microwave region.
TRAVELLING WAVE
• A traveling wave is a type of wave that
moves or propagates through a medium.
• Combination of forward and reverse
traveling waves giving rise to stationary waves.
STATIONARY WAVE
Animation of Standing and
Travelling wave

20. HISTORY OF LINACS
• 1ST GENERATION LINACS
• 1952: Hammersmith
Hospital,LONDON 1st LINAC was
installed.
• 1957: 1st Patient was treated
with LINAC
• Low energy <8 MV,Bulky,Limited
Gantry Movements. Historical image showing Gordon Isaacs, the first
patient treated for retinoblastoma with linear
accelerator radiation therapy (in this case an
electron beam), in 1957

21. HISTORY OF LINAC:2nd Gen LINACs
• Gantry can rotate 360 degrees.
• Was in action for 1962-1982.
• Improved precision and
accuracy.
• 1962: Kaplan and Saul used
LINAC for HODGKIN
LYMPHOMA.

22. History of LINAC:3rd Gen LINACS
• Better Accelarator Waveguide.
• Bending Magnets
• More Beam modifying devices.
• Wider range of beam energy,dose
rates,more modes.
• 4TH GEN LINACS: MLCs and
Electronic Portal Imaging.
• 5TH GEN LINACS full dynamic
conformal dose delivery.

23. PARTS OF LINAC
• 1.ELECTRON INJECTION SYSTEM.
• 2.MICROWAVE GENERATION SYSTEM
• 3.MODULATR POWER SUPPLY SYSTEM.
• 4.BEAM TRANSPORT AND MONITORING
SYSTEM.
• 5.AUXILLARY SYSTEM.
• 6.SAFETY INTERLOCK SYSTEM.
• 7.COMPUTER CONTROLLED FEEDBACK
SYSTEM.
• 8.BEAM COLLIMATOR AND APPLICATION
SYSTEM
• 9.COOLING SYSTEM.
• 10.CONTROL CONSOLE SYSTEM.

24. MEDICAL LINAC PARTS

25. 1.MODULATOR and POWER SUPPLY
• MODULATOR has 3 components.
1.Fan Control(Cool Power
Distribution units)
2.Auxillary Power distribution
system(Emergency Off Switch)
3.Primary Power distribution
system
• POWER SUPPLY
Provides DC current to modulator which throws
high voltage pulses to Klystron or Magnetron.

26. 2.Electron Injection System
• Mechanism:Thermoionic
Emission(Emission of charged
particle/Thermion from a surface of the
heated metal)
• In LINAC charged particle is Electron.
TWO TYPES of electron gun:
1.Diode Type ( Conducting current in one
direction,Rectifier present)
2.Triode Type (functions as amplifier,no
rectifier)

27. 3.Radio Frequency Power Generator
MAGNETRON KLYSTRON
Radiofrequency generator
High power oscilator,generates microwaves upto
3000 Hz.
Not a RF generator,rather AMPLIFIER,Driven by low
power microwave oscilator
1.First Electrons are accelarated in Buncher Cavity.Cavity
is energised by a low power microwave
2.Catcher Cavity Due to high electricl field the electrons
with high KE reach catcher cavity where it is converted
to RF
Peak Power upto 5 MW Peak Power 7MW or more( Used in high energy LINACS)
Small No requirement of RF driver Bulky Require RF Driver
Utilises lower DC voltage Utiises Higher DC voltage
can be mounted in rotating gantry. must be mounted within a tank of insulating oil. Placed
in gantry stand.

28. MAGNETRON KLYSTRON

29. 3.Wave Guide System
• Why do we need wave guide?
To focus the pathway of electron beam
• PLACEMENT
1.Horizontal(High Energy Machines)
2.Vertical (Low Energy Machine)
• Can be two types
1.Radiofrequency Transmission Wave Guide
2.Accelerating Wave Guide(Used for electrons)

30. ACCELERATING WAVE GUIDE
• Cylindrical Uniform Wave
guide,containing series of copper
disc/irises with central hollow.
• Discs are placed at equal distance
from each other which divide them
to multiple cylindrical cavity.
• accelerating electrons tend to
repulse each other due to coloumb
forces,which makes them diverge
from each other for which is
corrected by CO AXIAL MAGNETIC
FOCUSING FIELD produced by CO
AXIAL COILS.
• STERLING COILS: Coils keep the
pencil beam narrow.

31. TYPES OF WAVE GUIDES
• STANDING WAVE GUIDE
• Short Length
• Ends at a conductor disc,which
provides reflection,Both side this
will provide maximum deflection
causing wave to be stationary.
• Require a microwave peak
power of 2.5 MW
• TRAVELLING WAVE GUIDE
• Longer in length
• Require a terminating load to
absorb residual energy which
prevents backward reflection.
• Require microwave peak power
of 2MW

32. TREATMENT HEAD
• Bending Magnet.
• Shielding material.
• X Ray Target.
• Primary Collimator.
• Beam flattening filter
• Scattering Foil
• Beam monitoring device
• Secondary Collimator
• Field Light

33. Bending Magnet
• Changes the direction of the beam downwards to
isocentre.
• Needed only when the energy of the beam is more
than 6 MeV
• Three types:
1.90 degree: Simplest,Higher energy beans are bent
more,eliptical focal spot.
2.270 degrees( Achromatic):Achromatic electron
beam,refocuses spectral spread,BULKY.
3.112.5 degree( Salom):Combines usefulness of both 90
and 270 degree,Two 45 degree and one 112.5 degree at
end combines a total of 202.5 degrees,Less Vertical
space needed.
Electrons after exiting wave guide enter FLAY TUBE
where three pair of magnet bends the beam towards
isocentre and also focuses to diameter of 1 mm.

