For over 30 years, patients with a particular form of ocular tumour have been treated at PSI by means of proton irradiation. The tiny particles hit their target with millimetre precision, without endangering other structures of the eye. The irradiation facility OPTIS, developed at the PSI Center for Proton Therapy of the PSI, is a success story, considering that for more than 90 percent of the patients treated to date, the eye could be saved.