Linear accelerator

1. MEDICAL LINEAR
ACCELERATOR

2. INTRODUCTION
• The term radiation applies to the emission and propagation of energy through space or a
material medium.
• Broadly, it may be classified into electromagnetic radiation and particulate radiation.
• Radiation therapy basically uses ionizing radiation (X-rays, γ-rays and particulate radiation) for the
treatment of cancer.
• Radiation therapy, one of the three established cancer treatment modalities, is used to treat most
types of solid tumors and selected hematologic malignancies.
• Radiation therapy is routinely combined with surgery, chemotherapy, or both to improve
therapeutic results.
• Despite several decades of research, photon-beam still constitutes the main therapeutic modality
in RT, because of several unresolved technical problems with the use of particulate radiation

3. Ionizing Radiation
Directly Ionizing Radiation
• Electrons, Protons, Alpha particle & heavy charged
particles.
Indirectly Ionizing Radiation
• Photons (X-Ray, Gamma Rays) & Neutrons.

4. Ionizing Radiation In Clinic
Clinic
Diagnostic
Therapeutic
• Radiology (X-Ray, CT Scan etc.)
• Nuclear Medicine (Unsealed
Radiopharmaceutical)
• Brachytherapy (Sealed Radioactive
Source)
• Teletherapy ( Linac Produced
Radiation & Sealed Radioactive
Source.)
• Nuclear Medicine (Unsealed
Source.)

5. Teletherapy (External Beam Therapy)
X-Ray
Gamma Ray
Particle Beam
Linac (4- 22 MV)
Co-60 Sealed Source
(Avg. Energy 1.25 MeV)
Electron Beam- (4- 22 MeV)
Proton Beam (80-250MeV)
Helium Ion(30-250MeV)
Carbon Ion(200-500MeV), Neutron Etc.

6. Diagnostic Machine
Therapeutic Machine
CT
MACHINE
X-RAY
MACHINE
SPECT
MACHINE
HDR
Brachytherapy
MACHINE
Proton
Beamtherapy
MACHINE
Cobalt
Teletherapy

7. X-Ray Beam Therapy
Grenz-Ray Therapy
Contact Therapy
Superficial Therapy
Medium Voltage Therapy
Deep Therapy
Soft x-ray below 20KV, low penetrating power
Operate at potential 40-50 KV, very short distance treatment,
treatment depth not more than 1-2 mm
Operate at potential ranging from 50-150 KV, treatment distance
15-20 cm, useful for tumors confined to about 5mm depth.
Operate at potential ranging from 150-200 KV.
Operate at potential ranging from 200-300 KV, treatment distance
50 cm.
Supervoltage Therapy Operate at potential ranging from 500 to 1000 KV.

8. Grenz
contact
super
Ortho
Co-60
• Can not reach deep-seated tumors with an adequate
dosage of radiation.
• Do not spare skin and normal tissues.
• Although Cobalt therapy comes under MegaVoltage
therapy and being advantageous than other low
energy therapy, but it has separate technical reasons
to be disadvantageous in comparison to Modern
Linac.
LIMITATION OF LOW ENERGY THERAPY

9. Megavoltage Therapy(Medical Linac)
• Treatment machine that uses high-
frequency electromagnetic waves to
accelerate charged particles such as
electrons to high energies via a linear tube.
• Charged particles travel in straight lines as
they gain energy from an alternating
electromagnetic field.
• The resonating cavity frequency of the
medical linacs is about 3 billion Hertz
(cycles/sec).
• This is the most common device to treat
cancer with external beam radiation.

10. EARLY ACCELERATOR
• The first one was installed at Hammersmith
in 1952.
• In 1956, the first patient was treated at
Stanford University in the United States.
• The Linac had an 8 MV X-ray beam with
limited gantry motion.
• These linacs were large and bulky.

