Medical Oncologist
CION Cancer Clinics
One eye or both: what retinoblastoma says about the RB1 gene | CION Cancer Clinics
Whether retinoblastoma is in one eye or both says a lot about the RB1 gene. When both eyes are affected, the child almost always carries the fault in every cell, and it can be passed on. When one eye is affected, the fault is usually confined to that eye, but not always. This page explains what each pattern means for the child, their brothers and sisters, and their own future children. At CION Cancer Clinics, our oncologists explain what a gene result means for you and your family, and plan the checks that follow.
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Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed September 2026, next review due September 2027
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On this page
- Does it matter whether the cancer is in one eye or both?
- What does each pattern usually mean for the gene?
- How do doctors check whether a one-eye tumour is inherited?
- What do the words on an RB1 report mean?
- One eye and both eyes, compared
- What this page cannot tell you
- Four things parents tell us, and what is actually true
- Common questions about one-eye and two-eye retinoblastoma
The short answer
Does it matter whether the cancer is in one eye or both?
Yes, a great deal. When retinoblastoma is in both eyes, the child almost always carries a fault in the RB1 gene in every cell of the body. When it is in one eye, the fault is usually found only inside that eye. Usually is not always, which is why testing still matters.
Why both eyes points to an inherited fault
A tumour forms when both copies of RB1 stop working in one cell of the retina, the seeing layer at the back of the eye. A child born with one copy already broken in every cell needs only one more hit. That can happen in several places at once, in both eyes.
Why one eye usually does not
Without an inborn fault, both hits must happen by chance in the same cell. That is rare, so it tends to happen once, in one eye. But a minority of one-eye cases, often quoted as roughly one in seven, still turn out to be inherited.
The number of eyes is a strong clue. It is not a test result.Four patterns
What does each pattern usually mean for the gene?
Eye doctors look at more than one or two eyes. How many tumours there are, and who else in the family was affected, both count.
Both eyes
Treated as inherited, or heritable, until shown otherwise. The fault is in every cell, so it can pass to the child's own children later.
One eye, one tumour
The most common pattern. Most of these children have faults only in the tumour. Testing the tumour and the blood together is how you find out.
One eye, several tumours
Several separate tumours in one eye raise the chance of an inherited fault. Each one needed its own second hit, which is easier with the first already in place.
A parent or relative had it
Whatever the number of eyes, a family history makes an inherited fault likely. Some families carry milder faults that cause one-eye disease or none at all.
Ask about testing when
- A parent had retinoblastoma
- A parent lost an eye in early childhood
- A cousin or sibling was affected
Finding out properly
How do doctors check whether a one-eye tumour is inherited?
-
The tumour is tested if tissue is available
If the eye had to be removed, the laboratory looks inside the tumour for the two faults that switched off both copies of RB1.
-
The blood is checked for those exact faults
A blood sample is then searched for the same two changes. This is quicker and more reliable than searching the whole gene blind.
-
A fault in the blood means it is heritable
The child carries it in every cell. The other eye, the family and later health are then managed on that basis.
-
No fault in the blood is reassuring, not final
Occasionally the fault is present in only some cells of the body, called mosaicism. The risk to the child's own children is then much lower, but not zero.
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Without tissue, the blood is tested on its own
When the eye was saved, the whole RB1 gene is read from blood. A normal result lowers the chance of inheritance but cannot rule it out completely.
Not sure whether this applies to you?
Ask an oncologistOn the report
What do the words on an RB1 report mean?
- Unilateral and bilateral
- Unilateral means one eye is affected. Bilateral means both are.
- Unifocal and multifocal
- One tumour, or several separate tumours, inside the same eye.
- Germline
- Present in every cell from birth, so it can be passed on. The opposite is somatic, meaning found only in the tumour.
- Heritable retinoblastoma
- The form caused by a germline RB1 fault. It carries risks beyond the eyes.
- Mosaic
- The fault is in some cells of the body but not all, because it arose after conception.
- Trilateral retinoblastoma
- A related tumour in the pineal gland, a small gland in the brain. It is seen almost only in the heritable form.
Side by side
One eye and both eyes, compared
One eye (unilateral)
Both eyes (bilateral)
Usually not inherited
Almost always inherited
Usually found a little later in early childhood
Often found younger, sometimes in the first year
The other eye is watched until testing is clear
Both eyes are watched closely for new tumours
Brothers and sisters checked until results settle it
Brothers and sisters checked, and the parents tested
Later cancer risk rises only if the fault is inherited
A raised risk of some other cancers in adult life
Being straight with you
What this page cannot tell you
It cannot tell you whether your child's retinoblastoma is inherited. The pattern of eyes shifts the odds. Only testing, read by someone qualified, answers the question for your family.
