An energy filter allows, particularly at lower beam energies, a determined spread of the beam energies in order to preserve the beam edges and to spread the Bragg peak at each individual beam energy. The energy filter behaves similarly to a ridge filter and spreads out the Bragg peak of a low energy proton beam (70-100 MeV) so that energy modulation of the beam can be done with steps of 5 mm in one go over the full range of 235-70 MeV. Due to its mechanical flexibility, the energy filter can be placed on the skin of the patient which minimizes the effect of scattering in the filter due to the short distance behind a layer of a plurality of metal particles within a flexible soft plastic sheet to the skin of the patient.
