electron therapy

Question 1

Electrons can interact with either…

Answer 1

1. the nucleus of the atom
2. the atomic electrons

Question 2

what type of linac produces electrons as well as photons

Answer 2

dual modality linac

Question 3

what are the characteristics of electron therapy

Answer 3

1. non-isocentric
2. skin apposition as the patient is in contact with the applicator

Question 4

Describe the changes of a dual modality linac when changing from photons to electrons

Answer 4

1. the tungsten target is retracted
2. the flattening filter is retracted and a scattering foil is inserted.
3. an applicator is added to the head of the linac
4. gun current is reduced.
5. collimators are opened out wide

Question 5

what is the function of the primary coil

Answer 5

prevents leakage

Question 6

scattering foils scatter electrons, high z material scatter electrons more than low z materials. what is the problem with this?

Answer 6

bremsstrahlung is created in the foil which contaminates the foil with x-rays

Question 7

what is the collimator in electron therapy

Answer 7

the applicator

Question 8

why is skin apposition used in electron therapy

Answer 8

as electrons are easily scattered by air so the collimator has to go right down to the patient surface.

Question 9

what is the function of the endplate

Answer 9

located at the end of the applicator it defines the field size.

Question 10

graph wise what is the difference between a collimated and non collimated electron beam

Answer 10

with - dose increases and the dose fall of is rapid

without - less dose and dose falls of exponentially.

Question 11

why can’t lead be used in electron therapy to half-block the field

Answer 11

as lead has a high z number it will produce x-rays through bremsstrahlung

Question 12

describe the dosimetry of electrons

Answer 12

• deposit dose of a smaller range than photons
• sharp dose fall-off beyond the 80% isodose.

Question 13

in electron therapy what determines the Dmax

Answer 13

electron energy

Question 14

electron therapy treats/ superficial lesions, why

Answer 14

as the majority of the dose is deposited on the patients skin, there is little build up.

Question 15

Bremsstrahlung contamination in electron therapy should be kept at X% minimum

Answer 15

3%

Question 16

what is stand-off

Answer 16

when it is not possible to achieve skin apposition with the end plate - on curvy areas for example nose. Stand-off affects the dose by a factor of the extra distance squared.