RT Beam Basics
The radiation beam that leaves the collimator jaw opening of the linear accelerator is designed to be:
- flat (at a specified depth)
- spread out evenly from the central axis
The radiation can be thought of as coming out in a symmetrical uniform manner so that when it interacts with a perfectly flat surface, the radiation would give an even distribution in two dimensions (width and length).
In the 3rd dimension, depth, the radiation would decrease in strength as the beam is attenuated by the material it is interacting with:
Note how the beam falls off from the centre of the field as demonstrated by the decreasing isodose lines at the periphery.
This rapid decrease at the edges is called the penumbra region