Medical linear accelerator

Medical Linear Accelerator
Prof Amin E AAmin
Dean of the Higher Institute of Optics Technology
&
Prof of Medical Physics
Radiation Oncology Department
Faculty of Medicine
Ain Shams University

❖ A linear accelerator is a device that uses high Radio‐Frequency
(RF) electromagnet waves to accelerate charged particles (i.e.
electrons) to high energies in a linear path, inside a tube like
structure called the accelerator waveguide.
❖ The resonance cavity frequency of the medical linacs is about 3
billion Hertz (cycles/sec)
Introduction

Introduction
• Electron trajectories are linear in the
accelerator tube hence the name
‘LINEAR ACCELERATOR’.
• 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.

•Photon Beam (X-Ray):
•4 MV To22 MV.
•Single Beam.
•Duel Beams
•Electron Beam:
•Multi-Beams with energy
grope between: 4 - 22 MeV.
Radiation Protection in Radiotherapy
Introduction

Introduction
❖Linear accelerator is used to treat all parts and organs of
the body
❖High-energy electron beam – treating superficial tumors or
it can be made to strike a target to produce x rays for
treating deep seated tumors.
❖X-rays – treating deep-seated tumors

History Of Medical Linear Accelerators
❖ In 1928, R. Wideroe demonstrated that
electrons could be accelerated through a tube
by applying a radio frequency voltage to
sections of the tube
❖ In 1930, first linear accelerator principle
invented by Rolf Wideroe
❖ In 1949. idea of using LINAC in medical
application become interested
❖ Medical LINAC have been in clinical use since
the early 1950’s.
❖ First one was installed at Hammersmith in 1952.
Rolf Wideroe

❖ 1956, the first patient was treated at
Stanford university in U.S.
❖ There had an 8 MeV x ray beam and
limited gantry motion.
Radiotherapy (Stanford linac)
modern linac for therapy

A 2-year-old boy the first
patient to receive radiation in
operation therapy from the
medical linear accelerator at
Stanford

History Of The Medical Linear
Accelerator:
❖ 1952: Henry Kaplan and Edward Ginzton begin building a
medical linear accelerator.
❖ 1956: The first medical linear accelerator in the Western
Hemisphere is installed at Stanford Hospital in San Francisco.
❖ 1959: Stanford medical school and hospital move to the Palo Alto
campus, bringing the medical linear accelerator.
❖ 1962: Kaplan and Saul Rosenberg begin trials using the linear
accelerator with chemotherapy to treat Hodgkin's disease, an
approach that dramatically improves patient survival.

History Of The Medical Linear
Accelerator:
❖ 1994: First use of the CyberKnife, invented at Stanford, which
uses sophisticated computerized imaging to aim a narrow X-ray
beam precisely.
❖ 1997: Stanford pioneers the use of intensity-modulated radiation
therapy, which combines precise imaging with linear accelerators
that deliver hundreds of thin beams of radiation from any angle.
❖ 2004: Implementation of four-dimensional radiotherapy, which
accounts for the motion of breathing during imaging and radiation
delivery.
❖ Medical linear accelerators have become the backbone of radiation
therapy for cancer worldwide.

Linac generations
During the past 40 years medical linacs have gone through five
distinct generations, each one increasingly more sophisticated:
1. Low energy x rays (4-6 MV)
2. Medium energy x rays (10-15 MV) and electrons
3. (High energy x rays (18-25 MV) and electrons
4. Computer controlled dual energy linac with electrons
5. Computer controlled dual energy linac with electrons
combined with intensity modulation

Radiation Protection in Radiotherapy Part 5, lecture 2: Equ ipment - superficial, telecurie 20
First Generation
Early Accelerators
(1953-1961):
❖ The first one was installed in
Hammersmith in 1952.
❖ In 1956 ,the first patient was treated
at Stanford University in USA.
❖ The LINAC had an 8-MV Xray
beam with limited gantry motion.
❖ These LINACs were large and bulky.

Radiation Protection in Radiotherapy Part 5, lecture 2: Eq uipment - superficial, telecurie 21
Second Generations
(1962-1982):
❖ The second generation was
isocentric and could rotate 360o
around the
❖ gantry axis.
❖ They were built between 1962-
1982.
❖ They increased the accuracy and
precision of Dose delivery.
Second Generation

❖Better accelerator waveguides and
bending magnet systems and more
beam modifying accessories.
❖Wider range of energies , dose
rates, field sizes and operating
modes.
❖Higher reliability and computer
driven.

Radiation Protection in Radiotherapy Part 5, lecture 2: Equipment - superficial, telecurie 22
Third generation accelerators:
❖Improved accelerator guide
❖Magnet systems
❖Beam-modifying systems to provide
wide ranges of beam energy, dose rate,
field size
❖Operating modes with improved
❖beam characteristics
❖Highly reliable
❖Compact design
❖May include: dual photon energies, multileaf
collimation, several electron energies &
electronic portal verification systems
Third generation

Electron Accelerators
• Modern accelerators have a lot of
treatment options, for example
– X Rays or electrons (dual mode)
– multiple energies
• 2 or 3 X Ray energies
• 4 or more electron energies

Introduction
There are several types of linear accelerator designs, but the ones
used in radiotherapy accelerate electrons either by traveling or
stationary electromagnetic waves of frequency in microwave
region (≈ 3000 megacycles/sec).

Types Of EM Wave
1. Traveling EM wave
• Required a terminating (“dummy”) load to absorb the residual power at the
end of the structure
• Prevent backward reflection wave
2. Standing EM wave
• Combination of forward and reverse traveling waves
• More efficiency
–Axial beam transport cavities and the side cavities can be independently
optimized
• More expensive
–Requires installation of a circulator (or insulator) between the power
source
–the structure prevent reflections from reaching the power source

Types Of EM Wave
• Traveling wave
A Traveling wave is a wave traveling
(propagating) through a medium .
• Standing wave
A standing wave is a wave which
oscillates but does not propagate.

