8.4.3 - Compensation (Photon Beams)

An irregular patient contour causes shifting of isodose curves which may be undesirable. Inhomogeneity within part of the beam may also lead to shifting and unevenness of the isodose chart. Several methods of shifting isodose curves back to their desired profile are available. Compensation is usually available for megavoltage photons; treatment of superficial lesions in irregular areas is best served by electrons and bolus in most situations.


A physical wedge is a high attenuating material that is designed to correct the isodose lines by a certain amount. They are usually described by their degree of correction, eg ’30 degree wedge’. A physical wedge is loaded into the treatment head (in the path of the beam) typically below the secondary collimators. A physical wedge attenuates all of the beam to some degree, but has most of its effect at its ‘thick end’.
Modern linear accelerators use dynamic wedges. The secondary collimator, in this case an independent jaw, closes during the treatment to attenuate the dose in a similar way to the wedge. The jaw closes from the thick end of the wedge first. Unlike a physical wedge, the dynamic wedge performs no attenuation at the thin end and this must be accounted for when calculating monitor units.


Bolus (compensating bolus) is used when there is a more irregular contour that a wedge is not suitable for. It is made of a tissue equivalent material that is applied directly to the skin, forming a flat contour for the beam to enter through.
Bolus increases surface dose and negates the skin sparing effect of megavoltage beams. It is cheap and easier to apply than a compensator (below).


A compensator attenuates the beam based on the irregular contour of the patient. Like the physical wedge, it is placed in the treatment head of the machine, but functions in a similar way to bolus in evening out isodose lines at depth within the patient. It is usually made of a high attenuating material, and must be cast individually for each patient (and each beam). Unlike bolus, it does not increase skin dose as the scattered radiation generated in the treatment head is negligible at the patient’s skin.