A radiotherapy system comprising a plurality of particle beam sources (1a, 1b, 1c) arranged to radiate particle beams (6a, 6b, 6c) on a three-dimensional radiation target (3) located inside the body of a radiotherapy patient (4), the position in space and alignment of which particle beam sources are individually controllable and an imaging system (2) arranged to monitor the position and orientation in space of the three-dimensional radiation target including direction and speed of any movement of the radiation target, and also to monitor tissue characteristics of body tissue (5) surrounding the radiation target located in the radiation paths of the particle beams. The radiotherapy system comprises a particle beam control system (7) which, during a radiation treatment session, is arranged for receiving information on the position and orientation of the radiation target and on said tissue characteristics from the imaging system based on the received information on said tissue characteristics, identifying body tissue which shall not be exposed to the particle beams and in response to movement of the radiation target and/or of body tissue surrounding the radiation target, adjusting the individual positions and alignments of the particle beam sources and the individual characteristics of the particle beams so that (i) Bragg peaks or spread out Brag peaks (SOBP) of the particle beams are brought to intersect in a predetermined beam intersect region (8) inside the radiation target and (ii) the radiation paths of the particle beams do not travel through said body tissue identified as not to be exposed to the particle beams.
