Retinoblastoma

Genetic Basis — The RB1 Gene

Retinoblastoma is caused by biallelic loss of function of the RB1 tumour suppressor gene located on chromosome 13q14. Both alleles must be inactivated for tumour formation (Knudson two-hit hypothesis, 1971).

• Germline (heritable) retinoblastoma (~40%): One RB1 mutation is present in every cell from conception — inherited from an affected parent or arising as a new germline mutation. Only one additional somatic mutation (second hit) is needed in any retinal cell. Associated with bilateral, multifocal, and earlier-onset disease. Carriers have ~45% lifetime risk of retinoblastoma per eye
• Somatic (non-heritable) retinoblastoma (~60%): Both RB1 mutations occur independently in the same retinal cell after conception. Results in unilateral, unifocal disease. No risk to offspring
• RB1-negative retinoblastoma (~2%): Rare cases driven by MYCN amplification without RB1 mutation; typically unilateral, unifocal, rapidly growing, and occurring in younger infants

Knudson Two-Hit Hypothesis

Types of RB1 Mutations

• Point mutations: Nonsense, missense, or splice-site variants; most common
• Small insertions/deletions: Frameshift mutations causing premature truncation
• Large deletions: Including 13q14 deletion syndrome — associated with intellectual disability and dysmorphic features
• Promoter methylation: Epigenetic silencing of RB1 (somatic)
• Loss of heterozygosity (LOH): Second-hit mechanism; acquired deletion of the remaining wild-type allele

Role of the RB1 Protein (pRb)

The RB1 gene encodes pRb, a nuclear phosphoprotein that acts as a master regulator of the cell cycle. It functions primarily by binding and repressing the E2F family of transcription factors, which are required for S-phase entry and DNA replication.

• In normal cells, hypophosphorylated pRb sequesters E2F, keeping cells in G1 arrest
• Growth signals activate cyclin-CDK complexes that phosphorylate pRb, releasing E2F and allowing cell cycle progression
• Loss of both RB1 alleles eliminates this brake, allowing uncontrolled E2F activity and unchecked cell proliferation
• pRb also regulates apoptosis, differentiation, and genomic stability — loss promotes tumour progression beyond simple proliferation

Cell of Origin

Retinoblastoma originates from a population of developing retinal cone photoreceptor precursor cells. Evidence includes:

• Retinoblastoma cells express cone-specific markers (ARR3, RXRG, THRB)
• Human cone precursors are uniquely susceptible to RB1 loss due to their dependence on E2F-driven proliferation during development
• Mouse retinal cells are resistant to RB1 loss alone, explaining the lack of natural animal models without additional gene knockouts

Tumour Progression

• Endophytic growth: Tumour grows into the vitreous cavity; associated with vitreous seeding; high risk of spread
• Exophytic growth: Tumour grows into the subretinal space; associated with subretinal fluid and retinal detachment; seeds in subretinal space
• Mixed pattern: Both components present simultaneously
• Diffuse infiltrating: Rare flat growth pattern without discrete mass; older children; mimics uveitis; often misdiagnosed
• Extraocular extension: Via optic nerve to CNS, or via choroid/sclera to orbit; dramatically worsens prognosis
• Metastasis: Haematogenous spread to bone marrow, bone, CNS, and lymph nodes; rare in high-income settings with early treatment

International Classification of Retinoblastoma (ICRB) — Groups A–E

The ICRB (2005, updated 2006) is the current standard for intraocular retinoblastoma staging. It predicts globe salvage probability and guides treatment selection. Group E eyes are typically enucleated.

Laterality Classification

• Unilateral retinoblastoma (~60%): One eye affected; most commonly somatic; presents at a mean age of 24 months
• Bilateral retinoblastoma (~40%): Both eyes affected (synchronous or metachronous); almost always germline; presents at a mean age of 12 months; each eye staged independently
• Multifocal: Multiple tumour foci within one or both eyes; a feature of germline disease
• Trilateral retinoblastoma: Bilateral retinoblastoma + intracranial midline primitive neuroectodermal tumour (most commonly pinealoblastoma); associated with germline RB1 mutation; rare but rapidly fatal if undetected

Extraocular / Metastatic Staging (IICRB)

The International Intraocular Retinoblastoma Classification (IICRB) includes a staging system for extraocular spread used in high-risk pathology features and metastatic workup:

• pT1: Retina only (no high-risk pathology features)
• pT2: Intraocular — choroidal or optic nerve involvement
• pT3: Regional — optic nerve resection margin, scleral extension
• pT4: Extraocular — orbital or CNS extension
• M1: Distant metastasis (bone marrow, bone, CNS, lymph nodes)

Reese-Ellsworth Classification (Historical)

Genetic / Hereditary Risk Factors

• Family history of retinoblastoma: Child of an affected parent has ~45% risk of developing retinoblastoma; sibling of an affected child has ~6% risk (if parent unaffected)
• Germline RB1 mutation (confirmed): ~90% penetrance; mandates ophthalmological screening from birth
• New germline mutation: ~75% of bilateral cases arise from a new (de novo) germline mutation with no family history; clinical presentation cannot be distinguished from inherited cases
• 13q14 chromosomal deletion: Larger deletions may cause 13q deletion syndrome with intellectual disability and dysmorphic features in addition to retinoblastoma

Demographic Risk Factors

• Age: Peak incidence 1–2 years; rare after age 6; almost never in adults
• Sex: Equal incidence in males and females
• Race/ethnicity: No consistent predilection; incidence relatively uniform globally at ~1 in 15,000–20,000 live births
• Low- and middle-income country setting: Not a genetic risk factor, but associated with delayed diagnosis and worse outcomes due to limited screening access

Classic Presenting Signs

• Leucocoria (60–70% of cases): White pupillary reflex — most common presenting sign. Often first noticed by parents in flash photographs (the eye does not show the normal red reflex but appears white or pale). In clinical examination, the red reflex test (Brückner test) demonstrates absent or white reflex in the affected eye
• Strabismus (20–25%): Second most common presentation; caused by macular or perimacular tumour disrupting fixation; may be esotropia or exotropia; often the first sign detected at routine vision screening
• Rubeosis iridis (iris neovascularisation): New vessels on the iris surface; indicates advanced disease with significant ischaemia; may lead to neovascular glaucoma
• Hyphaema or hypopyon: Blood or white cells in the anterior chamber from neovascular or inflammatory response; can mimic uveitis and delay diagnosis
• Heterochromia iridis: Difference in iris colour from anterior segment involvement or rubeosis
• Buphthalmos / elevated IOP: Globe enlargement from neovascular or secondary glaucoma in advanced cases

Fundoscopic Signs

• White-cream retinal mass: Endophytic (projecting into vitreous) or exophytic (beneath retina); chalky-white surface; intrinsic vascularity
• Calcification: White chalk-like deposits within the tumour; visible on B-scan and CT; a hallmark feature distinguishing retinoblastoma from most differentials
• Vitreous seeding: Small tumour spheres or dust-like deposits floating in vitreous; an adverse feature for globe salvage
• Subretinal seeding: Tumour deposits in subretinal space; associated with exophytic tumours; a feature of ICRB Group C/D
• Retinal detachment: Exudative or tractional; may be total in advanced disease
• Dilated feeding vessels: Prominent retinal vessels supplying the tumour mass

Signs of Advanced / Extraocular Disease

• Proptosis / orbital cellulitis appearance: Orbital extension of tumour; most common presentation in low-income countries
• Preauricular or cervical lymphadenopathy: Regional metastasis
• Bone pain, pallor, hepatomegaly: Systemic metastasis (bone, marrow, liver)
• Phthisis bulbi: Shrunken, calcified end-stage globe

Caregiver-Observed Symptoms

• White glow in the eye in photographs: The most common parental observation leading to referral; visible particularly in flash photography when the red reflex is absent
• Crossed or wandering eye: Strabismus noticed by parents; often attributed initially to normal infantile development, causing diagnostic delay
• Difference in eye appearance: One eye larger or a different colour
• Poor visual behaviour / lack of tracking: In bilateral cases where both eyes are severely affected

Child-Reported Symptoms (Older or Verbal Children)

• Eye pain or redness: Occurs in advanced disease with secondary glaucoma or anterior segment involvement; not typical of early disease
• Blurred vision: May be reported if the tumour involves the macula or causes retinal detachment
• Floaters: From vitreous seeding in diffuse infiltrating retinoblastoma (rare pattern, older children)
• Headache: In trilateral retinoblastoma with intracranial involvement

