telecobalt machine

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Powerpoint Templates
Telecobalt machine

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Telecobalt machine
Main components of Telecobalt
machine are
• Radioactive source
• Source housing –includes
beam collimator and source
movement mechanism.
• Gantry
• Stand
• A patient support
assembly(Couch)
• Beam stopper

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Source Housing
• Housing for source is called “Source head”
• Contains Steel shell filled with lead for shielding purpose
and device to bring the source to on position(opening in the
head)
• Additional primary shield -Heavy metal alloy(copper-
tungsten) sleeve in the source off position.
• Shutter- device which brings source in front of an opening
from which the useful beam comes out.
• And shut off the beam when it is not needed.
• Four types of shutter mechanism basis of moving and
Fixed source
• 1.Rotating wheel
• 2.Sliding window
• 3.Mercury Shutter

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Source Head

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Moving source
• Moving source unit the source moves from a safe
position to a position just behind the primary collimator.
• Sliding window and rotating wheel will come under this
method
• Sliding window: Source mounted on a heavy metal
drawer is moved horizontally by pneumatic pressure
system through a hole running through the source head.
• In the on position source faces the aperture and in the
off position source moves to its shielded location.
• In the off condition light source on the same drawer
occupies the on position of the source.

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Shutter Mechanisam

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Rotating wheel
• Source is attached on a
rotating wheel
• Off condition source is
protected by
approximately 20 HVL of
lead
• When power is on the
wheel is driven by a
motor and its turning at
the direction 180 where
the diaphram .

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Fixed source
• In the fixed source unit the
cobalt source is stationary, the
primary collimator rotates
,opening up a passage for
gamma rays to emerge.
• Mercury Shutter: Mercury is
allowed to flow into the
container immediately below
the source to shutt off the
beam
• Moving Jaw:
• Source if fixed infront of the
aperture and the beam can be
turned on and off position by a
heavy metal Jaws

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Beam collimation and penumbra
• Collimators provide beams of desired shape and size
• Contains primary and secondary collimators
• Primary collimator is near to source which decides maximum
angle of the exiting beam.
• Primary collimator made of tungsten alloy has a conically
shaped hole and it is part of the shielding or shutter
mechanisam.
• Secondary collimator Consists of two pairs of heavy metal
blocks of tungsten to obtain square and rectangular field size.
• Secondary collimators are calibrated so that collimator reading
will
give corresponding field size at isocentre
• Collimators of telecobalt machine provides rectangular and
square shaped radiation fields typically ranging from 5X5 to

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• 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.

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Field size
• Field size may be defined either
geometrically or dosimetrically.
• Geometric field size: Defined as the
projection on the plane perpendicular to
the beam axis of the distal end of the
collimator as seen from center of the
source.
• It is correspond to the field size defined
by the light localizer.
• Dosimetric or Physical field size:As per
ICRU recommendation it is the Distance
intercepted by the given isodose curve
usually 50 % on a plane perpendicular to
the beam axis at a stated distance from
the source usually at isocentre and at

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Penumbra-Beam edge
• Penumbra refers to the region at the edge of the beam
where the dose rate changes rapidly as a function of
distance from the beam axis.
• The penumbra width of Ideal external beam should be
minimum.
• The physical penumbra is the penumbra measured in
the dose profile.
• Physical penumbra corresponds the distance between
20% and 80% isodose line at dmax
• Components to physical penumbra:
1.Geometric penumbra
2.Transmission penumbra
And scattered radiation from the patient.

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Geometric penumbra
• Geometric penumbra is caused
by source or focal spot having a
finite size.
• The width of the penumbra
=D(SSD-SDD)/SDD
Factors that increase penumbra:
• Increase in SSD
• Increase in Source diameter
Factors that decrease penumbra:
• Decrease in SSD& Source dia
• Increase in SCD (source to
collimator distance)
The distance between Pi and P0 is
called geometric penumbra width

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Transmission penumbra
• Transmission penumbra is
due to photons emitted by the
distal part of the source are
partially attenuated by the
collimator.
• These rays smear the
geometric penumbra
• Transmission penumbra can
be decreased when using
diverging block in which
block edges are parallel to the
beam divergence but this will
increase the geometric
penumbra.

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ODI and light field localizer
• Collimator also provided with
optical distance indicator(ODI)
and visual filed localizer.
• ODI used to measure the
distance of the patient surface
from the source.
• ODI consists of a light bulb
coupled to optical device
which displays the SSD
• Visual field localizer
comprises a lamp and mirror
,which can show the size and
position of the field in the
patient skin.
• ODI represents the exact SSD

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Gantry
• Gantry is attached to stand capable of rotating 360
degree
• 2 arms-Source head & beam stopper
• 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.
• Modern teletherapy machines have SADs of 80 or 100
cm

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Treatment couch:
• Integral part of the machine
• Has motorized movements –lateral,Longitudinal
,Rotational and vertical.
Stand:
• Base of unit and its fixed in floor has an electrical and
electronic circuits .
• Contains circular scale to indicate the position of gantry
during treatment.

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• Control console: outside the treatment room connected
to the machine by cables.
• Contains timer and on ,OFF switches with light indicator
• Prescribed target dose is delivered with help of two
timers
• Primary timer: Actually controls the treatment time.
• Secondary timer: Accounts for the source movement
from off to on position and again to off position
• Room Doors: Provided with inter locks: beam on when
the door is closed

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Beam Stopper
• Beam Stopper: It acts as counter weight and so
absorbs exit radiation
• Reduces primary radiation however patient setup
becomes difficult.