Protons Flashcards
why something other than x-rays or electrons
more ionising = more biological damage per unit dose (High LET advantage)
dose distribution advantage (more conformal)
boron neutron capture therapy (BNCT)
technique that was designed to selectively target high LET heavy charged particle radiation to tumours at the cellular level
Boron-10 has a high neutron absorption cross-section for thermal or slow neutron energies
immediately after capturing a thermal neutron boron-10 briefly becomes boron-11 before disintegrating to an energetic alpha particle and a recoil Li-7 ion
Protons - physics
positively charged
more conformal
the bragg peak
a pronounced peak on the Bragg curve which plots the energy loss of ionizing radiation during its travel through matter.
few cm wide
larger volumes in the depth direction are treated to a uniform dose through delivery of multiple pencil beams with different energy
energies and weights need to be optimised to create a uniform profile
broadened in depth by range straggling effects
range uncertainty is a problem
clinical benefits
target volumes typically range in size from a few mm to several litres - beams are narrow and deposit energy in a Bragg peak only 6mm wide, therefore, needs to be spread out in width and depth
improved target coverage and reduction of low doses to OARs –> reducing the risk of late toxicity
heavy ions
carbon ion
higher ionisation density
36x energy transfer
12x mass of protons
range is 3x less for the same velocity
smaller lateral deflections from nuclei
sharper lateral penumbra
much higher energy loss than protons
have less lateral spread
proton interactions
- dose of mono-energetic proton beam diminishes sharply downstream of the Bragg Peak (drops from 80-20% of the peak dose within a few mm)
- multiple scattering in the patient dominates how the dose falls off laterally - resultant penumbra is excellentfor low energy, very good for medium energy but less than ideal for high energy
- beam penetration within patient controlled by adjusting beam energy or putting attenuating material in the beam upstream
range
depth at which half of them come to rest
range straggling
individual protons with the same initial energy in the same material will have a slightly different range
fluctuation in the number of collision interactions and the energy loss per interaction
comparison of proton with electron
heavier mass of proton
travels in straight lines
continual energy loss along their path
end of range
proton fluence reduces significantly in a short distance as proton energy becomes low and protons stopping power increases
mean range
depth at which half of the incident protons have come to rest
proton beam energy and depth
reduction in mean energy - broadening of energy distribution
energy straggling
multiple coulomb scattering
causes a lateral broadening of the proton beam dose deposition
what does depth of bragg peak depend on
proton incident energy
how does protons deposit energy
through collision interactions with orbital electrons
active scanning
Different way of treating
- positively charged proton beam (can move it through magnetic field)
- select proton energy (position it in the X-Y)
- changes position using magnetic field
- mono-energetic pencil beam gets moved around to treat spots of the target
- start with distal edge and reduce energy as it goes
Most commonly used now
No wasted protons
passive scattering
Energy of the proton beam changed with range shifter wheel
2 scatterers to broaden beam
Narrow beam gets scattered and then gets collimated to treat only the target volume
Compensator used to treat distal edge of tumour
This can lead to wasted protons
is integral dose important
overall dose to NTT/RVR
challenges for proton therapy
- to use protons optimally
- to reduce costs
- to quantify proton RBEs for specific tumours and normal tissue
- to conduct clinical investigations of new treatment sites
- to build more proton facilities and train staff
spot scanning and organ motion
Breathing or other significant movement can cause challenge for spot scanning techniques
Gating can be used
stopping power of carbon ions
greater than for protons due to higher charge and mass
carbon ion range and Bragg peal
carbon ion rnage and bragg peak depth will be less than for protons of the same initial energy/nucleon
range uncertainty effect of carbon ion beams less than for protons
products of nuclear fragmentation interactions cause a low dose tail beyond the Bragg peak
What is a range shifter
a device used in the proton beam to reduce the beam energy when
treating superficially. The range shifter acts as a source of scattering and widens the
beam edge further.
what tasks are performed by planners
- choice of technique (SFO or MFO)
- Selectin of beam angles
- definition of plan objectives
- selection of beam modifers including pre-absorber
SFO
this is where the spot position and weights of each proton field
are optimised individually
MFO
where the spots from all fields are optimised together
generating a highly conformal dose distribution.
Also known as IMPT
What tasks are performed by the TPS
- spot weight
- spot placement
- definition of energy layers
what causes range uncertainty
CT calibration
heterogeneities
Anatomy changes
what are the overall benefits of protons
- little to no radiaiton behind the tumour
- lower integral dose per treatment
- potential to lower the risk of side effects
- may improve the quality of life during and after treatment
- reduces risks of secondary cancers
when should we use protons and what for?
