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Retinoblastoma can be either a hereditary condition or a sporadic condition. Bilateral retinoblastoma (both eyes are affected) is most often a hereditary condition, while only about 15% of unilateral retinoblastoma (one eye affected) is a heritable form. In hereditary retinoblastoma the patient carries one copy of the cancer-predisposing mutation in the RB1 gene. 10% of the time the mutation exists in the family. This means the mutation is passed down by one of the parents. If a parent has the heritable condition there is a 50/50 chance for each child to inherit the RB1 mutation. If the gene is inherited, it is likely but not certain that the child will develop retinoblastoma. Everyone who inherits the mutation does not develop tumors. This is why some parents can unknowingly pass on the mutation. The other 30% of retinoblastoma that is hereditary is caused by a new mutation in that child.
Sporadic, or the nonheritable form of retinoblastoma, is caused by both RB1 alleles in a single cell in the eye being knocked-out by a mutation. These mutations occur very rarely, and the chances of it occurring more than once in a person are unlikely. Therefore, a patient with a sporadic condition will usually only have one tumor (unilateral) and the average age on onset is later than infants with an inherited mutation. However, patients with hereditary retinoblastoma often have multiple tumors because the mutation is present in every cell of the body. Some patients with a unilateral tumor do have the heritable form of retinoblastoma. Identifying those patients may change care or screening they or family members receive.
Infants with the heritable form of retinoblastoma have a higher risk of developing second cancers. The most common tumors are sarcomas (cancerous tumors which affect bones and soft tissue), brain tumors and melanoma. This risk may be further increased in patients who receive radiation therapy. Patients with a heritable RB1 mutation also have an increased risk of developing trilateral retinoblastoma, which is the occurrence of both retinoblastoma and a pinealoma (an intracranial tumor).
The best way to determine which form of retinoblastoma a child is affected with is to consult with your doctor and perhaps have genetic counseling. Genetic counselors will look at family history and determine if genetic testing is important in your case. Genetic counselors can also assist in prenatal screening and make recommendations for further screening of other family members based on risk. Molecular genetic testing is not 100% accurate, but it can sometimes determine predisposition of other family members. This can aid in early detection
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