Current RT Developments - Proton Therapy Flashcards
What are the advantages of using Proton therapy over conventional photons?
1) No exit dose past target - spares healthy tissue beyond target
2) Reduced morbidity - reduced integral dose and second malignancies, especially for pediatric patients where longer timescales (longer life left) given rise to a greater risk of induced cancer; also less risk of treatment effecting natural development
3) dose escalation - can increase curative treatment options - “major motivation in adult indications”
Describe the bragg peak and the Spread Out Bragg Peak (SOBP).
When high energy/velocity charged particles enter the patient, their high velocity means there is a lower probability of interactions with the patient mass. As they penetrate deeper into the patient, interactions inevitably occur reducing the energy of the particles which in turn increases the probability of interaction and further energy loss.
Once the particles have slowed significantly they are highly likely to interact and lose their energy to the surrounding mass; therefore, most of the energy from the particles is deposited at the end of their path through the patient. This results in a peak of energy deposited with no further energy deposited in the patient beyond this point.
The SOBP occurs when particles of different initial energies are used/are incident on the patient. Each individual particle thus has a different maximum depth it will penetrate and therefore the bragg peak will occur at varying depths. if these energies are chosen so that the bragg peaks are adjacent to each other, this results in a range/spread of peaks.
What are the interaction types for a proton?
1) Coulomb interactions with electrons
2) Coulomb interactions with nuclei
3) in/Non elastic collisions with nuclei
For Pencil beam scanning systems, what are the typical deliverable energies?
70-230MeV (4-35cm deep in water).
What is range straggling?
Energy loss is a statistical process with each proton stopping at a slightly different range, hence range is ‘straggled’.
What is LET and what is it used for?
Linear Energy Transfer is the energy deposited per unit distance. It is used as a measure of beam quality. For multiple protons, the average is taken.
When a proton undergoes a Coulomb interaction with a nucleus, what happens to the proton?
It deposits some of its energy to the nuclei and changes direction with reduced velocity. The resultant spread of protons is gaussian in nature.
Describe a proton in-elastic interaction with a nucleus?
1) Nuclear fragments may be released
2) Original proton cannot be generally identified
3) produces a halo
Analytical modeling - lateral spread is a Gaussian function.
What are the characteristics of cyclotrons?
single energy stable beam energy high intensity beam produced not good for very high energies not good for heavier particles
What are the characteristics of synchrotrons?
Variable energy output
Pulsed beam
Can deliver at high energies and with heavier particles compared to cyclotrons.
What are the two types of proton treatment delivery?
Scattering and scanning.
Describe a proton delivery scattering system.
Monoenergetic protons are incident on a range modulator wheel (energy spreading); the output from this enters 2 scattering foils (lateral spreading); the output from this enters a perspex compensator to fine-tune the beam shape before the beam enters a brass collimator.
Note, the collimator and the compensator have to be custom made for each field; therefore time, resources, disposal/recyling needs to be considered.
Describe a proton delivery scanning system.
Monoenergetic protons are incident on an energy selection device (possibly a range modulator wheel); the output then traverses two pairs of orthogonal magnets acting as steering magnets to facilitate scanning of the pencil beam over the target, effectively dose painting the target.
What are the advantages and disadvantages of a scattering proton delivery system?
Advantages are:
dose is delivered to the entire target simultaneously (good for moving targets)
Disadvantages are:
Longitudinal length of SOBP is fixed; extra dose ust be delivered proximal to the target
Field-specific hardware needed:
1) additional source of neutron dose
2) manufacture, manual handling, storage, recycling, radiation protection implications
3) more difficult to adapt treatments
What are the advantages and disadvantages of a scanning proton delivery system?
Advantages are:
Improved ability to conform to target
No field-specific hardware required
Disadvantages:
Treating moving targets may be difficult due to interplay effects
Lateral edge of field is less sharp for shallow targets
How are targets less than 4cm from the patient surface treated with Proton Beam Therapy?
