Characteristics of Clinical Beams: Electrons

Question 1

What interaction occurs between an electron beam and the material it is going through?

Answer 1

Coulomb interaction with electrons and nuclei of the material.

Question 2

What can happen to electrons undergoing coulomb interactions?

Answer 2

• Loss of kinetic energy (described by the stopping power of the medium)
• Change in direction (described by the angular scattering power of the medium)

Question 3

What occurs during an elastic collision of the electron beam?

Answer 3

Change in direction, but no energy loss.

Question 4

What occurs during an elastic collision between an electron in the beam and an orbital electron of the material?

Answer 4

• Incident electron is deflected and loses part of it’s kinetic energy.

Question 5

What occurs during an elastic collision between an electron in the beam and an atomic nucleus of the material?

Answer 5

• Incident electron is deflected from its path and looses part of its kinetic energy in the form of bremsstrahlung.

Question 6

What happens in the Linac head when the modality is changed from photons to electrons?

Answer 6

• 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)

Question 7

What problems occur with the use of a closed applicator?

Answer 7

Additional scattering off walls degrades electron spectra and generates photon contamination.

Question 8

What is the function of the primary scattering foil?

Answer 8

• Produces a Gaussian spread from the focused beam that exits the waveguide.

Question 9

What is the function of the secondary scattering foil?

Answer 9

• Flattens the beam.

Question 10

How does the electron spectra change from the waveguide to the patient surface?

Answer 10

• 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.

Question 11

What is meant by R(50) for an electron beam?

Answer 11

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.

Question 12

What is meant by the Rp for an electron beam?

Answer 12

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.

Question 13

How is the mean energy at the surface, E(0), related to the R(50) of the beam?

Answer 13

E(0) = 2.5 x R(50)

Question 14

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?

Answer 14

E(d) = E(0)[1-(d/Rp)]

Question 15

What is the definition of the electron path length?

Answer 15

Total distance travelled by an electron before coming to rest.

Question 16

What is the range of an electron?

Answer 16

Sum of all the components of the individual path lengths in the original direction of travel.

Question 17

What happens to an electron PDD as energy increases?

Answer 17

• 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)

Question 18

How is the build-up region of an electron beam different to that of a photon beam?

Answer 18

Electrons deposit energy immediately
- larger surface dose than photons.
Electron path more oblique due to scattering

Question 19

How are R(90), R(80), and Rp related to the mean energy at the surface (rule of thumb)?

Answer 19

R(90) = E(0)/4
R(80) = E(0)/3
Rp = E(0)/2

Question 20

What advantages do electrons have over photon beams?

Answer 20

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.

Question 21

What is involved when planning with an electron beam?

Answer 21

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.

Question 22

Why are margins used in electron beam therapy?

Answer 22

Dose coverage is always less than the geometric size.

Question 23

What does the size of the margins depend on?

Answer 23

Beam energy

Geometry of field definition (cut-outs/applicators, surface field definition)

Question 24

What is mean by the Virtual source distance for an electron beam?

Answer 24

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.

Question 25

How does increasing the field size affect the PDD?

Answer 25

R(max) shifts away from surface
Spectrum changes due to scatter contributions along the central axis.
Practical range remains unchanges

Question 26

What happens if the field size in increased above the practical range of the electrons?

Answer 26

No change in PDD as electrons can’t reach central axis, and so can’t contribute to PDD dose.

Question 27

What happens to the penumbra if a gap is placed between the skin and the collimator?

Answer 27

Penumbra increase with increasing gap size.

Question 28

What is a rule of thumb for the margin size of an electron beam?

Answer 28

M ~ R(85)/2

Question 29

What advantages are offered by the use of lead cut-out to shape the beam?

Answer 29

• Spares adjacent normal tissue
• Improve dose-homogeneity
• Minimised effects of patient movement.

Question 30

What rule of thumb is used to estimate the thickness of the lead cut-out required?

Answer 30

Thickness > Initial energy (MeV)/2

Question 31

What happens to the isodose lines when the beam is incident on an irregular or curved surface?

Answer 31

Isodose line run parallel to the skin surface.

Question 32

What happens to the isodose curves when the beam is incident at an oblique angle to the surface?

Answer 32

Isodose lines are parallel to the surface but the direction is tilted.

Question 33

How does the dose distribution change near a bone?

Answer 33

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.

Question 34

How does the dose distribution change near lung/air?

Answer 34

Lung has a lower density than soft tissue.

• Lower attenuation
• Lower scattering power linear depth
• Increased dose beyond lung
• Decrease dose next to lung.

Question 35

What is the therapeutic range of an electron beam?

Answer 35

Maximum depth of penetration of the therapeutic dose.

Question 36

What is the therapeutic interval for an electron beam?

Answer 36

Depth of tissue treated at or above the therapeutic dose.

Question 37

Why are boluses used in electron beam therapy?

Answer 37

• Evens out irregular surfaces
• Increases surface dose
• Decreases penetration

Question 38

What are ideal characteristics for a bolus material?

Answer 38

Tissue equivalent in

• Mass stopping
• Mass angular scatter power
• Physical density

Question 39

At wat rate (MV/cm) do electrons loose energy?

Answer 39

2MeV/cm

Question 40

What is the optimum bolus thickness?

Answer 40

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.

Question 41

What advantages does a high-Z foil have over a bolus?

Answer 41

Increase of therapeutic range.
Sn and Pb foils a readily available
Cheap
Easy to work with.