Cobalt-60 is commonly used as a gamma ray source for teletherapy due to its suitable properties. A Co-60 unit contains a sealed radioactive Co-60 source that emits two gamma rays during decay. The source is moved between shielded and treatment positions using various mechanisms. Beam size and shape are controlled through collimation and additional devices can modify the beam. Precise patient and beam positioning is enabled through computer control and motorized components while shielding protects staff from radiation.

1. INTRODUCTION OF
CO -60 UNIT
NILESH KUMAR
PG RADIATION PHYSICS
DEPARTMENT OF RADIATION PHYSICS

2. • Overview
Radionuclide's such as radium-226, 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.

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.  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.
 often the output of teletherapy machine is started in Rmm ( roentgen per
minute at 1 m ) as a rough guide for source strength .
 Treatment head has the capacity to take a source with an activity of
10,000 roentgens per hour at a meter ( RHm ) .

7.  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.

9.  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.
 A typical teletherapy 60Co source is a cylinder of diameter ranging from 1.0 to 2.0
cm and is positioned in the cobalt unit with its circular end facing the patient

11.  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.

12.  A number of methods have been developed for moving the source from the
off position to the on position.
 These methods are -:
I. Rotating wheel
II. Sliding drawer
III. mercury shutter
IV. moving jaw

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.

14.  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 sourceautomatically
to radiation-off position in case
of any failure.

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

16.  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.

17.  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

18.  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.

19.  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

20.  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.

22.  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.

23.  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.

24.  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

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

27.  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

28.  Timer
 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 agin to OFF position ( shutter time )
 Formula of shutter time =

29.  where , R1 - meter reading for given time ( say 2 min )
R2 - Meter reading for two consecutive times ( 1+1 min )
eg - :
Meter reading
+ 300 v
Time 1 min
MR 1 MR 2 Mean
36.15 nc 36.14 nc 36.145 nc

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

31.  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.

32.  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.

33.  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 .

34.  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.

35.  Wedge Systems
 Wedge filters are of two main types -:
I. individualized wedge system,
 which requires a separate wedge for each beam width, optimally designed
to minimize the loss of beam output.
 A mechanism is provided to align the thin end of the wedge with the border
of the light field

37. 2 . The second system uses a universal wedge -:
 a single wedge serves for all beam widths.
 Such a filter is fixed centrally in the beam, while the field can be opened to
any size.
 only a small part of this wedge is effective in producing the given wedge
angle.
 The rest, being unwedged, does not contribute to the isodose tilt but
reduces the beam intensity .

39.  the individualized system reducing the beam output, it is preferred for use
in cobalt teletherapy.
 The universal wedge, is useful for linear accelerator beams where the
output is more.
 From the setup and treatment planning points of view, the universal wedge
is simpler to use than the individualized filter.

41.  Effect on Beam Quality
 The wedge filter alters the beam quality by preferentially attenuating the lower-
energy photons (beam hardening)
 to a lesser extent, by Compton scattering, which results in energy degradation (beam
softening).
 For the 60Co beam, because the primary beam is essentially monoenergetic, the
presence of the wedge filter does not alter the central axis percent depth dose
distribution.
 For x-rays, on the other hand, there can be some beam hardening , and as a result
the depth dose distribution can be somewhat altered, especially at large depths

42.  Breast Cone
 A beam modifying and directing device used for a tangential fields therapy
 Advantages: –
 Directs beam to the central axis of the area of interest, where a tangential beam is
applied to a curved surface
 Helps position, the patient with an accurate SSD
 Endplate provides compensation,
 which enhances surface dose and presses down the tissue.
 Effective shielding of lungs

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

44.  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

45. 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.

46.  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).

48.  Placing of shielding material
 In kilovoltage radiation shielding is readily achieved by placing sheets of lead on
the surface directly
 It is because of the lower penetrating power of beam
 In megavoltage radiation,
Thicker blocks used.
Placed higher up in shadows trays (15-20cm )

49.  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

51.  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.

53.  References -:
I. KHAN’S The Physics of Radiation Therapy
II. Radiation Oncology Physics: E.B. Podgorsak
III. Article - BARC DEVELOPS COBALT-60 TELETHERAPY MACHINE FOR CANCER TREATMENT