Ocular Complications

• Neovascular glaucoma: From retinal ischaemia and rubeosis; causes pain, corneal oedema, and vision loss
• Total retinal detachment: From exophytic tumour growth; leads to blindness in the affected eye
• Vitreous haemorrhage: From tumour vasculature; causes media opacity and may obscure diagnosis
• Phthisis bulbi: End-stage shrunken globe from untreated or severely advanced disease
• Amblyopia: From strabismus, anisometropia, or occlusion by tumour in the visual axis; occurs in successfully treated eyes if not managed

Complications of Treatment

• After intra-arterial chemotherapy (IAC): Ophthalmic artery occlusion, periocular oedema, forehead necrosis (rare), eyelash loss, transient chorioretinal vascular changes, stroke risk (very rare)
• After intravitreal chemotherapy (IVC): Periocular pigment clumping, vitreous haemorrhage, retinal pigment epithelial changes, retinal detachment risk if technique not meticulous
• After systemic chemotherapy: Myelosuppression, infection, hearing loss (carboplatin-related), secondary leukaemia risk (germline patients)
• After laser photocoagulation/thermotherapy: Retinal scarring, choroidal ischaemia, focal visual field loss
• After cryotherapy: Transient anterior segment inflammation, lid oedema, retinal detachment risk
• After external beam radiotherapy (EBRT — now rarely used): Facial bone hypoplasia, cataract, radiation retinopathy, radiation optic neuropathy, significantly increased secondary malignancy risk in germline carriers
• After enucleation: Enophthalmos, socket contracture, need for ocular prosthesis; psychological impact

Life-Threatening Complications

• Orbital and CNS extension: Via optic nerve; directly worsens survival prognosis
• Haematogenous metastasis: Bone, bone marrow, liver, lung; associated with high mortality
• Secondary malignancies (germline RB1 carriers): Osteosarcoma, soft tissue sarcomas, melanoma, brain tumours; cumulative lifetime risk ~35% by age 50 without radiotherapy; EBRT dramatically amplifies this risk
• Trilateral retinoblastoma: Rapidly fatal pinealoblastoma or suprasellar PNET in ~5% of bilateral/germline cases

Germline RB1 — A Systemic Cancer Predisposition Syndrome

Germline RB1 mutation is not limited to the eye. The pRb protein is a ubiquitous cell cycle regulator, and carriers face elevated lifetime risks of multiple cancers beyond retinoblastoma:

• Osteosarcoma: Most common secondary malignancy; peak risk age 10–20 years; 500× relative risk compared to general population
• Soft tissue sarcomas: Leiomyosarcoma, rhabdomyosarcoma, and others
• Melanoma: Uveal and cutaneous melanoma at elevated rates
• Lung and bladder carcinomas: Increased risk in adulthood
• Brain tumours: Including malignant glioma
• Cumulative risk: ~35% lifetime risk of a second primary cancer without radiotherapy; rises to ~70% with EBRT exposure — external beam radiotherapy is now avoided in germline carriers whenever possible

Trilateral Retinoblastoma

13q Deletion Syndrome

Large chromosomal deletions at 13q14 may encompass genes beyond RB1, resulting in:

• Intellectual disability and developmental delay
• Dysmorphic features (microcephaly, low-set ears, high-arched palate)
• Thumb hypoplasia or other limb anomalies
• Requires coordinated paediatric, genetics, and ophthalmology care

Long-term Surveillance Implications

• Germline RB1 carriers require lifelong cancer surveillance (annual clinical review, imaging when symptomatic)
• Genetic counselling for family planning — 50% transmission risk to offspring
• Avoidance of carcinogens (tobacco, ionising radiation) strongly advised
• Psychosocial support for survivors and families — critical component of long-term care

Examination Under Anaesthesia (EUA)

• The gold standard for diagnosis and staging of intraocular retinoblastoma
• Binocular indirect ophthalmoscopy with a 28D lens and scleral indentation to examine the retinal periphery
• Allows complete fundus documentation, measurement of tumour size, and assessment of vitreous and subretinal seeding
• Wide-field retinal imaging (RetCam) under anaesthesia for photographic documentation and serial comparison
• Intraocular pressure measurement; anterior segment assessment for rubeosis or hyphaema

Red Reflex Test (Brückner Test)

