A radiation therapy system for irradiating a target volume with a particle beam is provided. The radiation therapy system includes a radiation source that is operable to output an adjustable irradiation field that includes a particle beam that is scanned point by point over the target volume. A 3D imaging device may obtain a 3D radiation treatment data set in a radiation treatment phase. An adaptation unit adapts the irradiation field to a change in position and/or shape of the target volume in the radiation treatment phase. The adaptation unit compares the 3D radiation treatment data set with a 3D planning image data set furnished and obtains a transformation that describes the change in position and/or shape of the target volume. The adaptation unit transforms an irradiation field based on the transformation.