Figure 7.

Calculated peak patient dose (per proton incident on the face of the collimator) from neutrons originating in a pre-collimator + collimator configuration (see figure 1), as a function of the thickness of the pre-collimator. In each case the patient-specific collimator was 65 mm thick brass, and the pre-collimator was located immediately upstream of the patient-specific collimator. The points correspond to the pre-collimator thickness sufficient to stop 235 MeV protons (as in figure 6).