Charged Particle Interactions

Question 1

What is Bremsstrahlung?

Answer 1

The charged particle is deflected by the nucleus and changes direction
The excess energy is emitted as a photon

Question 2

Why is there more Bremsstrahlung in the target than in the patient?

Answer 2

Human tissues have a relatively low Z value

Question 3

When would a higher level of Bremsstrahlung than usual be seen in a patient?

Answer 3

eg. false hips

Question 4

What is stopping power?

What are its units?

Answer 4

The power of a material to ABSORB the energy of incident charged particles
(Charged particle equivalent of attenuation coefficient)

units = Jm^-1

Question 5

How can we find stopping power per unit mass?

What are its units?

Answer 5

Divide by the density

units = Jm^(2)kg^(-1)

Question 6

How can ionisation cause DNA damage?

Answer 6

Production of free radicals (molecules that are highly reactive due to the presence of unpaired electrons)
DNA chemical bonding (free radicals can form compounds, eg. hydrogen peroxide, which can initiate harmful reactions in the cell)

Question 7

What factors cause a charged particle to lose energy, and how does energy loss relate to them?

Answer 7

Charge:
Energy loss is proportional to the square of its charge

Velocity:
Energy loss is inversely proportional to the square of its velocity

Question 8

How is energy loss of a charged particle related to its charge?

Answer 8

Energy loss is proportional to the square of its charge

Question 9

How is energy loss of a charged particle related to its velocity?

Answer 9

Energy loss is inversely proportional to the square of its velocity

Question 10

How is energy loss of a charged particle related to its mass?

Answer 10

Energy loss is independent of its mass

Question 11

What is linear energy transfer?

What are its units?

Answer 11

The rate at which energy is deposited locally per unit length
(paintball gun analogy)

units = keV per micron

Question 12

What is a micron?

Answer 12

A micrometre

Question 13

What is the difference between stopping power and linear energy transfer?

Answer 13

Stopping power is the energy ABSORBED, linear energy transfer is the energy DEPOSITED

Question 14

Why do electrons have a fairly low depth of penetration in tissue?

Answer 14

They are small and charged, so easily deflected and scattered

Question 15

What is range straggling?

Answer 15

The point at which an incident electron has lost all of its energy identifies its range
Like any charged particle, it loses most of its energy at the end of its path (Bragg Peak)
Due to scattering, not all the electrons go in a straight line so the depths (therefore ranges) will very
This is range straggling

Question 16

How far into a patient will electrons travel before they begin to interact?

Answer 16

Electrons start interacting and losing energy as soon as they are incident on the material

Question 17

What happens to the surface dose of an electron beam as energy increases?

Answer 17

It increases

Question 18

What are the main features of an electron percentage depth dose curve?

Answer 18

High surface dose
Rapid fall off beyond dmax
Bremsstrahlung tail

Question 19

Why does an electron percentage dose depth curve have a Bremsstrahlung tail?

Answer 19

Generation of photons due to bremsstrahlung collisions with nuclei

Question 20

What is the rule of thumb for calculating the practical range (cm) of an electron beam?

Answer 20

0.5 x E

Question 21

What is the rule of thumb for calculating the therapeutic range of an electron beam?

Answer 21

0.3 x E

Question 22

What is meant by the therapeutic range of an electron beam?

Answer 22

The most useful treatment depth

Question 23

What part of the electron percentage dose depth curve is the therapeutic range usually found?

Answer 23

The distal 85-90%

Question 24

Does field size have a large or small effect on electron beam percentage depth dose curves?

Answer 24

Very big effect

Question 25

Is the effect of field size on an electron beam percentage dose depth curve most significant at large or small field sizes?

Answer 25

Small

Question 26

What effect does decreasing field size have on an electron percentage dose depth curve?

Answer 26

The dose drops off more quickly

Question 27

Why can treating at small field sizes with an electron beam be difficult?

Answer 27

The dose drops off very quickly

Question 28

Why do electron beam isodose lines 'balloon' out?

Answer 28

Scatter

Question 29

Why is range straggling not seen at lower energies?

Answer 29

The electrons do not have enough range

Question 30

Why do electron beam isodose lines 'balloon' out more at deeper depths?

Answer 30

The lateral scatter component increases with depth as electrons slow

Question 31

What does a bolus do?

Answer 31

Increases dose at the surface Reduces penetration to underlying tissue ''tricks'' dose into thinking there's more tissue

Question 32

What is the rule of thumb for the thickness of lead needed to shield an electron beam?

Answer 32

(0.5E) + 1mm

Question 33

When might backscatter be an issue with electron beams?

Answer 33

High density materials can produce a lot of back scatter (when the electrons hit the shielding they will produce xrays through bremsstrahlung) This means that shielding can present an issue if it behind tissue (eg. nostril shielding)

Question 34

Why is skin apposition important?

Answer 34

It is important that the dose to the treatment area is as expected If one area is further away from the source, the electrons will have been through more scattering events so will have less energy

Question 35

What is the issue if an area of skin is is further away from the electron beam than expected?

Answer 35

The electrons will have been through more scattering events so will have less energy

Question 36

Why may a dip in the beam occur? | What can this cause?

Answer 36

Scar, inframammary folds etc. | This can cause hotspots

Question 37

What are the main features of a heavy charged particle percentage dose depth curve?

Answer 37

``` Low entrance dose (they move very fast so don't deposit a lot of dose at the start) Bragg peak (the particles deposit most of their energy as they slow down and stop) Rapid dose fall off Fragmentation tail (heavy particles can crash into nuclei, splitting them into highly charged ions which carry on and do a lot of damage) Practically 0 exit dose ```

Question 38

Why do heavy charged particles have a low entrance dose?

Answer 38

They move very fast so don't deposit a lot of energy at the start

Question 39

What causes the Bragg peak?

Answer 39

The particles deposit most of their energy as they slow down and stop (high LET)

Question 40

What is a spread out Bragg peak and why is it necessary?

Answer 40

Proton beams are monoenergetic Bragg peak isn't wide enough to cover most treatment volumes Clinical beams use a range shifter to create a polyenergetic beam This effectively combines multiple Bragg peaks to cover a greater volume

Question 41

What is a range shifter?

Answer 41

A perspex wheel of varying thickness to attenuate the proton beam to different extents