Characteristics of Clinical Beams: Electrons

Question 1

What happens to an electron PDD as energy increases?

Answer 1

• 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)

Question 2

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

Answer 2

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

Question 3

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

Answer 3

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

Question 4

What advantages do electrons have over photon beams?

Answer 4

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 5

What is involved when planning with an electron beam?

Answer 5

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 6

Why are margins used in electron beam therapy?

Answer 6

dose coverage is always less than the geometric size

Question 7

What does the size of the margins depend on?

Answer 7

beam energy

geometry of field definition (cut-outs/applicators, suface field definition)

Question 8

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

Answer 8

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

Question 9

How does increasing the field size affect the PDD?

Answer 9

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

Question 10

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

Answer 10

No change in PDD as electrons cant reach central axis, and so cant contribute to PDD dose.

Question 11

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

Answer 11

Penumbra increases with increasing gap size

Question 12

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

Answer 12

M ~ R(85)/2

Question 13

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

Answer 13

• spare adjacent normal tissue
• improve dose-homogeneity
• minimised effects of patient movement

Question 14

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

Answer 14

Thickness > inital energy (MeV)/2

Question 15

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

Answer 15

isodose line run parallel to the skin surface

Question 16

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

Answer 16

isodose lines are parallel to the surface but the direction is tilted

Question 17

How does the dose distribution change near a bone?

Answer 17

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

Question 18

How does the dose distribution change near lung/air?

Answer 18

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 19

What is the therapeutic range of an electron beam?

Answer 19

maximum depth of pentration of the therapeutic dose

Question 20

What is the therapeutic interval for an electron beam?

Answer 20

depth of tissue treated at or above the therapeutic dose

Question 21

Why are boluses used in electron beam therapy?

Answer 21

• evens out irregular surfaces
• increases surface dose
• decreases penetration

Question 22

What are ideal characteristics for a bolus material?

Answer 22

Tissue equivalent in

• mass stopping
• mass angular scatter power
• physical density

Question 23

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

Answer 23

2Mev/cm

Question 24

What is the optimum bolus thickness?

Answer 24

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

Question 25

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

Answer 25

Increase of theraputic range.
Sn and Pb foild are readily avaliable
Cheap
Easy to work with

Question 26

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

Answer 26

Coulomb interaction with electrons and nuclei of the material.

Question 27

What can happen to electrons undergoing coulomb interactions?

Answer 27

• 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 28

What occurs during an elastic collision of the electron beam?

Answer 28

Change in direction, but no energy loss.

Question 29

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

Answer 29

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

Question 30

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

Answer 30

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

Question 31

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

Answer 31

• 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 32

What problems occur with the use of a closed applicator?

Answer 32

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

Question 33

What is the function of the primary scattering foil?

Answer 33

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

Question 34

What is the function of the secondary scattering foil?

Answer 34

Flattens the beam.

Question 35

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

Answer 35

• Broadens out towards tissue surface due to interactions with waveguide window, scattering foils, ionisation chamber, air, photon collimators, and electron applicators.

Question 36

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

Answer 36

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 37

What is meant by the Rp for an electron beam

Answer 37

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 38

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

Answer 38

E(0) = 2.5 x R(50)

Question 39

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 39

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

Question 40

What is the definition of the electron path length?

Answer 40

Total distance travelled by an electron before coming to rest.

Question 41

What is the range of an electron?

Answer 41

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