A method of radiotherapy includes irradiating a treatment volume containing a high atomic number contrast medium with a photon beam characterized by a peak energy of at least 1 MeV and a mean energy in excess of 250 keV. The irradiation may be provided by a beam from a linear accelerator operating without a flattening filter. The linear accelerator may have multiple modes including one or more modes for operating with a flattening filter and one or modes for operating without a flattening filter.