Protons

Question 1

What is LET?

Answer 1

• Linear Energy Transfer
• How much energy a particle transfers to the material it travels through
• per unit distance

Question 2

What is RBE?

Answer 2

• 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

Question 3

What are the properties of charged particles?

Answer 3

• 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

Question 4

What is a Bragg Peak?

Answer 4

• The point where the dose is highest

Question 5

What are issues with the Bragg Peak?

Answer 5

• It is very narrow, not broad enough to cover a tumour volume
• A spread out bragg peak must be created

Question 6

How do heavy ions act compared with protons?

Answer 6

• 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

Question 7

Why do we not use heavy ions for treatment?

Answer 7

• Higher energies are needed for depth penetration

- 150MeV protons = 16cm in water where 3600 MeV for carbon ions

Question 8

What technical considerations are necessary for proton therapy?

Answer 8

• How do we get a charged particle beam into the treatment room
• How do we produce the beam

Question 9

How are proton beams produced using a cyclotron?

Answer 9

• 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

Question 10

What are disadvantages of cyclotrons?

Answer 10

• 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

Question 11

How are proton beams produced using a synchotron?

Answer 11

• 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.

Question 12

What are disadvantages of synchotrons?

Answer 12

• Beam currents are lower than of a cyclotron, can limit doserate
• More expensive

Question 13

What are beamlines?

Answer 13

• Evacuated guides

- Often use magnetic and electric fields to steer and focus beam

Question 14

What are issues with the gantry system of setup in proton suites?

Answer 14

• Size and weight of gantry are a lot larger than typical photon linacs
• More difficult to use with necessary precision and accuracy

Question 15

What are the two main methods to shape a beam to target area?

Answer 15

• Passive beam shaping = straight forward

- Active beam shaping = more complex but more flexible

Question 16

How does passive beam shaping work?

Answer 16

• 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

Question 17

What are disadvantages of passive beam shaping?

Answer 17

• 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)

Question 18

How does active beam shaping work?

Answer 18

• 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.