Most kinds of cancer are named for the part of the body where the cancer starts. Retinoblastoma starts in the retina, the very back part of the eye. Retinoblastoma is the only common type of eye cancer in children. Rarely, children can have other kinds of eye cancer, but the information in this document is only about retinoblastoma and not other kinds of eye cancer.

To understand retinoblastoma, it helps to know something about the normal structures of the eye and how they work.

About the eye

The main part of the eye is the eyeball (also known as the globe), which is filled with a jelly-like material called vitreous. The eyeball has a lens with an iris (the colored part of the eye that acts like a camera shutter) on the front, which allows light to enter the eye and focuses it on the retina.

The retina is the inner layer of cells in the back of the eye. It is made up of specialized nerve cells that are sensitive to light. These light-sensing cells are connected to the brain by the optic nerve. The pattern of light (image) appearing on the retina is sent through the optic nerve to an area of the brain called the visual cortex, allowing us to see.

How retinoblastoma develops

The eyes develop very early as babies grow in the womb. During the early stages of development, the eyes have cells called retinoblasts that divide into new cells and fill the retina. At a certain point, the cells stop dividing and develop into mature retinal cells.

Rarely, something goes wrong with this process. Instead of developing into special cells that detect light, some retinoblasts continue to grow rapidly and out-of-control, and form a cancer known as retinoblastoma.

The chain of events that lead to retinoblastoma is rather complex, but it is always started by an abnormality (mutation or change) in a gene called the retinoblastoma (Rb or RB1) gene. When and where this gene change occurs causes 2 different types of retinoblastoma.

Congenital (hereditary) retinoblastoma

In congenital (hereditary) retinoblastoma, the abnormality in the Rb gene is present at birth (congenital) and is present in all the cells of the body (this is known as a germline mutation). This includes all of the cells of both retinas. About 1 out of 3 cases of retinoblastoma are congenital retinoblastoma. In most cases, there is no family history of this cancer. Only about 25% of the children born with this abnormality inherit it from a parent, whereas in about 75% of cases the gene change first occurs during early development in the womb. (The reasons for this are not clear.)

Children born with a mutation in the Rb gene usually develop retinoblastoma in both eyes (bilateral retinoblastoma). There are often several tumors (multifocal retinoblastoma) within the eyes.

Because all of the cells in the body have the changed Rb gene, these children also have a higher risk of developing cancers elsewhere in the body.

• A small number of children with this form of retinoblastoma will develop another tumor in the brain, usually in the pineal gland at the base of the brain (a pineoblastoma). This is also known as trilateral retinoblastoma.

• For survivors of hereditary retinoblastoma, the risk of developing another cancer later in life may be as high as 40%. (For more information, see the section, "What happens after treatment for retinoblastoma?")

Sporadic (non-hereditary) retinoblastoma

In about 2 out of 3 cases of retinoblastoma, the abnormality in the Rb gene develops on its own in only one cell in one eye. It is not known what causes this change. A child who has sporadic (non-hereditary) retinoblastoma develops only one tumor in one eye. This type of retinoblastoma is often diagnosed when the child is older.

Growth and spread of retinoblastoma

If treatment does not control the growth of retinoblastoma cells, 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 circulate fluid within the eye, the pressure inside the eye can rise. This can cause glaucoma, one of the serious complications of retinoblastoma, which 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 and 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 other organs such as the liver, bones, and bone marrow.

Medulloepithelioma

Medulloepithelioma is another type of eye tumor. It is not a type of retinoblastoma, but it is mentioned here because it also usually occurs in young children. These tumors are very rare.

Although most medulloepitheliomas are malignant, they rarely metastasize (spread). They usually cause eye pain and decreased vision. The diagnosis is made when a doctor finds a tumor mass in the eye by using an ophthalmoscope (instrument that helps doctors to look inside the eye). A biopsy (a procedure in which cells from the tumor are removed and looked at under a microscope) is often done to confirm the diagnosis. Treatment is with surgery. The doctor normally tries to remove only the tumor. If that is not possible, then the eye is removed.