Protons - 1 Flashcards
What is the rational for proton therapy?
No exit dose part the target
Reduced morbidity - good for paeds
Dose escalation - Can increase curative treatment options - good for adults
What interaction types do protons undergo?
Coulomb interactions with electrons
Coulomb interactions with nuclei
Inelastic collisions with nuclei
What is the result of a coulomb interaction with electrons for protons?
Secondary electron produced
Proton loses energy - slows down
Probability of further coulomb interactions increased
Produces the Bragg peak
What is the equation for the stopping power, mass stopping power and Bethe-Bloch equation for coulomb interactions with electrons?
S = -dE/dx
S/p = -1/p dE/dx
S/p prop 1/v^2 . Z/A . z^2 where v = velocity of proton, Z = atomic number of nucleus, A = relative atomic mass of nucleus, z = atomic number of proton
What is the source of the Bragg peak?
As protons slow, the rate of energy deposition increases - the range is therefore dependent upon the initial energy - range prop E^2
What is the equation for the velocity of a proton?
v = (c^2 - ((m0^2.c^6)/E^2))^1/2
What is the equation for the Lorentz factor?
gamma = 1/(1 - v^2/c^2)^1/2
What are the deliverable energies for pencil beam scanning systems?
70-230MeV
What is the source of range straggling?
The continuous slowing down approximation is only an approximation
Energy loss occurs at a very large number of discrete interactions with particles in the medium
Energy loss is a statistical process
Not every photon stops at the same point
The distal edge of the Bragg peak therefore has a distal edge
What is the result of coulomb interactions with the nuclei?
Proton direction is changed
Produces lateral scatter in beam
The spreading has a gaussian distribution
What is the result of inelastic collisions with the nuclei?
Nuclear fragments may be produced
Original proton can’t generally be identified
Produces the halo
What is the standard layout of a proton centre?
Accelerator
Proton beam line into multiple rooms - beam can only go to one room at a time
3 rooms if doing complex treatments, up to 5 if only doing simple treatments
What are the important factors for cyclotrons for proton production?
Single energy
Stable beam energy
Continuous beam
High intensity
What are the important factors for synchrotrons for proton production?
Variable output energy
Beam has dead time
For a varian probeam PTS what are the maximum field sizes and maximum depth tht can be treated at?
Max field size = 30x40cm
Treats up to 35cm depth
What are the two types of delivery systems used in proton centres?
Scattering systems
Pencil beam scanning systems
What is the equipment for a scattering system?
Range modulator wheel - energy spreading
Two scattering foils - lateral spreading
Perspex compensator
Brass collimator
What is the equipment for pencil beam scanning system?
Energy selection
Steering magnets
What factors affect choosing a scattering system?
Dose delivered to entire target simultaneously - good for moving targets
Longitudinal length of spread-out-Bragg-peak is fixed - extra dose must be delivered proximal to the target
Field-specific hardware is needed - additional source of neutrons, manufacturing, manual handling, storage, recycling, rad protection issues, more difficult to adapt treatments
What factors affect choosing a pencil beam scanning system?
Improved ability to conform to target
No field-specific hardware
Get interplay effects for moving targets
Lateral edge of field is less sharp for shallow targets
What is the purpose of a range shifter?
To treat targets at depths of <4cm which the deliverable energies wouldn’t allow
What are the characteristics of a range shifter?
Acts as a scatterer - spot size at shallow depths increases and spot has greater divergence
Air-gap should be minimised to reduce effect of increased spot divergence
Lateral edge is less sharp than for a scattering system with a collimator defining the field edge
What is the virtual source axis distance?
The distance from the isocentre to the apparent source of the proton beam - may differ in x and y depending on the design and position of the steering magnets
Divergent steering gives finite VSAD, Parallel steering gives infinite VSAD
What are the HU of a CT converted to for proton planning?
Relative proton stopping power - uncertainty due to imaging and treatment using different particles
What method is used to convert HU to relative proton stopping power?
Stoichiometric method
What measurements are needed to commission a beam model?
Bragg peaks in water - characterises range as a function of energy and dose delivered/MU to be defined
Lateral profiles in air - allows beam optics to be characterised
What chambers should be used for measuring Bragg peaks in water?
Large chamber with a small field
Small chamber with a large field
Chamber needs to be large enough to capture entire lateral profile of a single Bragg peak including the halo
Why is a 1D Bragg peak curve called an integrated depth dose curve?
Integrates the charge collected over the 2D plane of the detector
How can the halo be modelled?
MC dose calcs
Analysis of difference between large and small detector methods
What is the current code of practice for protons?
IAEA TRS-398 CoP - designed for scattering systems - primary standard: graphite callorimeter, reference beam: Co-60
How are the MUs defined for protons?
MU chambers located in nozzle, up-stream of range shifters
IDDs measured using traceable parallel plate chamber
Reference point is taken at a point in the entrance region of Bragg peak - 2cm
Charge collected by MU chamber related to dose at this point
Match gantries by adjusting gain on MU chambers - best fit for all energies
How are the beam optics commissioned?
Use film or scintillation screen for high spatial resolution
Measurement of lateral beam done in air at a range of distances from nozzle - at least 3 to fit hyperbola on
Dose outside chamber estimated by using MC dose calcs
Laterally beam has gaussian profile, longitudinally beam has hyperbola
Beam waist is at isocentre then focussing is weak
