electron therapy Flashcards
Electrons can interact with either…
- the nucleus of the atom
- the atomic electrons
what type of linac produces electrons as well as photons
dual modality linac
what are the characteristics of electron therapy
- non-isocentric
- skin apposition as the patient is in contact with the applicator
Describe the changes of a dual modality linac when changing from photons to electrons
- the tungsten target is retracted
- the flattening filter is retracted and a scattering foil is inserted.
- an applicator is added to the head of the linac
- gun current is reduced.
- collimators are opened out wide
what is the function of the primary coil
prevents leakage
scattering foils scatter electrons, high z material scatter electrons more than low z materials. what is the problem with this?
bremsstrahlung is created in the foil which contaminates the foil with x-rays
what is the collimator in electron therapy
the applicator
why is skin apposition used in electron therapy
as electrons are easily scattered by air so the collimator has to go right down to the patient surface.
what is the function of the endplate
located at the end of the applicator it defines the field size.
graph wise what is the difference between a collimated and non collimated electron beam
with - dose increases and the dose fall of is rapid
without - less dose and dose falls of exponentially.
why can’t lead be used in electron therapy to half-block the field
as lead has a high z number it will produce x-rays through bremsstrahlung
describe the dosimetry of electrons
- deposit dose of a smaller range than photons
- sharp dose fall-off beyond the 80% isodose.
in electron therapy what determines the Dmax
electron energy
electron therapy treats/ superficial lesions, why
as the majority of the dose is deposited on the patients skin, there is little build up.
Bremsstrahlung contamination in electron therapy should be kept at X% minimum
3%
