Medical apparatus that comprises:(A) a hadron therapy device adapted for directing a hadron beam having an initial beam energy, E0, along a beam path to a target spot located inside a subject of interest(B) a MRI for acquiring magnetic resonance (MR) image within an imaging volume, Vp, comprising the target spot(C) a prompt-γ system adapted for acquiring a signal generated by the hadron beam(D) a controller configured for:• computing an actual position of the Bragg peak of said hadron beam, based on the signal acquired by the PG system, and• locating the actual position of the Bragg peak on an MR image of the imaging volume, Vp, acquired with the MRI along the beam path from an outer surface of the subject of interest to the target spot.
