CLINICAL RADIATION GENERATORS IS USEFUL FOR RADIOTHERAPY AND NUCLEAR MEDICINE

2. The aim of radiotherapy is maximum and
uniform dose to the tumor volume and
minimal dose to the normal tissue or organ
at risk .
The successful of radiation therapy treatment
was depend on ability of radiation generating
equipment.

3.  High particle energy for penetration
 High particle flux for sufficient dose rate
 Energy efficient
 Compact
 Not too expensive
 Reliable
 Simple to operate
 Safe

4.  1895 K.Roentogen discovers X-rays.
 1913W.E.Coolidge develops vacuum X-ray tube.
 1931 E.O.Lawrence develops a cyclotron.
 1932 1MVVan de Graaff accelerator installed, Boston, MA (USA).
 1939 First medical cyclotron for neutron therapy, Crocker, CA
(USA).
 1946 20MeV electron beam therapy with a Betatron, Urbana, IL
(USA).
 1952 First Co-60 teletherapy units, Saskatoon (Canada).
 1956 First 6MeV linear accelerator, Stanford, CA (USA).
 1958 First proton beam therapy (Sweden).
 1959 First scanning electron beam therapy, Chicago, IL (USA).
 1976 First pion beam therapy, LAMPF, NM (USA).
 1990 First hospital based proton therapy, Loma Linda, CA (USA).

5.  kVp therapy machines
 Van de Graff accelerator
 Betatron
 Microtron
 Linear accelerator
 Cyclotron
 Radioactive source based therapy units

6. Photons are generated by hitting a solid
target with electrons via Bremsstrahlung
interactions.
 The electron energy is below 300 keV or the
acceleration potential is smaller than 300 kV.
 The photon energy spectrum is broad

7.  Grenz-ray therapy: < 20 kV
 Contact therapy: 40 kV ~ 50 kV
 Superficial therapy: 50 kV ~ 150 kV
 Orthovoltage (or deep) therapy: 150 kV ~ 500
kv
 Super voltage therapy: 500 kV ~ 1000 kV
 Megavoltage therapy: 1 MV

8.  Accelerating potential = 40 to 50 kV.
 Tube current = 2 mA
 SSD = 2 cm
 Beam hardening by 0.5-to 1.0-mm aluminum
filter.
 Useful for tumors not deeper than 1mm

9.  Accelerating potential = 50 to 150 kV.
 1-to 6-mm aluminum for beam hardening.
 Half-value layer (HVL) = 1-to 8-mm Al.
 Applicators or cones for field collimation.
 SSD = 15 to 20 cm.
 Tube current = 3 –8 mA.
 Useful for tumors confined to about 5-mm
dept

10.  Potential = 150 to 500 kV.
 Beam current = 10 to 20 mA.
 HVL = 1 to 4 mm Copper (Cu).
 SSD = 50 cm.
 Application: tumor located < 2 –3 cm in depth
 Limitation of the treatment:
 skin dose
 Depth dose distribution
 Increase absorbed dose in bone
 Increase scattering

11. A Grenz rays, HVL=0.0 4mm Al,
φ=33cm, SSD=10cm
B Contact therapy, HLV= 1.5mm Al,
φ=2cm, SSD=2cm
C Superficial therapy, HV L=3mm Al,
φ=3.6cm, SSD=20cm
D Orthovoltage, HVL=2mm Cu,
10x10cm2, SSD=50cm
E Co-60 γ-rays, 10x10cm2, SSD=80cm

12.  Energy: 500 –1000 kV
 Conventional transformer systems were not
suitable for producing potential > 300 kVp
 The problem solved by invention of resonant
transformer

13. Resonant transformer units Used to generate x-rays from 300 to
2000 kV
At resonant frequency
Oscillating potential attains very high amplitude
Peak voltage across the x-ray tube becomes very large

14.  X-ray beams of energy > 1 MeV
 Accelerators or γ ray produced by
radionuclides
 Examples of clinical megavoltage
machinesVan deGraaff generator
 Linear accelerator
 Betatron
 Microtron
 Teletherapy γray units (e.g. cobalt-60)

15.  TheVan de Graaff machine is an electrostatic
accelerator designed to accelerate charged
particles.
 In radiotherapy, the unit accelerates electrons to
produce high-energy x-rays, typically at 2 MV.
 In this machine, a charge voltage of 20 to 40 kV
is applied across a moving belt of insulating
material. A corona discharge takes place and
electrons are sprayed onto the belt.

