Pioneered by Lars Leksell during the 1960s, stereotactic radiosurgery (SRS) uses radiation to treat intracranial conditions through the intact skull. Early SRS focused on functional disorders and utilised protons from a synchrocyclotron, but the need for a solution better suited to routine use led Leksell to develop a dedicated device based upon narrowly-collimated photon beams from a large number of 60Co sources arranged around a hemisphere. All coincident through a common isocentre, these photon beams combine to produce a dose rate high at their intersection, but falling rapidly from that focus. The radiation is targeted by reference to a stereotactic coordinate system defined by a frame screwed to the skull, which rigidly immobilises intracranial anatomy to ensure accuracy. By combining multiples of these highly-focused radiation isocentres, treatments can deliver distributions of high dose that tightly conform to the target while sparing surrounding tissue.