Here the components of cobalt-60 teletherapy unit is discussed

1. COMPONENTS OF
CO -60 UNIT

2. • Overview:
Radionuclide's such as cesium-137, and cobalt-60 have been used as
sources of gamma rays for teletherapy.
These gamma rays are emitted from the radionuclide's as they undergo
radioactive disintegration.
Of all the radionuclide's, Co-60 has proved to be the most suitable for
external beam radiotherapy.
 A typical source activities are in the order of 5000-10,000 ci and provide
typical dose rate at 80 cm from the teletherapy source of the order of
100- 200 cGy/min.

3. The reasons for its choice over radium and cesium are higher
possible specific activity (curies per gram), greater radiation
output per curie, and higher average photon energy .

5.  COBALT-60 UNIT:
Source
 The 60Co source, usually in the form of a solid cylinder, disks, 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.
 A typical teletherapy 60Co source is a cylinder of diameter ranging
from 1.5 to 2.0 cm and is positioned in the cobalt unit with its
circular end facing the patient.

6.  The Co-60 source decays to Ni-60 with the emission of β particles (Emax = 0.32
MeV) and two photons per disintegration of energies 1.17 and 1.33 MeV.
 These gamma 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 – 0.1Mev
 25% of the primary photons are lost because of the self attenuation of
the source.

8.  Source Housing / source head
Functions of the head:
• To shield the source
• To expose the source as required
• To collimate the beam to correct size

9.  Source Housing / source head
 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.
 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 the off position to the on position, which are as follows:

10. Tungsten shield block

11.  Sliding drawer
A pneumatically driven source
drawer is used for moving the
source between shielded
position and treatment position.
The pneumatic cylinder will
return the source
automatically to radiation-off
position in case of any failure.

13.  Rotating wheel
The source mounted on a
rotating wheel inside the
sourcehead to carry the source
from the off position to the on
position.

15.  Mercury shutter
 mercury is allowed to flow into
the space immediately below
the source to shut off the beam.

17.  Moving jaw
 The 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.

19.  Beam Collimation
 The size and orientation of the radiation beam is controlled by the collimator
assembly.
 Two pairs of motorized jaws generate rectangular and square field of desired
size.
 The collimator can also be rotated around the radiation beam axis.
 The radiation field can be visualized by a high intensity light during patient
positioning.
 Multivane collimators are used as secondary collimators.

21.  The centre of the radiation field
is indicated by the collimator
cross hairs.
 Two sets of trimmers are
provided to decrease the
penumbra of the beam.
 The lower pad of the collimator
is prepared to receive wedge
filters and shadow tray for the
beam shaping lead blocks.

22.  Penumbra
 Penumbra refers the region at the edge of beam where dose rate changes
rapidly as function of distance from beam axis.
Types of penumbra :
Geometric
Transmission
For 1.5mm source diameter, penumbra will be <10mm at 5cm depth for
field area <400cm^2.

23.  Transmission penumbra
• It is the region irradiated by photons which are transmitted through the
edge of the collimator blocks
 Geometrical penumbra :
 formula =
• it is formed due to dimension of source.
• It is formed due to finite source size and must be concern in teletherapy
machines where the sources is between 1 – 2 cm in size.

25.  Conclusions :
 penumbra increases with increase in source diameter , SSD , and depth.
Penumbra width decreases with a increase in SDD
penumbra trimmers - :
it consists of extensible , heavy metal bars to attenuate the beam in the
penumbra region.
increase the source to diaphragm distance , reducing the geometric
penumbra.

26.  Gantry
 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.
 Most cobalt-60 unit machines have SAD of 80 cm.

27.  The axis of rotation of the three structures:
Gantry
Collimator
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

29.  Patient Support Assembly / Couch
 Treatment Bed has motorized movements
 Horizontal
 Vertical
 Lateral
 Base - 90° rotation to each side

30.  Control Console
 Control Console is situated outside the bunker
 Interlocks present on the console for- :
Air Pressure
 Door
Head Lock -Treatment Head has a swivel movement of +/- 180°.
 OFF Shield
 Treatment Mode.
 Wedge Filter.
Tray Interlock.
 Timer

31.  Warning lights
• Red light: Radiation present- do not enter room
• Green Light: treatment time elapsed-safe
• Yellow light: during the treatment
• Malfunction: both red and green lights still on- means that machine is still
in on position after prescribed dose has been delivered. Remove patient.

32.  Beam shaping and modifying devices
 The system has accessories like -:
wedge filters
Breast cone
Sheilding blocks, to modify the beam shape or beam attenuation.
 They are placed on the machine between the collimator and the patient.

33.  wedge filters
 special filters or absorbing blocks are placed in the path of a beam to
modify its isodose distribution.
 The most commonly used beam-modifying device is the wedge filter.
 This is a wedge-shaped absorber that causes a progressive decrease in the
intensity across the beam, resulting in a tilt of the isodose curves from their
normal positions.

34.  the isodose curves are tilted
toward the thin end, and the
degree of tilt depends on the slope
of the wedge filter.
 In actual wedge filter design, the
sloping surface is made either
straight or sigmoid in shape .

35.  The wedge is usually made of a dense material, such as lead or steel, and
is mounted on a transparent plastic tray,
 which can be inserted in the beam at a specified distance from the source .
 This distance is arranged such that the wedge tray is always at a distance of at
least 15 cm from the skin surface,
 so as to avoid destroying the skin-sparing effect of the megavoltage beam.

36.  SHIELDING BLOCKS
 Aims of shielding-
 protect critical structures ( organ )
 avoid unnecessary radiation to normal tissues
 matching adjacent fields

37.  An ideal shielding material should have the following characteristics :
 high atomic no
 high density
 easily available
 inexpensive
 easily modifiable
 The most commonly used shielding material for photons is Lead ( Pb )
The thickness of shielding block used depends upon the energy of the
radiation

38. The shielding material which reduces beam transmission to 5% of its original is
considered acceptable .
The term half value layer is an expression for the attenuation produced by any
material.
half-value layer (HVL) is defined as the thickness of an absorber of specified
composition required to attenuate the intensity of the beam to half its original
value.
Practically thickness of lead between 4.5 – 5 half value layers which results in
5% or less of primary beam transmission.

39.  Custom blocks
• The conformal blocks are made with low melting point alloy called Cerrobend.
• The Cerrobend material consists of 50.0% bismuth, 26.7% lead, 13.3% tin, and 10.0%
cadmium .
• The main advantage of Cerrobend over lead is that it melts at about 70 °C (compared with
327 °C for lead) and therefore, can be easily cast into any shape.
• At room temperature, it is harder than lead.
• The minimum thickness of Cerrobend blocks required for blocking may be calculated from
Table 13.1 using its density ratio relative to lead (e.g., multiply lead thickness by 1.21).

41.  T - ROD mechanism in cobalt-60 unit
 The external T-ROD shall be fitted with this indicator rod incase of failure of
Automatic source retracting system to push the source to the OFF condition.
 Source stuck during treatment – Emergency situation when the Cobalt-60
source gets stuck in between the “off” and “on” position .
 Rotate gantry to side opposite to maze wall – Insert “T-rod” and push it till the
source reaches “safe” position

43.  The beamstopper
 The beamstopper is a lead-filled steel assembly, which acts as a beam
absorber. The beamstopper attenuates 99.9% of the primary beam.
 A beam stopper is used to intercept the beam for additional shielding of the
exit beam. The beam stop also acts as a counterweight for the head of the
machine.

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