It is the objective of the present invention to provide an energy filter that 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. This objective is achieved according to the present invention by a flexible energy filter (2) for particle beam therapy, comprising: a) a cushion of plastic or water equivalent material (4) or a stack of at least two soft sheets of plastic or water equivalent material (4) and b) attached to the surface of the cushion or to the surface of a soft sheet of plastic or water equivalent material, or embedded in the cushion or embedded in the soft sheet or between the two soft sheets of plastic or water equivalent material (4), a layer (6) of a plurality of metal particles (8), preferably metal spheres said layer (6) having a cross sectional area corresponding at least to a cross section of a beam scanning area at an outlet of the particle beam equipment prior to its entry into a tissue. This energy filter behaves similar 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 the layer of the plurality of metal particles within the flexible soft plastic sheet (the flap) to the skin of the patient.
