1. STRUCTURE AND FUNCTION OF MEDICAL
LINEAR ACCELERATOR

 Presenter:- DR ABDUL WAHEED
 post graduate

2. LINAC
The linear accelerator (linac) is a device that uses high-
frequency electromagnetic waves to accelerate charged
particles such as electrons to high energies through a
linear mettalic tube.
The high-energy electron beam itself can be used for
treating superficial tumors, or it can be made to strike a
target to produce x-rays for treating deep-seated tumors.
There are several types of linear accelerator designs, but
the ones used in radiation therapy accelerate electrons
either by traveling or stationary electromagnetic waves of
frequency in the microwave region (~3,000
megacycles/sec).

3. HISTROY
The 1st
linear accelerator was developed by Wideroe in 1928 to
accelerate heavy ions
Electron linear accelerator were 1st
developed during late 1940s &
early 1950s by Fry, Ginzton & Chodorow.
In 1950 1st
linear accelerator designed for radiotherapy was
installed at Hammersmith hospital, London

4. NEED FOR LINACNEED FOR LINAC
o Higher energy photon beams.
o High dose rate.
o Electron therapy possible with linac.
o Multiple energies
- Photon : 6, 15 ; 6, 18 ; 6, 10, 15 ; 4, 15 MV
- Electron : 6, 9, 12, 16, 20 MeV.
o No need to change source.
o Modern RT possible
o No radiation leakage when the machine is off

5. During the past 40 yr. medical Linacs have gone through distinct generations
Low energy LINAC < 10MV Modern linac
 straight through beams
 fixed flattening filter
 external wedges
 symmetric jaws
 single transmission
ionization chamber
 isocentric mounting
• dual photon energy & multiple e-
energies
• achromatic bending magnet dual
• scattering foil or scanned e-
pencil
beams
• motorized wedge
• asymmetric or independent
collimator jaw.
• computer controlled operation
• dynamic wedge
• electronic portal imaging device
(EPID)
• MLC
• On board KV Imager
– full dynamic conformal dose
delivery.

6. PRINCIPLE
The basic principle involved is that e-
is injected into
beam of micro waves at an appropriate place & time.
Hence e-
will be acted upon by force applied by electric
field & carried along by the wave with increased velocity.

7. WORKING PRINCIPLE
A power supply provides A.C. power to Modulator that includes pulse
forming network.
High voltage pulses from Modulator section are DC pulses of a few
microseconds in duration & are delivered to Magnetron or Klystron &
simultaneously to e-
gun.
Klystron / Magnetron produce pulsed MW that are injected into accelerator
tube via waveguide.
At a proper instant e-
s produced by an e-
gun are pulse injected into
accelerator st. with an initial energy of about 50KeV.
Electrons are acted upon by e.m. field of microwaves & are accelerated by
force of electric field & are carried along MW.
High energy e-
emerges from exit window of accelerator st. in form of pencil
beam about 3mm. in diameter.

8. LINAC COMPONENTS
Linacs are isocentrically mounted.
Major components of linac are :-
Modulator cabinet
Gantry
Gantry stand or support
Patient support assembly or couch
Control consol
AUXILIARY SYSTEM

10. BEAM FORMATION
Beam forming components of linac are :-
Injection system
RF power generation system or MW power
source
Accelerating waveguide
Beam transport system

11. GANTRY STAND
Stand is anchored firmly to the
floor
Major components in stand are
Klystron :- source of microwave power to
accelerate electrons
Waveguide :- conveys MW power to
accelerator in gantry
Circulator :- to isolate klystron
from microwave reflected back from
accelerator.

12. GANTRY
Gantry rotates on bearings in the stand about a horizontal axis fixed by stand
Major components of gantry are:-
Accelerator structure
Electron gun (cathode)
Bending magnet
Treatment head
Beam stopper

13. MODULATOR
Modulator cabinet contains components that supply
high voltage pulses & distribute primary electric
power to all areas of machine from utility connection.
Pulsed power supply energizes the klystron &
electron gun when triggered by a timing pulse from
control console.