Types Of LINACS
• Medical linacs can be either type
– Traveling wave linac
– Standing wave linac

The Major Components Of Medical
Linear Accelerator
1. Power Supply
2. Modulator
3. Magnetron Or Klystron
4. Electron Gun
5. Wave Guide system
6. Accelerator Tube
7. Bending Magnet
8. Treatment Head
9. Treatment table(Couch)
A block diagram of typical medical
linear accelerator

The Major Components Of Medical
Linear Accelerator
A block diagram of typical medical linear accelerator

Linear Accelerator Systemss
I. Power supply system;
II. Electron injection system
III. RF power generation system;
IV. Accelerating waveguide;
V. Auxiliary system;
VI. Safety interlock system
VII. Computer controlled feedback system
VIII. Beam transport beam monitoring system;
IX. Beam collimator/applicator system.
X. Control console system
XI. Cooling system

Different Systems of Linear Accelerators
TYPE OF SYSTEM COMPONENTS
Power supply system Modulator to provide High voltage &
short duration pulses in synchronization.
Electron injection system Electron Gun, toprovide electrons for
acceleration.
Microwave system Magnetron or Klystron to provide
microwaves.
Beam transport beam
monitoring system
Accelerating waveguide ,provides
acceleration to electrons.

Different Systems of Linear Accelerators
TYPE OF SYSTEM COMPONENTS
Auxiliary system Vacuum pump, circulating cooling water ,RF
frequency tune, Pressurized dielectric gas for
RF transmission , RF isolator & Thyratron.
Safety interlock system Both hardware and software interlocking
system.
Computer controlled feedback
system
Monitor chamber, Hardware position encoders,
limiting micro switches
Beam collimator /applicator
system
Jaw collimators, multi-leaf collimators(MLC),
micro multi leaf collimators (mMLC).

The Process Within A Linear Accelerator
Starting From The Power Supply To The
Production Of A 3 Mm Pencil Beam.
e
HRFRF

Modulator and power supply
❖This important component of the linear
accelerator is usually located in the
treatment room In some Units.
❖The Modulator cabinet contains three
major components
• Fan control (cooling the power-
distribution system).
• Auxiliary power distribution system
)contains the emergency off button that
shuts off the power to the treatment unit ).
• Primary power-distribution system .

Modulator and power supply
❖ A power supply provides direct current
(DC) power to the modulator, which
includes the pulse-forming network and
a switch tube known as hydrogen
thyratron.
❖ High voltage pulses from the modulator
section are flat-topped DC pulses of a
few microseconds in duration.
❖ These pulses are delivered to the
magnetron or klystron and
simultaneously to the electron gun.

Radiofrequency Power Generation System
❖ Pulsed modulator produces the high voltage (100 kV), high
current (100 A), short duration (1s) pulses required by the RF
power source and the injection system.

Power Distribution
• Modulator cabinet: contains
components that distribute and monitor
primary electrical power and high-
voltage pulses to the magnetron or
klystron and electron gun
Modulator

Modulator Cabinet:
❖ It distribute primary electrical power and high voltage to
magnetron and klystron
❖ It located in treatment room
❖ It major components
➢ The fan control: need arises for cooling the power distribution
➢ Auxiliary system: contains the emergency off button
➢ Primary power dist: Primary power-distribution system

Production Of Electrons
• Electrons are produced in an electron
gun.
• A hot cathode emits electrons, which
are accelerated towards an anode,
passing through an aperture to reach
the accelerating waveguide.
Electron Gun

Electron Gun
• Electron gun: produce
electrons and injects
them into the
accelerator structure

Injection System
❖The linac injection system is the source of electrons, a simple
electrostatic accelerator referred to as the electron gun.
❖Two types of electron gun are in use in medical linacs:
❖Diode type
❖Triode type
❖Both electron gun types contain:
❖Heated filament cathode
❖Perforated grounded anode
❖Triode gun also incorporates a grid
ELEKTA Electron beam source

Injection System
Two types of electron gun producing
electrons in linac:

•Electrons are thermionically emitted from the heated cathode,
focused into a pencil beam by a curved focusing electrode and
accelerated towards the perforated anode through which they
drift to enter the accelerating waveguide.
Injection System

Electron Source
• Based on thermionic emission
• Cathode must be insulated because waveguide is at ground
• Dose rate can be regulated controlling the cathode temperature
– Direct or indirect heating
– The latter does not allow quick changes of electron emission but has a
longer lifetime

Electron Gun
• It is responsible for producing electrons and injecting them into the
accelerator structure .
• Tungsten Mesh/coil produces a stray of electrons due to thermionic
emission when voltage is applied in terms of “Filament current”.
• The electron gun and the source are pulsed so that the high velocity electrons
are injected into the accelerating waveguide at the same time as it is
energized by the microwaves.
• The number of electrons ejected depends upon the temperature of the
filament.
• The electron gun and waveguide system are evacuated to a low pressure to
make the mean free path of electrons between atomic collisions long
compared to path in the system.

Gantry
• Gantry: responsible for directing the photon (x-ray) energy or
electron beam at a patients tumor.
it rotates 360 degrees around a line/point, called the Isocenter.
• 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

Gantry
➢ Source of radiation
can rotate 360 degree.
➢ As the gantry rotates
collimator axis moves
in a vertical plane.
The point of
intersection of
collimator axis and
the axis of rotation of
the gantry is known
as iso center.

Radiofrequency Power Generation System
❖ The microwave radiation used in the accelerating waveguide to
accelerate electrons to the desired kinetic energy is produced by the RF
power generation system, which consists of two major components:
❖ A pulsed modulator.
❖ An RF power source.
❖ Power supply provides DC power to the modulator, which includes the
pulse forming network and a switch tube known as hydrogen thyratron.
❖ High voltage pulses from the modulator section are flat topped DC
pulses of few microseconds in duration.
❖ These pulses are delivered to magnetron or klystron and simultaneously
to the electron gun.
❖ The RF power source is either a magnetron or a klystron.

The radiofrequency power generation
system produces the microwave radiation
used in the accelerating waveguide to
accelerate electrons to the desired kinetic
energy and consists of two major
components:
❖Pulsed modulator
❖RF power sourc
(magnetron or
klystron)

Microwave Sources
Magnetron Klystron
❖ Low energy
Accelerator
❖ Less costly
❖ Smaller least
complicated
❖ Less reliable
❖ Shorter life span
Stable at higher
energies
Costly and
complex
Bulky

The Magnetron
• A device that produces high frequency microwaves
• Functions as a high-power oscillator
• Generating microwave pulses of several
microseconds with repetition rate of several hundred
pulses per second
• Frequency of microwave within each pulse is about
3000 MHz
• Peak power output:
– 2 MW (for low-energy linacs, 6MV or less)
– 5 MW (for higher-energy linacs, mostly use
klystrons)

Magnetron
• Magnetron is cylindrical construction consists of
evacuated central cathode and an outer anode
with resonant cavities machined out of a solid
piece of copper.
• The cathode is heated by an inner filament and
the electrons are generated by thermionic
emission.
• Both the electron gun and the Magnetron are fed
with High voltage power supply & short
duration pulses in synchrony with the
Modulated power supply system.

