In a beam filter assembly, a base filter is employed to modify a beam quality of a radiation beam of a base energy level and a first filter slice is stacked with the base filter to modify a beam quality of a radiation beam of a first energy level higher than the base energy level. In a beam filter positioning device, a base stage carries a base filter and a first stage carries a filter slice. The base stage is provided with a first engagement site and a second engagement site. The first stage is provided with a first engagement site, a second engagement site, and an open port. The first stage and the base stage are each independently movable relative to the beamline. The first stage is engageable with the base stage when at least one of the first and second engagement sites of the first stage is aligned with at least one of the first and second engagement sites of the base stage, and is further movable with the base stage in unison when engaged.