14. MAGNETRONCylindrical in shape
Employed to power low energy linac 12MeV or less but occasionally as high
as 20MeV.
Functions as high power oscillator or originator of MW power.
It is a diode
Central cylindrical cathode is surrounded by evacuated drift space & then by
an outer anode having 12 cavities
A static magnetic field H is applied perpendicular to plane of cross section
A pulsed electric field directed radially inward all around is applied b/w
cathode & anode

15. e-
emitted from cathode are
accelerated by pulsed electric
field Ep towards anode across
evacuated drift space.
Accelerated e-
s induce additional
charge on anode poles & an
electric field, Em, of MW
frequency b/w adjacent segments
of anode .
e-
s move in complex spirals
under combined influence of Ep,
H.
In the process 60% of K.E. of e-
beam is converted into MW
power

17. KLYSTRON
It is a MW amplifier linear
tube that has two cavities.
It is driven by low power
oscillator
On one side of linear tube is a
source of e-
i.e. cathode which is
given a negative pulse of
voltage
This accelerates e-
s into first
(buncher) cavity that is
energized by very low MW
power that sets alternating E
field across the gap b/w cavity
walls.
It is the (-)ve E field that
accelerates e-
s

18. As e-
bunches leave drift tube & traverse catcher cavity gap they generate
retarding E field by inducing charges on cavity ends & initiate energy
conversion process
K.E. of e-
s is converted into intense E field creating MW power used to
energize accelerator st.
Klystrons have 3-5 cavities & are used with high energy linacs e.g. 18MeV
& above . Additional cavities improve high current bunching & increase
amplification of the order of (100,000:1)

19. WAVEGUIDE
• MW power from Klystron/magnetron is conveyed to
accelerator by a system of hollow pipes called waveguide .
• These are either rectangular or circular in cross section
CIRCULATOR
• Circulator prevents MW power reflected from standing wave
accelerator st. from reaching Klystron/magnetron where it can
lead to instability & damage.

20. Electron Gun
Source of the electrons
Produced thermionically
Injected onto the central
axis of the waveguide.

21. ACCELERATOR WAVEGUIDE
It varies in length from 30cm for 4 MV unit to 1 or more meters for high energy
Linac
It consists of long series of adjacent, cylindrical, MW cavities.
Acc.st.is evacuated for free propagation of e-
It makes use of cavity principle for power generation but object is to transfer
energy from cavity E field to e-
beam for acceleration.
The cavities serve two purposes :-
Couple & distribute MW power b/w adjacent cavities
Provide a suitable pattern for acceleration of e-
s
1st
few cavities vary in size . They both accelerate & bunch e-
s just like Buncher
cavity of Klystron.

22. Only about 1/3rd
of injected e-
s are captured & accelerated by
MW E field. As they gain energy they travel faster until they
almost attain velocity of light.
1st
cavities are designed to propagate E field with increasing
velocity in order to stay in step with e-
s & to further bunch &
accelerate them.
Later cavities are uniform in size & provide constant velocity
traveling wave just less than velocity of light
Two types of accelerating waveguides have been developed to
accelerate e-
s :-
Traveling wave structure
Standing wave structure

24. Energy Change?
Energy switch– fields in the
accelerating cavities in section
D maybe varied in a controlled
amount relative to the fields
in the cavities in section U

25. Focusing
Focusing coils
 Aligned along the exterior of the waveguide.
 Magnetic fields parallel to the long axis of the
waveguide.
Steering coils
Independently of focusing coils
Ensure, electron beam is at the centre of WG
Entrance and exit electron beam as desired

