Protons Flashcards
1
Q
What is LET?
A
- Linear Energy Transfer
- How much energy a particle transfers to the material it travels through
- per unit distance
2
Q
What is RBE?
A
- Relative biological effectiveness
- Ratio of biological effectiveness between one type of ionising radiation to another, given the same amount of absorbed energy
- Higher RBE, the more damaging, per unit of energy deposited in biological tissues
- Protons are 0.1 more damaging that photons
3
Q
What are the properties of charged particles?
A
- As charged particles travel through tissue, energy is lost due to atomic or nuclear interactions
- reduces energy of the particle, this increases interactions with electrons
- maximum interaction occurs at the end of the range when the particle speed is reduced
- Means maximum energy transfer and therefore maximum dose to be deposited in the area
- No exit dose, finite range
- Energy required, depends on depth of target within tissue
4
Q
What is a Bragg Peak?
A
- The point where the dose is highest
5
Q
What are issues with the Bragg Peak?
A
- It is very narrow, not broad enough to cover a tumour volume
- A spread out bragg peak must be created
6
Q
How do heavy ions act compared with protons?
A
- Ratio of bragg peak dose versus dose in the entrance region is larger for heavy ions than for protons
- Heavy ions give improved dose conformation than photons and protons with better sparing of normal tissues close to the target
- Higher LET with heavy ions, compared to protons, in the bragg peak rather than the entrance
- High LET can be conformed to the tumour
7
Q
Why do we not use heavy ions for treatment?
A
- Higher energies are needed for depth penetration
- 150MeV protons = 16cm in water where 3600 MeV for carbon ions
8
Q
What technical considerations are necessary for proton therapy?
A
- How do we get a charged particle beam into the treatment room
- How do we produce the beam
9
Q
How are proton beams produced using a cyclotron?
A
- cyclotrons produce one, continuous fixed energy proton beam
- Straightforward beam production and application
- Creation of SOPB and energy range through the use of absorbers
- Multiple beam lines possible
- Need to produce higher energy with the knowledge that it can be reduced
10
Q
What are disadvantages of cyclotrons?
A
- Inefficient
- Not able to directly change energy of particles
- Degrading the beam energy using absorbers reduces the efficiency of the system
- More shielding is required due to secondary radiation produced
11
Q
How are proton beams produced using a synchotron?
A
- Produces a pulsed beam
- Higher energies are possible
- No absorbers are required for SOPB or energy range, this avoids scattering and degradation of beam.
- Acceleration more complicated than cyclotron and therefore more expensive.
12
Q
What are disadvantages of synchotrons?
A
- Beam currents are lower than of a cyclotron, can limit doserate
- More expensive
13
Q
What are beamlines?
A
- Evacuated guides
- Often use magnetic and electric fields to steer and focus beam
14
Q
What are issues with the gantry system of setup in proton suites?
A
- Size and weight of gantry are a lot larger than typical photon linacs
- More difficult to use with necessary precision and accuracy
15
Q
What are the two main methods to shape a beam to target area?
A
- Passive beam shaping = straight forward
- Active beam shaping = more complex but more flexible
16
Q
How does passive beam shaping work?
A
- Need to spread out the Bragg Peak (SOBP)
- Uses a variable degrader on a rotating wheel or spiral absorber
- This is preformed to produce specific depth-dose profile.
- SOBP must be at desired depth, need a range shifter. Plastic plates put into the beam to shift the range (stop beam earlier)
- Beam must be made clinically useful with a double scattering system and an orthogonal magnetic dipole wobbler.
- Need MLCs
- Intensity Modulation via a compensator which shapes the dose to the distal surface of the target volume. Means that the depth dose profile can only be shifted to smaller depths
17
Q
What are disadvantages of passive beam shaping?
A
- Depth dose is tailored to distal not proximal end
- higher dose region is located in normal tissue, proximal to the target volume
- Large amount of material is needed in the beam which can increase scatter and nuclear fragment production (neutrons)
18
Q
How does active beam shaping work?
A
- Utilises the charged nature of the particles to steer the beam with dipole magnets
- used with synchotron
- Target can be scanned in three dimensions allowing beam to be tailored to irregular shaping, no colls or compensator needed.
