Characteristics of Clinical Beams: Electrons Flashcards
What happens to an electron PDD as energy increases?
- surface dose increases
- depth of dose max increases
- R(50), R(80), and Rp increases 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, suface 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 scatter in the air and against the walls of the applicator etc.
From beam shape 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 unchanged
What happens if the field size in increased above the practical range of the electrons?
No change in PDD as electrons cant reach central axis, and so cant contribute to PDD dose.
What happens to the penumbra if a gap is placed between the skin and the collimator?
Penumbra increases 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?
- spare 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 > inital 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 attenutation 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 pentration 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 what rate (MV/cm) do electrons loose energy?
2Mev/cm
What is the optimum bolus thickness?
One that matches the surface dose to the theraputic dose and the theraputic interval to the theraputic range. A thicker bolus would only reduce the theraputic range
What advantages does a high-Z foil have over a bolus?
Increase of theraputic range.
Sn and Pb foild are readily avaliable
Cheap
Easy to work with
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?
- 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.