Magnetron
• Typical high voltage pulse of about 50kVp is a few micro seconds
long and is repeated a few hundred times per second.
• Pulse repetition frequency (PRF) OR Pulse per second differs
according to manufacturer but pulse width remains constant.(Pulses
are of about 4µs duration & are delivered at a PRF of 250Hz.)
• PRF or PPS determines the dose rate from a LINAC.

• A static magnetic field is applied perpendicular to the plane of the cross
section of the cavities and a pulsed DC electric field is applied between the
cathode and the anode.
• Magnetic field causes electrons to spiral outward
• Central cathode that also serves as filament
• The electrons emitted from the cathode are accelerated toward the anode by
the action of the pulsed DC electric field. Under the simultaneous influence
of the magnetic field.
• The electrons move in complex spirals toward the resonant cavities,
radiating energy in the form of microwaves. The generated microwave
pulses are led to the accelerator structure via the waveguide.
Magnetron Operation

The Magnetron
The cathode is heated by an inner filament
Electrons are generated by thermionic emission
Pulse E-field between cathode & anode Electron
accelerated toward the anod
Static B-field perpendicular to the plane of
cavities Electron move in complex spirals
toward the resonant cavities
Radiating energy in form of microwave

Magnetron
Device that
provides high-
frequency
microwave
Accelerate
the electrons
into the
accelerating
waveguide.
Preferred for
lower energies
<=6
Source of
microwave
about 3000
MHZ

The Klystron
• It is not a generator of microwaves
• It acts as a microwave amplifier
– Needs to be driven by a low-power microwave oscillator
• - suitable for high energy accelerators (> 10 MV)

Klystron
❖ The electrons produced by the cathode
are accelerated by a negative pulse of
voltage into buncher cavity which is
energized by low-power microwaves.
❖ The microwaves set up an alternating
electric field across the cavity.
❖ The velocity of the electrons is altered by
the action of this electric field to a
varying degree by a process known as
velocity modulation.

• Electrons form bunches due to variation in velocity resulting in bunching of
electrons as the velocity-modulated beam passes through a field-free space in
the drift tube.
• As the electron bunches arrive at the catcher cavity, they induce charges
on the ends of the cavity and thereby generate a retarding electric field.
• The electrons suffer deceleration, and by the principle of conservation of
energy, the kinetic energy of electrons is converted into high-power
microwaves.
Klystron

Klystron Operation
❖ DC beam produced at high voltage
❖ Low power RF excites input cavity
❖ Electrons are accelerated or deaccelerated
in the input cavity
❖ Velocity modulation becomes time
modulation during drift
❖ Bunched beam excites output cavity
❖ Spent beam is stopped

Electrons
produced by
the cathode
Electrons
are
accelerate
d by –ve
pulse into
buncher
cavity
Lower
level
microwave
set up an
alternating
E field
across the
buncher
cavity
Velocity of e-
is altered by
the action of E-
field (velocity
modulation)
1- Some e- are
speed up
2- Other are
slowed down
Passed in
the drift
tube
(field-free
space)
Electrons arrive
catcher cavity
1-Generate a
retarding E-
field
2-Electrons
suffer
deceleration
3-KE of
electrons
converted into
high-power
microwaves

Microwave Power Transmission
❖ The microwave power produced by the RF generator is carried
to the accelerating waveguide through rectangular uniform
waveguides usually pressurized with a dielectric gas (freon or
Sulphur hexafluoride SF6).
❖ Between the RF generator and the accelerating waveguide is a
circulator (isolator) which transmits the RF power from the RF
generator to the accelerating waveguide but does not transmit
microwaves in the opposite direction.

Accelerator Structure
➢ It consists of a copper tube with its interior divided by copper
discs or diaphragms of varying aperture and spacing.
➢ Waveguides are evacuated or gas filled to allow free propagation
of electrons.
➢ It has metallic structures of rectangular or circular cross-section
used in the transmission of microwaves.

Accelerator Structure
➢ This section is evacuated Electrons interact with the
electromagnetic field of microwaves.
➢ Electrons gain energy from the sinusoidal electric field by an
acceleration process.

• Two types of waveguide are
used in linacs: RF power
transmission waveguides
and accelerating waveguides.
• The power transmission
waveguides transmit the RF
power from the power source
to the accelerating waveguide in
which the electrons are
accelerated.
RF Power Transmission Waveguide

• The electrons are
accelerated in the
accelerating waveguide by
means of an energy
transfer from the high
power RF fields, which
are set up in the
accelerating waveguide
and are produced by the
RF power generators.
RF Power Transmission Waveguide

❑ Accelerating waveguide is obtained from a cylindrical uniform
waveguide by adding a series of disks (irises) with circular
holes at the centre, placed at equal distances along the tube to
form a series of cavities.
❑ The role of the disks (irises) is to slow the phase velocity of
the RF wave to a velocity below the speed of light in vacuum
to allow acceleration of electrons.
❑ The cavities serve two purposes:
• To couple and distribute microwave
power between cavities.
• To provide a suitable electric field
pattern for electron acceleration.
Accelerating Waveguide

Vacuum
• All electron paths, as well as the klystron or magnetron, must
be kept at high vacuum (10-7 torr level) (1 torr = 1 mmHg, 1
atm = 760 torr) to prevent electrical breakdown in the residual
gas for the high electromagnetic fields used to accelerate
electrons

Accelerating Waveguide
❑ Two types of accelerating waveguide
are in use:
• Traveling wave structure
• Standing wave structure

Standing Wave Accelerating Tube
❑ In the standing wave accelerating structure each end
of the accelerating waveguide is terminated with a
conducting disk to reflect the microwave power
producing a standing wave in the waveguide.
❑ Every second cavity carries
no electric field and thus
produces no energy gain for
the electron
(coupling cavities).