Use of a range shifter which is a perspex block in the beam path to reduce the energy (and therefore depth penetration) of the proton beam.
Note, additional scatter due to this block and more likely to scatter at lower energies; therefore, spot size is increased at shallower depths - therefore distance between range shifter and patient surface needs minimizing to reduce the impact of the increased spot divergence. Could use bolus on patient surface instead of range shifter.
Name a method used to calibrate a map of HU to relative proton stopping power.
Stoichiometric method.
How do you get dosimetry for a patient proton treatment plan?
Imaging with photons, treating with protons.
Phantom with varying known mass densities is CT scanned, from these known materials and densities, proton stopping power is derived. This is then used in TPS.
Note: in theory, stopping power changes with energy but in reality this change is negligable.
What are the main points to consider when commissioning a Proton Beam System (PBS)?
1) Depth dose profiles
2) Quantifying dose
3) Lateral profiles
4) Divergence of: a) individual spots and b) the steering system
All these are characterized by commissioning measurements.
What type of chamber would you use in PBS commissioning?
A parallel plate such as a PTW bragg peak chamber with a 8cm diameter. Note diameter of chamber must be»_space; than field size
(i.e. spot size).
What is Integrated Depth Dose (IDD)?
The charge in a parallel plate chamber is collected over a large chamber area.
How are IDDs acquired?
A parallel plate chamber is used to acquire the IDD profiles along the axis of the beam over the whole length of the bragg peak for a variety of incident energies that are used clinically. These allow the bragg peak to be characterised for each energy.
Note, due to range straggling, the bragg peak will broaden as depth (i.e. incident energy) increases.
Why are IDDs not able to determine absolute dose?
They miss the ionisation outside of the diameter of the chamber (caused by scatter due to nuclear interactions).
What is an MU?
As photons or charged particles pass through an ionisation chamber, they ionise the gas in the chamber. The chamber collects and counts the deposited charge.
An MU corresponds to a defined amount of charge collected at the chamber. As the energy of the ionising particle is directly related to the charge deposited, therefore the number of particles per MU depends on the energy of the ionising particle.
What is a typical reference depth for PBS commissioning?
2cm
What is the process for quantifying dose in PBS commissioning?
Measuring output
1) Use a small, e.g. roos 1.56 diameter, chamber
2) use a larger reference field (e.g. 10x10cm)
3) acquire data in a water tank at a reference depth of 2cm; a ‘grid’ of spots must be delivered at this depth across the whole 10x10 field; this will give a value for Gy mm2/MU. Note a grid of spots must be delivered to ensure charged particle equilibrium (for each spot, adjacent scatter from nuclear interactions will balance that lost).
This will give a dose area per MU at the specific incident energy of the proton, as the absolute dose will be a function of this energy. The process must be carried out for all energies to be used clinically and the TPS should be updated accordingly.
Outline the parameters that are configured during PB therapy planning.
Those chosen by the operator/planner:
1) Technique
2) Objectives
3) Beam angles
4) Beam modifiers (e.g. range shifters)
Those chosen by the optimiser:
5) spot positions
6) spot weights - to build up the SOBP
Define SFO and MFO.
SFO (Single Field Optimisation) and MFO (Multi Field Optimisation). These are how spots are put together to get dose spread.
What are the advantages and issues regarding MFO for PB therapy planning?
1) MFO allows greater control over dose distribution (only the combined dose is required to be uniform, not each field as in SFO)
2) This gives the optimiser greater flexibility when optimising the plan
Issues are:
1) assessing if the optimiser has arrived at a safe solution
2) assessing how sensitive the plan is to uncertainty
e.g. in H&N. if the CT to HU conversion is in error, this translates to a range error, therefore a greater dose to the spinal cord may be delivered; therefore even though the quality of the distribution is good, there may be a range error.
What is an established PB range uncertainty from literature?
