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 tumour is caused by an alteration (mutation) in both copies of the RB1 gene. In the case of non-hereditary retinoblastoma, also called sporadic retinoblastoma, the gene mutations occur sporadically in a single retinal cell. Sporadic retinoblastoma accounts for 50-60% of cases.

In hereditary retinoblastoma, the alteration in one copy of the RB1 gene is present in all cells of the body. The mutation to the other copy of the RB1 gene occurs sporadically. Only in about 10% of cases has a parent passed on a mutation in one of the patient’s copies of the RB1 gene. In the remaining cases, the mutation in the RB1 gene has been passed on by chance early in the child’s development (de novo mutation).