Lecture 13 and 14: Electron Beams

How is electron energy deposited through tissue?

coulomb interactions

Electron Beam PDD

high surface dose, dmax drops off rapidly, bremstrahlung tail

Bremstrahlung tail

photon contamination resulting from radiation loss experienced by incident electrons

As incident energy increase, is component of the bremstrahlung tail smaller or larger?

larger

What does penetration into tissue depend on?

Incident electron beam energy

TG25 recommendation for beam flatness for electron beams.

Flatness in plane pf 95% dose

TG25 recommendation for beam symmetry for electron beams.

no more than 2%

Virtual source

where the electron beam appears to diverge

effective SSD

the distance from the virtual source to the point of nominal SSD

biggest source for x-ray contamination for electron beams

scattering foil system

Air gaps decreases what in electron beams?

electron fluence

What effects the spatial distribution of scatter in electron beams?

beam obliquity

Why is a bolus used?

Flattens out an irregular surface, reduces penetration of electrons, builds up the surface dose

If an electron beam is abutted at the surface with a parallel photon field, a hot spot develops on what side?

Side of the photon field

To reduce the dose from electrons to an acceptable level, what thickness of lead is needed?

Approximately half the beam energy in mm

Between 1 and 40 MeV, what is the rate of energy loss for electrons?

2 MeV/cm

Where is the electron practical range on the PDD?