A miniature ripple filter for filtering a ripple found in the distal part of a proton SOBP

Ridge filters have been used to form spread-out Bragg peaks (SOBPs) for proton therapy. Their design is based on a pristine Bragg curve, which is dependent on the incident beam energy, to keep the sharp distal falloff. A proton synchrotron can supply a beam at any energy required for therapy. Thus, a large number of ridge filters are required in principle to obtain optimized SOBPs for different incident beam energies. As a compromise between need and practicality, a reduced number of ridge filters is usually imposed. When a ridge filter optimized at a certain energy is used for a beam at a lower energy, we observe a ripple in the depth-dose curve, especially around its distal part. This is due to the mismatch between the individual Bragg curves comprising the SOBP and the Bragg curves assumed in the design. We propose here an additional miniature ripple filter for filtering the ripple. The filter broadens the energy spread so as to match the energy of the incident beam with the used ridge filter. We machined a miniature ripple filter optimized for a 125-MeV beam and used with ridge filters optimized for a 155-MeV beam. The ripple filter consists of many identical units made from an aluminum alloy, and the spatial period and height are 2.4 and 2.85 mm, respectively. A cross-section of one unit consists of seven-step, up-and-down stairs. Since the ripple filter required precise machining, it took a few weeks to complete the machining. The filter is intended to be placed at 30–50 cm upstream from the patient surface. We compared measured depth-dose curves with and without the ripple filter and found that the ripple was cleanly filtered by the ripple filter. Two sets of ridge filters optimized at two energies and a few ripple filters are enough for the practical formation of the SOBP for protons at any energy between 70 and 250 MeV extracted from the synchrotron.