Teletherapy Cobalt-60 Machines

Teletherapy Cobalt-60
Machines
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

❖ Treatment machines incorporating γ ray sources for use in
external beam radiotherapy are called isotopic teletherapy
machines.
❖ For use in external beam radiotherapy, γ rays are obtained from
specially designed and built sources that contain a suitable,
artificially produced radioactive material. The parent source
material undergoes a β decay, resulting in excited daughter
nuclei that attain ground state through emission of γ rays (γ
decay).
Introduction

Megavoltage Treatment Machines
• Megavoltage treatment machines are the foundation of
modern radiotherapy techniques.
• Photon beams are produced in megavoltage treatment
machines.
• Their one difference from orthovoltage machines is the skin-
sparing effect (i.e., the maximum energy is delivered in the
subepidermal region).
• The skin-sparing effect is directly proportional to the energy
of the photons.

In 1949, Dr. Harold E. Johns , a Canadian
medical physicist sent a request to the
National Research Council(NRC) asking
them to produce Cobalt-60 isotopes for
use in a cobalt therapy unit prototype.
History of Cobalt 60 Teletherapy Machine

In 1951,oct 23rd; University of
Saskatchewan medical physicist
Dr. Harold Johns and his
graduate students became the
first researchers in the world to
successfully treat a cancer
patient using cobalt-60
radiation therapy.

➢ On October 27, 1951, the
world’s first cancer treatment
with COBALT 60 radiation
took place at Victoria Hospital
for a 43 year old cervical
cancer patient.
➢ This marked an important
milestone for the fight against
cancer.

• The unit consisted of a head pivoted
between the arms of a horizontal Y which
could be raised and lowered.
• The beam was turned on and off by an air
compressor that forced mercury in and
out of the reservoir, the radiation beam
being off when a pool of mercury was
between the source and a conical opening
in the head.
• Field size was varied by means of four
lead blocks at right angles to each other.

1951 : Cobalt Bomb (H.E.Johns)
First in SaskatoonCobalt 60 machine, Canada

Introduction
❖ The invention of the cobalt-60 teletherapy machine by
Harald E. Johns in Canada in the early 1950s provided a
tremendous boost in the quest for higher photon energies
and placed the cobalt unit at the forefront of radiotherapy
for a number of years.
❖ Most modern cobalt therapy machines are arranged on a
gantry so that the source may rotate about a horizontal axis
referred to as the machine isocentre axis.
❖ The source-skin distance (SSD) is either 80 cm or 100 cm.

Introduction
❑ Cobalt-60
isocentric teletherapy
machine built in the
1970s and 1980s by
Atomic Energy of
Canada, Ltd.
❑ Source-axis distance =
80 cm

Machines Using Radionuclides
➢Radionuclides have been used as source of γrays for
teletherapy
➢Radium-226, Cesium-137, Cobalt-60
➢60Co has proved to be most suitable for external beam R/T
➢Higher possible specific activity
➢Greater radiation output
➢Higher average photon energy

❑ Type equation here.Specific activity a is defined as the activity
A per mass m of a radionuclide) is linearly proportional to the
decay constant  and inversely proportional to the half-life t1/2
𝑎 =
𝐴
𝑚
=
𝜆𝑁
𝑚
=
ln 2 𝐴
𝑡 ൗ1
2
𝑚
❑ Specific activity
• Radium-226: a = 0.988 Ci/g (original definition: 1 Ci/g)
• Cobalt-60: a = 1130 Ci/g (carrier free); 300 Ci/g (in practice)
• Cesium-137: a = 80 Ci/g
Basic Properties Of Gamma Rays

• Air kerma rate in air (Kair)ai r is proportional to the specific air
kerma rate constant AKR and inversely proportional to d2, the
distance between the source and the point of interest
(𝐴 𝑎𝑖𝑟) 𝑎𝑖𝑟 =
𝐴Γ 𝐴𝐾𝑅
𝑑2
• Specific air kerma rate constant AKR in [Gy.m2
/ (GBq.h)]
• Cobalt-60: AKR = 309 Gy/m2
/ (GBq.h)
• Cesium-137: AKR = 78 Gy/m2
/ (GBq.h)
Basic Properties Of Gamma Rays

The important characteristics of radioisotopes in external
beam radiotherapy are:
1. High γ ray energy
2. High specific activity
3. Relatively long half-life
4. Large specific air kerma rate constant( ΓAKR )
5. Must be available in large quantities
Characteristics Of Radioisotopes

