Stock Photo - Photo essay at the regional hospital complex of Lille, France, hospital Roger Salengro, department of neurosurgery, Gamma knife. Helmet of Gamma knife. Gamma knife is a method of treatment by stereotaxic radiosurgery that allies stereotaxic neurosurgery very precise localization and in three dimensions of the lesion and and the multibeam radiotherapy concentration of very many beams, emitted from various sources, on a precise point. The helmet of the Gamma Knife contains 201 sources of Cobalt 60 of which the beams will converge to target the lesion to irradiate very precisely. These 201 sources of Cobalt 60, of about 30 curies each, are placed in five crowns around the helmet of Gamma Knife that is also used as a collimator. Depending on the irradiation to be made, a helmet with collimators of different diameters is used.