Wave Guide System
Accelerator Guide :
Also called as the accelerator
structure , mounted in the gantry:
i)Horizontally (High-energy
machines)
ii)Vertically (low-energy
machines).

Accelerator Structure/ Waveguide
➢ Microwave power (produced in the klyston) is transported to
the accelerator structure in which corrugations (wrinkle) are
used to slow up the waves synchronous with the flowing
electrons.
➢ Accelerating electrons tends to diverge, partly by the mutual
coulomb repulsion and mainly by the radial component of
electric field in waveguide structure.

Accelerator Structure/ Waveguide
➢ Electrons are focused back to their path by the use of co-axial
magnetic focusing field generated by the coaxial coils which
are coaxial with accelerating waveguide.(Also called as steering
coils)
➢ After the flowing electrons leave the accelerator structure, they
are directed toward the target (for photon production) or
scattering foil (for electron production) located in the treatment
head.

Beam Transport
❖The length of waveguides capable of generating high
megavoltage photon beams (> 6 MV) makes a straight beam
treatment head unfeasible.
❖High energy linacs use a beam transport system to deliver the
electrons to the treatment head.
❖This is accomplished using strong electromagnets which bend
the beam through 270o.
❖The three turns cause the electrons to initially diverge and then
converge upon the scattering foil as a pencil beam.

Waveguides for acceleration of electrons
using microwaves
Short standing wave guide
Buncher for initial acceleration
of electrons
Two pictures of accelerating waveguides - shown is a short guide for
standing wave acceleration with cavities placed aside and a travelling
wave guide (lower left).

Standing Wave Guide System
• Standing wave guide structure helps in reducing the accelerating
length due to option of side coupling cavities.

Standing Wave Guide System
• The standing wave structures provide maximum reflection of the
waves at both ends of the structure so that the combination of
forward and reverse traveling waves will give rise to stationary
waves as the microwave power is coupled into the structure via
side coupling cavities.
• Such a design tends to be more efficient than the traveling wave
designs since axial, beam transport cavities, and the side cavities
can be independently optimized.

Travelling Wave Accelerating Tube
❑ In the travelling wave
accelerating structure the
microwaves enter on the gun
side and propagate toward the
high energy end of the
waveguide.
❑ Only one in four cavities is at
any given moment suitable for
acceleration.

Travelling Waveguide System
❖ Travelling wave guide structure require relatively longer
accelerating waveguide.
❖ Functionally, traveling wave structures require a terminating,
or " dummy," load to absorb the residual power at the end of
the structure, thus preventing a backward reflected wave.

Traveling Wave Linac
• Notes
– Injection energy of electrons at 50 kV (v=0.4c)
– The electrons become relativistic in the first portion of the
waveguide
– The first section of the waveguide is described as the buncher
section where electrons are accelerated/deaccelerated
– The final energy is determined by the length of the waveguide
– In a traveling wave system, the microwaves must enter the
waveguide at the electron gun end and must either pass out at
the high energy end or be absorbed without reflection

Standing Wave Linac
• Notes
– In this case one terminates the waveguide with a conducting disc thus
causing a p/2 reflection
– Standing waves form in the cavities (antinodes and nodes)
– Particles will gain or receive zero energy in alternating cavities
– Moreover, since the node cavities don’t contribute to the energy, these
cavities can be moved off to the side (side coupling)
– The RF power can be supplied to any cavity
– Standing wave linacs are shorter than traveling wave linacs because of
the side coupling and also because the electric field is not attenuated

6 MV Short Waveguide
No bending
magnet

Accelerating Tube Parallel To The Beam

Accelerating Tube parapedicular To The
Beam

Accelerating Tube Outside The Room

Electron Beam Transport
❑ In medium-energy and high-energy linacs an electron beam
transport system is used to transport electrons from the
accelerating waveguide to:
▪ X-ray target in x-ray beam therapy
▪ Beam exit window in electron beam therapy
❑ Beam transport system consists of:
▪ Drift tubes
▪ Bending magnets
▪ Steering coils
▪ Focusing coils
▪ Energy slits

• The electrons exit the waveguide and enter the ‘flay tube’ where electron
beam is redirected towards the target, the electrons travel along a ‘Slalom’
path within the flay tube.
• Three pairs of magnets on the either side of the Flay tube, cause the electron
beam to bend through the turns of the Slalom.
• This process not only positions the beam to strike the target, but also focuses
the beam to a diameter of 1mm.
• The design of the magnets enables them to focus the electrons of slightly
different energies on to the same point on the target (Achromatic behavior)
Bending Magnet

Slalom 900 Bending
(Achromatic))
➢ In the higher-energy linac’s, however, the
accelerator structure is too long and, therefore,
is placed horizontally or at an angle with respect
to the horizontal.
➢ The electrons are then bent through a suitable
angle (usually about 90 or 270 degrees)
between the accelerator structure and the target.
➢ 90 degree magnets (Achromatic) have the
property that any energy spread
➢ results in spatial dispersion of the beam.
➢ 270 degree magnets (chromatic) designed to
eliminate spatial dispersion.
Chromatic 2700 Bending
Bending Magnet

Electron Beam Transport
Three systems for electron
beam bending have been
developed:
• 90o bending
• 270o bending
• 112.5o (slalom) bending

•Bending the electron beam
Achromatic
magnet:
All energies are
focused onto the
target
Slits for selection
of electron energy
A 270deg
bending magnet
is typically
better than a
90deg one

Radiation Protection in Radiotherapy Part 5, lecture 2: Eq uipment - superficial, telecurie 75
Bending Magnet
❖ Changes the direction of the
electron beam, downwards toward
the patient.
❖ Bends the pulsed electron beam
towards the target for X-rays or
toward the scattering foil for
electron treatments.
❖ Produces different beam paths for
different energies.
❖ Needed for energies greater than
6MeV.

