Cobalt-60 machines were introduced in the 1950s and were an early form of external beam radiotherapy. They emit high-energy gamma rays from radioactive Cobalt-60, which is produced in nuclear reactors. Though being replaced by linear accelerators, Cobalt-60 machines are still used in some developing countries due to their simpler design and lower costs. Strict quality assurance tests and regulations are required to ensure the safe operation of these radioactive sources and minimize radiation exposure.

1. RADIOTHERAPHY
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2. COBALT-60 MACHINES
• Introduced in the 1950’s, being replaced by linacs.
• The first practical radiation therapy treatment unit to
provide a significant dose below the skin surface and
simultaneously spare the skin the harsh effects of earlier
methods.
• Still used in developing countries: simpler design, cost,
little tech support.

3. Cobalt 60 Machines
• Constantly emit radiation.
• The 60Co source must be shielded in a protective housing
(source head).
• The source head is a steel shell filled with lead (may be up
to 2 ft in diameter).
• Uses a counterweight to balance the lead shielding in the
head of the machine housing, (similar to the beam stopper
on linac): absorbs a significant amount of radiation
transmitted through patient.

5. Cobalt-60 Production
• Cobalt: produced in nuclear reactors by the
irradiation of neutrons of the common stable
form of 59Co.
• The 59Co nucleus absorbs a neutron in the
reactor and becomes 60Co.

6. Cobalt 60
• Radioactive 60Co produces a useful therapy beam when it
undergoes beta decay.
• The nucleus emits a beta particle and then two photons,
1.17 MeV and 1.33 MeV for an effective energy of 1.25
MeV
60Co  60Ni+ + B- + neutrino (v) + gamma rays
• Radioactive 60Co emits radiation in the form of high energy
gamma rays in an effort to return to its more stable state.

8. 60Co Source
• The overall diameter of a 60Co source is 1 to 3 centimeters.
• Consists of pellets of radioactive 60Co encased in multiple
layers of welded metal to prevent contamination of the
environment and to absorb β- particles produced by the decay
process.
• Or 60Co sources are made with the 60Co fused into a solid
cylinder. Advantages:
– Smaller source with less penumbra for the same beam
intensity
– Less hazard of contamination should a source ever become
exposed to the environment.

9. •The 60Co source, usually in the form of a solid cylinder,
discs, 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

11. 60Co Activity
• 60Co activity may be expressed in curies (Ci), the historical
unit of radioactivity
– 3.7 x 1010 Becquerel (Bq)
– 1 Bq = 1 disintegration per second
• May also be defined in rhm units (roentgens per hour at 1
meter)
• Most sources have an activity of 750-9000 Ci, typically 3000-
9000 Ci used in radiation therapy

12. Half-life
• Half-life: the time necessary for a radioactive material to
decay to half or 50% of its original intensity.
– Requires a correction factor for this decay of about 1% per
month in all treatment calculations.
– Source must be replaced at about five year intervals.
• The half-life of 60Co is 5.26 years.

13. SOURCE HOUSING:
• The housing for the source is called the sourcehead.
• 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.
• 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 the off position to the on position.

14. IT WILL SUFFICE HERE TO MENTION BRIEFLY FOUR
DIFFERENT MECHANISMS:
• A) The source mounted on a rotating wheel inside the sourcehead to carry the source
from the off position to the on position.
• B) The source mounted on a heavy metal drawer plus its ability to slide horizontally
through a hole running through the sourcehead—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 a light source mounted on the same drawer occupies the on position of the
source;
• C) Mercury is allowed to flow into the space immediately below the source to shut off the
beam. and
• D) 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.
All of the above mechanisms incorporate a safety feature in which the source is returned
automatically to the off position in case of a power failure.

16. 60Co Shielding
• Cerrobend (Lipowitz metal): lower melting point than Pb,
cheaper
– 50% Bismuth
– 26.7% Lead
– 13.3% tin
– 10% Cadmium (a toxic metal can get into bloodstream)
• Density ratio of Cerrobend to Lead: 1.2 cm Cerrobend to 1 cm
lead.
– 5 HVL is needed to reduce intensity
– A thickness of 7.2 cm of Cerrobend needed, 6 cm lead.

