Retinoblastoma is the most common ocular tumor in children under 6 years of age. It may affect one eye (unilateral retinoblastoma) or both eyes (bilateral retinoblastoma). It originates in the developing retina and the average age of diagnosis is 2.5 years for unilateral retinoblastoma and 1 year for bilateral retinoblastoma. Most cases (75 %) are unilateral, whereas the remaining 25 % are bilateral.

The retinoblastoma tumor originates in an alteration (mutation) in both copies of the RB1 gene. These mutations can be inherited or they can occur by chance. Only in about 10 % of cases has one of the parents passed on a mutation in one of the copies of the RB1 gene to the patient. In the remaining cases, the mutation of the RB1 gene has been passed on by chance in the early development of the child. These two types of retinoblastoma are called inherited retinoblastoma, and the mutation is present in all or almost all the cells of the body. In the case of non-inherited retinoblastoma, also known as sporadic retinoblastoma, the mutations of the gene do not happen sporadically in a single cell of the retina. 90 % of the cases are sporadic retinoblastoma.