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Calculated patient dose (per proton incident on the face of the collimator) from neutrons originating in a pre-collimator + collimator configuration (see figure 1). In each case the patient-specific collimator was 65-mm thick brass; the four pre-collimators were each of sufficient thickness to stop 235 MeV protons (see table 1), and were located immediately upstream of the patient-specific collimator.
