Retinoblastoma is caused by mutations in a gene called RB1, which normally keeps cell growth in check. When both copies of this gene stop working inside a developing retinal cell, that cell loses its ability to control division and grows into a tumor. About 40% of cases are hereditary, meaning the child is born with one faulty copy already present in every cell, while the remaining 60% are sporadic, with both mutations occurring by chance in a single retinal cell after birth.
The RB1 Gene and How It Fails
RB1 is a tumor suppressor gene, meaning its job is to act as a brake on cell division. When functioning normally, it produces a protein that prevents cells from multiplying too quickly. Most retinoblastoma-causing mutations stop the gene from producing any functional protein at all. Without that brake, immature cells in the retina (the light-sensitive layer at the back of the eye) can divide unchecked and form a cancerous mass.
The critical detail is that humans carry two copies of RB1, one inherited from each parent. A single working copy is enough to keep cell growth under control. A tumor only forms when both copies are knocked out in the same cell. This is the foundation of what’s known as the “two-hit hypothesis,” first proposed by geneticist Alfred Knudson in 1971, and it explains why retinoblastoma takes two distinct forms.
Hereditary vs. Sporadic Forms
In the hereditary form, a child inherits one damaged copy of RB1 from a parent (or develops a new germline mutation very early in embryonic development). That first “hit” is present in every cell in the body from the start. All it takes is one additional mutation in a single retinal cell to disable the remaining working copy, and a tumor begins. Because every retinal cell already carries the first hit, the odds of a second hit occurring are relatively high, which is why hereditary retinoblastoma often affects both eyes and can produce multiple tumors.
In sporadic cases, a child is born with two perfectly normal copies of RB1. For a tumor to develop, both copies must be independently disabled by random mutations within the same retinal cell. The probability of this happening is far lower, which is why sporadic retinoblastoma almost always affects only one eye and produces a single tumor. These two spontaneous mutations typically occur during fetal development or the first months of life, while retinal cells are still actively dividing.
A Rare Cause Beyond RB1
Fewer than 2% of retinoblastomas develop without any defect in RB1 at all. In these cases, the driver is a different genetic event: high-level amplification of a gene called MYCN. Instead of losing a growth brake, these cells gain a massively overactive growth accelerator. The MYCN-driven form tends to appear in very young children, affects one eye, and carries aggressive features. It represents a newer discovery that challenges the traditional two-hit model, essentially showing that a single powerful “hit” to a growth-promoting gene can sometimes be enough on its own.
Why Children and Not Adults
Retinoblastoma is a childhood cancer because it arises from retinal cells that are still immature and rapidly dividing. Once retinal development is complete (typically by age 3 to 5), those cells stop proliferating and become much less vulnerable to cancerous transformation. Hereditary cases are usually diagnosed before age 2, while sporadic cases tend to appear slightly later, often between ages 2 and 5. The global incidence is roughly 6,275 new cases per year, based on 2021 estimates.
How Retinoblastoma Is Detected
The most common sign parents notice is a white glow in the pupil, called leukocoria. Normally, when light enters the eye, it reflects off the blood-rich tissue behind the retina and produces the familiar red-eye effect in flash photography. A tumor growing in the retina blocks that reflection, replacing the red glow with a white or yellowish one. Parents often first spot this in photographs or in dim lighting when the pupil is dilated. Other signs include a misaligned eye (strabismus) or, less commonly, redness and swelling.
Long-Term Risks of the Hereditary Form
Children who carry a germline RB1 mutation face risks beyond the eye. Because the faulty gene is in every cell, they have an elevated chance of developing other cancers later in life, particularly bone and soft tissue cancers during adolescence and early adulthood. A specific concern is trilateral retinoblastoma, where a tumor develops in a structure deep in the brain called the pineal gland. This occurs in 3 to 9% of children with the hereditary form and is more common in those with a family history than in children whose germline mutation arose spontaneously.
Survival Rates
When the tumor is confined to the eye, the prognosis is excellent. Five-year survival rates in the United States exceed 95% for intraocular retinoblastoma. If the cancer spreads outside the eye but not to the brain, survival ranges from 60 to 90% depending on how far it has traveled. Spread to the brain or spinal fluid drops survival to less than 20%, but this outcome is uncommon in countries with access to early screening and treatment.
Genetic Testing and Family Planning
When a child is diagnosed with retinoblastoma, genetic testing of the tumor and the child’s blood can determine whether the mutation is hereditary or sporadic. This distinction matters enormously for the family. If the child carries a germline RB1 mutation, each sibling and future offspring has a 50% chance of inheriting it. Siblings who test positive are monitored with regular eye exams under anesthesia, typically until age 5, to catch any tumors early enough to preserve vision.
If genetic testing confirms the mutation is limited to the tumor (no germline involvement), siblings and future children generally do not need ongoing surveillance. For families where a known RB1 mutation has been identified, options like preimplantation genetic testing during IVF or prenatal diagnosis are available to guide reproductive planning. Genetic counseling is strongly recommended for any family with an affected child, particularly before planning additional pregnancies.