It cannot read a report for you
RB1 faults come in many forms, and a few are milder than others. What your child's specific variant means is a question for the counsellor or eye cancer specialist who ordered the test. Please do not search the variant name online and draw conclusions.
Who this does not apply to
This page is about children who already have retinoblastoma, and their close relatives. Most families will never need an RB1 test. A squint or a white glow in one photograph has many ordinary causes, though it should be shown to an eye doctor promptly.
Families travelling from Telangana districts can ask for counselling in Telugu, and for results to be explained with both parents present.Commonly believed
Four things parents tell us, and what is actually true
"It is only in one eye, so it cannot be genetic."
Most one-eye cases are not inherited, but some are. The only way to know is to test, ideally the tumour and the blood together.
"Nobody in our family had eye cancer, so it is not in the genes."
Most children with the heritable form are the first in their family. The fault usually arose new, in an egg or sperm cell, and neither parent did anything to cause it.
"Once the eye is removed, the gene problem is over."
Removing the eye treats the tumour. If the fault is inherited, it remains in every other cell, so the other eye, later health and future children still need planning.
"Her brother is fine, so he does not need eye checks."
Early tumours cause no signs a parent can see. Until testing shows the fault did not pass to him, regular eye examinations are the safe course.
Questions we are asked
Common questions about one-eye and two-eye retinoblastoma
My child has retinoblastoma in one eye. Could the other eye get it?
If the fault is inherited, yes, a new tumour can appear in the other eye, mostly in the early years. That is why the other eye is examined regularly until genetic testing clarifies things. If the fault is found only in the tumour, the risk to the other eye is very low.
Does bilateral retinoblastoma always mean the gene is inherited?
Almost always. It means the child carries the fault in every cell, or in many of them. It does not mean a parent must carry it. Most such faults arose new in that child, which is why both parents are usually offered a blood test too.
What is the chance my child passes it on later?
If the child carries the fault in every cell, each of their future children has a one in two chance of inheriting it. If it is found only in the tumour, the chance is very small. Mosaic results sit in between, and a counsellor will explain what they mean for your family.
Why test the tumour if we are worried about the blood?
The tumour holds both faults. Once the laboratory knows exactly what they are, it can search the blood for those two changes very sensitively. This catches mosaic cases that a general blood test can miss, and it makes testing relatives far simpler.
The eye was saved with treatment. Can we still find out?
Yes. The RB1 gene can be read from a blood sample alone. A fault found in the blood settles the question. A normal result is reassuring but slightly less certain, so eye checks for brothers and sisters may continue a little longer.
Do the brothers and sisters need eye examinations?
Usually yes, until testing shows they did not inherit the fault. Young children are often examined under a short anaesthetic. Once a sibling's blood test comes back clear for the family's known fault, those examinations can normally stop.
Does the heritable form affect anything beyond the eyes?
It brings a raised lifetime risk of some other cancers, including bone and soft tissue cancers and melanoma. Avoiding unnecessary radiation, not smoking, and knowing which symptoms to report all help. A survivor's follow-up plan should say this plainly.
Will it matter when my child is older and getting married?
If the fault is inherited, it matters for their own children, and many families choose to discuss it before marriage. Options such as early testing of a baby, or testing during pregnancy, exist. A counsellor can talk this through with the young person when they are ready.
Your Specialists
Meet CION's oncologists. Bring your family history or genetic report to them.
Our medical oncologists see people with a strong family history of cancer, arrange genetic counselling and testing where it fits, and plan the checks that follow.
Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
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Sources
- GeneReviews (NCBI) — Retinoblastoma
- National Cancer Institute — Retinoblastoma Treatment (PDQ) – Patient Version
- MedlinePlus Genetics — Retinoblastoma
- MedlinePlus Genetics — RB1 gene
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
Testing the family for RB1Who is tested, in what order, once a fault is foundScreening siblings and children of a survivorHow eye checks for brothers, sisters and babies are plannedSecond cancer surveillance for survivorsWhat the heritable form means in adult lifeRB1 and family planningChoices for a survivor who wants children
Talk to us
Not sure whether your child's retinoblastoma is inherited?
Tell us which eye was affected and what testing has been done so far. We will explain what is still unanswered and arrange genetic counselling if it would help. One helpline serves every CION centre.