16. • These electrons are carried to the
top where they are removed by a
collector connected to a spherical
dome.
• As the negative charges collect
on the sphere, a high potential is
developed between the sphere
and the ground.
• This potential is applied across
the x-ray tube consisting of a
filament, a series of metal rings,
and a target.

• The rings are connected to
resistors to provide a uniform
drop of potential from the
bottom to the top

17. X-rays are produced when the electrons strike the
target.Van de Graaff machines are
 capable of reaching energies up to 10 MV,
limited only by size and required high-voltage
insulation.
 Van de Graaff and resonant transformer units
for clinical use are no longer produced
commercially.
 The reason for their demise is the emergence of
technically better machines such as cobalt-60
units and linear accelerators.


18.  Betarton was the first electron accelerator
used for radiotherapy in early 1950s.
 An electron in a changing magnetic field
experiences accelerator in a circular orbit.
 Can produce 6 MeV to 40 MeV electrons.
 Electron beam current is low for photon
therapy (or low dose rate).
 The field is small.

20. he 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
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

21.  LOW ENERGY PHOTONS (4–8 MV):
 straight-through beam; fixed flattening filter;
external wedges; symmetric jaws; single
transmission ionization chamber; isocentric
mounting.
 MEDIUM ENERGY PHOTONS (10–15 MV) and
electrons: bent beam; movable target and
flattening filter; scattering foils; dual
transmission ionization chamber; electron cones
LINAC GENERATIONS

22.  HIGH ENERGY PHOTONS (18–25 MV) AND
ELECTRONS: dual photon energy and multiple
electron energies; achromatic bending magnet;
dual scattering foils or scanned electron pencil
beam; motorized wedge; asymmetric or
independent collimator jaws
 High energy photons and electrons: computer
controlled operation; dynamic wedge; electronic
portal imaging device (EPID); multileaf
collimator (MLC.

23.  Linacs are usually mounted isocentrically and
the operational systems are distributed over
five major and distinct sections of the
machine, the
● Gantry;
● Gantry stand or support;
● Modulator cabinet;
●Patient support assembly (i.e. treatment
table);
● Control console

25.  The main beam forming components of a
modern medical linac are usually grouped
into six classes.
(i) Injection system
(ii) RF power generation system
(iii) Accelerating waveguide
(iv) Auxiliary system
(v) Beam transport system
(vi) Beam collimation and beam monitoring
system.

26.  The injection system is the source of electrons;
it is essentially a simple electrostatic
accelerator called an electron gun.
 Two types of electron gun are in use as sources
of electrons in medical linacs:
 Diode type;
 Triode type

27.  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.

28.  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
An RF power source;
A pulsed modulator.

29.  The RF power source is either a magnetron
or a klystron.
 both are devices that use electron acceleration and
deceleration in a vacuum for the production of high
power RF fields.
MAGNETRON
The magnetron is a device that produces
microwaves.

31.  It functions as a high-power oscillator,
generating microwave pulses of several
microseconds' duration and with a repetition
rate of several hundred pulses per second.
 The magnetron has a cylindrical construction,
having a central cathode and an outer anode
with resonant cavities machined out of a solid
piece of copper

32.  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.

33.  The generated microwave pulses are led to
the accelerator structure via the waveguide.
 Typically, magnetrons operate at 2 MW peak
power output to power low-energy linacs (6
MV or less).
Although most higher-energy linacs use
klystrons, accelerators of energy as high as 25
MeV have been designed to use magnetrons
of about 5 MW power.


34.  The klystron is not a generator of microwaves
but rather a microwave amplifier.
 It needs to be driven by a low-power
microwave oscillator.
 The electrons produced by the cathode are
accelerated by a negative pulse of voltage
into the first cavity, called the buncher cavity,
which is energized by low-power microwaves.

36.  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.
 Some electrons are speeded up while others
are slowed down and some are unaffected.

37.  This results in bunching of electrons as the
velocity-modulated beam passes through
a field-free space in the drift tube.
The electrons suffer deceleration, and by
the principle of conservation of energy, the
kinetic energy of electrons is converted
into high-power microwaves.

