Absorbed Dose Distribution - Electrons Flashcards
1
Q
What changes need to be made to a Linac for electron beams?
A
- Target removed (no photons produced)
- Flattening filter removed
- Scattering foil put in (produces a larger beam)
- Electron applicator placed externally (produces good field delineation)
2
Q
Electron beam overview (2)
A
- reduced skin sparing (advantage when treating close to surface)
- Finite range (dose to deeper tissue minimized)
3
Q
Why treat with electrons? (4)
A
-region of fairly uniform dose then a rapid fall off
- treat superficial legions
- avoid dose to deep/adjacent tissues
- skin lesions, scar boosting, avoiding sensitive deep structures
4
Q
Electron Interaction Types
A
- electrons are almost monoenergetic
- elastic coulomb force interactions
inelastic coulomb force interactions
5
Q
Elastic Collisions
A
- conservation of energy and momentum
- kinetic energy of particle after collision equal incident particle
- electron direction may change
- collisions with: outer shell electrons, nuclei (bounces off)
6
Q
Inelastic Collisions
A
- conservation of momentum but not energy
- kinetic energy of incident particle lost to other forms of energy (excitation of struck atom, x-ray)
- collisions with: inner shell electrons, nuclei (x-ray emission)
7
Q
Electron beam CAX PDD
A
- relatively high surface dose (75-100%)
- Max dose occurs at depth of z-max
- dose drops off rapidly after and levels off at a small low level dose called bremsstrahlung tail (few %)
8
Q
Electron vs photon PDD
A
- reduced skin sparing -\ surface dose increases with energy (opposite to photons)
- choice of beam energy is more critical due to rapid fall off beyond 90%
- decreased slope at greater depths
- x-ray contamination gives a tail to the curves
9
Q
Ranges
A
- maximum range - Rmax - max dose - E/4
- Practical range - Rp - at rest - E/2
- Therapeutic range - R80-90 - Treatment - E/3
10
Q
Effect on field size
A
- Large field sizes = PDD distribution at a given energy is independent of field size
- Small fields = when side of electron field is smaller than Rp, more scattering occurs resulting in a reduction in depth dose. Dmax decreases and surface dose increases.
11
Q
Electron beam isodose curves
A
- beam expands rapidly below surface due to electron scattering on atoms of the medium
- spread depends on: isodose level, beam energy, field size, beam collimation
12
Q
High energy electron isodose curves
A
- low level curves bulge out
- high level curves laterally constrict
12
Q
Surface Irregularities
A
- Surface contour changes
- Causes hot or cold spots
12
Q
Low energy electron isodose curves
A
All isodose levels bulge out
13
Q
Oblique incidence
A
- beam is not at a right angle to surface
- Dose increases (hot spot) close to surface