26. DIFFERENCE b/W TRAVELING & STANDING WAVE ST.
TRAVELING WAVE STANDING WAVE
MW enters on gun side MW can enter from any where along
acc. st.
MW exits acc. st. to be absorbed in a
resistive load
Acc. st.is terminated with a conducting
disc for reflection
Hence only one advancing incident
wave
Hence two waves incident & reflected
wave
No cavity can be moved out as they
provide E field in direction of
propagation i.e. no side coupling
Cavity with no E field can be moved
out to side i.e. side coupling

27. ELECTRON BEAM TRANSPORT
Acc. wave guide are long & are
mounted parallel to gantry rotation
axis.
To make e-
beam strike target
bending magnets are used.
Three types of banding system have
been developed :-
90° bending
270° (achromatic) bending
112.5° (slalom) bending
The 270° bending is achromatic in
sense that e-
of variable energy will
enter the beam defining system at
same point & in same direction.
For 90° bending magnet system
energy, position & direction of e-
entering bending system needs to be
accurately regulated

28. ELECTRON BEAM TRANSPORT

29. Treatment head
• The important components of linac treatment head are :-
Retractable x-ray targets (photon beam forming)
–Flattening filters & e-
scattering foils (beam shaping)
–Primary & secondary adjustable collimators & optimal MLC
(beam defining)
–Dual transmission ionization chamber (beam monitoring)
–Field defining light & a range finder (beam localizing)

30. TARGET
Transmission targets are used as in high energy range photons
produced are directed in direction of incoming e-
.
 Efficiency of x-ray production ↑with ↑in e-
energy. Hence
target heating is not a serious problem & can be cooled by cold
water flowing through a copper block into which target is
fitted.
up to 10 MeV, a thick tungsten target is employed,
Thick aluminum target being used for energies greater than
this.
Retractable for electron beam therapy.
As a result bremsstrahlung type interaction, as electron energy
is converted into a spectrum of x ray energy equal to incident
electron
Average photon energy of the beam is approximately one third

31. FLATTENING FILTER
High energy X-rays emerging from
target are forward-peaked in intensity
along beam CAX & of progressively
less intensity away from it.
To make intensity uniform across
beam a conical metal absorber called
flattening filter is placed in beam path.
High Z filter would soften the beam
due to pair production.
Al flattening filters are used in low
energy linacs & copper or steel in high
energy linacs because Al filter would
be large enough to be accommodated
in treatment head of high energy linac.

32. BEAM MONITORING
Most common dose monitors are transmission
I.C. permanently embedded in linac treatment
head b/w flattening filter or scattering foil &
photon beam secondary collimator.
Sealed parallel plate I.C. are used to make their
response independent of ambient temp. &
pressure.
For pt . Safety two I.C. are used, one serving as
check on another, with completely independent
biasing power supplies & readout electrometers.
During pt. treatment if primary chamber fails
the secondary chamber will terminate irradiation
with additional dose of few percent above
prescribed dose .
In event of simultaneous failure of both
chambers, timer will shut the machine down
with minimal overdose to pt.

33. BEAM LOCALIZING
The FS definition is provided by light localizing
system in treatment head .
Consists of mirror & light source
Located in space b/w chambers & jaws
Projects light beam as if emitting from x-ray focal
spot.
Provides an intense light field, duplicating in size &
shape the radn
field incident on the pt.as defined by
collimators & other beam limiting devices.
Facilitates positioning of pt. for treatment
A range finder light projects a numerical scale on pt.’s
to define SSD

34. BEAM COLLIMATION
Beam collimation is achieved by two or three collimator devices :-
Primary collimator
Secondary moveable collimator
MLC (optional)
To provide sharp edges for treatment fields or to reduce
transmission penumbra , movement of blocks is confined to arcs so
that block faces present flat edge to beam diverging from target .
 Blocks are adjustable in pairs & provide max. FS of 40x40cm2
at
isocenter. Secondary collimator rotates about beam axis allowing
angulations of field.
Accessories to modify emergent x-ray beam externally e.g. wedges
& compensators can be slided into a slot on treatment head.

