Embodiments of the present invention disclose methods and systems for producing an adaptive pencil beam having an adjustable lateral beam size and Bragg-peak width. According to one disclosed embodiment, an apparatus for producing an adaptive pencil beam is disclosed. The apparatus includes a set of momentum band expanders configured to widen a momentum spread of a pencil beam, where a momentum band expander is selected from the set of momentum band expanders to receive the pencil beam, and a slit at dispersive focus of two dipole magnets to adjust a width of a Bragg-peak of the pencil beam. According to another disclosed embodiment, a method for producing an adaptive pencil beam with an adjustable lateral beam is disclosed. The method includes selecting a scatter foil, or setting of a defocusing/focusing magnet, and adjusting a lateral size of the pencil beam.
