This document provides information about external beam therapy and linear accelerators used for radiation therapy. It discusses various components of linear accelerators including the electron injection system, microwave system, and beam collimation/applicator system. It also describes accessories used with linear accelerators like wedges (physical, motorized, dynamic), blocks, and multi-leaf collimators. Portal imaging devices are discussed as well as on-board imaging capabilities. Console control and safety features are also summarized. Key linear accelerator concepts like isocenter, treatment head, and beam flattening filters are defined.

1. Lecture 3
External Beam Therapy (EBT)

2. Lecture 3

11. • Special cones (applicators are used to collimate the
electron beams.

12. Treatment table

15. console control

16. Console controls is the place from which, we
control the movements of the machines, while
delivering the radiation to the patients
Accessories required for console control:
In general to treat the patient in the console
control should have control station from which
we can set the treatment time, interrupt,
terminate and some basic options would be
there.

17. In Linear Accelerator based console control:
1. The work for the technologist is to enter the correct data and
treat the patient.
2. The patient should be monitored during the treatment; this is
the work of radiation therapist.
3. Emergency stop button should be present.
4. Record and verify systems should be present.
5. Audio and visual communication should be present.
6. In linac Green, Red, and Blue lights should be present. Blue will
indicate the machine is in the operation mode, green will
indicate that, there is no treatment going on, Red light will
indicate that, radiation is on inside or treatment is going on.
7. Console control door should be interlocked with treatment
room for safety purposes.
8. inside the console control other than radiation workers should
not present for safety purposes. Especially children’s, women,
and old age peoples.

18. The main beam forming components of a modern
medical linac are usually grouped
1)Electron injection system
2) Microwave system
3) Power supply system
4) Beam transport beam monitoring system
5) Auxiliary system
6) Safety interlock system
7) Computer controlled feedback system
8) Beam collimator/applicator system (for electron )
9) Cooling system
10) Control console system

20. Isocentre Of The Linac

21. The isocentre is the point in space about which the gantry
of the linear accelerator, the treatment head of the linear
accelerator, and the couch rotate.
1- The mechanical isocentre is the point in space about
which the linear accelerator and couch rotate.
2-The radiation isocentre is the point where the radiation
beams intersect if the gantry, collimator or couch is rotated.
These two points need not be the same although ideally
they should be.

22. Accessories used with lINAC

23. Wedge
Called Wedge filters may be used to even out the isodose surfaces for
photon beams striking relatively flat patient surfaces under an oblique
beam incidence.
Wedges are commonly used as beam-modifying devices in radiation
therapy to optimize the target volume dose distribution.
In this context, variety of wedge filters is available with modern linear
accelerators.
There there types of wedge filter are in use:
1. physical wedge filters
2. Motorized wedge filter
3. dynamic wedge filters.
.

24. Wedges
Physical Motorized Dynamic
Universal
Individualized

25. 1-Physical wedge
• Physical wedges are made of lead, brass or steel.
• The physical wedges (PW) can be inserted in the treatment head of
linear accelerator in four different orientations (in, out, left and
right), they cause a progressive decrease in the intensity across the
beam and a tilt of isodose curves under normal beam incidence
• Angle physical wedges (i.e. 15°, 30°, 45° and 60°)

26. Physical Wedge
Gantry
100
90
80
70

27. 2- Motorized wedge
Motorized wedge which is integrated into the head of the unit and
operated remotely.
Motorized wedge is a single physical wedge (60º universal wedge)
which could generate desired angle from 0 º to 60º.

28. Motorized Wedge
100
90
80
70
100
90
80
70
Gantry
Beam on
100
90
80
70
Beam off

29. 3- Dynamic wedge
• In case of Enhanced Dynamic Wedges (EDW), the
required dose distribution can be achieved by one of the
collimator jaws motion in two different directions (in and
out)
• Dynamic wedges provide the wedge effect on isodose
curves through a closing motion of a collimator block
during irradiation.
• This wedge was moved into the field for part of the time
to create the wedge beam profile desired.
• dynamic enhanced wedges are moving jaws that are
moved by computer control to create wedge beam
profiles.

