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.