11. SECOND GENERATION
• 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.

12. THIRD GENERATION
• 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.

13. Components
1. Gantry
2. Gantry/Drive stand
3. Modulator cabinet
4. Patient Support Assembly (PSA)
5. Control Console

14. Main Beam forming Components
1) Electron injection system
2) RF power generator
3) Accelerating waveguide
4) Auxiliary system
5) Beam transport system
6) Beam collimation system, and
7) Beam monitoring system

15. MODULATOR
• Modulator cabinet: contains components that distribute and monitor primary electrical
power and high-voltage pulses to the magnetron or klystron.
• This is the noisiest of the Linac systemcomponents and is located inside the
treatment room.
• Contains 3 subcomponents:
1) Fan Control, cooling the power distribution system.
2) The auxiliary power distribution system, contains the emergency off button
that shuts the power to the linac.
3) Primary power distribution system.

16. DRIVE STAND
• Drive Stand: a stand containing the apparatus that drives the linear accelerator
• Open on both sides with swinging doors for easy access to gauges, valves, tanks, and
buttons
• Klystron/Magnetron: power source used to generate electromagnetic waves for the
accelerator guides
• Waveguide: hollow tube-like structure that guide the electromagnetic waves from the
magnetron to the accelerating guide where electrons are accelerated
• Circulator: directs the RF energy into the waveguide and prevents any reflected microwaves
from returning to the klystron
• Water-cooling system: allows many components in the gantry and drive stand to operate at
a constant temperature

17. Klystron
• Provides the source of microwave power
to accelerate the electrons.
• This is done by amplifying introduced
Radio Frequency (RF) electromagnetic waves.
• Based on velocity and current modulation
Principle.

18. Magnetron
• Electron tube that provides microwave
• power to accelerate the electrons.
• Preferred for lower electron energies,
4 MeV to 6 MeV linacs.
• For higher energies the Klystron is a better
choice.

19. GANTRY
• Gantry: responsible for directing the photon (x-ray)
energy or electron beam at a patients tumor.
Electron gun: produce electrons and injects them into the accelerator structure.
Accelerator structure: a special type of wave guide in which electrons are accelerated.
Treatment head: components designed to shape and monitor the treatment beam

20. PFN
• A pulse forming network (PFN) is an electric circuit that
accumulates electrical energy over a comparatively long time, then
releases the stored energy in the form of a relatively square pulse of
comparatively brief duration for various pulsed power applications.
• The Thyratron uses these pulses as a high-‐tech switch to deliver these pulses to
the electron gun.

21. WATER COOLING SYSTEM
• Located in the Drive Stand and Gantry
• Provides thermal stability to the system.
• Allows many components in the Drive Stand and
Gantry to operate at a constant temperature.

22. ELECTRON GUN
• The electron gun is responsible for injecting electrons into the
accelerating waveguide.
• It accomplishes this using a heated cathode which
thermoinically "boils-off" electrons.
• These electrons are then accelerated toward a grounded
Perforated anode and are focused into a pencil beam by a
focusing coil.
• Finally, electrons pass through the anode into the
Accelerating waveguide where they are accelerated.

23. ACCELERATING GUIDE
• This is sometimes called the accelerator structure
or accelerator wave guide.
• It can be mounted in the gantry horizontally for high
energy single or dual energy machines with klystrons.
• It can be mounted vertically for low energy machines
with magnetrons.
• The microwave power produced in the klystron or
magnetron is transported to the accelerator structure
to accelerate the pulsed electron bunches.

24. TREATMENT HEAD
• Treatment head: components designed to shape and
monitor the treatment beam
• Bending magnet: direct the electrons vertically toward the patient
• X-ray target, Primary collimator: designed to limit the maximum
field size, Beam flattening filter: shaped the x-ray beam in its cross
sectional dimension, Ion chamber: monitors the beam for its symmetry
in the right-left and inferior-superior direction,
Secondary collimators: upper and lower collimator jaws,
Field light: outlines the dimensions of the radiation field as it appears on
the patient, allows accurate positioning of the radiation field in relationship
to skim marks or other reference points

25. BEANDING MAGNET
• Changes the direction of the electron beam, downwards
toward the patient.
• Bends the pulsed electron beam towards the
target for X-‐raysor toward the scattering foil for
electron treatments.
• Produces different beam paths for different
energies.
• Needed for energies greater than 6 MeV.