Beam Transport
Effect of 90o dipole
magnet on exit beam
having
a) Energy spread,
b) Radial displacement,
c) Radial divergence

Linac Treatment Head
The important components in a typical head of
a modern linac )fourth or fifth generation)
designed to shape and monitor the treatment
beam includes:
1. Bending magnet;
2. Shielding material;
3. Several retractable x-ray targets (one for
each x-ray beam energy);
4. Primary collimator;
5. Flattening filters (one for each x-ray beam
energy);

6. electron scattering foils (also called
scattering filters);
7. Beam monitoring devices (Dual
transmission ionization chambers);
8. Adjustable secondary collimator with
independent jaw motion.;
9. A field defining light and range finder;
10.Retractable wedges;
11.Multileaf collimator (MLC)

Part 5, lecture 2: Equipment - superficial, telecurie 103Radiation Protection in Radiotherapy

• Shielding material :The treatment
head consists of a thick shell of high-
density shielding material such as
lead, tungsten, or lead-tungsten alloy.
• Shielding material is used to avoid the
unnecessary irradiation to the
surroundings, patient as well as the
radiation workers.
Treatment Head Shielding

The Linac X-Ray Beam
• Production of x-rays
– Electrons are incident on a target of a high-Z material (e.g. tungsten)
– Target – need water cooled & thick enough to absorb most of the
incident electrons
– Bremsstrahlung interactions
• Electrons energy is converted into a spectrum of x-rays energies
• Max energy of x-rays = energy of incident energy of electrons
• Average photon energy = 1/3 of max energy of x-rays
• Designation of energy of electron beam and x-rays
– Electron beam - MeV (million electron volts, monoenergetic)
– X-ray beam – MV (megavolts, voltage across an x-ray tube,
heterogeneous in energy)

122
❖ Electrons are incident on a target of a high-Z material (Tungsten).
❖ A typical spectrum of a clinical X-ray beam consists of line
spectra that are characteristic of the target material and that are
superimposed on to the continuous bremsstrahlung spectrum.
❖ The bremsstrahlung spectrum originates in the X ray target,
while the characteristic line spectra originate in the target and in
any attenuators placed into the beam
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Production Of X-rays Beam

• X-ray target: The pencil electron
beam strikes on the x-ray target to
produce photons.
• X-ray target used is transmission
type target.
• It is mainly made of Tungsten due
to its high atomic number (Z = 74)
& High melting point 33700C.
• Increasing its thickness maximize
the x-ray output and minimize
electron contamination
X-Ray Target

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-ray beam in the direction of the patient’s tumor.
The X-ray target is located at the focus of the bending magnet.
94% of the electrons energy goes into heat.

Beam Collimation
In a typical modern medical
linac, the photon beam
collimation is achieved with
two or three collimator
devices:
➢ Primary collimator;
➢ Secondary movable beam
defining collimators;
➢ An MLC.

Beam Collimation
The electron beam collimation
is achieved with:
• Primary collimator.
• Secondary collimator.
• Electron applicator (cone).
• Multileaf collimator (under
development).

• The treatment beam is first collimated by a fixed
primary collimator located immediately beyond
the x-ray target.
• It is designed to limit the maximum field size
• It absorbs scatter from the target
Primary Collimator

Primary Collimator
➢ The radiation beams are collimated by
adjusting the upper and lower collimator
jaws.
➢ It be made up of two orthogonal sets of 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-ray beams.
➢ the transmission through the collimators
should be less than 2% of the primary (open)
beam

Asymmetric Collimator
• The asymmetric collimator is a very useful feature on
linacs which now also becomes available on some
Cobalt units.
• The idea is that the two jaws in a collimator set can
move independently.
• This is obviously also a prerequisite for the dynamic
wedge.
• Asymmetric collimation is most commonly used for
‘half beam blocking’ where a beam is blocked at
central axis with the resulting field edge having no
divergence.
• This is very useful for beam matching and functioning.

X-Ray Emission
• x-rays produced from
high energy electrons
impinging on a target
tend to be scattered in the
forward direction
• x-rays produced by
lower energy electrons
tend to be scattered at
right angle to the direction
of the electron beam

Spatial Distribution of X-Rays Around a
Thin Target

Flattening Filter
• It modifies the narrow,
non-uniform photon
beam at the isocenter
into a clinically useful
beam through a
combination of
attenuation of the
center of the beam and
scatter into the
periphery of the beam

Flattening Filter
• Measured in percent at a particular depth in a
phantom (10 cm)
• Must be carefully positioned in the beam or the
beam hitting the patient will be non-uniform,
resulting in hot and cold spots

Beam Flattening Filter
• The flattening filter is a cone
shaped made of lead, steel,
copper etc.
• Change the beam profile at depth
• Absorbs photons on the central
axis
• Producing a more uniform
beam profile at the treatment
distance.

Flattening Filter
• Flatness: a wide beam that is
nearly uniform in intensity
from one side to the other
(+/- 6%)
• Symmetry: the
measure of intensity
difference between
its opposite sides
(+/- 4%)
This is a photo of a Varian flattening filter
for a 24 MV x-ray photon beam on the left
and a 6 MV x-ray photon beam on the right

Treatment Head Components
X-Ray Mode Electro Mode

Electron Beams
• No target required
• Scattering foil used to produce
larger beam - alternative would
be to scan the pencil beam
using electromagnetic fields
• Applicator required to produce
good field delineation on the
patient
Electron
Beam
Scattering
Foil Ion
Chamber
Secondary
Collimator
Electron
applicator
Patient
Primary
Collimator

❖ The Electron beam exits the window of accelerator tube is
narrow pencil beam about 3 mm in diameter.
❖ In electron mode, instead of striking the target, is made strike
an electron scattering foil in order to spread the beam as well as
get a uniform electron fluence across the treatment field
Electron Beam

Scattering Foil
➢ Scattering foil: thin metal sheets
provide electrons with which they
can scatter, expanding the useful size
of the beam.
➢ Typically consist of dual lead foils
with a thickness that ensure
minimize the bremsstrahlung x-rays
➢ narrow beam is usually spread by
two scattering foils
➢ This converts the beam from a pencil
beam to a usable wide beam
Photo of Elekta electron
scattering foils mounted on
moveable carousel

Carrousel
• Carrousel is a device in treatment
head which helps in the movement
of ’Flattening filters of different
energies as well as Scattering foils’.

Dose Monitoring System
• Radiation exposure is controlled by two independent
integrating transmission ionization chamber systems.
• One of these is designated as the primary system and should
terminate the exposure at the correct number of monitor units
• These also steer the beam via a feedback loop

❑ To protect the patient, the standards for dose monitoring
systems in clinical linacs are very stringent.
❑ The standards are defined for:
• Type of radiation detector.
• Display of monitor units.
• Methods for beam termination.
• Monitoring the dose rate.
• Monitoring the beam flatness.
• Monitoring beam energy.
• Redundancy systems.

