Why use protons?
- more ionising = more biological damage per unit dose
- more conformal dose distribution
What increases as neutron energy increases?
- the possibility of particle emission
What is the interactions to a secondary chared particle?
- a neutron is absorbed into the target nucleus to form a compund nucleus
- the compound nucleus decays to produce a product nucleus (Y) and emission of an energetic particle b
What are B, X and Y?
- the secondary particles set in motion by neutron interactions
What is the weight comparison of secondary charged particles and secondary electrons?
- secondary charged particles are at least 1835 times heavier than the secondary electrons produced by photon interactions
What are some general facts about neutrons?
- much higher ionisation density along the tracks
- high LET radiation
- greater incidence of directly damaging ionising events with biological targets
- compare to indirectly ionising events (free radicals) causing chemical damage
What is relative biological effectiveness?
- ratio of photon dose to the neutron dose required
- to achieve a given biological effect (same level of cell kill less neutron dose than a photon dose is required)
What is the RBE of neutrons compared to photons?
- high LET nature of neutrons results in more efficient cell kill per unit dose than for photons
What is oxygen enhancement ratio?
- the ratio of dose required to kill the hypoxic cells compared to the dose required for aerated cells
What is the benefit of neutron for OER?
- neutrons have higher cell kill for hypoxic cells compared to photons
- OER reduced to 1.5 from 2.5
What are the neutron producing interactions?
- proton or duetron beam in the energy range 50-70 MeV with a thick beryllium target
- beryllium gives a high neutron flux and also has the advantage of being a solid with excellent mechanical and thermal properties
What is boron neutron capture therapy?
- designed to selectively target high LET heavy charged particle radiatino to tumour at the cellular level
- boron-10 has an unusually high nuetron obsorbtion cross-section for thermal or slow neutron energies (<0.01 eV)
- immediatley after capturing a thermal neutron boron-10 briefly becomes boron-11 before disintegrating to an energetic a-particle and a recoil Li-7 ion
What are the three proton interactions that have consequences for proton RT?
- the dose of mono-energetic proton beam diminishes sharply downsteam of the bragg peak (drops from 80-20% of he peak in few mm)
- mutiple scattering in the patient dominates how the dose falls of laterally
- beam penetration within the patient can easily controlled either by adjusting the beam energy or putting attenuating material in the beam upstream
What are the three levels of dose fall off laterally (penumbra) for protons?
- resultant penumbra excellent for low energy <100MeV protons
- very good medium energy (100-150MeV) dose fall off from 80-20% in 6mm
- larger penumbra ideal for protons of higher energy)
What are some design delivery features of a proton machine?
- accelerator in centre of facility
- cyclotrons or synchrotrons can be used
- costly and complex compenets of proton facility is mechanism for rotating beam around patient
- moving system (100 tones) must be controlled with sub-mm precision
- target volumes range from few mm to seveal litres
What is the physical feautre of a proton beam?
- beam produced are narrow and deposit their energy in a bragg peal only 6mm wide
- beam therefore needs to be spread out in width and depth
What is passive scattering?
- protons only produce one energy so the range shifter can degrade the energy
- cannot control the dose distribution on the front edge (compensator)
- get a lot of protons on the edge of the target which only give dose to normal tissue
What is active scanning?
- using varying dynamically sweeping magnets to shape dose to target
What is the PSI spot scanning technique?
- a pencil beam (7mm diameter) of protons is regulated by computer controlled magnets in such a way that the high-dose spot can be positioned very precisley for an exactly specified period of time and at any desired location within a tumour
- by superimposing many individual spots the desired radiation dose can be delivered uniformaly within a tumour, with the dose being individually monitored for each single spot
What does the PSI spot scanning technique enable?
- extrememly precise and homogenous irradiation, ideally adapted to the shape of the tumour, which is in most cases irregular
What tumours is spot scanning best for?
- well-immobilised tumours located in the head and neck and lower pelvis
- technique is sensitive to motion of the target volume
What are the challenged of proton therapy?
- to use protons optimallly
- to reduce the cost of proton therapy
- to quantify proton RBEs for specific tumours and normal tissue
- to conduct clincal investigations of new treatment sites
- train physicians, physicits, therapsits and dosimetrists in proton therapy
- build more proton facilities
What is heavy-ion therapy?
- carbon, nitrogen, argon and silicon
- rate of energy loss is similar to protons
- coulomb force interactions with nuclei and electrons plus nuclear reactions giving rise to radioactive nuclides
- combine the dose localisation properties of the Bragg peak and a high relative biological effectiveness
What is the clinical application for SCC head and neck?
- disappointing results
- comparison of photon to mixed beam neutons and photons for inoperable tumour
- no improvement in local control or survival
- adding photons increasing treatment time and diluted benefits of neutrons
