2 The amount of skin sparing with electron treatments decreases as the energy of an electron increases (higher electron energy means higher skin dose). This concept is the opposite of skin sparing with photons (higher photon energy means less skin dose). Explain what happens in electron interactions that causes an increase to the skin surface dose.
The dose build-up of an electron beam is much different than a megavoltage beam. Where megavoltage beams tend to increase skin sparing with increasing energy the opposite is true of electron beams, the higher the energy the more skin dose the patient will receive. This is a product of the lower energy electrons are scattered and a more rapid rate than that of higher energies electrons over a shorter distance.1
When an electron beam enters the body the beam expands rapidly below the surface due to scattering. The lower electron energies create a bowing effect with isodose curves, whereas the higher energy electrons tend to only bow out in the lower energy range, showing lateral compression of the higher energy range.1 With lower energy electrons they are able to scatter more easily through larger angles because of their shorter dmax.2 Thus not eliminating skin dose but pulling some dose away from the skin. Whereas higher energy electrons, having a more lateral compression keeps the higher dose for compact not allowing it to scatter laterally and causing more bremsstrahlung interactions of the electrons in the tissue and more of these interactions happening closer to the skin’s surface.
The dose build-up of an electron beam is much different than a megavoltage beam. Where megavoltage beams tend to increase skin sparing with increasing energy the opposite is true of electron beams, the higher the energy the more skin dose the patient will receive. This is a product of the lower energy electrons are scattered and a more rapid rate than that of higher energies electrons over a shorter distance.1
When an electron beam enters the body the beam expands rapidly below the surface due to scattering. The lower electron energies create a bowing effect with isodose curves, whereas the higher energy electrons tend to only bow out in the lower energy range, showing lateral compression of the higher energy range.1 With lower energy electrons they are able to scatter more easily through larger angles because of their shorter dmax.2 Thus not eliminating skin dose but pulling some dose away from the skin. Whereas higher energy electrons, having a more lateral compression keeps the higher dose for compact not allowing it to scatter laterally and causing more bremsstrahlung interactions of the electrons in the tissue and more of these interactions happening closer to the skin’s surface.
- Kahn FM. The Physics of Radiation Therapy. 5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2014.