38.  Waveguides are evacuated or gas filled
metallic structures of rectangular or
circular cross-section used in the
transmission of microwaves.
 The length of the accelerating waveguide
depends on the final electron kinetic
energy, and ranges from ~30 cm at 4 MeV
to ~150 cm at 25 MeV.
Two types of waveguide are used in linacs:
1. RF power transmission waveguides
2. accelerating waveguides.

39.  The power transmission waveguides transmit
the RF power from the power source to the
accelerating waveguide in which the
electrons are accelerated.
 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.

40.  The simplest kind of accelerating waveguide
is obtained from a cylindrical uniform
waveguide by adding a series of discs (irises)
with circular holes at the centre, placed at
equal distances along the tube.
 These discs divide the waveguide into a series
of cylindrical cavities that form the basic
structure of the accelerating waveguide in a
linac.

41.  Two types of accelerating waveguide have
been developed for the acceleration of
electrons:
 Travelling wave structure
 Standing wave structure
 In the travelling wave structure the
microwaves enter the accelerating waveguide
on the gun side and propagate towards the
high energy end of the waveguide.

42.  These guides have relatively low shunt
impedances compared to standing
waveguide systems therefore, they need to
be physically longer to achieve the same
output energy.
 Electrons from the gun end enter a velocity
of 0.8 c (at 80 kV) where c is the velocity of
light.

43.  After the first 30 cm, they are travelling at
velocities close to c.
 This first part of the guide is called the
buncher section.
 Thereafter, further energy gain results in a
relativistic mass increase, and the iris
separation remains constant.

44. WAVE GUIDE

45.  In this configuration only one in four
cavities is at any given moment suitable
for electron acceleration, providing an
electric field in the direction of
propagation.

46.  In the standing wave structure each end of
the accelerating waveguide is terminated
with a conducting disc to reflect the
microwave power, resulting in a buildup of
standing waves in the waveguide.
 In this configuration, at all times, every
second cavity carries no electric field and thus
produces no energy gain for the electrons.

47.  These cavities therefore serve only as
coupling cavities and can be moved out to the
side of the waveguide structure, effectively
shortening the accelerating waveguide by
50%.

48.  In low energy linacs the target is embedded in
the accelerating waveguide and no beam
transport between the accelerating
waveguide and target is required.
 Bending magnets are used in linacs operating
at energies above 6 MeV, where the
accelerating waveguides are too long for
straight-through mounting.

49.  The accelerating waveguide is usually
mounted parallel to the gantry rotation
axis and the electron beam must be bent
to make it strike the X ray target or be able
to exit through the beam exit window.
● 90º bending;
● 270º bending (achromatic);
● 112.5º (slalom) bending

50.  Electrons moving in a magnetic field will
be bent in a trajectory dependent on the
energy of the individual electron.
 The requirement for the generation of
satisfactory clinical beams is that all the
electrons shall exit from the accelerating
tube within a small focal spot and at the
correct angle.

51.  the electron beam at the input to the bending
chamber will have a very narrow energy
spread so that all the electrons will
automatically follow the same trajectory.
 precise control of beam energy is difficult to
achieve and a more stable solution is to design
a so-called achromatic bending system where
electrons of different energies exit at the same
point and in the same direction in spite of their
having followed different trajectories

52.  more energetic electrons (larger radii) enter
closer spaced regions with higher bending
fields, and less energetic electrons encounter
lower bending fields between the wider pole
spaces.
 all electrons that entered on axis at 08 should
converge again at the same point at 2708.
The deflection is without dispersion with
energy (i.e. achromatic).

53.  The linac head contains several components
that influence the production, shaping,
localizing and monitoring of the clinical
photon and electron beams.
 The important components found in a typical
head of a fourth or fifth generation linac
include.

54.  Electrons originating in the electron gun are
accelerated in the accelerating waveguide to
the desired kinetic energy and then brought,
in the form of a pencil beam, through the
beam transport system into the linac
treatment head, where the clinical photon
and electron beams are produced.

55.  linac include:
 Several retractable X ray targets;
 Flattening filters and electron scattering foils
(also called scattering filters);
 Primary and adjustable secondary
collimators;
 Dual transmission ionization chambers;A
field defining light and a range finder;
 Optional retractable wedges;
 Optional MLC

56.  Clinical photon beams are produced with a
target–flattening filter combination.
 Clinical electron beams are produced by
retracting the target and flattening filter from
the electron pencil beam and Either
scattering the pencil beam with a single or
dual scattering foil .