MachinesUsing Radionuclides
❖ Radium-226
❖ Cesium-137
❖ Cobalt-60

Machines Using Radionuclides
Radionuclide
Half-Life
(Years)
γRay
Energy
MeV
Exposure Rate
constant
(I- Value) (Rm2/Ci.h)
Specific Activity
Achieved in
Practice (Ci/g)
Radium-226
(filtered by 0.5 mm
Pt)
1622 0.83
(avg.)
0.825 ~ 0.98
Cesium-137 30.0 0.66 0.326 ~ 50
Cobalt-60 5.26 1.17,
1.33
1/30 ~ 200

MachinesUsing Radionuclides
Cobalt-60 is suitable for EBRT:
1. Higher possible specific activity (curie/gram)
2. Greater Radiation Output per curie
3. Higher average Photon energy

What is Cobalt?
❖Natural Cobalt (chemical symbol Co) is a hard, stable, bluish-gray,
easily breakable metal that may be stable (nonradioactive, as found
in nature), or unstable (radioactive, man-made).
❖Cobalt has properties similar to iron and nickel. Its atoms contain
27 protons, 32 neutrons, and 27 electrons.
❖Its atoms contain 27 protons, 32 neutrons, and 27 electrons.
❖The most common radioactive isotope of cobalt is cobalt-60.

Cobalt-60
❖This particular radioisotope is historically important for
several reasons. It is involved in the radioactive fallout from
nuclear weapons.
❖For many years, the gamma radiation from this decay was
the main source for radiation therapy for cancer.

Who discovered Cobalt and Cobalt-60?
❖In 1735, a Swedish scientist, George Brandt,
demonstrated that a blue color common in colored glass
was caused by a new element, cobalt.
❖Radioactive cobalt-60 was discovered by Glenn T.
Seaborg and John Livingood at the University of
California - Berkeley in the late 1930's.

COBALT -60
❖ A radioactive form of
cobalt -60 prepared by
exposing cobalt-59 to the
neutrons in nuclear.
❖ Source is usually a solid
cylinder (1 x 2.5cm), and
encapsulated in a stain-less
steel capsule.

Cobalt-60 Source
❑Cobalt-60 ( 60Co) can be produced by placing cobalt-59 in a
strong neutron field, the nucleus absorbing a neutron to form
60Co.
❑As soon as it is formed 60Co starts to undergo radioactive
decay to nickel-60 with a half -life of 5.26 yrs.
❑The emissions are a β−particle with an energy of
0.31MeV(max) and two γ rays with energies of 1.17 MeV
and 1.33 MeV.

Cobalt-60 Source
• Source
• From 59Co(n, γ) nuclear reactor
• Stable 59Co → radioactive 60Co
• In form of solid cylinder, discs, or pallets
• Treatment beam
60Co →60Ni + 0β(0.32 MeV) + γ(1.17 & 1.33 MeV)
• Heterogeneity of the beam
• Secondary interactions
• β absorbed by capsule → bremsstrahlung x-rays (0.1MeV)
• scattering from the surrounding capsule, the source housing
and the collimation system (eletron contamination)

Decay of Cobalt-60
60 𝐶𝑜 → 60 𝑁𝑖 + 𝑒− + 𝜐 𝑒

Decay of Cobalt-60
❖ Teletherapy Cobalt-60
decay through beta
minus decay with a half-
life of 5.26 y.
❖ The beta particles
(electrons) are absorbed
in the source capsule.

What Are The Properties Of Cobalt-60?
• Cobalt (including cobalt-60) is a hard, brittle, gray metal with
a bluish tint.
• It is solid under normal conditions and is generally similar to
iron and nickel in its properties.
• In particular, cobalt can be magnetized similar to iron.

❖ Treatment machines used for external beam radiotherapy with
gamma ray sources are called teletherapy machines. They are
most often mounted isocentrically with SAD of 80 cm or 100
cm.
❖ The main components of a teletherapy machine are:
❖ Radioactive source
❖ Source housing, including beam collimator and source movement
mechanism.
❖ Gantry and stand.
❖ Patient support assembly.
❖ Machine control console.
Components Of A Teletherapy Machine

Patient support
assembly
Source
head
Gantry
Main
frame
Collimator
Components Of A Teletherapy Machine

❖ The 60Co source, usually in the form
of solid cylinder, discs, or pellets, is
contained inside a standard stainless
steel capsule and sealed by welding.
❖ The 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.
COBALT -60 Source

Teletherapy sources are cylinders with
height of 2.5 cm and diameter of 1, 1.5, or
2 cm.
• The smaller is the source diameter, the
smaller is the physical beam penumbra
and the more expensive is the source.
• Often a diameter of 1.5 cm is chosen as a
compromise between
the cost and penumbra.
Teletherapy Cobalt 60 Source

❖ Typical source activity: of the order of 5 000 – 10 000 Ci (185 –
370 TBq).
❖ A source with an activity of less than 3,000 Ci is replaced with a
new one; this is necessary after 5–7 years of use.
❖ Typical dose rates at 80 cm from source: of the order of 100 –
200 cGy/min
❖ Teletherapy source is usually replaced within one half-life after
it is installed. Financial considerations often result in longer
source usage.

