Figure 2.

Beam shaping of a pristine pencil beam (entering from the left) to an extended target volume (spread out Bragg peak, SOBP) by passive devices. The devices are schematically shown in the middle and their effects on the beam are shown above and below, respectively, for the lateral and longitudinal enlargement. Scattering by a multi-step device of heavy and light atomic material broadens the beam to a non-Gaussian shape with flat top. A range modulator is used to extend the Bragg peak longitudinally and produce the requested flat depth profile. With the range shifter the complete profile is shifted to the tumor depth. An adaption to the maximum contours of the tumor volume is achieved laterally by patient specific collimators and in depth by compensators. Using the passive technique, the resulting SOBP can only be tailored to the distal edge of the target volume at the expense of a corresponding high dose area in the proximal normal tissue (reprinted from [10]).