1.3 - Linac Accessories

Linac accessories are components that are added to the treatment head to further modify the beam.

Custom blocks

Blocks are thick pieces of lead that are used to shape the field. They can be loaded into the accessory tray as an additional means of collimating the beam. Custom blocks have fallen out of use due to multileaf collimation, which is faster and less taxing on staff. They are still important when an island block is required as MLCs are not capable of emulating this.

Physical wedges

Physical wedges are graduated pieces of lead that have a thick end and a thin end. The thin end causes less attenuation than the thick end; this causes a shift in the isodose curves within the treated volume. The wedge is denoted by the angle it tilts the isodose curves - eg. a 30o wedge would cause a 30o tilt in the isodose curves.
Physical wedges are not in common use due to the ability of the independent jaws to perform dynamic wedging.


Compensators are used when an irregular patient contour causes an undesirable isodose distribution. They have an advantage over compensating bolus in that skin sparing is maintained. They are loaded into the accessory tray in a similar way to the custom blocks or physical wedges. Compensators are very infrequently used due to the time required in construction.

Electron cone / applicator

The electron cone is a device used for electron treatments only. It provides additional collimation with the aim of keeping a sharp beam edge within the treatment volume. The electron cone typically has an additional tray for the insertion of electron cutouts. Unlike the custom blocks for photon treatments, the electron cutouts are much thinner (about 1 cm of lead) and lie very close to the patient surface (to reduce the size of the penumbra).