❖Often the output of a teletherapy machine is state d in Rmm
(roentgens per minute at 1 m) as a rough guide for the source
strength.
❖Treatment Head has the capacity to take a source with an
activity of 10 000 Roentgens per hour at a meter(RHm) (165
Roentgens per minute at a meter (Rmm)).

The source head consists of:
• Steel shell with lead for shielding purposes
• Mechanism for bringing the source in front of the collimator
opening to produce the clinical gamma ray beam.
Teletherapy Source Housing

Head
Shields the source, Exposes the source as required, and
Collimates the beam to the correct size.

❖The housing for the source is called the “source head”.
❖It consists of a steel shell filled with lead for shielding
purposes and 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.
Source Housing

Treatment Head And Collimator
1. The cobalt source (orange) is situated in a drawer, and
surrounded by lead
2. The collimator system;
3. manual system that can pull the source to
the resting position in case of emergency
4. pneumatic system
5. link between the head and the
6. rotating part of the machine that is used to change the source
when its activity is no longer sufficient for treatment.

A number of methods have been developed for moving the source
from OFF position to ON position
1. Source mounted on a rotating wheel inside the source head to
carry the source from OFF to On position
2. Source mounted on a heavy metal drawer is moved horizontally
by pneumatic system through a hole running through the source
head. 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 the light source mounted on the same
drawer occupies the ON position of the source.
Cobalt-60 Source Housing

3.Mercury is allowed to flow into a container immediately below
the source to shut OFF the beam.
4.Source is fixed in front of the aperture and the beam can be
turned ON and OFF by a shutter consisting of heavy metal jaws.

A number of
methods for
moving the
source so that the
useful beam may
emerge from the
unit have been
developed.

In one of the simplest arrangements, the
source is mounted on a heavy metal wheel
which may be rotated through 180o to carry
the source from the “off” position to the
“on” position.

Another arrangement employs
mercury, which is allowed to
flow into the region immediately
below the source to shut off the
beam.

In this arrangement the source is
mounted on a heavy metal plug slides
horizontally from the “off” position to
the “on” position.

In this arrangement the source is placed at
the center of a sphere, and the beam is shut
off by a set of heavy metal jaws which
moves laterally to intercept the.

In all the source housing arrangements, all
machines are arranged so that they fail “safe”,
that is, if the power fails, the sources
automatically goes to “off” position.

Currently, two methods are used for moving the teletherapy
source from the BEAM-OFF into the BEAM-ON position
and back:
• Source on a sliding drawer
• Source on a rotating cylinder

Methods for moving the teletherapy source from the BEAM-
OFF into the BEAM-ON position and back:
Source on a sliding drawer Source on a rotating cylinder

Teletherapy source housing
When the source is in the BEAM-OFF position, a
light source appears in the BEAM-ON position
above the collimator opening, allowing an optical
visualization of the radiation field, as defined by
the machine collimator.

❖ Some radiation (leakage radiation) will escape from the
teletherapy machine even when the source is in the BEAM-
OFF position.
❖ Head leakage typically amounts to less than 1 mR/h
❖ (0.01 mSv/h) at 1 m from the source.
❖ International regulations require that the average leakage of
a teletherapy machine head be less than 2 mR/h (0.02
mSv/h).

Collimator
Collimators of teletherapy machines provide square and
rectangular radiation fields typically ranging from 5×5 to
35×35 cm2 at 80 cm from the source.

Penumbra
• The region, at the edge of a radiation beam, over which the
dose rate changes rapidly as function of distance from the
beam axis (off-axis distance).
• Types of penumbra:
1. Transmission penumbra
2. Geometric penumbra
3. Physical Penumbra

• It is transmission through the edge of the
collimator block.
• The region irradiated by photons which are
transmitted through the edge of the
collimator block
• The inner surface of the blocks is made
parallel to the central axis of the beam
• The extent of this penumbra will be more
pronounced for larger collimator opening
• Minimizing the effect
• The inner surface of the blocks remains always
parallel to the edge of the beam
Collimator
Source
SSDSDD
Transmission Penumbra

Geometric Penumbra
The geometric penumbra resulting from
the finite source diameter, may be
minimized by using:
• Small source diameter
• Penumbra trimmers
as close as possible
to the patient’s skin
(z = 0)

