Retinoblastoma is diagnosed primarily through a clinical eye examination, not a biopsy. Because this cancer develops in the retina of young children, doctors rely on a combination of physical examination under anesthesia, imaging studies, and genetic testing to confirm the diagnosis and plan treatment. Biopsy is contraindicated because piercing the eye risks spreading tumor cells outside it.
First Signs Parents and Doctors Notice
The most recognizable sign of retinoblastoma is leukocoria, a white or yellowish glow in the pupil that often shows up in flash photographs. Across studies, leukocoria is the presenting symptom in roughly 50 to 98 percent of cases. It appears when light reflects off the surface of the tumor instead of the retina’s normal blood-rich tissue. Strabismus, where one eye turns inward or outward, is the second most common sign, reported in about 5 to 26 percent of cases.
The average time between a parent first noticing something unusual and receiving a clinical diagnosis is about 3.4 months, though many children are diagnosed within the first month. Longer delays matter: children whose diagnosis took more than five months were significantly more likely to have tumors that had spread beyond the eye. When the tumor remained inside the eye, the average delay was about 2.9 months. When it had already grown outside the eye at diagnosis, the average delay stretched to 8.7 months.
Examination Under Anesthesia
The definitive clinical evaluation is an examination under anesthesia, or EUA. Young children cannot hold still or cooperate with a thorough retinal exam while awake, so general anesthesia is used. The ophthalmologist fully dilates the pupil, then uses indirect ophthalmoscopy and scleral indentation (gently pressing on the outer wall of the eye) to inspect the entire retina from front to back.
During this exam, the doctor maps out the number, location, and size of any tumors. They also document whether the retina has detached, whether fluid has collected beneath it, and whether tiny clusters of cancer cells (called “seeds”) are floating in the vitreous gel or sitting under the retina. These details determine the tumor’s classification and directly shape the treatment plan. Children who are being monitored for retinoblastoma typically undergo repeated EUAs at regular intervals.
Imaging: Ultrasound and MRI
Two imaging tools play central roles in diagnosis. Ultrasound of the eye is fast, noninvasive, and especially good at detecting calcifications inside a tumor. Calcifications appear in about 95 percent of retinoblastoma cases and are one of the strongest clues that a mass in a child’s eye is retinoblastoma rather than something else. On ultrasound, these show up as bright, highly reflective spots within the tumor.
MRI provides a more detailed picture. It shows the full extent of the tumor, whether the retina is detached, and critically, whether cancer has invaded the optic nerve or spread beyond the eye into the orbit or brain. On MRI, retinoblastoma tumors typically appear bright on certain sequences and dark on others, a pattern that helps distinguish them from other conditions. Doctors optimize the MRI by using small surface coils placed near the eye and contrast-enhanced sequences with fat suppression to get the clearest possible view of the retinal tissue and any areas of tumor growth.
CT scans can also detect calcifications but are generally avoided in children with suspected retinoblastoma. The radiation exposure from CT may increase the risk of secondary cancers, particularly in children who carry a genetic predisposition to tumor development.
Ruling Out Conditions That Mimic Retinoblastoma
About 16 to 22 percent of children referred with suspected retinoblastoma turn out to have a different condition. The two most common mimics are Coats’ disease (responsible for about 40 percent of these lookalike cases) and persistent fetal vasculature, or PFV (about 26 percent). Both can cause leukocoria in young children, which is why imaging is so important.
Several MRI features reliably separate retinoblastoma from its mimics. Retinoblastoma eyes tend to be larger than normal, and the retinal detachment typically forms a sharp V-shape. Floating tumor seeds in the vitreous are almost exclusively a retinoblastoma finding, with a specificity of 96 percent. In contrast, conditions like PFV and Coats’ disease tend to produce a smaller eye, Y-shaped retinal detachment, deformities of the lens or the structure behind it, and no calcifications. A visible stalk running from the optic disc to the lens is a hallmark of PFV, while fluid-filled cysts within the retina point toward Coats’ disease. The absence of calcifications on ultrasound is 100 percent specific for pseudoretinoblastoma in these studies, meaning if there are no calcifications, the diagnosis is very unlikely to be retinoblastoma.