17. Penumbra
• Penumbra: the area at the edge of the radiation beam at which
the dose rate changes rapidly as a function of distance from the
beam axis.
• Describes the edge of the field having full radiation intensity for
the beam compared with the area at which the intensity falls to 0.
• The larger the source size, the larger the penumbra
P = S(SSD-SDD)/SDD
• When depth is given:
P = S(SSD + D -SDD)/SDD
• Larger field sizes are necessary to cover the same amount of
tissue adequately compared to the linac.

18. Diagram for calculating
geometric penumbra. SDD,
source to diaphragm distance;
SSD, source to surface
distance.

19. Penumbra
• Geometric penumbra: the place where a lack of sharpness or
fuzzy area occurs at the edge of the beam
– Occurs at the skin surface and greater depths in tissue.
• Transmission penumbra: occurs as the radiation passes
through the edge of the primary collimators
– Occurs at the edge of the patients shielding blocks mounted
or placed below the collimator
– Correlates with the size of the collimator opening- larger field
sizes have more transmission penumbra
– Penumbra width depends upon:
– Source diameter
– SSD
– Depth below the skin and SDD(Inversely)

20. Reducing Penumbra
• The transmission penumbra can be reduced by using satellite
collimators, penumbra trimmers or trimmer bars.
• Trimmers are metal bars that attenuate the edge of the beam
providing a sharper field edge.
– Should be placed no closer than 15 cm from the patients
skin to reduced electron contamination (increased skin dose)
by metal devices.
• Provides enough distance for the secondary electrons
produced by the trimmer bars to lose sufficient energy

21. Dose Maximum:
• Dose maximum (Dmax): when a greater percentage of
dose occurs below the skin surface
• Dmax is the depth of maximum buildup, in which 100%
of the dose is deposited.
• For 60Co, Dmax occurs at 0.5 cm below the skin surface.
• Electron equilibrium is another term used to describe
Dmax. As energy increases, so does the depth of electron
equilibrium.

22. Beam On/Beam Off
• Turning the beam on requires physically exposing the source
either by moving it into position or by removing shields
around the source.
• Air pressure: the compressor generates air pressure by
pushing the source horizontally into position over the
collimator opening.
• Rotating wheel: the motor rotates a wheel 180 degrees by
placing the source over the collimator opening.

23. 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.

24. Timer Error
• Timer error (TE, or travel time correction, shutter error): is
a consequence of the physical motion of the source. Even if
its less than 1 second.
• The difference between the beam on time setting and the
time that the source is in the treatment position, the time it
takes to advance and retract the source
• Timer error is determined at the time of machine beam
calibration and is checked monthly.
TE = (R1t1-R2t2)/R1-R2
• The timer setting necessary for a treatment is the time
calculated plus the timer error.

25. Quality Assurance
• Required be the Nuclear Regulatory Commission (NRC)
• A qualified radiation/medical physicist must perform full
calibration testing annually. More frequent if:
– The source is replaced
– A 5% deviation is noticed during a spot check
– A major repair requiring the removal or restoration of major
components is done.

26. Quality Assurance
• Written directive: the prescription, must be clear, unambiguous,
and signed by a licensed authorized user.
• Recordable event must be reported to the RSC-Radiation Safety
Committee:
– A delivered dose more than 15% in excess of the weekly
prescription
– Lack of daily recording of dose
• Misadministration must be reported immediately to the NRC
– Treatment of the wrong patient, wrong site
– Weekly dose more than 30% in excess of the prescription
– The delivery of more than 20% (25%???) total dose

27. Tests
• The integrity of the source must be tested twice a year to
ensure the sealed source continues to be totally sealed.
– Wipe tests: wipe collimator edges with a filter paper,
background radiation reading done, activity is
determined (acceptable under 0.005 mCi)
• Radiation and light field coincidence
• Timer accuracy
• Dose rates
• & more.

28. Leakage Limits
• Leakage in the off position:
– Cannot exceed 2 mrem/hr at 1 meter
• Leakage in the on position:
– Cannot exceed 0.1% of the useful beam at 1 meter from
the source.