❑ Transmission ionization chambers, permanently embedded in the
linac’s x-ray and electron beams, are the most common dose
monitors.
❑ They consist of two separately sealed ionization chambers with
completely independent biasing power supplies and readout
electrometers for increased patient safety.

Dose monitoring system
➢ Most linac transmission ionization chambers are permanently
sealed, so that their response is not affected by ambient air
temperature and pressure.
➢ The customary position for the transmission ionization chamber
is between the flattening filter (for x-ray beams) or scattering foil
(for electron beams) and the secondary collimator.

❑ The primary transmission ionization chamber
measures the monitor units (MUs).
❑ Typically, the sensitivity of the primary chamber
electrometer is adjusted in such a way that:
• 1 MU corresponds to a dose of 1 cGy
• delivered in a water phantom at the depth of dose maximum
• on the central beam axis
• for a 10x10 cm2 field
• at a source-surface distance (SSD) of 100 cm.

❑ Once the operator preset number of MUs has been reached,
the primary ionization chamber circuitry:
• Shuts the linac down.
• Terminates the dose delivery to the patient.
❑ Before a new irradiation can be initiated:
• MU display must be reset to zero.
• Irradiation is not possible until a new selection of MUs and
beam mode has been made.

Ionization Chamber
• Photos of Elekta Ionization chamber

Vertical And Horizental Chambers
This system is not identical for all manufacturers - important is that
a variety of feedback loops allow to steer the beam and turn it off if
certain parameters are not met.

Secondary Collimators:
• Place Away from the x-ray target
• Secondary collimators are typically
independent jaws
• It allow the field to be shaped into a
variety of rectangular shapes.
• There are two sets of jaws higher jaws
(y-axis) lower jaws (x-axis)
• independent jaws to perform dynamic
wedging.

• The beam is further collimated by a
continuously movable x-ray collimators.
• This collimators consists of two pairs of lead of
tungsten blocks (jaws) which provide a
rectangular opening (from 0X0 to 40X40 cm2)
projected at a standard distance such as 100 cm
from the x-ray source.
• The collimator blocks are constrained to move
so that the block edge is always along a radial
line passing through the x-ray source position.
Secondary Collimators

• Field light: The field size definition is
provided by a light localizing system in the
treatment head.
• It is a Field localizing device, Used to display
the position of the radiation field on the
patient skin.
• A combination of mirror and a light source
located in the space between the chambers
and the jaws projects a light beam as if
emitting from the x-ray focal spot.
Field light

• An high accuracy bulb is
placed at 450 angle with the
Mercury mirror placed in the
path of the beam
(Transmission type mirror) .
• Thus the light field is
congruent with the radiation
field. allows accurate
positioning of the radiation
field in relationship to skim
marks or other reference
points.
Field light

Wedge
• It is a tool that cause
a progressive
reduction in intensity
across the beam,
resulting in tilting the
isodose curves from
their normal position.

Wedge Angle
• The wedge isodose angle (q) is the
complement of the angle through which the
isodose curve is tilted with respect to the
central ray of thr beam at any specified depth.
• This depth is important because the angle
decrease with increasing depth.
• The choice of reference depth varies;
– 10 cm depth
– ½ - 2/3rd of the beam width.
– At the 80% isodose curve (MV)
– At the 50% isodose curve (KV)

Wedges Systems
There are three different types of wedges systems;
➢ Physical, conventional, manual, or external wedges
➢ Universal, motorized or internal wedges
➢ Dynamic or virtual wedges

Physical (Manual Or External) Wedges
It is an angled piece of lead or steel or copper or tungsten that is placed in
the beam to produce a gradient in radiation intensity (at a distance of at
least 15 cm from theskin)

Physical Wedge
• The thin end causes less attenuation than the thick end; this causes a shift in
the isodose curves within the treated volume.
• The wedge is denoted by the angle it tilts the isodose curves
• eg. a 30o wedge would cause a 30o tilt in the isodose
curves.
• Physical wedges are not in common use due to the ability of the independent
jaws to perform dynamic wedging.

Physical (Manual Or External) Wedges
• Manual intervention
is required to mount
physical wedges on
the gantry head’s
collimator assembly
(VARIAN,
SIEMENS)

Physical Wedge
Typically linacs are
equipped with 4
wedges: 15, 30, 45
and 60deg.
The wedge angle can
be defined in
multiple ways - it is
typically the angle of
the 80% isodose line.

Universal (Motorized Or Internal) Wedges
• It is a single wedge if it is introduced
in the beam for a full time the isodose
curve will be tilted by 60o.
• If the wedge is positioned for a
shorter time the wedge angle (isodose
tilting) will be smaller.
• Any angle between 10o to 60o can be
obtained.
Universal wedges by ELEKTA

Universal (motorized or internal) wedges
• A single wedge serves for all beam widths
• On ELEKTA machines, a single wedge of 60° is
permanently mounted inside the linac head and automatically
inserted into the treatment beam during beam delivery.
• Other wedge angles less than 60° can be obtained by
combining the 60° wedge field and the open field with proper
weights depending on the desired wedge angle.

Dynamic Wedge
• The wedge shape is generated by moving
one jaw (hot jaw) while the beam is on (the
other is static: cold jaw)
• The resultant wedged beam is clean, more
flexible in terms of field size and wedge angle,
and does not require manual loading/unloading

Dynamic Wedges
• Dynamic wedge most popular
because:
– no weight
– any wedge angle possible
– but difficult to commission

Virtual Wedge Beamline
Dose Rate Control
Jaw Speed Constant
MU/min
mm/sec

Multi Leaf Collimator
• A multileaf collimator (MLC) for
photon beams consists of a large
number of collimating blocks or leaves
that can be driven automatically,
independent of each other, to generate
a field of any shape.
• Typically the MLC systems consists 60
to 120 pairs, which are independently
driven.
• The individual leaf has a width of 1 cm
or less as projected at the isocenter.

• The leaves are made of tungsten alloy (r
=17.0 to 18.5g/cm3) and have thickness along
the beam direction ranging from 6cm to
7.5cm, depending on the type of accelerator.
• The leaf thickness is sufficient to provide
primary x-ray transmission through the
leaves of less than 2%.
• The primary beam transmission may be
further minimized by combining jaws with
the MLC in shielding areas outside the MLC
field opening.

