Proton Therapy Flashcards
What is the rationale for proton therapy?
No exit dose past the target volume (most important).
Reduce morbidity - including integral dose and second malignancy (important for paediatric).
Dose escalation - can increase curative treatment options (motivation for adult treatments).
What does the proton depth dose curve look like?
Surface dose to just before the peak, the dose is approximately constant, the dose curves up to a point at the peak, after the peak there is a sharp fall off to zero dose.
Photon depth dose has additional dose outside of the target at depth and superficially.
What is the peak of the proton depth dose called?
Bragg peak
What is a major technique called that delivers protons?
Pencil beam scanning
What are the 3 proton interaction types?
Coulomb interactions with (orbital) electrons - (stopping)
Coulomb interactions with nuclei - (scattering)
In-/Non-elastic collisions with nuclei - (halo)
Describe the process of coulomb interactions with electrons.
Secondary electron(s) released. Proton loses energy and slows down. As the proton slows down, the rate of energy deposition increases. Probability of further Coulomb interactions increased which produces the Bragg peak. The range is defined by the initial energy of the beam.
What is the stopping power for a proton beam?
S = - dE/dx
Stopping power, S, is the rate of change of energy over distance
What is the mass stopping power for protons?
S/ρ = -(1/ρ)(dE/dx)
where ρ is mass density
and S/ρ is mass stopping power
What is the Bethe-Bloch equation (protons)?
S/ρ ∝ 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 = atomic number of target nucleus
What does CSDA stand for, and what does it mean? (protons)
Continusous Slowing Down Approximation.
The approximation states that the rate of energy loss at each point along the track is assumed to be equal to the total stopping power. Energy loss fluctuations are neglected. This assumes energy deposition is a smooth process instead of a discrete process when the proton interacts with particles in the medium. This makes energy loss a statistical process; not each proton stops at the same range. This results in a finite slope of the distal edge of the Bragg peak.
The CSDA range is a close approximation to the average path length as it slows to rest calcuated by the CSDA.
The range is the integral of the reciprocal of the total stopping power wrt energy.
What determines the range of a proton beam?
The initial energy.
What is the total energy of a proton?
E = rest mass energy + kinetic energy
E = mc^2
E = γm(0)c^2
Where γ is the Lorentz factor
What is the equation for the Lorentz factor? (proton)
γ = 1 / √(1 – v^2/c^2)
What is the proton rest mass?
m(0) = 1.67 E-27 kg
= 9.38.28 MeV/c^2
(1 MeV = 1.602 E-13 J)
What is the equation for the proton velocity?
v = = √[ c^2 – (m(0)^2 c^6) / E^2 ]
What does the proton velocity vs. Energy curve look like?
As energy increases, velocity increases.
The graph is a curve (almost logarithmic increase).
Describe proton coulomb interactions with nuclei.
Proton direction is changed.
This produces a lateral spread of the beam with a Gaussian profile.
Describe proton inelastic/non-elastic interactions with nuclei.
Nuclear fragments may be released.
The original proton cannot generally be identified.
This produces the halo.
What are the provisions from the DoH regarding proton therapy?
They have facilitated the setting up of a clinical reference panel to approve referrals of appropriate NHS patients to proton therapy centres outside of the UK in a fair and equitable manner.
Furthermore, they have developed a business case for at least one modern proton treatment facility in England.
What is the National Proton Therapy Service?
Two NHS proton developments: The Christie (due to open Aug 2018), UCLH (due to open 2020).
Each centre aims to treat approx. 750 patients per year.
This will stop the overseas referring for proton therapy eventually as the 2 year ramp up for each centre occurs.
Name 5 indications for paediatric referral for proton therapy. (as per Final Business Case 2015)
Any from: •Very Young Age •Rhabdomysarcoma Orbit •Rhabdomysarcoma Parameningeal/Paraspinal •Rhabdomyosarcoma Pelvis •Ewings •PPNET (extra osseus) •NGGCTs (Germinoma) focal RT •Nasopharyngeal (H&N) •Chordoma/Chondrosarcoma •Osteosarcoma •Adult Type Sarcoma (Bone/ST) •Ependymoma •LGG •Optic Pathway Glioma •Craniopharyngioma •Meningioma (excluding G3) •Esthesioneuroblastoma •Pituitary Gland Tumours •Juvenile Angiofibroma •*Retinoblastoma •*Medullo (PNET) •*Hodgkins •*Selected Neuroblastoma •*Selected Wilms Tumour
Where * are UK service expansion criteria
Name 3 TYA indications for referall for proton therapy.