Why Biopsy Is Not Used
Unlike most cancers, retinoblastoma is never biopsied before treatment. Inserting a needle into the eye to sample tumor tissue carries a real risk of seeding cancer cells into the orbit or bloodstream. The diagnosis is made entirely through clinical examination and imaging. The only time tumor tissue is examined under a microscope is after an eye has been surgically removed (enucleated), which happens in advanced cases where the eye cannot be saved.
When pathologists examine enucleated eyes, they look for distinctive cellular structures called Flexner-Wintersteiner rosettes: rings of tumor cells arranged around a central open space. These rosettes represent the tumor’s attempt at forming retinal tissue and are found almost exclusively in retinoblastoma. A second type, Homer Wright rosettes, can also appear. The presence of these structures confirms the diagnosis and helps pathologists assess how differentiated (or organized) the tumor cells are.
Genetic Testing for RB1 Mutations
Retinoblastoma is caused by mutations in the RB1 gene, and genetic testing is a critical part of the diagnostic workup. About 40 percent of retinoblastoma cases involve a germline mutation, meaning the genetic change is present in every cell of the body and was either inherited from a parent or occurred very early in development. The remaining 60 percent are somatic, meaning the mutation happened only in retinal cells and is not heritable.
Testing uses blood samples to look for germline mutations and, when available, tumor tissue from enucleated eyes. Labs use next-generation sequencing to identify small mutations, deletions, duplications, and larger structural changes in the RB1 gene. These methods can detect mosaic mutations, where only a fraction of cells carry the change, at levels as low as 2.5 percent of cells in tumor samples. A rapid screening panel can also check for eleven of the most commonly recurring RB1 mutations and can detect mosaicism at levels as low as 1 percent.
Identifying a germline mutation changes the clinical picture significantly. It means the child is at risk for tumors in the other eye, for new tumors developing over time, and for certain other cancers later in life. It also means siblings and future children may need screening.
Screening for At-Risk Children
Children with a family history of retinoblastoma or a known RB1 mutation in the family undergo serial dilated eye exams starting in infancy, ideally performed by an ophthalmologist experienced in retinoblastoma. The frequency depends on the child’s estimated risk level. High-risk children, such as those who have inherited a confirmed RB1 mutation, require more frequent exams, sometimes under anesthesia to ensure a complete view of the retina.
Screening does not stop in early childhood. RB1 mutation carriers are recommended to continue eye exams every one to two years after age 7, because the genetic predisposition persists even after the typical window for retinoblastoma has passed.
How Tumors Are Classified After Diagnosis
Once retinoblastoma is confirmed, the tumor is classified using the International Intraocular Retinoblastoma Classification, which assigns a grade from Group A (very low risk) through Group E (very high risk). This system guides treatment decisions.
- Group A: All tumors are 3 mm or smaller, confined to the retina, and located away from the most sensitive visual structures. No seeding is present.
- Group B: Tumors can be any size or location but remain confined to the retina with no seeding. A small amount of fluid beneath the retina (extending no more than 5 mm from the tumor base) is permitted.
- Group C: Tumors of any size with localized seeding, meaning small clusters of cancer cells near the tumor in the vitreous or under the retina. Up to one quadrant of the retina may be detached.
- Group D: Diffuse or widespread seeding, large tumors growing into or out of the retina, or retinal detachment spanning more than one quadrant.
- Group E: The eye is essentially destroyed by tumor, with features like uncontrollable pressure buildup in the eye, massive bleeding inside the eye, or tumor extending forward to touch the lens.
Groups A and B are typically treated with focal therapies that preserve the eye and vision. Groups C and D often require more intensive treatment, including chemotherapy delivered directly into the eye’s blood supply. Group E eyes usually cannot be salvaged and are removed surgically to prevent the cancer from spreading.