Retinoblastoma is a cancer that starts in the retina, the very back part of the eye. It is the most common type of eye cancer in children. Rarely, children can have other kinds of eye cancer, such as medulloepithelioma, which is described briefly below, or melanoma.hey usually cause eye pain and loss of vision. The diagnosis is made when a doctor finds a tumor mass in the eye by using an ophthalmoscope (an instrument that helps doctors to look inside the eye). Like retinoblastoma, the diagnosis is usually made based on where the tumor is inside the eye and how it looks.