Particle-induced visual sensations in heavy-ion tumor therapy

Many patients with tumours located in the skull base reported visual sensations (phosphenes) during radiation therapy with 12C ions at GSI Darmstadt. These effects, mostly described as streaks moving through the field of vision, occur only during well-defined time-phases of the raster-scan irradiation and they are related to the actual position and stopping range of the pencil-like ion beam. An attempt was made to correlate the phosphene observations with temporary local dose deposition near the eyes or sensitive structures of the visual system. First results based on a pushbutton study with a total of 39 patients indicate that phosphenes are mainly stimulated by local dose deposition in the eye or substructures in the eye (presumably the retina). No phosphenes were observed in a number of cases, where the optic nerves were located partly in the treatment volume but the eyes were clearly outside the radiation field