Health Talk: Retinoblastoma
Retinoblastoma is a type of cancer that develops in the retina — the lining of the eye that senses light as it comes through the front of the eye. Though it is a rare form of cancer, it is the most common form of ocular cancer (cancer affecting the eye) in children, and according to the National Cancer Institute, about 300 children per year in the United States are diagnosed with this form. About 40 percent of retinoblastoma cases are inherited, and this form of cancer was actually the first to be associated with a genetic abnormality.
It is also possible, however, for retinoblastoma to occur sporadically, or without a family history. If a genetic mutation is found, the chance of having a second child with retinoblastoma is 40 to 50 percent, while if no genetic mutation is present, the chance drops to 2 to 5 percent. Retinoblastoma is rarely found in adults.
Retinoblasts are rapidly growing cells that mature to form the retina. However, sometimes these cells do not stop growing when they should, and instead keep reproducing; these excess cells then form a tumor in the retina. These tumors can continue to grow — some tumors almost completely fill the vitreous humor, the jelly-like substance that fills the eyeball, and others may break off and spread to other parts of the eye, eventually traveling outside the eye to the lymph nodes and other parts of the body.
Retinoblastoma has been linked to mutations in a gene called RB1. RB1 is a tumor suppressor gene that normally prevents cells from undergoing uncontrolled division.Mutations in RB1 give rise to the production of a faulty protein that is unable to regulate the division of cells. The cells thus rapidly multipy, forming tumors.
Retinoblastoma can occur in either one or both eyes. A common symptom is leukocoria, or white pupil. This occurs because of the tumor that is present in the retina — the pupil is not actually white, but when a light is shown into the eye, it appears to be white. Another common symptom is that the eyes will appear to be looking in different directions, along with eye redness, eye swelling, and a constant dilation of the pupil.
There are a variety of tests and treatments available for retinoblastoma; the treatments used differ depending on the location and size of the tumor. Chemotherapy is often used to shrink the size of the tumor on the retina so that another type of treatment can be used to completely remove the tumor. In addition, chemotherapy can be used when the tumor has spread from the retina to other parts of the body.
There are two types of radiation therapy that are used to treat retinoblastoma as well — internal radiation, or brachytherapy, and external beam radiation. In any type of radiation therapy, high energy beams, such as X-rays, are used to kill the cancerous cells. In internal radiation, a small disk made of radioactive material is stitched into the eye near the site of the tumor and left for a few days while the radiation is released from the disk. This allows a higher concentration of radiation to be used and reduces the chance that the radioactive beams will damage healthy cells. In external beam radiation, a machine outside the body delivers the high-powered light beams; however, this type of treatment can have side effects if the radiation reaches the brain, and thus this treatment is reserved for patients with advanced forms of retinoblastoma.
Additional treatments include laser therapy, or laser photocoagulation, where a laser is used to destroy the blood vessels that supply oxygen and nutrients to the tumor — without a fuel source, the cancerous cells die. Cryotherapy, or cold therapy, uses extreme cold to repeatedly freeze and thaw the cancerous cells, which causes them to die. Finally, in hyperthermia, or heat therapy, extreme heat is directed at the cancerous cells through the use of ultrasound, microwaves, or lasers. If the cancer has not spread beyond the eye, survival rates are very high. If it has spread, survival rates depend on how far the tumor has spread.
However, in very severe cases, the affected eye must be removed. When this occurs, a ball made of plastics and other biocompatible materials is placed in the empty eye socket, and the muscles that control the movement of the eye are attached to this implant. Although the implant cannot see, the eye muscles adapt to the implant and control its movement just as they controlled the movement of the natural eye. A few months after the surgery, an artificial eye can be placed over the implant, custom-made to match the healthy eye. The artificial eye sits behind the eyelid and attaches to the implant, so that as the eye muscles move the implant, it appears as though they are moving the artificial eye.