The present invention relates to a particle therapy system that comprises a spot scanning system to irradiate with a particle beam a plurality of spots in a layer of a target with prescribed spot doses for each spot of the layer. The therapy system is further adapted to perform multiple paintings of the layer and to deliver partial spot doses to selected spots of the layer during each painting time so that each spot of the layer will have received its prescribed dose after completion of the multiple paintings. The therapy system further comprises means for setting the partial spot doses and the number of times that a spot will be selected for irradiation in the course of the multiple paintings to such values that any spot of the layer will never have to be irradiated with a partial dose which would fall below a minimum dose deliverable by the system, and this whatever the number chosen for the number of layer paintings.The invention also relates to a corresponding irradiation method.