57.  The primary collimator defines a maximum
circular field, which is then further truncated
with an adjustable rectangular collimator
consisting of two upper and two lower
independent jaws and producing rectangular
and square fields with a maximum dimension
of 40 × 40 cm2 at the linac isocentre.

58.  IEC specifies in detail the standards for
radiation monitors installed in clinical
electron linacs.
 It deals with standards for the type of
radiation detectors, display of monitor units
(MUs),

59.  Most common dose monitors in linacs are
transmission ionization chambers
permanently imbedded in the linac clinical
photon and electron beams to monitor the
beam output continuously during patient
treatment.
 Most linacs use sealed ionization chambers
to make their response independent of
ambient temperature and pressure.

60.  For patient safety, the linac dosimetry
system usually consists of two separately
sealed ionization chambers with completely
independent biasing power supplies and
readout electrometers.
 if the primary chamber fails during patient
treatment, the secondary chamber will
terminate the irradiation, usually after an
additional dose of only a few per cent above
the prescribed dose has been delivered.

61.  Charged particles are accelerated by electric
field cyclically.
 Charged particles fly in circular orbits in
magnetic field.
 The radius of the orbits increases as the
particle speed increases.
 Synchrotron was invented to overcome the
energy limit.

62. Used to generate high energy protons and
heavy ions for therapy.
 Used to accelerate deuterons to produce
neutrons.
 Used for the production of radionuclide's . i.e.
for PET

64. Radionuclides Half-life
[years]
energy
[MeV]
Γvalue
[Rm2/Ci hr]
[Ci/g]
Cs -137 30 0.66 0.325 50
Co 60 5.26 Avg 1.25 1.33 200

65. Co-60 unit
 60Co is generated from 59Co via 59Co(n,γ) 60Co
reactions.
 The half-life for 60Co is 5.26 years.
 60Co decays to 60Ni with the emission of
βparticles and two photons (1,17 MeV and 1.33
MeV) per disintegration.
 The Co source is a cylindrical capsule of 1 cm
diameter and 2 cm long, causing relative large
penumbra.
 β particles are absorbed in the Co-metal and the
stainless steel capsule.

68. 1 Cobalt-
60source;
2 tungsten
cylinder;
3 enriched
uranium;
4, lead;
5 laser source;
6collimator;
7 g-rays

69.  The housing for the source is called the
sourcehead .
 It consists of a steel shell filled with lead for
shielding purposes and a device for bringing
the source in front of an opening in the head
from which the useful beam emerges. Also, a
heavy metal alloy sleeve is provided to form
an additional primary shield when the source
is in the off position.

70.  A number of methods have been developed for
moving the source from the off position to the on
position.
 (a) the source mounted on a rotating wheel inside the
sourcehead to carry the source from the off position
to the on position;
 (b) the source mounted on a heavy metal drawer plus
its ability to slide horizontally through a hole running
through the sourcehead in the on position the source
faces the aperture for the treatment beam and in the
off position the source moves to its shielded location
and a light source mounted on the same drawer
occupies the on position of the source

71.  The 60Co source, usually in the form of a solid
cylinder, discs, or pallets, is contained inside a
stainless steel capsule and sealed by welding.
 This capsule is placed into another steel capsule,
which is again sealed by welding.The double-
welded seal is necessary to prevent any leakage
of the radioactive material.

72.  All of the above mechanisms incorporate a
safety feature in which the source is returned
automatically to the off position in case of a
power failure.

73.  A collimator system is designed to vary the
size and shape of the beam to meet the
individual treatment requirements.
 The simplest form of a continuously
adjustable diaphragm consists of two pairs of
heavy metal blocks

74.  Each pair can be moved independently to obtain
a square- or a rectangle-shaped field. Some
collimators are multivane type (i.e., multiple
blocks to control the size of the beam).
 The term penumbra, in a general sense, means
the region, at the edge of a radiation beam, over
which the dose rate changes rapidly as a
function of distance from the beam axis.

77.  the penumbra width increases with an increase
in source diameter, SSD, and depth but
decreases with an increase in SDD.
 The geometric penumbra, however, is
independent of field size as long as the
movement of the diaphragm is in one plane;
that is, SDD stays constant with an increase in
field size.

78. Thank you