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Radiotherapy Year 3 > Protons > Flashcards

Protons Flashcards

1
Q

What is the rationale for proton therapy?

A

No exit dose past the TV
Reduce morbidity (including integral dose and second malignancy) - major motivation for paediatric indication
Dose escalation can increase curative treatment options - motivation in adult indications

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2
Q

Why are protons especially beneficial for paediatric patients?

A

Reduced risk of growth/development problems
Reduced risk of induced cancers

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3
Q

What are the three types of proton interactions?

A

Coulomb interactions with electrons
Coulomb interactions with nuclei
Non elastic collisions with nuclei

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4
Q

How do protons interact with electrons

A

Secondary electron(s) released
Proton loses energy but doesn’t change direction
Probability of more interactions is increased. Rate of energy deposition increases.
Produces Bragg peak

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5
Q

What is Bethe-Bloch equation?

A

S/rho proportional to 1/v^2 . Z/A . z^2

Where:
v = Velocity of incident proton
z = Atomic number of incident proton
Z = Atomic number of target nucleus
A = Mass number of target nucleus

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6
Q

What is range of proton defined by?

A

Initial energy of beam
Range ~ E^2 approx

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7
Q

Where does range straggling come from

A

Energy loss occurs at a very large number of discrete interactions in medium
Energy loss is statistical process, each proton stops at slightly different range

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8
Q

What is LET

A

Linear energy transfer, energy transferred per unit distance.
Measure of beam quality
For beam of many protons must be calculated by averaging over all protons
Dependent on proton’s energy (velocity), deposits more as it slows down

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9
Q

How do protons interact with nuclei?

A

Proton direction is changed
Produces lateral spreading
Has gaussian profile

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10
Q

How do protons interact with nuclei in non elastic way?

A

May release nuclear fragments
Can no longer identify original proton
Produces halo

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11
Q

What needs to be considered about plotting lateral spread?

A

Gaussian function gives good fit on linear scale
On logarithmic scale, has parabolic shape due to halo
This can have difference of up to a few percent

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12
Q

What are logistical considerations of having multiple proton accelerators?

A

Most cost effective approach is to have one accelerator for multiple rooms
Beam can only be delivered to one room at once
3 is optimal

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13
Q

How do cyclotrons work

A

There is a fixed magnetic field, the alternating electric field between the Ds accelerates the proton beam

Have two semi circular electrodes, Dees, with space between them.
Magnetic circuit, set of coils used to create strong B field perpendicular to Dees. Strong electric field between Dees. Proton injected into space between Dees travels along trajectory radius r. Accelerating E field reverses as proton completes half circle and accelerates it across gap. Radius increases until proton beam ejected from Dees. Single energy produced.

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13
Q

What are the key characteristics of cyclotrons?

A

Single energy
Stable beam energy
High intensity

Not good for high energies or heavier particles

used in both UK centres

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14
Q

How do synchotrons work?

A

Magnetic field is adjusted as the protons are accelerated
Electric field in cavity is timed to accelerate proton beam

Ring of cavities. Protons accelerated up to few MeV and then injected into B field of Synchrotron. Bending magnets accelerate protons. These are interspersed with linear focussing sections and RF cavity for acceleration. Acceleration is synchronised with angular frequency of protons. Can produce different energies.

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15
Q

What are the key characteristics of synchrotrons?

A

Variable output energy
Pulsed beam
Can do higher energies and heavier particles

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16
Q

What are the energies and depths delivered by the ProBeam

A

70-240MeV, treating up to 35cm, max field size 30x40

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17
Q

What beam is produced and how is the beam shaped?

A

A mono energetic pencil beam is produced
The beam must be spread and shaped in longitudinal and lateral directions
Scattering or scanning

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18
Q

What makes up a scattering system?

