Fig. 2
From: Applications of various range shifters for proton pencil beam scanning radiotherapy
A schematic diagram of a pencil beam scanning proton beam snout with a machine-related range shifter (MRS: 40 cm × 32 cm). The air gap is defined as the distance from the isocenter to the MRS. The snout position is defined from the isocenter to the bottom edge of the snout. A 2.5 cm gap exists between the MRS and the edge of the snout. The snout can be extended towards the isocenter. The range of the snout extensions is from 10 to 54 cm with a fully retracted position at 54 cm