Disclosed is a method of determining information regarding the location of energy deposition of an ion beam, in particular a proton beam, in an absorptive medium, in particular in the tissue of a patient undergoing radiation therapy, comprising the following steps:generating an intensity modulated ion beam, wherein the intensity modulation comprises one or more modulation frequency components,detecting an acoustic signal attributable to the time dependent energy deposition in said absorptive medium by said intensity modulated ion beam using at least one detection apparatus, said detection apparatus being preferably configured for extracting at least one modulation frequency component of the acoustic signal corresponding to a respective one of the one or more modulation frequency components of said intensity modulation, or a harmonic thereof, andderiving information regarding the location of the energy deposition based, at least in part, on a time lag between the timing of the intensity modulation of said ion beam and said acoustic signal.
