Eye cancer occurs when certain factors cause the cells in the eye to grow out of control and become malignant. For instance, a weakened immune system increases a person’s risk for developing a cancer called primary lymphoma of the eye. Another eye cancer called retinoblastoma, which typically affects children, is caused by a genetic mutation that can sometimes be inherited from a parent. Unlike normal cells, which divide and grow in an organized manner, cancer cells continue to divide, until they form a growth or tumor (a mass of excess tissue). In some cases, cancer cells in the eye may become invasive, spreading to tissues outside of the eye (metastasis). Any form of cancer in the eye can affect its function and the patient’s ability to see.

The eye is an unusual location for cancer to develop. The structure of the eye and its specialized tissues make it more likely for cancer to develop in only a few parts of the eye. The eye and the surrounding tissues are composed of three main parts:

• Eye (globe).  The center of the eye is filled with vitreous (a jelly-like substance). The back portion of the eye is lined by the retina, the light sensitive nerve tissue that transmits images to the optic nerve and brain. The uvea is a layer of pigmented tissues that contains three parts:
  
  
• The iris, the colored part surrounding the pupil that regulates the light entering the pupil.
  
  
• The choroid, which lines the eye under the retina and delivers blood to the retina and the front of the eye.
  
  
• The ciliary body, which allows the lens to focus on near and distant objects and produces the aqueous fluid that bathes the front of the eye.
  

Cancers that affect the eye are called intraocular cancers and most commonly affect the retina or the uvea.

• Orbit. Contains the tissues surrounding the eye. These tissues include the nerves attached to the eye as well as the muscles that control the movement of the eyeball. Cancers of the orbit are called orbital cancers.
  
  
• Adnexal (accessory) structures. Include the eyelids and tear glands. Cancers that begin in these tissues are known as adnexal cancers.
  

Cancers of the adnexa and orbit develop from nerve, muscle and skin tissues, just like their counterparts in other parts of the body. For example, a cancer of the eyelid is typically a form of skin cancer. This guide focuses on intraocular cancers, which are unique to the eye.

Intraocular cancers can originate inside the eyeball (primary eye cancers) or spread (metastasize) to the eyeball from other locations in the body (secondary eye cancers). The most common secondary cancers that invade the eye are breast and lung cancers. These cancers frequently spread to the uvea.

Secondary cancers are more common than primary eye cancers, which are considered rare. The American Cancer Society (ACS) estimates that 2,340 new cases of primary or intraocular eye cancer will be diagnosed in 2007. Although primary eye cancers can appear at any age, most cases occur in children under age 6 or adults over age 50. The ACS estimates that 220 Americans will die from eye cancer during 2007.