Abstract: 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.