A retinoblastoma is a dangerous cancer often occurring in very young children, under 2, which can cause blindness or the loss of the eye. The tumor is a diffuse, gluey mass spread all over the eye. It also spreads easily. As a result of this, direct biopsy is impossible, unlike with most other tumors. Instead, eye examination by ophthalmologists (eye specialists) and ultrasound imaging are needed to make a diagnosis.