Particle Therapy Flashcards
Describe the process of Neutron Therapy?
As neutron energy increases the possibility of inelastic or particle emission increases.
A neutron is absorbed into a target nucleus to form a COMPOUND nucleus.
The COMPOUND nucleus decays to produce a PRODUCT nucleus and the emission of a particle (proton, neutron, alpha particle etc.).
These particles and compound/ product nuclei are called secondary particles.
Compare secondary particles (neutrons, protons etc.) with secondary electrons produced by photon interactions.
Secondary particles are at least 1835 times heavier than secondary electrons
Have much higher ionisation density along the tracks
Neutrons have high LET radiation
There is a greater incidence of directly damaging ionising events with biological targets (more single strand and double strand breaks in DNA)
Secondary electrons produce more indirect damage (formation of free radicals that damage cells). - not as efficient as secondary particles.
What is RBE?
Relative
Biological Effectiveness.
Definition: The ratio of photon dose to the neutron dose required.
The high LET nature of neutrons results in more efficient cell kill per unit dose than for photons. (So to achieve the same level of cell kill less neutron dose than photon dose is required).
What is OER?
Oxygen Enhancement Ratio.
Definition: The ratio of dose required to kill the hypoxic cells compared to the dose required for aerated cells.
Typically 2.5 - 3.5 for low LET. For neutrons: 1.5 -1.7.
Neutrons have a higher cell kill for hypoxic cells compared to photons.
How are neutrons produced?
A proton or deutron beam (in the energy range 50-70 MeV) interacts with a thick beryllium target.
Beryllium gives a high neutron flux and also has excellent Mechanical and Thermal properties.
Clinical Trials for Neutron Therapy: SCC head and neck
Disappointing results
RCT compared photons with mixed beams of photons and neutrons.
Found no improvement in local control or survival.
Addition of photons and additional treatment time diluted the benefits of neutrons.
Clinical Trials for Neutron Therapy: Salivary gland tumours
Long term loco-regional control was 67% for neutrons versus 17% for photons and electrons.
Clinical Trials for Neutron Therapy: Prostate
Study 1:
10 year local control for mixed beams (40% neutrons and 60% photons) was 70% vs 56% for photons only
Study 2:
No survival benefits
Higher complication rate with neutrons
What is Boron Neutron Capture Therapy?
Was developed to target tumours with high LET radiation.
Boron 10 has a high neutron absorption.
After capturing a thermal neutron, boron-10 briefly becomes boron-11 before disintegrating into an alpha particle and a recoil Li-7 ion.
List various proton interactions
- Characterised by low dose in shallow regions of their path and high dose near the end of their path
- Dose of mono-energetic proton beam diminishes sharply downstream of the Bragg peak (80%-20% within few mm)
- Multiple scattering in the patient especially laterally (large penumbra for high energies)
- Beam penetration can be controlled and adjusted using attenuating material
- Potentially superior dose distributions
- Less integral dose
What are challenges of implementing proton therapy?
• Need to use optimally
• Proton technology very expensive need to reduce cost.
(More patients must be treated to reduce the cost per patient).
• Difficult to quantify proton RBEs for specific tumours and tissues.
(Doing this will allow us to optimise biological dose and improve treatment outcomes)
• Need to conduct investigation on new treatment sites
• Training
(physicists, doctors and therapists need to be trained in photon therapy. Also different QA process).
• Need to build more proton facilities. (At the moment proton therapy is not available to patients who could benefit).
What is Heavy Ion Therapy?
The use of Carbon, Nitrogen, Argon, Neon and Silicon in radiation therapy.
Rate of energy loss and stopping power similar to protons.
Coloumb force interactions with nuclei and electrons created nuclear reactions that give rise to radioactive nuclides.
Heavy ion therapy combines the dose localisation of the Bragg Peak and a high relative biological effectiveness.
Beams have been produced in research facilities - not much clinical evidence. Thus clinical value is difficult to assess.
What are the indications and contra-indications of particle therapy
Particle therapy has a real potential for improving the efficacy of RT.
The negative is the high cost for generally unproven techniques (not much extensive literature).
Need to provide clinical evidence that proton therapy, neutron therapy can improve the clinical outcome of treatment.
