Most kinds of cancer are named for the place where the cancer starts. Retinoblastoma starts in the retina, the very back portion of the eye. Retinoblastoma is the only common type of eye cancer in children. Rarely, children can have other kinds of eye cancer. The information here applies only to retinoblastoma and not to other kinds of eye cancer.

The eye consists of a sphere called the globe, which is filled with a gelatinous material called vitreous; a lens with an iris (the colored part of the eye that acts like a camera shutter) on the front; and the retina in the back. The lens and the iris focus incoming light on the retina. The retina is like the sensor array in the back of a digital camera.

The retina is connected to the brain (the “memory” in a digital camera) by the optic nerve. The pattern of light appearing on the retina is transmitted from the retina to the brain, allowing us to see. The eyes develop very early as babies grow in the womb. During the early stages of development, the eyes contain cells called retinoblasts that divide into new cells and fill the retina. Later on, the cells stop dividing and develop into mature retinal cells.

Rarely, something goes wrong during development of retinal cells. Instead of developing into cells specialized for detecting light, some retinoblasts continue to grow rapidly and out-of-control, and form a cancer known as retinoblastoma. The sequence of events that lead to the development of retinoblastoma is rather complex, but it is always started by an abnormality (mutation) in a gene called the retinoblastoma gene. About one-third of the time, this abnormality is congenital (present at birth) and it is present in all the cells of the body (and therefore in all the cells of both retinas), and two-thirds of the time the abnormality in the retinoblastoma gene develops on its own only in one cell in one eye without a known cause. In the first form of retinoblastoma (where the child is born with the mutation) there may not always be a family history of this cancer; in fact, only 25% of the children born with this abnormality have an affected parent, whereas 75% of them acquire this abnormality while in the womb. 

The 2 different types of abnormalities (all cells in the body or only one cell in the eye) lead to 2 different forms of retinoblastoma. In the first one, children born with the genetic defect develop more than one retinoblastoma, usually in both eyes (bilateral or multifocal retinoblastoma). Also, within the eyes there are often several small tumors (multifocal retinoblastoma). In the second form, children develop only one tumor (unilateral and unifocal retinoblastoma). These 2 forms are treated very differently.

If growth of these cells is not controlled by treatment, they can form a tumor that fills much of the globe (eyeball). The cells may break away from the retinal tumor and float through the vitreous to reach other parts of the eye, where they form more tumors. If these tumors block the channels that are important in circulation of fluid within the eye, the pressure inside the eye can rise, resulting in glaucoma. Glaucoma is one of the serious complications of retinoblastoma that can lead to pain and loss of vision in the affected eye.

Most retinoblastomas are found and treated before they have spread outside the globe. But retinoblastoma cells can occasionally spread to other parts of the body. The cells sometimes grow along the optic nerve to reach the brain. Retinoblastoma cells can also grow through the covering layers of the globe into the eye socket, eyelids, and nearby tissues. Once tissues outside the globe are affected, the cancer may then spread to lymph nodes (small bean-shaped collections of immune system cells) and to internal organs and to bones.