Retinoblastoma

Definition

Retinoblastoma is a rare childhood cancer of the eye. It is curable if detected early, but often requires surgical removal of the eye.

Description

Retinoblastoma is a malignant tumor which usually appears in infants or young children. It occurs at a frequency of about one in every 15,000 births. In some cases, there is a family (familial) history of the disease.

Causes & symptoms

The genetic cause of retinoblastoma has been extensively studied. It is described as a "two-hit" process. Normally, individuals have two good copies of the retinoblastoma gene (RB-1) on chromosome 13. The disease develops in individuals in which mutation has occurred in both copies of RB-1.

It appears that about 40% of patients are born with a defective copy (first "hit"), inherited from one parent. The second copy is rendered defective by a separate mutation (second "hit") that occurs in the eye. Individuals with an inherited RB-1 defect have high likelihood of developing retinoblastoma in both eyes. For these individuals, diagnosis occurs at age one. These patients also have increased risk of developing other types of cancer.

The other 60% of patients inherit two normal copies of RB-1 and develop the disease only after each copy experiences an independent mutation. The likelihood of two independent "hits" is lower, and these individuals are less likely to develop retinoblastoma in both eyes. For these individuals, average age of diagnosis is 2.1 years.

Chances of developing retinoblastoma decline sharply after age five. For those individuals who have had retinoblastoma in one eye, there is some possibility of the disease appearing in the other eye at any age into adulthood.

In cases with a family history of retinoblastoma, the child inherits a defective chromosome 13 from one parent. The defective alternative (allele) behaves dominantly, in that the victim needs only to inherit one defective gene. The tumor arises, however, only after a second, spontaneous mutation occurs in one of the cells of the retina; therefore, a situation then exists in which both copies of chromosome 13 carry defective alleles.

In the majority of cases, spontaneous mutations appear to occur in both copies of chromosome 13.

The RB-1 gene carries the information for making a protein called pRB. This protein regulates cell division. When pRB is absent or defective due to defective copies of the gene, uncontrolled cell division occurs, and cancer results. pRB appears to be involved in many types of cancer besides retinoblastoma.

Diagnosis

Diagnosis is usually made in early childhood. A white reflection in the pupil of the eye is often the first sign of the disease. The presence of a tumor can be confirmed by an ophthalmologist directly examining the retina through the pupil.

Genetic testing may be recommended to see if a defective gene has been inherited.

Treatment

Treatment for retinoblastoma depends on the size and number of tumor locations (foci) in the eye, as well as whether the disease is found in one or both eyes.

When only one eye is involved, it is surgically removed (enucleated). If both eyes are involved, one eye can sometimes be saved by treating the tumor with radiation, photocoagulation (use of intense laser light to destroy cancer cells), or cryotherapy (use of intense cold to kill cancer cells). Chemotherapy is increasingly used as a follow-up to one of these treatments. However, some forms of radiation therapy have been shown to promote other cancers, especially of the bone.

Because many patients have a strong predisposition to this disease, frequent eye exams are recommended. Close monitoring is important. Even after successful treatment, retinoblastoma patients should receive frequent eye examinations in order to get the earliest possible warning if the disease recurs.

Prognosis

Individuals with familial tumors in only one eye have a high incidence (70%) of recurrence in the other eye. Some patients experience secondary tumors in other non-ocular tissues of the body.

In a small percentage of cases, retinoblastoma is fatal because it has already spread through the optic nerve to the brain. However, if diagnosis occurs early, when the tumor is restricted to the eye, 90% of patients can be cured.

Prevention

No preventative measures are possible for a genetic condition, such as retinoblastoma. When expectant parents are related to anyone who has had retinoblastoma, they should receive genetic counseling.

Key Terms

Cryotherapy

Cancer treatment in which the tumor is destroyed by exposure to intense cold.

Photocoagulation

Cancer treatment in which the tumor is destroyed by an intense beam of laser light.

Further Reading

For Your Information

Books

• Brunner, Sharon H. Perfect Vision: A Mother's Experience with Childhood Cancer. Fuquay-Varina, NC: Research Triangle Publishing, 1998.

  Periodicals

• Quesnel, Susan, and David Malkin. "Genetic Predisposition to Cancer and Familial Cancer Syndrome." In Pediatric Oncology 44 (August 1997): 791-795.

  Organizations

• Institute for Families with Blind Children. PO Box 54700, Mail Stop 111, Los Angeles, CA 90054-0700. (213) 669-4649.

Gale Encyclopedia of Medicine. Gale Research, 1999.