First Year Exam: Electrons

Question 1

What effect does decreasing field size below side scatter equilibrium condition have on depth dose curve? What is a consequence of this?

Answer 1

Makes everything more shallow (but still has the same Rp)

Consequence: Loss in both lateral and depth coverage

Question 2

How much does depth dose vary with changing applicator size (after equilibrium condition is met)

Answer 2

Minimally

Maybe 1-2% difference

Question 3

Is penumbra larger or smaller at surface as you increase energy?

Answer 3

Smaller at surface for higher energies

Question 4

If penumbra larger or smaller at deeper depths as you increase energy?

Answer 4

Larger deeper in tissue for higher energy

Question 5

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:

Answer 5

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

Question 6

Roughly what margin do you want around your PTV for your cutouts (projected to PTV)?

Answer 6

1 cm margin all around

Question 7

In choosing the best beam energy, what are your goals with R90 and Rp?

Answer 7

You want your R90 > max depth of PTV while your Rp < minimum depth of critical structures

Question 8

As a general rule of thumb, how do you want to orient your electron fields relative to patient surface? Why?

Answer 8

Perpendicular for best possible penetration

Question 9

How thick does a lead collimator need to be for electrons?

What about cerrobend?

Answer 9

Lead(mm): Most probable energy / 2 + 1 (or not +1, either or)

Cerrobend: 1.2*thickness of required lead

Question 10

To sharpen penumbra, where do you want to put your collimation?

Answer 10

As close to surface as possible

Question 11

Why is it unbeneficial (besides penumbra), to position collimator far from patient skin?

Answer 11

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.

Question 12

How do you want to construct an internal collimator? Why?

Answer 12

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)

Question 13

What kind of material do you usually use for bolus?

Answer 13

Tissue-like material

Wax, water, superflab, plastic sheets, etc

Question 14

How do you want to orient your bolus?

Answer 14

On patient skin with no sharp edges within the field (will result in hot and cold spots otherwise)

Question 15

What 5 possible interactions with the medium can electrons undergo?

Answer 15

1. 2. Elastic collisions with electrons or nuclei
2. 4. Inelastic collisions with atomic electrons (ionization and/or excitation)
3. Inelastic collisions with atomic nuclei (bremsstrahlung)

Question 16

In low Z absorbers, what is the most common electron interaction?

Answer 16

Ionizations and excitations

Bremsstrahlung doesn’t really become significant until higher Z materials

Question 17

Why does surface depth dose increase with increase electron energy?

Answer 17

Higher energies have less scatter and are predominantly forward, therefore the surface dose relative to max dose is similar

Question 18

Why does the slope of depth dose falloff decrease with energy?

Answer 18

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

Question 19

What effect does a smaller field size have on dmax and slope of falloff?

Answer 19

Decreases dmax

Decreases slope of falloff (falls off slower)

Question 20

Explain the polarization density effect for high energy electron beams?

Answer 20

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

Question 21

Why do we have to directly measure a cutout’s percent depth dose curve?

Answer 21

Because there is no such thing as equivalent square fields for electrons. That only applies to photons

Question 22

How much does output vary from smallest to largest cone size?

Answer 22

15% increase

Question 23

How much does output vary from smallest to largest jaw size?

Answer 23

Upwards of 200% increase

Question 24

Why does output vary so drastically with changing jaw size for electrons?

Answer 24

Because it gives a large amount of scattering surfaces in the LINAC head as compared to the cone

Question 25

Why don’t electrons follow an inverse square relationship with distance from the actual source?

Answer 25

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

Question 26

How is the virtual source distance derived?

Answer 26

1. Take a charge reading at point A
2. Take a charge reading at point B
3. Do an inverse square equation from A to B, with the SSD replaced by VSD
4. Rearrange and solve for VSD using arithmetic

Question 27

What does it mean to feather an electron beam?

Answer 27

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

Question 28

What is the spectrum of an electron beam like before it enters the patient?

Answer 28

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

Question 29

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?

Answer 29

1. Polarization density effect (majority reason)
2. 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)

Question 30

What is the equation for minimum cutout radius needed for lateral scatter equilibrium?

Answer 30

R <= 0.88*sqrt(E0)

Question 31

What affect does changing SSD have on EDD?

Answer 31

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