• Radiation source: not a point source
• e.g. 60 Co teletherapy → cylinder of diameter ranging from 1.0 to 2.0 cm
From considering similar triangles ABC and DEC
DE = CE = CD = MN = OF + FN – OM
AB CA CB OM OM
AB = s (source diameter)
OF = SSD
DE = Pd ( penumbra)
Pd = s (SSD + d – SDD)
SDD
Parameters determine the width of penumbra
Geometric Penumbra

Geometric Penumbra
• Solutions
• Extendable penumbra trimmer
• Heavy metal bars to attenuate the beam in the penumbra region
• Increase the source to diaphragm distance, reducing the geometric penumbra.
• Secondary blocks
• Placed closed to the patient for redifining the field
• Should not be placed < 15 – 20 cm, excessive electron contaminants
• Definition of physical penumbra in dosimetry
• Lateral distance between two specified isodose curves at a specified depth
• At a depth in the patient, dose variation at the field border
• Geometric, transmission penumbras + scattered radiation produced in the
patient

Physical penumbra: Lateral distance between
to specified isodose curves at a specific depth
(90% & 20% at Dmax). Takes scattered
radiation into account.
Physical Penumbra

Collimators
• Collimators provide beams of desired
shape and size.
• They provide square and rectangular
radiation fields typically ranging from 5
× 5 to 35 × 35 cm2 at 80 cm from the
source.
• The rotational movement of the
collimator is continuous, and it can rotate
360° about its own axis.
• The collimator system can move to any
position when the gantry is rotated.

Gantry
❖The gantry can rotate by 360°. The rotational movement of the
gantry is motorized and controlled in two directions continuously.
❖Teletherapy machines are most often mounted isocentrically,
allowing the beam to rotate about the patient at a fixed SAD.
They can be used either as fixed field machines or rotation units.
It is Made so because once set, the SSD doesn't change with the
circular movement of Gantry.
❖ Modern teletherapy machines have
❖ SADs of 80 or 100 cm.

Patient Support Assembly
Treatment Bed has
motorized movements
1. Horizontal
2. Vertical
3. Lateral

Isocenter
❖ The axis of rotation of the three structures: Gantry,
Collimator, and Couch coincide at a point known as
the Isocenter.
❖ Isocentric Mounting
❖ Enhances accuracy.
❖ Allows faster setup and is more accurate than older non
isocentrically mounted machines.
❖ Makes setup transfer easy from the simulator to the treatment
machine.

Control Console
❖Control Console is situated outside the bunker
❖Interlocks present on the console for
❖Door
❖Head Lock
❖OFF Shield
❖Treatment Mode
❖Wedge Filter
❖Tray Interlock
❖Timer

Timer
❖ The prescribed dose is delivered to the patient with the help of
two treatment timers: primary and secondary.
❖ The primary timer actually controls the treatment time and turns the
beam off upon reaching the prescribed beam-on time.
❖ The secondary timer serves as a backup timer in case of the primary
❖ timer’s failure to turn the beam off.
❖ The set treatment time should incorporate the shutter correction
time to account for the travel time of the source from the
BEAM-OFF to the BEAM-ON position at the start of the
irradiation and for the reverse travel at the end of irradiation.

❖ γ rays constitute the useful treatment beam.
❖ The β particles are absorbed in the cobalt metal and the stainless
steel capsules resulting in the emission of bremsstrahlung x-
rays and a small amount of characteristic x-rays.
❖ However these x-rays of average energy 0.1 MeV do not
contribute appreciably to the dose in the patient because they
are strongly attenuated in the material of the source and the
capsule.
Treatment Beam

• The lower energy γ rays produced by the interaction of the
primary γ radiation with the source itself, the surrounding
capsule, the source housing and the collimator system are also
contaminants of the treatment beam.
• The scattered components of the beam contribute significantly
( approx. 10% to the total intensity of the beam.
Treatment Beam

❖Electrons are also produced by these interactions and constitute
electron contamination of the photon beam.
❖Electron contamination can reverse the skin sparing effects of
cobalt60 treatment beam, if severe.
❖Electron contamination is greater for very short
diaphragm to skin distances and for large field sizes.
Treatment Beam

Beam characteristics for photon beam energy 60Co, SSD = 80 cm
1. Depth of maximum dose = 0.5 cm
2. Increased penetration (10-cm PDD = 55%)
3. Beam edge not as well defined—penumbra due to source
size
4. Dose outside beam low because most scattering is in
forward direction
5. Isodose curvature increases as the field size increases
Treatment Beam

Teletherapy Cobalt-60 Machines

Teletherapy Cobalt-60 Machines

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