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Cancer starts when cells begin to grow out of control. Cells in nearly any part of the body can become cancer, and can spread to other areas of the body. To learn more about how cancers start and spread, see What Is Cancer? For information about the differences between childhood cancers and adult cancers, see Cancer in Children.

Retinoblastoma is a cancer that starts in the retina, the very back part of the eye. It is the most common type of eye cancer in children. Rarely, children can have other kinds of eye cancer, such as medulloepithelioma, which is described briefly below, or melanoma.

To understand retinoblastoma, it helps to know about the parts 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 humor. The front of the eyeball has a clear lens with an iris (the colored part of the eye that acts like a camera shutter), 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 special nerve cells that are sensitive to light. These light-sensing cells are connected to the brain by the optic nerve, which runs out the back of the eyeball. The pattern of light (image) that reaches the retina is sent through the optic nerve to an area of the brain called the visual cortex, allowing us to see.

How does retinoblastoma develop?

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, these cells stop dividing and develop into mature retinal cells.

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

The chain of events inside cells that leads to retinoblastoma is complex, but it almost always starts with a change (mutation) in a gene called the retinoblastoma (RB1) gene. The normal RB1 gene helps keep cells from growing out of control, but the change in the gene stops it from working like it should. Depending on when and where the change in the RB1 gene occurs, 2 different types of retinoblastoma can result.

Congenital (hereditary) retinoblastoma

In about 1 out of 3 children with retinoblastoma, the abnormality in the RB1 gene is congenital (present at birth) and is in all the cells of the body, including all of the cells of both retinas. This is known as a germline mutation.

In most of these children, there is no family history of this cancer. Only about 25% of the children born with this gene change inherit it from a parent. In about 75% of children 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 RB1 gene usually develop retinoblastoma in both eyes (known as bilateral retinoblastoma), and there are often several tumors within the eye (known as multifocal retinoblastoma).

Because all of the cells in the body have the changed RB1 gene, these children also have a higher risk of developing cancers in other areas as well.

• 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 other cancers later in life is also higher than average. (For more information, see the section “What happens after treatment for retinoblastoma?”)

Sporadic (non-hereditary) retinoblastoma

In about 2 out of 3 children with retinoblastoma, the abnormality in the RB1 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 found at a later age than the hereditary form.

Children with this type of retinoblastoma do not have the same increased risk of other cancers as children with congenital retinoblastoma.

How does retinoblastoma grow and spread?

If retinoblastoma tumors are not treated, they can grow and fill much of the eyeball. Cells might break away from the main tumor on the retina and float through the vitreous to reach other parts of the eye, where they can form more tumors. If these tumors block the channels that let fluid circulate within the eye, the pressure inside the eye can rise. This can cause glaucoma, 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 eyeball. 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 eyeball and into the eye socket, eyelids, and nearby tissues. Once the cancer reaches tissues outside the eyeball, it can 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 (the soft, inner part of many bones).

Intraocular medulloepithelioma

Medulloepithelioma is another type of tumor that can start in the eye. 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.

Medulloepitheliomas start in the ciliary body, which is near the front of the eye (see image above). Most of these tumors are malignant (cancerous), but they rarely spread outside the eye. They usually cause eye pain and loss of vision.

The diagnosis is made when a doctor finds a tumor mass in the eye by using an ophthalmoscope (an instrument that helps doctors to look inside the eye). Like retinoblastoma, the diagnosis is usually made based on where the tumor is inside the eye and how it looks. A biopsy (removing cells from the tumor to be looked at under a microscope) to confirm the diagnosis is almost never done because it might harm the eye or risk spreading the cancer outside of the eye.

Treatment for medulloepithelioma is almost always surgery to remove the eye. This usually gets rid of all of the cancer, as long as it was still only in the eye.

The rest of this document refers only to retinoblastoma.