wedges and shi

wedges and shi

what is the purpose of using a wedges and a wedged field

to alter dose distribution so that isodose curves are no longer flat but tilted

wedges are tissue compensators used to alter isodose distribution to a defined angle

wedges to dose distribution

designed to modulate intensity of radiation across the field

eh—> more dose under thin end, less dose under thick end

if normalized:
- isodose under thin end higher than 100%
- isodose under thick end are lower than 100%

what is the purpose of using a wedge

• beam arrangements where beams overlap

• to achieve dose homogeneity across the ptv

• account for density variations

• account for change in contour

how wedges affect dose uniformity

there is a potential to get hot spots under the thin end of wedges

• this increases as wedge angle and field size increases

the advantage is that there’s a rapid dose fall off after this region of overlap

3 types of wedges and their uses

physical wedge (no longer in use)
- wedge effect achieved by physical attenuator in way of beam
- bad because attenuator lowers amount of radiation along the central axis (i.e. radiation less intense) —> to get same dose, monitor units have to increase —> increases treatment time

universal wedge
- single wedge fixed in the field, built into linac
- moved in and out of field
- selection of effective wedge angles done by splitting beam into two components or factors
- also results in reduced output (as its a physical wedge)

enhanced dynamic wedge
- jaws move dynamically across radiation field
- speed of this motion determines wedge angle
- wedging effect is given by moving one of the jaws with variable speed (depending on wedge angle required) while opposite jaw remains steady

explain enhanced dynamic wedges

wedge effect achieved by moving collimator and changing dose rate —> change in MUs not as great as physical wedges as there is no attenuator in beam path
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explain what MLCs are

multi-leaf collimators are beam modifiers that enable the beam to be shaped
they have movable leaves —> blocks some part of radiation beam
positioning of leaves allow a shaped field to be generated

explain the purpose of MLCs

• creates conformal dose distribution around the PTV

• shielding and shaping of OARs, avoids unnecessary dose to normal tissue

• can optimise dose distributions to deliver tumorcidal dose of radiation to PTV whilst minimising radiation to normal tissue