A method for treating or irradiating a target volume (10) with a charged particle beam (100), to the target volume being associated three coordinates according to a X, Y and Z direction, the Z coordinate corresponding to the beam direction while the X and Y coordinates correspond to directions perpendicular to the Z direction, said charged particle beam producing in said target volume an irradiation spot (101), the method comprising the following steps: - continuously scanning said spot in the X and Y directions and applying a continuous movement to said spot in the Z direction by seamlessly modifying the energy of said beam, the step of applying a continuous movement in Z direction and the step of continuous scanning in X and Y direction being performed simultaneously, thereby performing a continuous 3D scanning of the target volume.