35. Shielding blocks may be mounted on a tray that can be slided into
an aperture on accessory mount.
Electron applicators can be slided into same aperture.
Modern linacs incorporate independent (asymmetric) jaws to
provide asymmetric fields, blocking ½ or ¾ field.
Asymmetric jaw can be used as dynamic wedge.
MLCs are recent addition to linac dose delivery & consist of
tungsten leaves(40-120pairs) with individually computer controlled
motors
These leaves are made of tungsten alloy & have thickness about 6-
7.5cm along direction of beam . Each leave has a width of 0.5-1cm
as projected at isocenter.

36. o MLCs are recent addition to linac dose delivery & consist of
tungsten leaves (40-120pairs) with individually computer
controlled motors
o These leaves are made of tungsten alloy & have thickness
about 6-7.5cm along direction of beam . Each leave has a width
of 0.5-1cm as projected at isocenter
o Specifically, conformal RT and IMRT can be delivered using
MLC’s.

37. Material
Tungsten alloy (tungestan,Fe,Cu,Ni) is the
material of choice for leaf construction because:
High density
Hard
Inexpensive
low coefficients of expansion

40. MLC Leaves -TypesMLC Leaves -Types

41. Advantages of Multi leaf collimators
1.Beam shaping is simple and less time
consuming.
2. Can be used without the need to enter
treatment room.
3. Correction and changing of field shape is
simple.
4. Overall treatment time is shortened.
5. Constant control and continuous

42. DisadvantagesDisadvantages
1. Step edge effect
2. Radiation leakage between the leaves
Intra leaf < 2%
Inter leaf < 3%
3. Wider penumbra

43. Production of clinical photon beam
Photon beam emanating from
medical linac are produced in
an x-ray target & are flattened
with flattening filter
Each clinical beam has its own
target-flattening filter
combination. The flattening
filters & scattering foils are
mounted on a rotating carousel
or sliding drawer for ease of
mechanical positioning into
beam

44. PRODUCTION OF CLINICAL ELECTRON BEAM
Majority of high energy
linacs, in addition to
providing dual photon
energies, also provide e-
beams with energies ranging
from 4 to 30 MeV
To activate e-
beam mode
both target & flattening filter
of x-ray mode are retracted
from e-
beam

45. PRODUCTION OF CLINICAL ELECTRON BEAM
Techniques used for clinical e-
beam production are :-
Pencil beam scattering – e-
pencil beam over relatively large
area used in RT (upto 25x25cm2
) is achieved by placing thin
foils of high Z material (cu or pb) into pencil beam at level
of flattening filter in X-ray mode.
Pencil beam scanning – is alternative method used
infrequently for producing clinical e-
beam. It has two
computer controlled magnets which deflect pencil beam in
orthogonal planes.
Special applicators are used to collimate e-
beam.

46. ISOCENTRE
o The is the point in space about
which the gantry , the
treatment head and the couch
rotate
o The mechanical isocentre is the
point about which the linear
accelerator and couch rotate
o The radiation isocentre is the
point where the radiation
beams intersect if the gantry,
collimator or couch are rotated.
o These two points should ideally be
the same

47. Auxiliary system
These systems are essential for operation , control & monitoring of linac
treatment unit & consist of following systems :-
Vacuum system :- provides vacuum for operation of e-
gun, accelerator st. &
bending magnet system.
 Without vacuum e-
gun would burn out just like a light bulb filament
exposed to air.
accelerated e-
s would collide with air molecule deflecting them & reducing
their energy , pencil like e-
beam would be diffused & broken up.
The vacuum is maintained by electronic ion pump . Use of this pump
transformed linac from a laboratory instrument to a practical clinical tool.
Earlier oil based rotatory & diffusion pumps were used which required
significant maintenance.

