Beam Characteristics Flashcards
Why can we not just state a beam energy in MeV to know how it interacts?
Beams of the same maximum energy can have radically different energy spectra and interact differently
What does a percentage depth dose curve show
It illustrates how the beam interacts with tissue
What is the difference between MV and MeV?
What is the clinical convention?
MV is a unit of potential difference
MeV is a unit of energy
All our clinical beams have MeV energies but we say MV for photon beams and MeV for electron beams
What is one electronvolt?
The amount of energy one electron gains when it is accelerated through 1 volt
What part of the beam is the percentage depth dose curve plotted through?
The central axis
What are the key features of a photon beam percentage depth dose curve?
Surface dose
Build up region
dmax
Near exponential dose fall off
Where can surface dose for a photon beam come from?
Xrays contributind head scatter
Secondary electrons from interactions in collimators, MLCs etc
Electron transport from primary photon interactions
Electron transport from photons interacting at patient surface
Some backscatter photons from the patient
Some secondary electrons from photons interacting in patient
How far do compton electrons usually travel in tissue?
About 2cm
Why does the build up region occur for photon beams?
Compton scatter is the dominant interaction process at MeV energies
Most compton electrons continue in the same direction as the primary beam, and most ionisation occurs at the END of their track when they’re stationary in tissue
Why can the dose build up for photon beams be good and bad?
Good: Skin sparing
Bad: If we want to treat something in the build up region
What is KERMA?
What are its units?
Kinetic Energy Released per unit Mass
units = Jkg^-1
Where is maximum KERMA in a patient?
At the surface
If maximum KERMA is at the surface of the patient, why do we get a build up region?
Compton scatter
Maximum energy is released at the surface, but the compton electrons released from the interactions move forward and deposit most of the energy at the end of their path
What is charged particle equilibrium?
The energy released to electrons in a given area is equal to the energy absorbed in that area from electron interactions
What is the difference between KERMA and absorbed dose?
KERMA is the amount of energy RELEASED
Absorbed dose is the amount of energy ABSORBED
What is the rule of thumb for dmax of a photon beam
About a quarter of the beam energy
What is dmax for:
6MV
10MV
15MV
6MV = 1.5cm 10MV = 2.4cm 15MV = 3cm
Does dmax increase or decrease with increasing energy for a photon beam?
Increase
Is there charged particle equilibrium in the build up region of a photon beam?
No
Is there charged particle equilibrium in the fall off region of a photon beam?
Some
BUT
Energy is being lost all the time due to attenuation
What is dose fall off in a photon beam due to?
Exponential attenuation & the inverse square law
What does the rate of dose fall off in a photon beam depend on?
Beam energy & field size
Does surface dose for a photon beam increase or decrease with increasing energy?
Decrease
What is the rate of fall off (approximately) for a photon beam JUST due to the inverse square law at 100cm SSD?
2% per centimetre
What are the factors affecting percentage depth dose of a photon beam?
Depth
Beam quality
Field size
FSD
How does depth affect percentage depth dose for a photon beam?
Why?
Percentage depth dose DECREASES with increasing depth
Increasing depth has 2 effects:
Increased chance of attenuation
Increased effect of ISL
Both of these reduce the number of photons reaching a particular point and so reduce absorbed dose
How does beam quality affect percentage depth dose for a photon beam?
Why?
Percentage depth dose INCREASES with increasing energy
Beam quality has 2 factors:
Higher quality photons have a decreased chance of interacting with matter (so will get further)
Higher quality photon interactions will produce higher quality scatter (so will get further)
How does field size affect percentage depth dose for a photon beam?
Why?
Percentage depth dose INCREASES with increasing field size
It has NO EFFECT on the primary beam (chance of interactions is related to linear attenuation, so each photon has the same chance)
It DOES AFFECT the amount of scatter (more neighbours being irradiated means more scatter generated & increased contribution of dose)
How does FSD affect percentage depth dose for a photon beam?
Why?
Percentage depth dose INCREASES with increasing FSD
This is due to the decreased effect of the inverse square law at larger distances
Moving 1cm away from the source when at 1cm doubles the distance
Moving 1cm away from the source when at 10cm only increases the distance by 10%
Why do we measure the beam profile?
Percentage depth dose tells us how the beam behaves along the central axis, but we also need to know how it behaves along the beam width
How are the curved edges of the beam profile made less curved?
Flattening filters
What is the penumbra region of the beam profile?
The area of gradual dose drop off at the edge of the beam (dose does not stop completely at the beam edges)
What is the treatment width on the beam profile?
The distance between the two 95% points
What is the geometric beam width (light field) on the beam profile?
The distance between the two 50% points
What is the penumbra width usually defined as on the beam profile?
The space between the 20% and 80% isodose lines
What does the light field show?
The geometric field size (50% isodose line)
What is the jaw setting on the linac?
The size of the 50% isodose line at dmax
What are penumbra caused by?
Why?
TRANSMISSION PENUMBRA
When edges of the beam pass through the ‘corner’ of the collimators and are incompletely attenuated
GEOMETRIC PENUMBRA
Relates to the size of the initial focal spot
Smaller focal spot = less penumbra
SCATTER
There is reduced scatter contribution at the field edge compared to the centre of the field
There is scatter beyond the field edge due to primary interaction processes and head scatter
INCREASED DEPTH
Divergence increases field size
Larger proportion of scattered radiation
Wider penumbra
Why can the shape of the %DD curve not be used to specify beam energy?
It depends on scatter (field size, FSD etc)
Why can the energy of the beam at production not be used to specify beam energy?
It doesn’t tell us how the beam will behave in tissue
Do paired linacs produce identical percentage depth dose curves?
No necessarily
They may produce xrays in the same way, but if the heads differ slightly, the head scatter will be different, so the percentage depth dose curve will be different
How can we specify energy for a photon beam?
There are many different ways eg. Nominal MV D10(%) d80(cm) dmax
What is an isodose chart?
It is constructed by combining measured data
The lines connect equal dose
Why is the energy specification for kV xrays in HVL of Al or Cu, not in MV?
The filter will dramatically change the shape of the percentage depth dose curve, so must be specified in the energy specification
SO
It is specified in Half Value Layer
