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RADT229 > Charged Particle Interactions > Flashcards

Charged Particle Interactions Flashcards

1
Q

What is Bremsstrahlung?

A

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

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2
Q

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

A

Human tissues have a relatively low Z value

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3
Q

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

A

eg. false hips

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4
Q

What is stopping power?

What are its units?

A

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

units = Jm^-1

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5
Q

How can we find stopping power per unit mass?

What are its units?

A

Divide by the density

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

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6
Q

How can ionisation cause DNA damage?

A

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)

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7
Q

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

A

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

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

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8
Q

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

A

Energy loss is proportional to the square of its charge

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9
Q

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

A

Energy loss is inversely proportional to the square of its velocity

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10
Q

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

A

Energy loss is independent of its mass

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11
Q

What is linear energy transfer?

What are its units?

A

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

units = keV per micron

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12
Q

What is a micron?

A

A micrometre

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13
Q

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

A

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

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14
Q

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

A

They are small and charged, so easily deflected and scattered

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15
Q

What is range straggling?

A

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

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16
Q

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

A

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

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17
Q

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

A

It increases

18
Q

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

A

High surface dose
Rapid fall off beyond dmax
Bremsstrahlung tail

19
Q

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

A

Generation of photons due to bremsstrahlung collisions with nuclei

20
Q

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

A

0.5 x E

21
Q

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

A

0.3 x E

22
Q

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

A

The most useful treatment depth

23
Q

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

A

The distal 85-90%

24
Q

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

A

Very big effect

25
Q

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

A

Small

26
Q

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

A

The dose drops off more quickly

27
Q

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

A

The dose drops off very quickly

28
Q

Why do electron beam isodose lines ‘balloon’ out?

A

Scatter

29
Q

Why is range straggling not seen at lower energies?

A

The electrons do not have enough range

30
Q

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

A

The lateral scatter component increases with depth as electrons slow

31
Q

What does a bolus do?

A

Increases dose at the surface
Reduces penetration to underlying tissue

'’tricks’’ dose into thinking there’s more tissue

32
Q

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

A

(0.5E) + 1mm

33
Q

When might backscatter be an issue with electron beams?

A

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)

34
Q

Why is skin apposition important?

A

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

35
Q

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

A

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

36
Q

Why may a dip in the beam occur?

What can this cause?

A

Scar, inframammary folds etc.

This can cause hotspots

37
Q

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

A 
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
38
Q

Why do heavy charged particles have a low entrance dose?

A

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

39
Q

What causes the Bragg peak?

A

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

40
Q

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

A

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

41
Q

What is a range shifter?

A

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

RADT229 (6 decks)

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