8.4.5 - Modification Of Skin Dose Photons

Bolus (build-up bolus) is material directly applied to the surface to increase the dose at the skin surface for megavoltage beams. It is usually tissue equivalent but may be made of another material such as tin foil.
In a non-bolus treatment, surface dose is low due to the relatively low production of electrons in the air and some contribution from the treatment head. Dose increases with depth rapidly as more photons are attenuated in the tissue.
Bolus causes increased photon attenuation to begin at some point above the skin. This has the result of shifting the maximum dose superficially and increasing skin dose.
The choice of bolus thickness is dependent on the beam quality and the desired depth of treatment. For a 10 x 10 cm field, 6 MV photons achieve a pre-max 90% dose at about 0.6 cm and to treat subcutaneous tissue 0.6 or greater bolus would be acceptable. The equivalent field with 18 MV photons would not achieve 90% dose until 1.5 cm and thicker bolus would be required.

Bolus and Superficial X-Rays

Superficial x-rays already have 100% dose at the skin surface and do not require build up bolus.