A

Range modulator for energy spreading
Typically two scattering foils for lateral spreading
Compensator and collimator
Compensator adjusts beam edge, collimator shapes beam laterally

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19
Q

What makes up a pencil beam scanning system

A

Energy selector (could be done range modulator wheel or shark tooth method)
Steering magnets for steering

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20
Q

What are the advantages and disadvantages of scattering systems

A

+ dose delivered to entire target simultaneously which is good for moving targets

  • longitudinal length of SOBP is fixed, extra dose delivered proximal to target
  • field specific hardware is necessary (additional source of neutron dose, manual handling issues, storage and recycling issues, radiation protection issues as they become activated, more difficult to adapt treatments)
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21
Q

What are the advantages and disadvantages of scanning systems

A

+ improved ability to conform to target
+ no field specific hardware

  • treating moving targets could be harder due to interplay
  • lateral edge of field less sharp
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22
Q

What is the minimum deliverable proton energy and depth and how is this adjusted

A

70MeV, 4cm
Use range shifter to treat shallower depth (block of material inserted into beam path and attached to end of nozzle)

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23
Q

How does range shifter impact spot size and how can it be altered

A

Increased divergence for range shifted spots, as range shifter acts as scatterer. Lateral edge is less sharp.

Spot size depends on air gap between range shifter and patient, reduce this to reduce spot size

