RT Beam Basics
Beam Modification Devices
Beams can be further shaped and modified by other techniques.
Two common techniques are using wedges and blocks.
A wedge is a piece of impenetrable material (usually iron) that is shaped such that is thick on one side and thin on the other, much like a piece of pizza. By placing this in front of a beam it will cause a progressive decrease in beam intensity across the field.
A block is a full piece of impenetrable material that has a distinct border that causes the beam intensity to rapidly fall off a the edge of the block. This is useful to avoid critical structures or limit the amount of dose at different edges of a field.
A multileaf collimator is a set of two opposing thin blocks within the head of a linear accelerator and each block can be moved independently. This allows for more concise blocking. If these leaves are moved during treatment they create dynamic beam modification. This type of modification is used in technologies like intensity modulated radiation therapy (IMRT).