cobalt unit
TRANSCRIPT
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External Beam Radiotherapy
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py
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17-May-133
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17-May-134
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17-May-136
:
SSD = Fixed.
SAD = Fixed
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:
.
SAD = Fixed.
.
.
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17-May-138
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primary photons + scattered photons + e- contamination
P
Collimator
Accessoire
XRTube
inhrentFiltre additionnelFiltre
X
e-
P
60Co
g
e-
Source
e-
P
Accelarator
Monitor
e-
FlqtteningFiltre
Tqrget
e-
e-
Yph Kerma
Dose
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-60
Cobalt-60 unit
:1.25 MeV
:80cm or100cm
:
2 to 2.5 Gy/min at SSD
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Why the Cobalt-60
]...[112
BqsmGyg
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-60
n+ 59Co 60Co + g
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SOURCEof COBALT-60
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SOURCE HOUSING
The housing for the source
is called the sourcehead
THERATRON 780THERATRON 780
the sourcehead
consists of
a steel shell filled withlead for shielding purposes
a device for bringing the
source in front of an
opening in the head from
which the useful beam
emerges.
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the source mounted on a rotating wheel inside the sourceheadto carry the source from the off position to the on position
thesource 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 mercury is allowed to flow into the space
immediately below the source to shut off the beam the source is fixed in front of the aperture and the beam can
be turned on and off by a shutter consisting ofheavy metal jaws
THE SOURCE MOVING METHODS
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. 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
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Specific activity large enough for FSD of 80cm or even 100cm
Therefore, isocentric set-up possible
Isocentric set-upIsocentric set-up allows movement of all components
around the same centre
collimator
gantry
couch
Control area of a 60-Co unit
Dual timer control
Patient monitoring video system
Source head and transfer mechanism
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-60
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Beam Collimation and Penurnbra
A collimator system is designed to vary the size and shapeof the beam to meet the individual treatment requirements.
The simplest form of a continuously adjustable diaphragm
consists of two pairs of heavy metal blocks
Each pair can be moved independently to obtain a square
or a rectangle-shaped field
Some collimators are multivane type, i.e.,multiple blocks tocontrol the size of the beam
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transmission penumbra +geometric penumbra
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transmission penumbra if the inner surface of the blocks is made parallel to the central axis
of the beam the radiation will pass through the edges of the
collimating blocks
The extent of this penumbra will be more pronounced for larger
collimator openings because of greater obliquity of the rays at the
edges of the blocks
This effect has been minimized in some designs by shaping thecollimator blocks so that the inner surface of the blocks remains
always parallel to the edge of the beam
In these collimators, the blocks are hinged to the top of the
collimator housing so that the slope of the blocks is coincident withthe included angle of the beam. Although the transmission
penumbra can be minimized with such an arrangement,
it cannot be completely removed for all fieldsizes.
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The term penumbra, in a general sense, means the region, at
the edge of a radiation beam, over which the dose rate
changes rapidly as a function of distance from the beam axis
The transmission penumbra,, is the region irradiated by photonswhich are transmitted through the edge of the collimator block.
The geometr icpenumbra, isThe geometric width of the
penumbra (Pd) at any depth ( d ) from the surface of a
patient can be determined by considering similar triangles
ABCand DEC. From geometry, we have:
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D0
DmDs
De
rate
tmt
strat endSource
D0 =Dm
tm + tio
Dm
tm
tes est un temps fictif, < ou > 0
Determination of tio
D0
D1
tt1
D0
D2
tt2
D1 = D0 (t1 + tio) D2 = D0 (t2 + tio)
tio =D1 t2D2 t1
D2D1
I nput output time
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