48. • Pressure system :- pressurizes waveguide with dielectric Freon & SF6 to
prevent electrical breakdown from high power MW E fields.
• Cooling system :- provides temperature controlled water
– Establishes operating temp. of sensitive components & operates primarily to
remove residual heat dissipated in other components
– Temp. control is particularly critical for acc. st. Otherwise cavities will
change dimension slightly resulting in detuning & impairment of their
acceleration capabilities
• Automatic frequency control system :- Senses the optimum operating
frequency of acc. St. to maximize radn output. It uses this information to
klystron/magnetron to this MW frequency.

49. TREATMENT COUCH
It supports the patient during treatment hence also called pt.
support assembly .
It is controlled by a hand pendent/ couch thumb wheels
Couch can be moved up-down, in-out, left-right for positioning
of pt. during each treatment session.
Couch can be rotated about a vertical axis passing through
isocenter.

50. CONTROL CONSOLE
It is operation centre for linac
It supplies timing pulses to initiate each pulse of radiation.
It provides visual & electronic monitor for linac operating
parameters including individual pt.’s dose prescription.
The control console provides status information on treatment
modality, accessories in use, prescribed dose & dose delivered,
interlock status, emergency off, & other data pertinent to linac
operation & pt. treatment.
A closed circuit TV system provides visual contact

51. LINAC CONFIGURATION
Acc. st. aligned directly with linac
isocenter.
Simplest & most practical
configuration
Used for low energy linacs (4-6MV)
X-ray target & e-
gun form part of acc.
waveguide.
No need for beam transport system.
Straight through photon beam
RF power source mounted in gantry

52. LINAC CONFIGURATION
• Acc. Waveguide for intermediate
(8-15MeV) & high (15-30MeV) e-
energies are too long for direct
isocentric mounting.
• Acc. Waveguide located in gantry
stand or in gantry parallel to
gantry axis of rotation.
• A beam transport system
transports e-
beam from acc. st. to
x-ray target.
• RF power source is in gantry
stand.

53. ADVANTAGE OF LINAC OVER Co-60
High dose rate.
Higher PDD hence good for deep seated tumors.
Sharp beam with less penumbra as focal spot size is small
For Co-60 source size is 1.5cm & for linac Focal spot size 2.5-3mm.
Small FS for precision therapy possible.
Large fields can be treated as max. FS on linac is 40x40cm2
while on Co-60
max. FS is 35x35cm2
.
Linacs are safer than Co-60 from radiation protection point of view.
No chances of accidental exposure.

54. Build-up de pth is m o re fo r linac as co m pare d to Co -
6 0 .
Ele ctro n the rapy po ssible with linac.
Linac with MLCs can be use d fo r co nfo rm althe rapy
i. e . le ave s o f MLC can be co nfirm e d to shape o f
tum o r e le ctro nically.
IMRT can be de live re d with dynam ic m o ve m e nt o f
le ave s.
Linac are available with Dual e ne rg y pho to n be am
so e ne rg y can be se le cte d as pe r re q uire m e nt.
Since do se rate is hig h m o re pt. can be tre ate d in
le ss tim e

55. DISADVANTAGE
Output may vary due to voltage fluctuations
Requires more electrical backup.
More liable to breakdown because of complicated electronic circuits.
Down time more as compared to Co-60
Complex to operate.
Costly.
Requires proper maintenance
Total life less (max, 15yr.)
Requires daily dosimetric checks e.g. output constancy is checked daily
before treating patients.

56. FLATTENING FILTER FREE (FFF) LINEAR ACCELERATORS
Flattening filter free (FFF) linear accelerators can increase
treatment efficiency and plan quality.
Such beams differ from the standard flattened beams (FF)
in the high dose rate and the profile shape peaked on the
beam central axis.
Overall treatment time reduced by 40 – 50%.
 Softening of the x-ray spectra
Reduction in head scattered radiation
Nonuniform beam profile.

57. CONCLUSION
LINAC is highly sophisticated machine used for radiotherapy
Periodic Q.A. of linac is very essential as it may expose pt. to
danger if it fails to deliver the required dose to the pt., or if
does not satisfy standards of mechanical & electrical safety.

58. THANK YOU