34. BEAM COLLIMATION and MONITORING SYSTM
• Shielding Material:Thick shell of high density material like
Lead, Tungsten or Lead-Tungsten Alloy.
• Target: Transmission Type target,High Atomic number and
High Melting Point(Z 74, 3370 degree MP in Tungsten)
• Primary Collimator
• Flattening Filter, Scattering Foils
• Dual Ion Chamber
• Secondary Collimator
• Multileaf Collimator
• Wedges

35. • PRIMARY
COLLIMATORS
1. Located
immediately below
X Ray Target
2. Fixed
• FLATTENING FILTER
• Makes narrow noon
uniform photon
beam to clinically
useful beam.
• Attenuation at centre
and scattering at
periphery.
• Lead ,steel or copper.

36. • Scattering Foil:
1. In electron therapy of LINAC.
2. Instead of hitting the target directly the 3
mm beam first hits the scattering foil
3. this scattering foil spreads the beam
uniformly across the field.
• Dual Ion Chamber
1.aka Beam monitoring
device/chamber,monitors and measures the
Beam.
2.Transmission type,flat parallel type plate type
ionisation chamber.
3.1st Chamber:1st Dosimeter,It can measure
and stop radiation if needed.
4.Second chamber is backup,when 1st chamber
fails.

37. • Secondary Collimator
1. 4 blocks,2 upper,2 lower( X/lower and Y /upper axis)
2. Provide Rectangular or Square(upto 40 x 40 cm2) field at LINAC isocentre.
3. Movable
4. Lead or Tungsten made.
Tertiary Collimators/MLCs(Multi leaf
Collimator):
A multileaf collimator (MLC) is a Collimator or beam-limiting device that is
made of individual "leaves" of a high atomic numbered material, usually
tungsten, that can move independently in and out of the path of a radiotherapy
beam in order to shape it and vary its intensity.
MLCs are used in external beam radiotherapy to provide conformal shaping of
beams.
120 Millenium MLC:60 pairs/120 number of leaves.( middle 40 of 0.5 cm and
outer 10 on both side of 1cm)

38. • Light Localising System
Field light source and a mirror to deliniate the field
margins and borders,Field Light borders will be
congruent with Field borders.
High Efficiency Bulb placed at 45 degree with a
mercury mirror system.
• LASER
Laser system to pin point ISO CENTRE.(where Gantry
Couch and Collimator axis meet).
2 Side lasers(X Axis) and 1 coronal(Y and Z both axis
covered) laser is used.

39. • TREATMENT COUCH
• Mechanically moving motorised couch.
• Pt is positioned according to desired co
ordinates.
• Immobilised using immobilisers.
• 4D couch:Can move in lateral vertical
and longitudinal axis.
• 6D:horizontal ,vertical, longitudinal,Yaw
,Roll,and PITCH
HAND PENDENT:
• Hand Control for couch collimator
gantry movement

41. Auxillary System
• VACCUM PUMPING SYSTEM (Essential for the electron acceleration,Reduce
collison with Gas molecules)
• Cooling System: (Water with fixed temperature 1 degree C and fixed flow
cools RF Generator,Beam Transport System,Target)
• High Pressure Gas System: ( Between Magnetron and Wave Guide filled with
gas like N2,Freon,SF6 in high pressure for cooling and reduce spark)
Radiation Safety/Interlock System
1. LAST MAN OUT SWITCH(LMO)
2. BEAM INTERLOCK
3. ON/OFF SWITCH
4. EMERGENCY OFF SWITCH

42. • EPID(ELECTRONIC PORTAL
IMAGING)
1. Viewing portal images instanteniously.
2. Real Time images before initiating
treatment.
3. Can be saved in database for future.
4. Flat pannel arrays of solid state drive
based on amorphus silicon technology.
5. Photodiodes detect the photos and display
in System.
• CBCT
• detectors embeded in flat panel instead of circular ring
like diagnostic CTs.
• Uses CONE BEAM,so aka CBCT Cone Beam CT.
• Flat Beams are used in Tomotherapy and Diagnostic CTs
• Cone Beam Computed Tomography (CBCT) is a
variation of traditional computed tomography, Unlike a
CT scanner, it uses an X-ray tube and detector panel
which rotates around the patient capturing data in
cone-shaped X-ray beam instead of slices as captured
by Computed Tomography (CT)

43. KV Imaging
• Conventional X Ray source attached at aretractable arm of newer LINACS.
• Separate source for beam generation.
• Flat panel of XRay detectors on opposite side attached.
• Versetile Imaging, can take
1.CBCT
2.2D Imaging
3.Flouroscopy
• Produces good quality clear images with great resolution.
• can be used for patient positioning,motion management,dosimetric adjustments
before treatment.

44. Megavoltage CBCT
• Uses the megavoltage XRay beam of LINAC. EPID is mounted
in opposite side.
• Good Image quality for bony landmarks.
• Useful for patient position verification.
• Identification of implanted metal markers can be done.

45. BIBLIOGRAPHY
• 1.Fundamental Physics of Radiology -
Meredith
• 2.Khan's the Physics of Radiation Therapy
by Faiz M Khan and John P. Gibbons.
• 3.Johns and Cunningham's the Physics of
Radiology
• 4.https://oncologymedicalphysics.com
• 5.https://www.wikipedia.org
• 6.https://www.researchgate.net