Retinoblastoma: A malignant eye tumor in children, usually under age 5, that arises in cells in the developing retina containing cancer-predisposing mutations in both copies of the gene RB1. The most common sign of retinoblastoma (RB) is a white pupillary reflex to light (leukocoria). Strabismus (a "lazy eye") is the second most common sign.

There are two forms of RB. One is hereditary while the other is sporadic:

• The inherited form of RB is usually present at birth as multiple tumors (multifocal) in both eyes. It is due to the transmission of an RB1 (germline) mutation followed by an acquired (somatic) RB1 mutation.

• The sporadic form of retinoblastoma has later onset and typically leads to a single tumor in only one eye. It is due to acquired (somatic) mutations in both RB1 genes.

Patients with hereditary RB are also at increased risk of developing tumors outside the eye, including pinealomas (in the pineal gland of the brain), osteosarcomas, soft tissue sarcomas, and melanomas. These RB-related tumors usually are diagnosed in adolescence or adulthood.

When RB is detected at an early stage, it can sometimes be treated locally but often requires removal of the eye (enucleation). Early diagnosis and treatment of RB and RB-related tumors can reduce morbidity and increase longevity.