Retinoblastoma

01Pathophysiology

Retinoblastoma arises from immature retinal cells, most likely cone precursors, when both copies of the RB1 tumor suppressor gene are inactivated. RB1 normally encodes the pRB protein, which binds and sequesters the E2F transcription factor, holding the cell at the G1/S checkpoint. When pRB is lost, E2F is freed, the checkpoint collapses, and the precursor cell proliferates uncontrollably to form a retinal tumor.

The Knudson two-hit hypothesis is central to how this disease is tested. In the hereditary form (around 40% of cases), the child inherits one defective RB1 allele in every cell of the body. Only a single somatic "second hit" is then needed for tumor formation, which is why hereditary cases tend to be bilateral, multifocal, and present earlier (often under 12 months). In the sporadic form (around 60%), both hits must occur in the same retinal cell after birth, so the tumor is typically unilateral, unifocal, and presents later (around 24 months).

As the tumor grows, it disrupts the normal retina and replaces the choroidal pigment behind the pupil with a chalky white mass. Because light entering the eye is reflected off this pale tumor instead of the vascular retina, the red reflex is replaced by a white pupillary reflex (leukocoria). If the tumor distorts central vision, the affected eye loses fixation and drifts, producing strabismus, which is the second most common presenting sign. Advanced tumors can seed the vitreous, invade the optic nerve (a route to the central nervous system), or extend through the sclera into the orbit.

A clinically important point is that children with the germline RB1 mutation carry a lifelong risk of second primary malignancies, most notably osteosarcoma, soft tissue sarcomas, and melanoma, because every cell already harbors one inactivated allele.

02Classification and Clinical Manifestation

The most widely tested staging scheme for intraocular disease is the International Classification of Retinoblastoma (ICRB), which guides eye salvage decisions.

For extraocular spread, the International Retinoblastoma Staging System (IRSS) is used (Stage 0 through IV), with Stage IV indicating distant metastasis.

Clinical manifestations by pattern:

03Diagnostic Workup

The workup begins with the simplest tool available to any clinician, which is the red reflex test. An abnormal or asymmetric red reflex in a child should never be dismissed as a camera artifact. Once leukocoria or strabismus raises suspicion, the child is referred urgently to pediatric ophthalmology for an examination under anesthesia (EUA) with dilated indirect ophthalmoscopy. This is the best initial diagnostic test because it allows full visualization of the retina, mapping of all tumor foci, and assessment of vitreous seeding.

B-scan ultrasound is performed at the same time. The presence of an intraocular mass with calcifications in a child under 3 years old is essentially diagnostic of retinoblastoma, and this is the imaging finding most often tested. MRI of the orbits and brain with contrast is then obtained to evaluate optic nerve and extraocular involvement and to screen for trilateral retinoblastoma, the association of bilateral disease with a midline intracranial neuroblastic tumor. CT is generally avoided because radiation can induce second tumors in children with germline RB1 mutations.

A test the exam loves to penalize students for ordering is biopsy. Transscleral biopsy is contraindicated because it can seed tumor cells into the orbit and convert a curable intraocular disease into a metastatic one. Diagnosis is therefore clinical and radiologic.

Once the diagnosis is established, RB1 genetic testing is offered to the patient and first-degree relatives, and bilateral or multifocal disease is treated as hereditary even if no mutation is identified.

04Management and Treatment

Management is individualized according to laterality, tumor group, vision potential, and germline status, and is always coordinated by an experienced pediatric oncology and ocular oncology team.

For early intraocular disease (Groups A and B), vision-preserving focal therapy is the cornerstone. Laser photocoagulation and cryotherapy are used for small posterior and peripheral tumors, respectively, often after a few cycles of systemic chemoreduction with VEC (vincristine, etoposide, and carboplatin) to shrink the tumor first.

For advanced intraocular disease (Groups C and D), the modern standard in many centers is intra-arterial chemotherapy, in which melphalan is infused directly into the ophthalmic artery, sometimes combined with topotecan or carboplatin. Intravitreal melphalan or topotecan is added when vitreous seeds are present, since systemic agents penetrate the vitreous poorly.

For Group E disease, an eye filled with tumor, neovascular glaucoma, hyphema, or no salvageable vision, primary enucleation is the treatment of choice. The surgeon harvests as long an optic nerve stump as possible (around 10 to 15 mm) so the pathologist can assess invasion. If pathology reveals post-laminar optic nerve invasion, massive choroidal invasion, or anterior chamber involvement, adjuvant systemic chemotherapy is given to reduce metastatic risk.

The "next best step" logic that examinations reward is straightforward. A toddler with leukocoria on photographs goes for urgent ophthalmology referral and EUA, not biopsy. A child with confirmed retinoblastoma needs MRI of the brain and orbits before any therapeutic decision, both for staging and to screen for trilateral disease. A child with a Group E eye and no vision potential should proceed to enucleation rather than indefinite chemotherapy attempts.

Contraindications and cautions: external beam radiotherapy is avoided whenever possible in children with germline RB1 mutations because it dramatically increases the risk of radiation-induced sarcomas in the field. CT imaging is similarly minimized for the same reason. Live vaccines should be deferred during active chemotherapy.

Long-term management centers on lifelong surveillance. Survivors of hereditary retinoblastoma have a substantial cumulative risk of second primary malignancies, most classically osteosarcoma of the femur in adolescence, as well as soft tissue sarcomas and cutaneous melanoma. Genetic counseling is offered to the family, and siblings undergo serial fundoscopy in infancy.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The classic trap is the parent who brings a photograph showing one eye glowing white while the other glows red, and the clinician reassures them that it is a camera flash artifact. Asymmetric or absent red reflex in a child is retinoblastoma until proven otherwise, and the correct next step is urgent referral for examination under anesthesia, not observation. A second common mistake is choosing biopsy to confirm the diagnosis. Transscleral biopsy seeds tumor and is contraindicated; the diagnosis is made by fundoscopy plus B-scan ultrasound showing a calcified intraocular mass, with MRI for staging. A third pitfall is forgetting that bilateral or multifocal disease is hereditary by definition, even when family history is negative, which means lifelong surveillance for second malignancies (especially osteosarcoma) and genetic counseling for the family. A fourth pitfall is reflexively ordering a CT scan for calcifications; in a child with a possible germline RB1 mutation, this needlessly raises the lifetime risk of radiation-induced tumors, so MRI is preferred. Finally, do not overlook trilateral retinoblastoma, the association of bilateral retinoblastoma with a midline intracranial neuroblastic tumor, usually pineal, which is why every child with bilateral disease needs brain MRI at diagnosis.

The core competency being tested is recognizing leukocoria as a red flag in a young child, ordering the right sequence of noninvasive investigations (red reflex, then EUA with B-scan, then MRI), avoiding harmful steps (biopsy, CT, external beam radiation when avoidable), and understanding the genetic logic that links hereditary retinoblastoma to bilateral disease, early onset, and lifelong cancer risk.