Abstract: Apparatus for treatment by radiation of an intracranial portion disposed at a predetermined region, comprises a linear accelerator for supplying a source of energetic electrons, and a collimator disposed adjacent the predetermined region for supplying an X-ray source of radiation to the predetermined region. The collimator includes a plurality of channels oriented toward the predetermined region. Each channel has a target which produces the X-ray radiation when the respective target is selectively exposed to the energetic electrons. The collimator also has a cover layer of radiation attentuation material having a low atomic or Z number and that extends over the outer surface of the collimator but leaves unexposed the plurality of channels. Preferably, the cover layer has a thickness at least equal to the range of the energetic electrons.A method of treating an intracranial portion with the apparatus of the present invention is also disclosed.