Characteristics of Clinical Beams: Electrons Flashcards
What interaction occurs between an electron beam and the material it is going through?
Coulomb interaction with electrons and nuclei of the material.
What can happen to electrons undergoing coulomb interactions?
- Loss of kinetic energy (described by the stopping power of the medium)
- Change in direction (described by the angular scattering power of the medium)
What occurs during an elastic collision of the electron beam?
Change in direction, but no energy loss.
What occurs during an elastic collision between an electron in the beam and an orbital electron of the material?
- Incident electron is deflected and loses part of it’s kinetic energy.
What occurs during an elastic collision between an electron in the beam and an atomic nucleus of the material?
- Incident electron is deflected from its path and looses part of its kinetic energy in the form of bremsstrahlung.
What happens in the Linac head when the modality is changed from photons to electrons?
- Reduction in beam current in waveguide.
- X-Ray target retracts
- X-Ray flattening filter is changed to scattering foil(s)
- Applicator is attached (done manually)
What problems occur with the use of a closed applicator?
Additional scattering off walls degrades electron spectra and generates photon contamination.
What is the function of the primary scattering foil?
- Produces a Gaussian spread from the focused beam that exits the waveguide.
What is the function of the secondary scattering foil?
- Flattens the beam.
How does the electron spectra change from the waveguide to the patient surface?
- At waveguide: almost mono-energetic
- Broadens out towards tissue surface due to interactions with waveguide window, scattering foils, ionisation chamber, air, photon collimators, and electron applicators.
What is meant by R(50) for an electron beam?
Depth in water along the beam central axis in a
10x10 cm^2 or larger beam of electrons at a source-to-surface distance (SSD) of 100 cm at which the absorbed dose is 50% of the maximum value.
What is meant by the Rp for an electron beam?
Practical range of an electron beam, determined from the depth-dose curve as the depth of the point where the tangent at the inflexion point of the falloff portion of the curve intersects the bremsstrahlung background.
How is the mean energy at the surface, E(0), related to the R(50) of the beam?
E(0) = 2.5 x R(50)
How is the mean dose at depth d, E(d), related to the mean energy at the surface, E(0), and the practical range of the electrons, Rp?
E(d) = E(0)[1-(d/Rp)]
What is the definition of the electron path length?
Total distance travelled by an electron before coming to rest.
What is the range of an electron?
Sum of all the components of the individual path lengths in the original direction of travel.
What happens to an electron PDD as energy increases?
- Surface dose increases
- Depth of dose max increases
- R(50), R(80), and Rp increase in depth.
- Gradient of fall off increases
(X-Ray contamination increases)
How is the build-up region of an electron beam different to that of a photon beam?
Electrons deposit energy immediately
- larger surface dose than photons.
Electron path more oblique due to scattering
How are R(90), R(80), and Rp related to the mean energy at the surface (rule of thumb)?
R(90) = E(0)/4 R(80) = E(0)/3 Rp = E(0)/2
What advantages do electrons have over photon beams?
Better depth-dose curve -- Rapid build up/Steep drop-off Single Treatment field Applied orthogonally to skin surface Not computer planned Same RBE as MV X-Rays Electrons (and therefore dose distribution) suffer significant perturbation in presence of inhomogeneities.
What is involved when planning with an electron beam?
Selecting a field size - field margins and/or use of cut-out. Choosing a beam energy - Depth of penetration - use of bolus or foils Prescribing a dose schedule.
Why are margins used in electron beam therapy?
Dose coverage is always less than the geometric size.
What does the size of the margins depend on?
Beam energy
Geometry of field definition (cut-outs/applicators, surface field definition)
What is mean by the Virtual source distance for an electron beam?
Electrons do not originate from a source like photons do, due to the scatter in the air and against the walls of the applicator etc.
From beam shape and applicator position electrons appear to have originated from a virtual source.
How does increasing the field size affect the PDD?
R(max) shifts away from surface
Spectrum changes due to scatter contributions along the central axis.
Practical range remains unchanges
What happens if the field size in increased above the practical range of the electrons?
No change in PDD as electrons can’t reach central axis, and so can’t contribute to PDD dose.
What happens to the penumbra if a gap is placed between the skin and the collimator?
Penumbra increase with increasing gap size.
What is a rule of thumb for the margin size of an electron beam?
M ~ R(85)/2
What advantages are offered by the use of lead cut-out to shape the beam?
- Spares adjacent normal tissue
- Improve dose-homogeneity
- Minimised effects of patient movement.
What rule of thumb is used to estimate the thickness of the lead cut-out required?
Thickness > Initial energy (MeV)/2
What happens to the isodose lines when the beam is incident on an irregular or curved surface?
Isodose line run parallel to the skin surface.
What happens to the isodose curves when the beam is incident at an oblique angle to the surface?
Isodose lines are parallel to the surface but the direction is tilted.
How does the dose distribution change near a bone?
Bone has a higher density than soft tissue
Similar mass stopping power and mass angular scattering power.
- Increased attenuation of the beam
- Greater scattering per linear depth
- Dose beyond bone decreases
- Dose next to bone increases.
How does the dose distribution change near lung/air?
Lung has a lower density than soft tissue.
- Lower attenuation
- Lower scattering power linear depth
- Increased dose beyond lung
- Decrease dose next to lung.
What is the therapeutic range of an electron beam?
Maximum depth of penetration of the therapeutic dose.
What is the therapeutic interval for an electron beam?
Depth of tissue treated at or above the therapeutic dose.
Why are boluses used in electron beam therapy?
- Evens out irregular surfaces
- Increases surface dose
- Decreases penetration
What are ideal characteristics for a bolus material?
Tissue equivalent in
- Mass stopping
- Mass angular scatter power
- Physical density
At wat rate (MV/cm) do electrons loose energy?
2MeV/cm
What is the optimum bolus thickness?
One that matches the surface dose to the therapeutic dose and the therapeutic interval to the therapeutic range.
A thicker bolus would only reduce the therapeutic range.
What advantages does a high-Z foil have over a bolus?
Increase of therapeutic range.
Sn and Pb foils a readily available
Cheap
Easy to work with.
