A system for radiation therapy includes an ionizing radiation source for producing a beam oriented along an axis aligned with a target volume for delivering ionizing radiation to the target volume along a beam path and at a dose rate, wherein the beam includes a soft spectrum and a hard spectrum. The system also includes a filter arranged within the beam path and including a central aperture that is free of beam-filtering material, wherein the central aperture is surrounded by a beam-filtering material having a thickness dimension selected and positioned in the beam path to highly attenuate the soft spectrum of the beam to reduces external scatter of the beam and allow a majority of the hard spectrum of the beam to pass through the central aperture unfiltered. The beam path is free of beam-filtering material arranged to attenuate a central portion of the beam.
