A compact lightweight gantry for a proton therapy system that has a source-to-axis distance (SAD) of less than 2 m and can deliver a proton beam of superior quality. The reduced SAD leads to reduced requirements on the maximum magnetic fields that can be generated by the bend magnets in the gantry beamline. Correspondingly, lightweight bend magnets can be used. The various components in the gantry beamline are optimized to achieve a beam spot size of approximately 4 mm sigma or less through a pencil beam scanning nozzle disposed downstream of the final bending magnet. In addition, the proton therapy system is configured to operate at a maximum beam energy in the range of 220-230 MeV.