Ninety percent of all children who develop retinoblastoma are the first one in their family to have eye cancer. When retinoblastoma is passed from parent to child, the child usually develops retinoblastoma in both eyes.
Although it is not exactly understood why retinoblastoma occurs, it is known that in all cases this cancer is caused by an abnormality in chromosome 13, in which a piece of the chromosome is nonfunctional or missing. (Chromosomes are the components of a cell that contain genetic information.)
Chromosome 13 is responsible for controlling retinal cell division. In children with retinoblastoma, retinal cell division continues unchecked, causing retinal tumors.
In 60 percent of retinoblastoma cases, the abnormality is only found in cells of the eye; in 40 percent of the cases, the abnormality is present in every cell of the body including the eye.
There are many options available for genetic testing of retinoblastoma. These tests can modify the risk or can predict whether or not a child in a family with bilateral retinoblastoma is at risk to develop the disease. Our genetic counselor, Katherine Beaverson, MS, CGC, is the world’s only genetic counselor dedicated to retinoblastoma.
At the present time, the genetic test is most accurate if the lab can study a specimen of tumor from the surgically removed, or enucleated, eye of the patient together with a blood sample. If the patient has the inherited genetic abnormality, other members of the family may be screened or observed for development of the disease.
The most common situation is where neither parent has the gene for retinoblastoma but has a child born with retinoblastoma. If the parents are genetically normal, the chance of another child having retinoblastoma is one in 15,000 to 20,000.
If a parent had unilateral retinoblastoma, 7 to 15 percent of their offspring will have retinoblastoma. When a parent with unilateral retinoblastoma has a child who develops retinoblastoma, that child will develop bilateral retinoblastoma 85 percent of the time.
Every time the unilateral retinoblastoma parent has another child, the chance of that child developing retinoblastoma is 7 to 15 percent.
If a parent has been treated for bilateral retinoblastoma, almost half (45 percent) of their children will develop retinoblastoma in their eyes. At birth, the child may have tumors in the eye or elsewhere in the body. Other children may not have tumors at birth but will begin to develop them by 28 months and can continue to form them for up to seven years.
Many of these children do not have the tumor present at birth. In our experience, if the child is going to develop retinoblastoma, he or she will begin to develop tumors in the eyes by 28 months and can continue to form them until the age of seven years.
The majority of children with retinoblastoma born to a parent with bilateral retinoblastoma will also have bilateral retinoblastoma, but about 15 percent will develop unilateral retinoblastoma.
The situation where neither parent has had retinoblastoma is the most common and the most difficult to explain. When this occurs, both parents are encouraged to have a dilated retinal exam.
One percent of the time, there is evidence of a cured or limited form of retinoblastoma in the eye of a parent who was never aware of having had retinoblastoma and was never treated for it. In this case, 45 percent of that parent’s children will develop retinoblastoma.
More confusing, and rarer, is the possibility that a parent has the gene for retinoblastoma but no evidence of retinoblastoma in their eyes. This is called the carrier state, meaning the parent carries the gene but not the disease. Again, 45 percent of their children will be affected with retinoblastoma.