4.6 Electron Beam physics

Question 1

What are the key features of electrons?

Answer 1

1. because electrons are charged particles they interact with electrons and nuclei due to their coulomb field as they pass through matter. This means the lose energy as they travel. (unlike photons which aren’t charged)
2. Electrons have low mass and therefore scatter easily
3. Electrons have a steep drop off and are therefore useful in treating superficial targets

Question 2

Describe the electron path

Answer 2

The electron path is very tortous - ‘drunken man’

For electrons of the same energy the length of the path is always the same but the range (how far into the material the electron gets) varies (range straggling)

Question 3

How does the energy of an electron beam change with depth?

Answer 3

Mean energy reduces by 2MeV per 1cm in water/ soft tissue

Question 4

What happens when the energy of an electron beam increases?

Answer 4

• Mean path length increases
• Scattering is less likely
• Greater energy means more penetrating (harder to defelct) so fewer electrons are absorbed at the surface
• Beam is forward peaked - harder to deflect and tend to travel in straighter lines

Question 5

What is Dmax?

Answer 5

Depth of 100% max dose

Question 7

What is R90?

Answer 7

Depth of 90% max dose (therapuetic range)

Question 8

What is the clinical significance of R90?

Answer 8

Distal tumour margin should be no deeper than R90 to ensure it is all covered

Question 9

What is R50?

Answer 9

Depth of 50% max dose

Question 10

What is the clinical signficance of R50?

Answer 10

Used to measure electron beam quality

Question 11

What is Rp (practicle range)?

Answer 11

Range of electrons that would have scattered the least

i.e. the equivallent if an electron had only travelled in straight line.
Constant for same energy, regardless of field size

This is where the curve continues and intercepts the axis of the Bremsstrahlung tail

Question 12

What is the 2, 3, 4, 5 rule?

Answer 12

Dmax is 2x energy (mm)
D90 is 3x energy (mm)
D50 is 4x energy (mm)
Rp is 5x energy (mm)

Question 13

What is the Bremsstrahlung tail?

Answer 13

Dose deposited by electrons created by contamination in the head of the linac - not from the primary beam
Secondary electrons that will then deposit their dose

Question 14

What happens if the energy of an electron beam is increased?

Answer 14

• surface dose increases
• PDD at depth increases (increased dose penetration)
• Dose fall off is less steep
• Bremsstrahlung tail increases (more interactions in the head of the Linac)

Question 15

What happens to the electron beam if the field size (applicator) is increased?

Answer 15

Do not use the equivalent square for electrons!

• Dmax depth increases
• Surface dose is reduced

Over 10x10cm the PDD is independent of the field size - this is because of Lateral Scatter Equilibrium

Question 16

What is Latteral Scatter Equilibrium?

Answer 16

• same number of electrons being scatter towards the central axis as away from the axis
• the distance from the edge of the field to the cental axis is greater than the electron lateral scatter range

Question 17

What happens to the electron beam if the field size (applicator) decreases?

Answer 17

<10x10cm PDD is strongly dependent on applicator size and shape

This is because the range of electrons is greater than the distance from the edge of the field to the central axis

Question 18

What happens to build up and skin sparing as electron energy increases?

Answer 18

Absolute surface dose does not signficiantly change
Relative surface dose increases - relatvie to the Dmax of that beam
Dmax increases - distance over which the build up occurs increases - lower energy electrons scatter more sharply. Higher energy beams don’t scatter as much so are more like the original electrons so can keep penetrating

Question 19

What does a bolus do?

Answer 19

• Moves build up region into bolus - so high dose at skin
• Can bring Dmax to a superficial tumour

Question 20

What does altering the surgace obliquity do?
Increase the angle of incidence

Answer 20

• Surface dose increases
• Dmax comes closer to the surface (increases)
• Penumbra widens
• Isodose lines follow surface controur

Question 21

How does inhomogeneity of tissue perturb dose distribution?

Answer 21

When electrons are scattered from high density tissue (e.g. chest wall) to low density tissue (e.g. lung) a higher dose is given to the low density area because the electrons can travel further without scatter interactions. A lower dose is given to the higher density area e.g. chest wall due to the lack of backscatter.

Question 22

What factors contribute to dose perturbation in inhomogenous tissue?

Answer 22

• Effect increases with increasing beam energy
• The shape of the tissue
• The composition of the tissue
• The size of the inhomogeneity relative to the field size

Question 23

What is shielding used for?

Answer 23

Shielding spares irradiation of distal tissues e.g. can be put inside a cheek or under an eyelid

Question 24

What thickness is needed to shield from a beem?

Answer 24

Thickness of lead (mm) = Energy (MeV)/2

Question 25

Why does thickness of a shield matter?

Answer 25

SHield has to be thick enough to fully absorb the treatment electrons otherwise you unintenionally increase the skin dose by adding electrons from within the lead

Question 26

What is the downside of a shield?

Answer 26

• Electron bckscatter off of the lead shield increases near tissue being shielded.
• The electrons are of low energy because they have already interacted with the lead shield so are absorbed near the skin. Exit surface dose can increase to 130-140%.
• Wax is added to the shield to absorb these electrons before they depost dose on the surface

Question 27

What shape is electron dose deposition?

Answer 27

A narrow electron beam has a teardrop shaped dose deposition
In clinical practice when you put a lot of narrow beams next to eachother you get multiple teardrops all overlappng. The teardrp shape gives a lateral bulge outside of the applicator field

Question 28

How do electrons scatter as they get deeper?

Answer 28

Electrons scatter away from the central axis. Therfore lower isodoses widen laterally with depth in tissue whereas the useful high isodoses e.g. 95 and 90 constrict at the peripheray.

Question 29

What happens to the 90% isodose as the electron field gets bigger?

Answer 29

It gets flatter

Question 30

What is the approximate width of the penumbra?

Answer 30

1cm

Question 31

How big should the applicator be?

Answer 31

Beam can be constricted at the treatment 90% isodose
Therfore the applciator should be 1cm larger either side of the treatment volume

Question 32

What is the risk of bulging isodose?

Answer 32

Can start to encroach on OARs

Question 33

What hapens if you increase the FSD (standard 100cm)?

Answer 33

• Broader penumbra - due to beam divergence
• Beam is more penetrating (relative impact of ISL is less)
• Absolute dose rate decreases
• Absolute surface dose decreases
• Relative surface dose increases - like adding 15 min to a 5 min and 60 min car journey

e.g. for 6MeV with a Dmax of 1.2cm

At 10cm deep
100cm FSD: (110/101.2)^2 = 1.18
120cm FSD: (120/121.2)^2 -1.15