• The transmission through the
collimators should be less than
2% of the primary (open) beam.
• The transmission between the
leaves should be checked to
ensure that it is less than the
manufacturer’s specification

Multi Leaf Collimator (MLC)
• Used to define any field shape for
radiation beams
• The number of leaves in
commercial models with 120
leaves (60 pairs) covering fields up
to 40 × 40 cm2
• MLCs are becoming valuable in
supplying intensity modulated
fields in conformal radiotherapy,
either in the step and shoot mode or
in a continuous dynamic mode.

MLC
• The quality of the field definition
depends on the width of the leafs
• There is always some interleaf leakage
• Typically the transmission through the
MLC is larger than through a standard
collimator

Dynamic MLC
• Concept similar to dynamic wedge
• The field shape is altered step-by-
step or dynamically while dose is
delivered
• When MLC moves during
treatment different parts of the field
are shielded resulting in different
overall radiation levels delivered in
different parts of the beam:
Intensity modulated radiotherapy

Electron Applicators
• may be
– open sided for modern accelerators using double scattering
foils or scanned beams
– enclosed for older accelerators using single scattering foils
• must be checked for leakage
– adjacent to the open beam
– on the sides of the applicators

Electron Applicator
An example of an electron applicator attached to the treatment head

Electron Applicator
Base of electron
applicator Indicates
field size at surface
Enables insertion of
individual low
melting point alloy
cut-outs for beam-
shaping

Lasers
➢ The accuracy of the laser guides in
determining Isocenter position.
➢ Isocenter is a virtual point where the
central axis of Gantry, Collimator and
couch meets.
➢ 2 Side lasers, saggital and Ceiling
lasers are mounted on walls of
LINAC unit.
➢ Tolerance of laser position is 2 mm

Linear Accelerator
Mechanical System

Treatment Couch
➢ Treatment couch: mounted on a rotational
axis around the isocenter
➢ Also called patient support assembly (PSA)
➢ It is a mechanically movable motor driven
couch in a horizontal and lengthwise
direction- must be smooth and accurate
allowing for precise and exact positioning of
the isocenter during treatment positioning
➢ Support up to 120-160 kgm.
➢ Range in width from 45-50 cm

Treatment Couch
➢ Racket-like frame should be periodically
tightened to provide more patient
support and reduce the amount of sag
during treatment positioning.
➢ Immobilization devices can be hold in
the table.

Treatment Table(couch)
• Patient is positioned over the treatment table
according to the desired co-ordinates of
planning.
• Patient is immobilized using the
Immobilization devices.
• Hand Pendent: It contains all the control
switches which can be used to access the
movement of Gantry, Couch, Collimator
jaws(Field size),SSD etc.,

Electronic Portal Imaging
➢Imaging device at the
beam exit side of the
patient to record the
treatment field
➢Allows to verify that the
field was delivered to
the correct location in
the patient
➢Many different systems
available...
Siemens Varian

Electronic Portal Image
Comparison of
simulator and
portal image
(right)

Electronic portal imaging overcomes two
problems by making it possible to view the
portal images instantaneously
1) real time images can be displayed on computer
screen before initiating a treatment.
2) Portal images can also be stored on computer
for later viewing or archiving.

• EPIDs use flat panel arrays of solid state detectors based on
amorphous silicon (a-Si) technology .Flat panel arrays are
compact, & easier to mount on a retractable arm for positioning
in or out of the field.
• A scintillator converts the radiation beam into visible photons.
The light is detected by an array of photodiodes implanted on an
amorphous silicon panel.

On Board Imaging (OBI)
• CT scans acquired with detectors imbedded in a flat panel
instead of a circular ring is known as Onboard Imaging .
• CT scanning that uses this type of geometry is known as
cone-beam computed tomography (CB CT) .
• In cone-beam CT, planar projection images are obtained from
multiple directions a s the source with the opposing detector
panel rotates around the patient through 1800 degrees or more.

On Board Imaging (OBI)
• These multidirectional images provide sufficient
information to reconstruct patient anatomy in 3D ,
including cross-sectional, sagittal, and coronal planes.
• A filtered back-projection algorithm is used to reconstruct
the volumetric images.
• They are mounted on the accelerator gantry and can be
used to acquire volumetric image data under actual
treatment conditions.
• They enable the localization of planned target volume and
critical structures before each treatment.
• The system can be implemented either by using a
kilovoltage x- ray source or the mega voltage therapeutic
source.

• Kilovoltage x-rays for a kilovoltage CBCT (
kVCBCT) system are generated by a
conventional x-ray tube that is mounted on a
retractable arm at 900 to the therapy beam
direction.
• A flat panel of x-ray detectors is mounted
opposite the x-ray tube.
• This imaging system is versatile and is
capable of cone-beam CT as well 2-D
radiography and fluoroscopy.
Kilovoltage CBCT

1. Produce volumetric CT images
with good contrast an sub
millimeter spatial resolution.
2. acquire images in therapy room
coordinates, and
3. Use 2-D radiographic and
fluoroscopic modes to verify portal
accuracy, management of patient
motion, and making positional and
dosimetric adjustments before and
during treatment.
Advantages of Kilovoltage CBCT

Megavoltage CBCT
• MVCBCT uses the megavoltage x-ray beam of the
linear accelerator and its EPID mounted opposite the
source.
• EPIDs with the a-Si flat panel detectors are sensitive
enough to allow rapid acquisition of multiple, low-dose
images as the gantry is rotated through 1800 or more.
• Multidirectional 2-D images, volumetric CT images are
reconstructed from these.
• MVCBCT system has good image quality for the bony
anatomy and, in even for soft tissue targets.

Megavoltage CBCT
MVCBCT is a great tool for;
➢ On-line or pretreatment
verification of patient positioning,
➢ Anatomic matching of planning
CT,
➢ Pretreatment CT, avoidance of
critical structures such as spinal
cord, and
➢ identification of implanted metal
markers if used for patient setup.

Advantages Of MVCBCT Over KVCBCT
❖ Less susceptibility to artifacts due to high-Z objects such as
metallic markers in the target, metallic hip implants, and dental
fillings
❖ No need for extrapolating attenuation coefficients from kVto
megavoltage photon energies for dosimetric corrections
• From images we can say that MV-CBCT image of 2.5cGy is
sufficient for bony anatomy verification during patient positioning.