To spare side effects
caused by x-ray properties
* Minimize ‘wasted dose’ to
healthy tissue
* ‘Complex’ tumors
Clinical Applications
- paediatric cases
- cranial/head and neck
- reirradiation
- hypo-fraction
what are some important considerations for beam positioning
exit target- if there is an OAR on the exit path to beam
beam must be positoned through well-immobilised anatomy
beam path - should be homogenous
what are the two types of uncertainties
- setup uncertainties: patient motion/position
- range uncertainties
SOBP
Spread out brag peak
Where the radiation is distributed to the target
Parts of the proton treatment unit
gantry - 220 degree range
snout
range shifter/preabsorber
imaging - at scanning nozzle
Defintion of energy layers
The TPS identifies the energy layers required to allow Bragg peaks to be placed at the range
of depths necessary to cover the target. The deliverable energy typically ranges from
approximately 70-230 MeV, which corresponds to physical depths of 4-25 cm in water.
During delivery, each energy layer is delivered in sequence. Within each layer, the steering
system moves the beam to each x,y spot position (termed spot) and delivers the required
dose (quantified in Monitor Units (MUs) to each spot.
Spot placement
The TPS defines the locations of all spots in each energy layer, distributing the spots so that
they cover the entire volume of the target. Within each layer, spots are arranged in either a
square or hexagonal grid, and the TPS defines the order in which the spots are delivered.
Spot Weight
The TPS defines the dose to be delivered to each spot in terms of the number of MUs. This
relates to the charge measured by the monitor unit ionisation chamber which is part of the
delivery system and is located within the nozzle. The number of MUs can be directly related
to the delivered dose via the calibration of the delivery system
What is the air gap
In proton beam therapy the air gap is defined as the distance between the end of the proton
beam compensator and the body of the patient
what is the snout
The snout is a physical attachment that can be fitted on to the compact nozzle and would
typically be used to place an additional range shifter and aperture closer to complex patient
anatomy.
it dictates the maximum field size
WET
Water equivalent thickness values (for range shifters)
ranges from 2-5cm
Used when energies cannot get low enough to treat
IMPT
IMRT’ equivalent
* Each field delivers a heterogenous
dose to target
* Fields are strongly coupled
* Less robust
* More opportunity for complex
dosimetry
- used for head and neck and multiple dose levels
SFUD
3DCRT equivalent
Each field achieves a uniform dose over target
- fields are decoupled
- more robust plans
- less opportunity for complex dosimetry
- can be used for prostates and lungs and tumours far from OAR
RBE
Relative biological effectiveness
Constant of 1.1 is assumed
Relative measure of the damage done by proton per unit of energy deposited to biological tissues
High LET radiation will have high RBE
OER
Ratio of the radiation dose needed to cause the same biological damage when there is oxygen absent to when there is oxygen present
Ranges from 2.5-3
Indicates radio resistance due to hypoxia
Beam angles for prostate
2 Lats
Beam angles for lung
Beam angles for breast
Beam angles for R sided brain lesion
PSI spot scanning technique
Pencil beam (7mm diameter)
By using magnetic field, the protons are controlled and positioned very precisely within a desired position in the target
How does increasing proton energy change distribution
Increased depth with increased energy
More lateral scatter
Do we need to be mindful of OER for proton treatment and why?
Not as much as photon treatment due to high LET damage
Low LET vs High LET in tumour control
High LET (protons/carbon ions)
* low energy deposition upon entry and max deposition at Bragg peak
* Can be effecitve in tumours with long cell cycles, as it is not dependent on cell oxygenation
* As LET increases, generally so does RBE –> associated with greater tumour kill in comparison to Low LET radiation
* Can be beneficial in relatively radioresistant tumours
Low LET
*
Clinical benefits of neutrons
- Neutrons are classified as a form of High LET radiation
- High RBE of neutronns associated with greater tumour kill rates –> particularly beneficial in slow growing tumours or those radioresistant to low LET.
- Can cause more damage to cells with an OER between 1.5-2.0 –> may be more effective in treating hypoxic tumours than conventional RT
- Not cell cycle dependent
Limitations of neutrons
- In comparison to conventional RT, neutrons dose distributions show a broadrer penumbra
- Acute and late side effects from photons are expected with neutrons, however they may be more exacerbated.
- Studies have show that although locoregional control may be increased, increased toxicities may be experienced (e.g., prostate)