30. Dynamic Wedge
B
A
100
90
80
70

31. Blocks
• blocks are the most commonly used field shaping
device in radiotherapy
• A. Block Thickness Shielding blocks are most commonly
made of lead.
• The thickness of lead required to provide adequate
protection of the shielded areas depends on the beam
quality and the allowed transmission through the
block. A
• primary beam transmission of 5% through the block is
considered acceptable for most clinical situations.
• The benefit of the blocks is to protect the healthy
organs near the tumor area from radiation.

33. Block Tray
Gantry
100
90
80
70

34. Multi Leaf Collimator (MLC)

37. Electronic Portal Imaging Devices –
EPID-
• Electronic portal imaging devices – EPID- can
use for daily imaging for treatment localization
and verification.
• The portal image can be analyzed with
computer software before the treatment.
• EPIDs use flat panel arrays of solid state
detectors based on amorphous silicon (a-Si)
technology

39. Advantages of Portal Imaging:
1. Treatment field verification during the treatment
gives the quality treatment.
2. Images will be available immediately for
reference.
3. While x-ray imaging patient have to be shift from
treatment room to x-ray room it leads to
unnecessary error in the setup.

40. Disadvantages of Portal Imaging:
1. Bulky attachment is one of the major
disadvantages.
2. Poor image quality.
Because in diagnostic imaging photoelectric effect is
predominant but, in therapy Compton Effect is more
predominant than other effects. In Compton Effect
the scattering of the photon beam would be there
because of its high energy. Not only that but also
Compton Effect doesn’t depend on atomic number of
the material.
3. Not cost effective.

42. On Board Imaging (OBI)
Now a day this trend has been modified by setting
the x – ray tube in the gantry or changing the
energy of linear accelerator to ordinary KV imaging
energy.
In all the latest linear accelerator are coming with
Electronic Portal Imaging Device (EPID).
In addition to that, on-board imager (OBI) is
attached with the machine to verify the treatment
field. But still EPID is in practice in all the linear
accelerators.

45. Choose the correct answer
The machine used for radiotherapy is called as _________
a) Therapeutic X – Ray machine
b) Linear Accelerator
c) Radioactive Ultrasound Machine
-------- result from Coulomb interactions between the incident electron and the nuclei of
the target material
a) Characteristic x-ray
b) Gamma Rays
c) Bremsstrahlung Radiation
The radiation comes from a machine outside the body and is focused on the cancer called
a) External radiotherapy
b) Internal radiotherapy
c) Thermotherapy
Result from Coulomb interactions between the incident electrons and atomic orbital
electrons of the target material
a) Characteristic x-ray
b) Gamma Rays
c) Bremsstrahlung Radiation

46. The bremsstrahlung spectrum produced in a given X ray target depends on
A. The kinetic energy of the incident electron .
B. The thickness and atomic number Z of the target.
C. All above
Why image quality in portal imaging are very poor?
a) Compton is predominant in therapy
b) Because of high energy
c) Photoelectric effect is dominant
d) A and B is correct
e) A and C is correct
f) All
Q2. Explain The Following
1- Superficial X rays, 2- Orthovoltage 3- Megavoltage X rays
4. Linear Accelerator and How does it work?
5. Linear generation
6. Components of modern linacs
7. Treatment head of linacs and component it
8- wedge filter and types of it
9- console control
10 – beam flatting filter and scattering foil

47. Question : Arrange the given words on the suitable space of the following figure?
(Gantry, electron gun, Isocenter, Accelerating wave and X-ray target)

48. Use some videos to explain the linear
accelerator
• https://www.youtube.com/watch?v=jSgnWfb
Ex1A
• https://www.youtube.com/watch?v=lZ9cGVax
Oes
• https://www.youtube.com/watch?v=hy9atKA
qAf4&list=PLk59CaRJX5aZWsX5hyLmQQNGfKE
H5iY-J&index=3

49. Thank you for your attention