26. BENDING MAGNET
• Bending magnet: bends the electron beam through a right angle, so it ends up
pointed at the patient
1) 90 degree magnets (chromatic) have the property that any energy spread results in spatial
dispersion of the beam.
• Electrons are bent in proportion with their energy, the lower energy electrons are bent more, the
higher energy electrons less
• Results in a beam that is spread from side to side according to energy
• Energy sensitive, act as energy differentiators
2) 270 degree magnets (achromatic) designed to eliminate spatial dispersion
• So not significantly disperse the different electron energies in the beam.

27. X-RAY TARGET
• The collision of the electrons with the high density transmission target
creates the X-‐Rays (photons), forming a forward peaking shaped
• X-raybeam in the direction of the patient’s tumor.
• The X-raytarget is located at the focus of the Bending Magnet.
70-80%(check) of the electrons energy goes into heat.

28. FLATTENING FILTER
• It is a conical shaped metal absorber, that absorbs
more forward peaking photons than the ones in the
periphery.
• Shapes the X-‐r a y s in their cross sectional shape.
• It is required to create a flattened beam of
sufficient area, uniformity and symmetry.
• It is usually made of Tungsten, Steel, Lead,
Uranium and Aluminum.
• In dual energy photon linacs, two flattening filters are
required for the low and the higher photon energies.

29. SCATTERING FOIL
• The electron beams have pencil-‐likeshapes.
• These narrow pencil beams need to be broadened to clinical useful
beams and need to be made uniform.
• There is a different scattering foil for each electron beam
energy produced.
• Made out of Aluminum or Cooper.
• A thin foil (or multiple ones)are used, they are measured in mils,
i.e. 8 mils of Al is about 0.2 millimeters.

30. MONITOR IONIZATIONCHAMBER
• They monitor integrated Dose, Dose Rate
and Field Symmetry.
• The radiation that leaves the X-‐RayTarget
or the electron Scattering Foils will pass
through dual monitor ionization
chambers, and they produce an
ionization current.
• This ionization current is proportional
to the X-‐rayof electron beamintensity.
• In Varian sealed, Elekta unsealed
monitoring chamber used.

31. COLLIMATOR
• The radiation beams are collimated by
adjusting the upper and lower
collimator jaws.
• The jaws are made of High Z
number, like Tungsten or Lead.
• The jaws can define a rectangular
shaped beam up to 40 cm by 40 cm
for X-‐raybeams.

32. MLC
• They are heavy metal field-‐shapingdevices
with independent moving mechanisms
used to create a custom like block to
spare normal tissue and direct the
radiation dose to the tumor.
• The MLC’s became a key element in the
treatment delivery of X-‐raybeams with
IMRT (Intensity Modulated Radiation
Therapy).

33. TREATMENT COUCH
• The treatment couch or table is where
the patient lays still to receive the
radiation treatment.
• It moves Up/Down, Right/Left and
In/ Out.
• Robotic couches are being used in some
linacs nowadays for
3 more degrees of freedom.
• PITCH: rotation around the X-axis
• ROLL: rotation around the Y-axis
• YAW: rotation around the Z-axis

34. TPS

35. 1) SIEMENS 2) ELEKTA 3)VARIAN

36. Beam Generation In Medical Linac
Photon Beam Generation Electron Beam Generation

37. Therapeutic Beam
Beam Profile Percentage Depth Dose Distribution
Relative
Dose
Off Axis Distance

38. Thank you
Any Questions ??