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24
How does proton planning differ to photon planning in terms of imaging?
Electron density would no longer be representative of dose, map HU to relative proton stopping power Uncertainty arises because imaging and treatment uses different particles
25
What measurements are used to commission a PBS system?
Depth dose profiles Quantifying dose Lateral profiles Divergence (of individual spots and steering system)
26
How are depth dose profiles measured
Single spot Large diameter parallel plate chamber Chamber diameter much greater than spot size, dose deposited outside of chamber up to approximately 1% Insensitive to chamber positioning Charge integrated over chamber area to give integrated epth dose curve Chamber scanned in depth direction to measure IDD over full bragg peak Done for each energy
27
How do bragg peaks vary with energy
Sharpest for lower energy and broader for high, high energy protons undergo larger number of interactions and come to rest, so range straggling increases with depth
28
Why are IDDs not suitable for absolute dose measurements
Miss dose scattered laterally due to nuclear interactions
29
How is dose quanitified?
A large field is used with a small chamber - charged particle equilibrium is met Reference depth defined, field consists of large number of spots with defined MU and dose is measured in terms of Gy/ (MU/area) (or Gy.mm^2 / MU) Absolute dose calibration is dependent on energy, charge is dependent on energy of proton
30
How are lateral profiles measured
Single spot measured in air with IBA lynx each spot has gaussian profile Measured for full range of energies at a number of positions up/down stream of isocentre
31
What are the features of the lateral profile of the beam?
Beam envelop forms a curve which can be fitted using hyperbola Waist is narrowest part of beam Far field divergence angle is theta
32
What is the VSAD?
Virtual source axis distance Distance from isocentre to apparent source of proton beam May differ in x and y depending on design and position of steering magnets Some systems use divergent steering and some use parallel steering
33
What are the features of PBS planning?
(similar to step and shoot photon IMRT) Inverse planned PTV and OAR objectives Discrete beam angles, no arcs Typically 1-5 fields per plan Beams are not necessarily coplanar
34
What parameters are configured during PBS planning?
Defined by the planner: Technique Objectives Beam angles Beam modifiers (range shifters etc) Computer: Spot positions Spot weights
35
What is the planning technique dependent on?
Anatomy of targets/OARs Type of cancer Delivery technology On treatment imaging technology TPS features Department protocols
36
What are the two distinct types of planning?
Single field optimisation Multi field optimisation
37
How does MFO differ from SFO?
SFO optimises the two fields independently, MFO optimises them together MFO provides more control over combined dose distribution (uniform dose for each field not required, only combined dose distribution must be uniform)
38
How do energy layers build up dose?
Each field consists of a number of energy layers which cover the range of the target Each energy layer consists of a number of spots
39
What is uncertainty vs error?
Error is the mistake in a measurement but it is unknowable Uncertainty expresses the chance that an error of a given magnitude has been made
40
What is range uncertainty?
The uncertainty in where the protons actually stop in the patient 2.7 - 4.6% + 1.2 mm
41
What are some clinical sources of range uncertainty?
CT calibration Beam paths passing through inhomogeneities Patient anatomy changes from planning scan
42
How does CT calibration impact range uncertainty?
Unrelated to patient setup Systematic CT HU to stopping power table has associated uncertainty, gives uncertainty in proton range Could have shift in bragg peak if region has different stopping power
43
How do inhomogeneities impact range uncertainty?
PBS more sensitive than photon RT to inhomogeneities: an offset which means field is delivered in a different position then means the depth the spots are delivered to vary
44
What could be issues wrt inhomogenities?
Beams passing along edges of inhomogeneities Cavities which could fill with gas/liquid Dense target in low density surrounding Moving targets
45
How will anatomy changes impact range uncertainty?
Weight loss eg: less tissue so dose will overshoot
46
What is a robust plan?
One which remains under tolerance under possible error scenarios Large degree of degeneracy in proton planning, but each plan will react differently under error conditions
47
How can we cope with uncertainties?
Consider beam directions Consider target definitions Robust optimisation
48
How could we change beam directions to reduce uncertainties?
Avoid beam directions with OAR behind target Using lateral edge avoids range uncertainty problems Additional fields/patched fields may help
49
How could we consider target definition to reduce uncertainties?
Use beam specific PTVs where expansion from CTV to PTV depends on direction (along or lateral to beam) and heterogeneities in beam path Bc due to energy dependent range of protons, DD is not well approximated by static dose cloud approximation, uniform expansion does not necessarily mean good CTV coverage
50
How does robust optimisation minimise uncertainties?
Use CTV for optimisation, information describing uncertainties is supplied to optimiser, which looks for plan which meets objectives in nominal case and error scenarios Robustness is not a global property: robustness is a property under specified conditions: maximum dose to spinal cord under 3mm shift for example
51
What is the cost of robustness?
Plan quality in the absence of error (this is the same for photon RT, where we irradiate a larger volume of healthy tissue for target coverage and compromise target coverage for OARs at times
52
How can treatment plan verification be done?
Physical measurment (like photon IMRT. must verify fluence and range, can take 1-3 hours) Independent software calculation (ability to assess plan in CT not just water, potential to decrease workload)
53
How is physical plan verification typically done?
Deliver each field at gantry 0 to 2D array in water Measure proton fluence to compare to TPS calc. Do for 2-3 depths to validate range Compare to TPS dose Some centres adjust MU to scale dose as required due to difficult modelling halo in TPS or modelling range shifter in TPS This validates range and dose in water but not patient
54
How is software plan verification usually done?
Use a different calculation method for independence from planning algorithm MC has better accuracy in some scenarios (dose deposition in heterogeneous materials, full modelling of interactions, better handling of implants) MC also allows calculation of some metrics not in TPS: neutron dose, out of field dose, LET distribution..
55
When could MC be done and what does that verify?
After planning checks dose calculation After transfer to delivery equipment checks transfer After delivery checks delivered dose
56
How are shape of dose distribution and absolute dose checked?
Shape: gamma analysis of normalised dose Absolute dose: evaluation of normalisation factor
57
What is RBE?
Relative biological effect The biological effect due to 1Gy of charged particles is not the same as that from photons RBE = D_photon/D_ion to get the same isoeffect
58
What does RBE depend on?
Tissue type Tissue oxygenation Endpoint (toxicity etc) Proton energy/LET
59
What RBE is used clinically?
1.1 This is intentionally conservative
60
What is LET?
Linear energy transfer Measure of energy deposited over a distance LET is highest at distal end of Bragg peak where protons deposit most of their energy All protons contribute to LET
61
How can LET be calculated?
Track averaged LET Dose averaged LET Direct measurement is difficult/impossible, calculated by MC or analytical methods
62
What current research is going on wrt LET?
Optimisation of LET weighted dose Aim to avoid LET hot spots in OARs

Radiotherapy Year 3 (12 decks)

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