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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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17-May-1310

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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17-May-1313

Why the Cobalt-60

]...[112

BqsmGyg

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17-May-1314

-60

n+ 59Co 60Co + g

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17-May-1315

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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17-May-1317

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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17-May-1318

. 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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17-May-1319

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17-May-1320

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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17-May-1321

-60

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17-May-1322

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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17-May-1323

transmission penumbra +geometric penumbra

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17-May-1324

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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17-May-1325

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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17-May-1327

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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