Eye cancer. Coloured axial magnetic resonance imaging (MRI) scans comparing a healthy eye (bottom) with one showing a malignant melanoma cancer (yellow) at the back of the eyeball. Below each eye, the optic nerve (orange) is seen running back to the brain. Melanoma is a cancer of melanocytes (pigment-producing cells). It is a highly malignant cancer, and can spread rapidly to other parts of the body (metastasis). In the eye, it can cause retinal detachment and visual disturbances. If it is diagnosed early, the eye can be removed, but the prognosis is poor if it has spread. MRI uses radio waves and a powerful magnetic field to obtain slice images through the body.