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Most cancers 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 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. But the
information in this document focuses on 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) 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 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. The normal Rb gene helps keep cells from
growing out of control. Depending on when and where the change in the
Rb gene occurs, 2 different types of retinoblastoma can result.
Congenital (hereditary) retinoblastoma
In about 1 out of 3 retinoblastomas, the abnormality in the Rb
gene is congenital (present at birth) and is in all the cells of the
body (known as a germline mutation). This includes all of the cells of
both retinas.
In most of these children, there is no family history of this
cancer. Only about 25% of the children born with this gene 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). Within the eyes
there are often several tumors (multifocal retinoblastoma).
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.
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 at a later age than the
congenital form.
Growth and spread of retinoblastoma
If retinoblastoma tumors are not treated, they can continue to
grow and may fill 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 (cancerous),
they rarely spread outside the eye. 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 (an instrument that helps
doctors to look inside the eye). As with retinoblastoma, the diagnosis
is usually made based on the appearance and location of the tumor
inside the eye. A biopsy (a procedure in which cells from the tumor are
removed and looked at under a microscope) to confirm the diagnosis is
almost never performed.
Treatment is almost always surgery to remove the eye.
Last Medical Review: 10/26/2009
Last Revised: 10/26/2009
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