2.7% - 4.6% +1.2mm
What are the clinical sources of range uncertainty?
1) CT calibration and artifacts (unrelated to patient setup, systematic)
2) Beam paths traverse inhomogeneous regions (patient setup, patient motion, gas/liquid filled cavities, etc)
3) Patient anatomy changes (weight gain/loss, tumour regression, etc)
What are the potential consequences of range uncertainty in PB therapy for brain tumours?
1) dose to cord may increase and may exceed tolerance, leading to patient paralysis below the affected part of the cord
2) underdosing of the PTV and increase in radiation to the surrounding, healthy, tissue. This could lead to the cancer not being cured as well as cognitive impairment for the patient ( i.e. a sub-optimal outcome).
What is more sensitive to inhomogeneties, photon or proton therapy? What are the potential issues faced?
Proton Therapy!!!
1) Moving targets
2) beams passing/grazing inhomogeneous regions
3) cavities (air or liquid filling)
4) Dense targets in low density surroundings (e.g. Lung)
How many fields does a typical PB therapy plan have? Roughly how many spots per field?
1-5 fields, each field has several 1000 spots.
What makes a plan robust?
The ability to remain within tolerance whatever the possible error scenarios.
What could the planner do to avoid/minimise issues with uncertainties?
1) avoid having beam directions where an OAR is directly behind the target
2) use lateral beams (avoids range uncertainty issues) - avoids brain stem, eyes, optic chiasm, etc
3) use additional/patched fields (e.g. 2 lat and one sup)
4) use beam specific PTVs
5) robust optimisation by disregarding the PTV and using the CTV instead with uncertainties being given/entered into the TPS so the optimiser can take them into account. The optimiser thus finds plans satisfying the objectives for a nominal plan + plans with a number of error scenarios
Note: robustness is a tradeoff with plan quality!
Define robustness (in terms of planning).
A plan is robust to specified objectives under specified conditions, i.e. a plan cannot be ‘globally’ robust!
Is an independent check of treatment plans mandatory in the UK?
Yes, specified in Towards Safer Radiotherapy (2008), i.e. guidance.
Outline the typical procedure for PB physical plan verification.
1) deliver each field at gantry angle 0 to a 2D array in water or a water equivalent material
2) measure photon fluence for comparison to TPS calculation
3) measure @ 2-3 depths covering the range required clinically
4) compare absolute dose vs TPS
List some advantages of using Monte Carlo for software plan verification.
1) independent validation of spot weights
2) independent validation of Bragg Peak range
3) improved dose calculation w.r.t. inhomogeneities
4) full modeling of nuclear interactions
What is the Relative Biological Effect (RBE)?
RBE = Dphoton/Dion
With Isoeffect being defined as achieving the same biological effect (i.e. RBE * Dion gives the same biological effect as Dphoton).
What does Relative Biological Effect (RBE) depend on?
Some factors are:
1) Photon energy/LET
2) Tissue type
3) Endpoint (cell survival, toxicity, etc)
4) Tissue oxygenation (hypoxic cells more radio-sensitive)
What general Relative Biological Effect (RBE) value is used clinically?
RBE = 1.1 (conservative value).
What are the two methods of calculating LET?
1) Track average LET
2) Dose average LET
Measuring LET directly is impossible/difficult. Therefore, Monte Carlo or analytical methods are used.
What is the summary of experiences of planning and delivering PB therapy at the Christie?
1) wide range of cases have been treated, each involved adapting the planning approach to suit each individual case but following common principles
2) becoming clearer on what is and is not possible with PBT in terms of both dose escalation and OAR sparing
3) multi-modality imaging is essential
4) close management with consultants has been extremely beneficial in terms of workflow and mutual learning.
5) No in-vitro dosimetry performed as no exit dose
6) Verification takes longer than with photon treatments
7) proton therapy more sensitive to changes in anatomy and setup errors than photon treatments, therefore tend to do more imaging than with photon treatments.
