4.7 - Bolus

What is meant by ‘bolus’?
Why is it used?
What bolus materials are used at your linac?
How is it applied and what are the possible errors in its application?


Bolus is a material applied to the surface of a patient for one of three reasons:

  • To negate the skin sparing effect of megavoltage photons and some megavoltage electron fields
  • To compensate for an irregular patient contour to achieve flat isodose lines at depth
  • To smooth out sharp edges on the patient surface to prevent hot spot formation (particularly for electron beams)

Bolus materials

Bolus should be:

  • Tissue equivalent
  • Easy to apply
  • Comfortable for the patient
  • Inexpensive

Commonly used bolus materials at my department include:

  • Wet gauze or combine
    • Wet gauze is the least accurate of the bolus types, but is capable of conforming to unusual shapes such as the ear or nose with relative ease. Care must be taken to ensure that air gaps within the wet gauze are minimised.
  • Super-flab (reusable)
    • Super-flab comes as a sheet of flexible plastic material which can be moulded over gentle patient contours. It is often used when a large amount of bolus is needed, such as for chest wall treatments following mastectomy.
  • Wax blocks
    • Wax provides structural support which is important in some areas, eg. for penile treatments. Wax blocks are difficult to alter once they are cast and are usually custom made for each application