Hadron therapy is an innovative mode of radiotherapy (RT) for cancer treatment which enables tumor cells to be more effectively destroyed than conventional RT using photons. The precise knowledge of the lineal energy of particles is used in the field of microdosimetry (MKM model) as a fundamental parameter in the prediction of the relative biological efficiency (RBE) of clinical beams. Based on single‐crystal CVD (scCVD) super‐thin diamond membranes obtained using deep Ar/O2 plasma etching, prototypes of solid‐state microdosimeters for lineal energy measurements are produced at the Diamond Sensors Laboratory of CEA‐LIST. The response of a diamond membrane microdosimeter to single projectiles is investigated in ion microbeams. The microdosimeter is irradiated using a raster scanning method and the charge transport properties of the device are determined with sub‐micron precision by measuring the charge collection efficiency (CCE), the μSVs 3D spatial definition and the pulse‐height spectra. A prototype of this novel microdosimeter is then tested in a 100 MeV therapeutic proton beam at the Institute Curie − Proton Therapy Centre in Orsay. All results effectively demonstrate the great potential for this device to be used for studies of the RBE in clinical applications.Read less <