Cooling System
• Heat dissipation in linear accelerator is an
important step in maintenance in large setup
and heavy patient load in hospitals.
• The x-rays produced are almost the 1 percent
of the electron energy which is striking on the
target.
• Hence 99% of the energy is converted to heat.
• This heat is needed to be cooled and that is
achieved by the ‘Cooling system’.
• It is located in the drive stand and gantry.

Cooling System
• It provides thermal stability to the
system.
• It allows many components in the drive
stand and gantry to operate at a constant
temperature

Cooling System
• Cooling system consists
of ‘water chiller’ for
cooling the water and
water inlets and outlets to
various parts of LINAC
including X-ray target.

Radiation Safety/Interlock System
• Safety from radiation also plays an important role
in Radiotherapy.
• Various Interlocks are present in LINAC to avoid
the mis-happens or wrong treatment to the patient.
• Interlocks indicates the problem in particular
device in the LINAC assembly and interlocking
system helps in solving the particularly and easily.

Radiation Safety/interlock system
• Safety Interlocks include:
1)Last Man Out Switch(LMO)
2)Door interlock
3)Beam ON/OFF Key etc.,
• Emergency switches are provided at all the systems
of an LINAC unit to completely turn Off the entire
Unit with only single switch during emergency
situations.

Field Blocking
• Significant irradiation of the normal tissue outside the target
must be avoided as much as possible.
• These restrictions can give rise to complex field shapes, which
require intricate blocking.
• Field blocking and shaping devices are:
➢Shielding blocks
➢Custom blocks
➢Asymmetric Jaws
➢Multileaf collimators

Shielding Blocks
• Aims of Shielding:
• Protect critical organs
• Avoid unnecessary radiation to surrounding normal tissue
• Matching adjacent fields
• An ideal shielding material should have the following
characteristics:
• High atomic number.
• High-density.
• Easily available.
• Inexpensive.
• The most commonly used shielding material for photons is Lead (Pb).

Custom Blocks
• Custom blocking system uses a low melting point alloy,
Lipowit metal ( Cerrobend) , which has a density of 9.4 g/cm3
at 20°C (83% of the lead density)
• This material consists of,
1) 50.0% bismuth
1) 26.7 % lead,
2) 13.3 % tin, and
3) 10% cadmium
• The main advantage of Cerrobend over lead is that it melts at
about 70°C and can be easily cast into any shape.
• At room temperature, it is harder than lead.

Custom Blocks

Field Blocking
Beam Energy Required Lead
thickness
Co-60 (1.25
MeV
5 cm
4 MeV 6 cm
6 MeV 6.5 cm
10 MeV 7 cm
15 <eV 8 cm

Compensators
➢ A beam modifying device which evens
out the skin surface contours, while
retaining the skin-sparing advantage.
➢ It allows normal depth dose data to be
used for such irregular surfaces.
➢ Compensators can also be used for
➢ Tocompensate for tissue heterogeneity.
➢ Tocompensate for dose irregularities
arising due to reduced scatter near the
field edges (example mantle fields), and
horns in the beam profile.
Notice the reduction in
the hot spot
Without compensator with compensator

Compensators
h'
h
d
d
h’/h
Radiation Protection inSeRmadinioathreornapByeamModification Devices. Moderator : Dr. S.C. Sharma. Department of Radiotherapy.
1
• Can be constructed using thin
sheets of lead, lucite or
aluminum.
• This results in production of a
laminated filter

Compensators
Radiation Protection inSeRmadinioathreornapByeamModification Devices. Moderator : Dr. S.C. Sharma. Department of Radiotherapy.

Bolus
• A tissue equivalent material used to reduce the depth of the
maximum dose (Dmax).
• Better called a “build-up bolus”.
• Commonly used materials are:
– Cotton soaked with water.
– Paraffin wax.
• Other materials that have been used
– Mix- D (wax, polyethylene, mag oxide)
– Temex rubber (rubber)
– Lincolnshire bolus (sugar and mag carbonate in form of spheres)
– Spiers Bolus (rice flour and soda bicarb)

Bolus
➢Commercial materials:
➢Superflab: Thick and doesn't undergo elastic deformation. Made of
synthetic oil gel.
➢Superstuff: Add water to powder to get a pliable gelatin like material.
➢Bolx Sheets: Gel enclosed in plastic sheet.
➢ A bolus can be used in place of a compensator for kilovoltage
radiation to even out the skin surface contours.
➢ In megavoltage radiation bolus is primarily used to bring up
the buildup zone near the skin in treating superficial lesions.

Beam Spoiler
• A beam spoiler is a piece of
material, placed into the path of the
photon beam in radiotherapy.
• The purpose of the spoiler is to
reduce the depth of the maximum
radiation dosage.
• The relative surface dose increases
when the surface to tray (spoiler)
distance is reduced.

Beam Spoiler
• The beam spoiler is composed of a sheet of material which has
a low atomic number, typically lucite, the thickness of which
is varied according to the beam energy and the distance by
which the radiation dose must be shifted.
BEAM
SPOILER

 Advantages: particles are able to reach very high energies
without the need for extremely high voltages
 Linear accelerators attack the affected area with higher doses
of radiation than other machines
Advantages

 Disadvantages: A linear accelerator can cost anywhere between
onemillion and three million dollars. Operating the machine
costs about $900,000 annually.
 The particles travel in a straight line, each accelerating segment is
used only once. The segments run in short pulses, limiting the
average current output and forcing the experimental detectors to
handle data coming in short bursts, thus increasing the
maintenance expense
Disadvantages

Advantages And Disadvantages Of
Treatment By Photon
• Advantages:
• High penetration depth
• Applicable for deep tumor treatment
• Low dependency with heterogeneity and flatness of surface
• Low dependency of beam profile penumbra with depths
• Disadvantages:
• Dose delivery to normal tissues beyond of tumor
• Low dose level for superficial parts (for superficial tumors treatment).

Advantags And Disadvantages Of
Treatment By ElectronAdvantages:
• Energy deposition in a small volume
• Applicable for shallow tumor treatment
• Lesser dose delivery to normal tissues beyond of the tumor
Disadvantages:
• Small penetration depth
• Dependency of the dose distribution to the density heterogeneity
• Dependency to the flatness of the irradiated surface
• Increment of the penumbra with depth
• Small fields problem

Medical linear accelerator

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