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• Source of radiation is
at a distance from the
• Treatment machines
incorporating γ ray
sources for use in
Until 1951, all isotope machines produced were tele-radium units
( radium bomb ).
The source to skin distance was usually not greater than 10 cm in
Major drawbacks of these machines were-i) high risk of radiation
hazard due to radon gas leak produced as a by product, ii) high
cost of radium, iii) large self absorption, iv) low exposure rate
constant and v) low output.
Bryant Symons radium "bomb" at Westminster Hospital,
London, England, in the 1930s.
For tele-cesium units the source to skin
distance is 20cm to 40cm.
They have not been very popular because of
relatively low exposure rate constant and low
Dr. Harold E. Johns - Invented the cobalt-60 unit in
First Tele-Cobalt machines –Saskatoon Cancer
Clinic , Canada and the Victoria Hospital, London
JANUS Co-60 machine.
Ulrich Henschke, Bronx, New York
Co-60 source of about 2000 Ci.
Two specially designed rooms were built
adjacent to each other
Two heads looking in opposite directions
The gift to see both future and past
Iso-centric Cobalt-60 Teletherapy Machine,
Atomic Energy of Canada, Ltd.
SSD : 60cm
Source activity :low
Time taken MORE
The main components of a teletherapy machine are:
1. radioactive source;
2. source housing, including beam collimator and source
3. a gantry and stand in isocentric machines or a housing
support assembly in stand-alone machines;
4. a patient support assembly
5. a machine console.
Cylindrical double-welded stainless-steel capsule
Diameter ranging from 1.0 to 2.0 cm
Discs / pellets
Positioned in the cobalt unit with its circular end
facing the patient
• The Cobalt-60 source is produced by irradiating ordinary, stable 59Co with
neutrons in a nuclear reactor.
The nuclear reaction is represented as
• The nuclei of 60Ni will be in the excited states following this decay and the
de-excite to the ground state by emitting two γ ray photons of energy 1.17 MeV and
1.33 MeV in cascade.
• The decay half-life is 5.26 years and the average photon
energy is 1.25 MeV.
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
Also a heavy metal alloy sleeve is provided to form an additional primary shield
when the source is in the OFF position.
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.
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.
Collimators provide beams of desired shape and size.
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
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.
The gantry can rotate by 360°. The rotational movement
of the gantry is motorized and controlled in two directions
continuously; its rotation speed can be adjusted.
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.
Treatment Bed has motorized movements
Table Top – 90 rotation to each side
Base - 110 rotation to each side
Control Console is situated outside the bunker
Interlocks present on the console for
1. Air Pressure
3. Head Lock -Treatment Head has a swivel movement
of +/- 180
4. OFF Shield
5. Treatment Mode
6. Wedge Filter
7. Tray Interlock
The prescribed target dose is delivered with the help
of two treatment timers:
Primary Timer - the primary timer actually controls
the treatment time.
Secondary Timer - accounts for the source
movement from OFF to ON
position and again to OFF
position (shutter error).
Source ON/OFF Indicator –
Red- ON Green- OFF Amber- TRANSIT
• 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.
• 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
• The scattered components of the beam contribute significantly ( approx.
10% to the total intensity of the beam.
BHABHATRON - The indigenous Cobalt-60 Teletherapy
Machine has a capacity of 200 RMM source and its source
to iso-centre distance is 80 cm. The system has unique user-friendly
features and fully closable collimator for improved radiation safety.