TYA = Teenage; Young Adult
Any from: •TYA satisfies OP paediatric criteria •TYA satisfies OP adult criteria •*TYA satisfies UK paediatric criteria •*TYA satisfies UK adult criteria •*Lymphoma (selected) •*Breast Cancer (selected) •*Ano-Rectal Cancer •*Seminoma •*Gynae Cancers (selected)
Where * are UK service expansion criteria
Name 3 Adult indications for referral for proton therapy.
Any from: •Chordoma BoS •Chondrosarcoma BoS •Para Spinal/Spinal Sarcoma •*Meningioma •*Orbital/Skull Base NOS •*CSI - Curative •*Skull base H&N e.g. Paranasal
Where * are UK service expansion criteria
Name 3 trials for referral for proton therapy.
Any from: •*Lung Ca St3 •*Recurrent Ano-Rectal •*Oesophageal Ca/Nasopharynx •*Nasopharynx Ca •*Mediastinal rare - Thymoma •*Gynae - Ca Cx nodal, Adv Vaginal •*Selected Hodgkins/Non-Hodgkins
Where * are UK service expansion criteria
What are the drawbacks of having multiple rooms in a proton centre?
If there are multiple rooms, the accelerator can only provide a beam to a single room at a time, thus limiting the number of rooms that can be served by an accelerator.
If the cyclotron breaks, or anything connecting it to the rooms, it is likely that more patients will be affected due to the likely higher throughput.
Due to the timing of beam provision to rooms, careful planning of patient timing is required.
How does a cyclotron work?
A uniform magnetic field is applied perpendicular to an electric field. The electric field is applied across two dees (semi circles). Electrons are injected in the centre of the dees. The electric field is reversed just as the electron finishes the half circle, causing an acceleration across the gap. With the increased speed, the electrons move in a larger circle, so the electric field switches with constant frequency. This process is repeated many times, resulting in an electron exiting the port at high speed.
Name the 4 key points of a cyclotron.
Single energy.
Stable beam energy.
Continuous beam.
High intensity.
Name the 2 key points of a synchrotron.
Variable output energy.
Beam has a dead time.
How does a synchrotron work?
The particle beam travel around a fixed closed-loop path. A magnetic field is created through use of electromagnetics. The magnetic field which bends the particle beam increases with time during the accelerating process (synchronized with the kinetic energy of the beam). This increase in magnetic field strength allows a constant trajectory of the particle as its momentum is increased. This accelerates the beam until the particles reach almost the speed of light.
In addition the frequency of the accelerating electric field is increased in synchrony with the orbital frequency of the charged particle.
What are the components of a synchrotron?
Schematic of a weak-focusing electron or proton synchrotron includes:
(1) injector, (2) injection system, (3) vacuum chamber, (4)
electromagnet sector, (5) straight section, (6) accelerating device.
The magnetic field is perpendicular to the plane of the figure of an aerial view.
Name the 2 types of proton delivery technologies.
Scattering System
Pencil Beam Scanning System
Name the components in a scattering system.
Range Modulator Wheel - this does the energy spreading.
Typically 2 scattering foils (double scattering) - this provides lateral scattering.
Perspex Compensator.
Brass Collimator.
Name the components in a pencil beam scanning system.
Energy selection (the location of this differs for each system). Steering magnets.
List the advantages/disadvantages of using a scattering system over a pencil beam scanning system.
Adv:
•Dose delivered to entire target simultaneously (good for moving targets)
Disadv:
•Longitudinal length of SOBP (spread out Bragg peak) is fixed (extra dose is required proximal to the target)
•Field specific hardware is needed (collimators, compensators.)
- This is an extra source of neutron dose
- Has implications for manufacture, storage, handling
- Harder to adapt treatments.
List the advantages/disadvantages of using a pencil beam scanning system over a scattering system.
Adv:
• Improved ability to conform to target
• No field specific hardware
Disadv:
•Potential for interplay effects causing difficulty in treating moving targets
•Lateral edge of field is less sharp for shallower targets
What is the typical energy range of a PBS system? What depth does this correspond to?
70-100 MeV.
Depth of: 4.0-7.5 cm
What options are available to enable treatments at shallower depths?
Range shifter (block of material attached to nozzle which broadens the spot as it introduces extra scatter) Bolus (block of material on patient)
What are the features of a range shifter?
Acts as a scatterer; spot size at shallow depths is increased and has greater divergence
Air gap between patient and range-shifter should be minimised to minimise the spot divergence
The lateral edge is less sharp than for a scattering system (where a collimator defines the egde).
Define VSAD.
Virtual Source Axis Distance:
The distance from isocenter to the apparent source of the proton beam. It may vary in x and y dependent on the design and position of the steering magnets.
