Protons Flashcards
What is the rationale for proton therapy?
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
Why are protons especially beneficial for paediatric patients?
Reduced risk of growth/development problems
Reduced risk of induced cancers
What are the three types of proton interactions?
Coulomb interactions with electrons
Coulomb interactions with nuclei
Non elastic collisions with nuclei
How do protons interact with electrons
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
What is Bethe-Bloch equation?
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
What is range of proton defined by?
Initial energy of beam
Range ~ E^2 approx
Where does range straggling come from
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
What is LET
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
How do protons interact with nuclei?
Proton direction is changed
Produces lateral spreading
Has gaussian profile
How do protons interact with nuclei in non elastic way?
May release nuclear fragments
Can no longer identify original proton
Produces halo
What needs to be considered about plotting lateral spread?
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
What are logistical considerations of having multiple proton accelerators?
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
How do cyclotrons work
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.
What are the key characteristics of cyclotrons?
Single energy
Stable beam energy
High intensity
Not good for high energies or heavier particles
used in both UK centres
How do synchotrons work?
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.
What are the key characteristics of synchrotrons?
Variable output energy
Pulsed beam
Can do higher energies and heavier particles
What are the energies and depths delivered by the ProBeam
70-240MeV, treating up to 35cm, max field size 30x40
What beam is produced and how is the beam shaped?
A mono energetic pencil beam is produced
The beam must be spread and shaped in longitudinal and lateral directions
Scattering or scanning
What makes up a scattering system?
Range modulator for energy spreading
Typically two scattering foils for lateral spreading
Compensator and collimator
Compensator adjusts beam edge, collimator shapes beam laterally
What makes up a pencil beam scanning system
Energy selector (could be done range modulator wheel or shark tooth method)
Steering magnets for steering
What are the advantages and disadvantages of scattering systems
+ 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)
What are the advantages and disadvantages of scanning systems
+ 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
What is the minimum deliverable proton energy and depth and how is this adjusted
70MeV, 4cm
Use range shifter to treat shallower depth (block of material inserted into beam path and attached to end of nozzle)
How does range shifter impact spot size and how can it be altered
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
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
What measurements are used to commission a PBS system?
Depth dose profiles
Quantifying dose
Lateral profiles
Divergence (of individual spots and steering system)
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
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
Why are IDDs not suitable for absolute dose measurements
Miss dose scattered laterally due to nuclear interactions
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
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
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
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
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
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
What is the planning technique dependent on?
Anatomy of targets/OARs
Type of cancer
Delivery technology
On treatment imaging technology
TPS features
Department protocols
What are the two distinct types of planning?
Single field optimisation
Multi field optimisation
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)
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
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
What is range uncertainty?
The uncertainty in where the protons actually stop in the patient
2.7 - 4.6% + 1.2 mm
What are some clinical sources of range uncertainty?
CT calibration
Beam paths passing through inhomogeneities
Patient anatomy changes from planning scan
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
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
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
How will anatomy changes impact range uncertainty?
Weight loss eg: less tissue so dose will overshoot
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
How can we cope with uncertainties?
Consider beam directions
Consider target definitions
Robust optimisation
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
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
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
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
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)
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
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..
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
How are shape of dose distribution and absolute dose checked?
Shape: gamma analysis of normalised dose
Absolute dose: evaluation of normalisation factor
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
What does RBE depend on?
Tissue type
Tissue oxygenation
Endpoint (toxicity etc)
Proton energy/LET
What RBE is used clinically?
1.1
This is intentionally conservative
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
How can LET be calculated?
Track averaged LET
Dose averaged LET
Direct measurement is difficult/impossible, calculated by MC or analytical methods
What current research is going on wrt LET?
Optimisation of LET weighted dose
Aim to avoid LET hot spots in OARs
