First Year Exam: Electrons Flashcards
What effect does decreasing field size below side scatter equilibrium condition have on depth dose curve? What is a consequence of this?
Makes everything more shallow (but still has the same Rp)
Consequence: Loss in both lateral and depth coverage
How much does depth dose vary with changing applicator size (after equilibrium condition is met)
Minimally
Maybe 1-2% difference
Is penumbra larger or smaller at surface as you increase energy?
Smaller at surface for higher energies
If penumbra larger or smaller at deeper depths as you increase energy?
Larger deeper in tissue for higher energy
What occurs to the following as you increase angle of obliquity?
Penumbra: Surface dose: Depth of max dose: Max dose: Therapeutic depth: Depth of maximum penetration:
What occurs to the following as you increase angle of obliquity?
Penumbra: Decreases Surface dose: Increases Max dose: Increases Depth of max dose: Decreases Therapeutic depth: Decreases Depth of maximum penetration: Increases
Roughly what margin do you want around your PTV for your cutouts (projected to PTV)?
1 cm margin all around
In choosing the best beam energy, what are your goals with R90 and Rp?
You want your R90 > max depth of PTV while your Rp < minimum depth of critical structures
As a general rule of thumb, how do you want to orient your electron fields relative to patient surface? Why?
Perpendicular for best possible penetration
How thick does a lead collimator need to be for electrons?
What about cerrobend?
Lead(mm): Most probable energy / 2 + 1 (or not +1, either or)
Cerrobend: 1.2*thickness of required lead
To sharpen penumbra, where do you want to put your collimation?
As close to surface as possible
Why is it unbeneficial (besides penumbra), to position collimator far from patient skin?
Because it does literally nothing. This is because the side scattering in the phantom will all make it into the region you were trying to protect. This won’t happen if the collimator does its job closer to the surface though.
How do you want to construct an internal collimator? Why?
Lead inside, covered with a layer of non dense material
Lead does the attenuation of the beam, the layer of non-dense material gets rid of the backscatter (and also protects your internals from lead toxicity)
What kind of material do you usually use for bolus?
Tissue-like material
Wax, water, superflab, plastic sheets, etc
How do you want to orient your bolus?
On patient skin with no sharp edges within the field (will result in hot and cold spots otherwise)
What 5 possible interactions with the medium can electrons undergo?
- Elastic collisions with electrons or nuclei
- Inelastic collisions with atomic electrons (ionization and/or excitation)
- Inelastic collisions with atomic nuclei (bremsstrahlung)
In low Z absorbers, what is the most common electron interaction?
Ionizations and excitations
Bremsstrahlung doesn’t really become significant until higher Z materials
Why does surface depth dose increase with increase electron energy?
Higher energies have less scatter and are predominantly forward, therefore the surface dose relative to max dose is similar
Why does the slope of depth dose falloff decrease with energy?
Due to the randomness of electron paths
At higher energies, they have a longer path length and therefore more opportunity for their energy spectrum to spread out
What effect does a smaller field size have on dmax and slope of falloff?
Decreases dmax
Decreases slope of falloff (falls off slower)
Explain the polarization density effect for high energy electron beams?
In a dense medium (water) when an electron interacts it creates ions. Due to a high density of the medium, the created ions are in a dense distribution, so ions screen the incident electron’s electric field from interacting with distant particles, effectively reducing the interaction rate
In a low density medium (air), the ionization track has a low density of ions, and therefore screening of distant particles is negligible
Why do we have to directly measure a cutout’s percent depth dose curve?
Because there is no such thing as equivalent square fields for electrons. That only applies to photons
How much does output vary from smallest to largest cone size?
15% increase
How much does output vary from smallest to largest jaw size?
Upwards of 200% increase
Why does output vary so drastically with changing jaw size for electrons?
Because it gives a large amount of scattering surfaces in the LINAC head as compared to the cone
Why don’t electrons follow an inverse square relationship with distance from the actual source?
Because there is significant amounts of scatter from many materials in the LINAC head
Therefore, instead of a point source, there are multiple effective sources for electrons
How is the virtual source distance derived?
- Take a charge reading at point A
- Take a charge reading at point B
- Do an inverse square equation from A to B, with the SSD replaced by VSD
- Rearrange and solve for VSD using arithmetic
What does it mean to feather an electron beam?
If you have two adjacent electron beams, the bulges will cause hot or cold spots
Feathering is the process of, fraction by fraction, moving the fields either further or closer together in order to reduce the hot or cold spot
What is the spectrum of an electron beam like before it enters the patient?
It’s nearly monoenergetic out of the bending magnet
On its way to the patient, it starts to broaden the spectrum a bit as some electrons interact at different rates with the collimation and the air
But for the most part, it’s still close to monoenergetic as it hits the patient
What are the two reasons why the stopping power ratios in electrons are not constant as they progress through medium, and hence, we can’t directly measure a %DD curve? Which reason is the biggest contributor?
- Polarization density effect (majority reason)
- Change in electron energy (not as major, recall the collisional stopping power remains fairly constant until under 0.5 MeV. The radiative stopping power will decrease as electrons travel through medium, but it’s magnitude is small to begin with in water, especially compared with collisional. So energy in water doesn’t cause a huge effect until 0.5 MeV or below)
What is the equation for minimum cutout radius needed for lateral scatter equilibrium?
R <= 0.88*sqrt(E0)
What affect does changing SSD have on EDD?
Very minimal effect because depth of penetration is typically small and divergence is minimal
But for higher energies, the R90 can be shifted a few mm distally
