A silicon-on-insulator diode array with a sensitive depth of 10 microns has been developed for microdosimetry in proton therapy. The detector was coupled to a radiation-hard charge sensitive amplifier with the probe assembly capable of measuring an LET down to 1.2 keV/μm. The device has been successfully tested at two proton therapy centers. The 230 MeV Northeastern Proton Therapy Center, Boston and the 250 MeV Proton Medical Research Center at Tsukuba, Japan. The device offers much improved spatial resolution compared with a proportional gas counter particularly in the critical high dose region around the proton Bragg peak. Due to its small cross-sectional area (0.04 cm2) measurements may also be made in facilities with short high intensity beams.