Week 5 - Particle Therapy Flashcards
boron neutron capture therapy
The more ionising a particle is, the more biological damage per unit dose it can deliver.
- a technique designed to selectively target high LET heavy charged particle radiation to tumours at the cellular level
- boron-10 has an unusually high neutron absorption cross-section for thermal neutron energies
- immediately after capturing a thermal neutron, boron-10 becomes boron-11, before disintegrating to an alpha particle and Li-7 ion with gamma rays
Explain the Bragg peak
Amount of radiation delivered increases to a certain depth, at which after the Bragg peak dose dramatically declines (only 6mm wide)
Describe the concept of range straggling in protons
This suggests protons with the same initial energy in the same material may have a slightly different range due to the fluctuation in the number of collision interactions and the energy loss per interaction.
Discuss the disadvantages/challenges of proton beam delivery
- Space needed for the installation of the proton accelerator
- Expensive technique (using protons optimally)
- Not widely available in Australia
- Need more evidence on the RBE for specific tumours (clinical investigations)
- Training is needed + extra facilities
- highly sensitive to uncertainties (i.e., set up error or motion, or CT hounsfield units)
- more complex treatment delivery and planning
- cost
- lack of randomised data
Explain the PSI spot scanning technique
Pencil beam is planned to a position for an exact specified period of time and at a desired location within a tumour. Many individual spots are chosen within the tumour to treat, dose is monitored for each spot and delivered uniformly.
Explain 4 differences between heavy ions and protons
- Heavy ions consist of carbon, neon, argon, and helium ions which have higher LET.
- Bragg peak entrance dose is higher in protons
- Improved dose distribution and sparing
- As these heavy ions have a higher mass, they require stronger magnets in the gantry so the cost of equipment is more and challenge of engineering is higher
Define boron neutron capture therapy
Targets high LET particles to tumours at cellular level by giving patient boron-10 drug which will accumulate in tumour cells and when irradiated with neutrons a reaction occurs resulting in lithium ions and alpha particles which destroy the cell
What cases broadening of a proton beam?
- Multiple colomb scattering
- Energy straggling
- Higher energy beams
passive scattering
Passive scattering uses a scattering foil to spread out the protons combined with a compensator to adjust the beam shape to treat the tumour. This causes increased dose on the anterior edge of the tumour.
active scanning
Active scanning bends protons to deposit dose to a defined location within the volume. Meaning no compensator needed, no wasted protons.
What causes higher exit dose in carbon ions?
Nuclear fragmentation from nuclear interactions
List 3 sources of range uncertainty
- CT calibration
- Heterogeneities
- Anatomy changes
List 5 benefits of the Bragg peak
- Little to no radiation behind the tumour
- Lower integral dose per treatment
- Potential to lower the risk of side effects
- May improve QoL
- Reduces risks of secondary cancers
discuss the advantages of protons
- low entrance dose
- no exit dose
- highly conformal dose distributions superior to photons
- improved QOL
- improved tumour control
- due to less dose to OARs, side effects are minimised
- Homogenous dose to a target volume while minimising dose to normal tissue
- Lower integral dose for protons - good for paeds
What is the rationale for protons opposed to x-rays?
- to spare side effects caused by x-ray properties (normal tissue is spared beyond the target)
- to minimise “wasted dose” to normal tissue
- complex (i.e., large or irregular shaped) tumours
What is the rationale for protons opposed to electrons?
- heavier mass
- travel in straight line
- continual energy loss along their path (coulomb interactions with orbital electrons)
- some scattering from the nucleus (repulsive coulomb force due to positively charged protons and positively charged nucleus)
What is the effect of increasing proton energy?
increasing proton energy, increases the depth of the Bragg peak
What is the rationale for active or pencil beam scanning (opposed to passive scattering)?
- unable to conform proximal edge of proton beam to shape of target volume which increases dose to normal tissue superior to target volume (i.e., normal tissue proximal to the proton beam and in “spread out Bragg peak”)
- wasted protons where higher energy produces protons outside of target volume which need to be collimated
What are the clinical applications of proton therapy?
(1) paediatric
(2) head and neck - to spare brain stem or spinal cord, parotid glands or optic structures
(3) breast
(4) prostate
(5) liver (hypo-fractionation)
(6) re-irradiation
What is the rationale for proton therapy in paediatrics?
to reduce dose normal tissue to reduce risk of secondary malignancy
IMPT vs IMRT
- reduction in normal dose
- less beams
the effect of the bragg peak with higher proton energy
- the depth of the bragg peak is different
- higher beams are more penetrating
- bragg peak is broader
advantage of higher LET particles
The more ionising a particle is, the more biological damage per unit dose it can deliver.
Explain the Bragg peak
Amount of radiation delivered increases to a certain depth, at which after the Bragg peak dose dramatically declines (only 6mm wide)
Describe the concept of range straggling in protons
This suggests protons with the same initial energy in the same material may have a slightly different range due to the fluctuation in the number of collision interactions and the energy loss per interaction.
differences between heavy ions and protons
- Heavy ions consist of carbon, neon, argon, and helium ions which have higher LET.
- Bragg peak entrance dose is higher in protons
- Improved dose distribution and sparing
- As these heavy ions have a higher mass, they require stronger magnets in the gantry so the cost of equipment is more and challenge of engineering is higher
high LET particles
- heavy, charged particles such as protons, heavy ions
- transfers most of their energy within the tissue over a short distance
- less penetrating
low LET particles
- x-rays or gamma rays
- less energy to the tissue per unit length
- more penetrating
RBE
relative biological effectiveness
OER
- oxygen enhancement ratio
- the presence or absence of molecular oxygen within a cell influences the biological effect of ionising radiation
- more pronounced for low LET radiations
OER equation
OER = Dose required under hypoxic conditions / Dose required under aerobic (well-oxygenated) conditions
high and low OER values
high = indicates that more radiation is needed to achieve the same biological effect in hypoxic tissue, making it more radioresistant
low = indicates that the tissue is more radiosensitive in well-oxygenated conditions.
range shifter
a device used in proton beam to reduce the beam energy when treating superficially. Acts as a source of scattering and widens the beam edge further
SFO
single field optimisation
- where the spot position and weights of each proton field are optimised individually
MFO
multi-field optimisation
- where the spots from all fields are optmised together generating a highly conformal dose distribution
