Schema of collimator and compensators

By passing each beam through an aperture shaped to correspond to the cross-sectional area of the lesion in the beam's bird's eye view, the cross-section of each proton beam is conformed to the corresponding silhouette of the lesion (collimation). By interposing specific materials in the beam's path, energy attenuation is achieved to account for the effects of sloping skin surfaces and tissue non-homogeneities (compensation).