• Performed at neonatal examination and routine well-child visits using a direct ophthalmoscope
• Abnormal: asymmetric, absent, white, or pale reflex — requires urgent paediatric ophthalmology referral
• Recommended by the American Academy of Pediatrics (AAP) and Singapore MOH child health guidelines as a routine screening test

Imaging

• Ocular ultrasound (B-scan): First-line imaging. Highly sensitive for detecting calcification (hyperechoic foci with posterior acoustic shadowing) — present in ~95% of retinoblastoma. Measures tumour dimensions. Detects retinal detachment
• MRI orbits and brain (without and with gadolinium): Essential for ruling out extraocular extension and trilateral retinoblastoma (pineal/suprasellar tumour). Preferred over CT in young children due to absence of ionising radiation. Does not reliably detect calcification
• CT scan: Rarely used now due to radiation exposure concerns in children who may carry germline RB1 mutations. Can detect calcification but inferior to MRI for soft tissue detail and CNS assessment
• Fluorescein angiography (FA): Used in selected cases to assess tumour vascularity and response to treatment; not part of routine diagnostic workup in young children

Genetic Testing

• Peripheral blood RB1 sequencing and deletion/duplication analysis: Identifies germline mutations; recommended for all retinoblastoma patients (particularly bilateral, multifocal, or family history cases)
• Tumour DNA testing (enucleated specimens): Identifies somatic mutations; can confirm non-hereditary status if both mutations are somatic
• Family cascade testing: Offered to first-degree relatives of germline mutation carriers
• Prenatal testing / preimplantation genetic diagnosis (PGD): Available for families with known germline mutations

Metastatic Workup (Selected Cases)

Indicated for Group D/E eyes, optic nerve involvement, or extraocular features:

• Bone marrow biopsy and aspirate
• Lumbar puncture for cerebrospinal fluid cytology
• Technetium bone scan or PET-CT
• Full blood count, liver function tests

Principles of Management

Management is guided by three priorities in order: (1) saving the child's life, (2) saving the eye, (3) preserving vision. Treatment is individualised based on ICRB group, laterality, germline status, and systemic extent.

1. Intra-Arterial Chemotherapy (IAC)

• Indication: First-line for Groups B, C, D; rescue therapy for Group D/E eyes failing systemic chemotherapy
• Route: Selective ophthalmic artery catheterisation via femoral artery under general anaesthesia (interventional radiology)
• Agents: Melphalan (primary), carboplatin, topotecan — delivered directly into the eye's blood supply at high local concentration
• Efficacy: Globe salvage rates of ~80% for Group D, ~60–70% for Group E (select cases)
• Cycles: Typically 3–6 monthly cycles; combined with focal consolidation (laser/cryo)
• Advantages over systemic chemo: Higher local drug concentration, lower systemic toxicity, no significant myelosuppression

2. Intravitreal Chemotherapy (IVC)

• Indication: Treatment of vitreous seeds — the most challenging aspect of globe salvage
• Agent: Melphalan 20–30 mcg; occasionally topotecan
• Technique: Direct intravitreal injection with strict antiseed precautions (cryotherapy at the injection site, no reflux, immediate pressure application) to prevent extraocular seeding
• Efficacy: Highly effective for vitreous seeds; complete seed regression in ~70–80% of cases
• Combined use: Often used alongside IAC for Group C/D eyes with significant vitreous seeding

3. Systemic Chemotherapy (Chemoreduction)

• Indication: Bilateral retinoblastoma (second eye preservation), trilateral disease, metastatic disease, high-risk pathological features post-enucleation
• Regimen: Vincristine, etoposide, carboplatin (VEC) — 6 cycles over 6 months
• Role: Reduces tumour size to allow focal consolidation; does not reliably eliminate vitreous seeds alone — replaced by IAC+IVC for advanced intraocular disease
• Adjuvant post-enucleation: High-risk features (massive choroidal invasion, optic nerve margin involvement, extraocular extension) require systemic chemotherapy ± orbital radiotherapy

4. Focal Consolidation Therapy

• Laser photocoagulation (transpupillary thermotherapy — TTT): For small tumours ≤3 mm; destroys residual active tumour after chemoreduction; targets feeding vessels
• Cryotherapy: For peripheral tumours ≤3 mm not amenable to laser; triple freeze-thaw cycle; destroys tumour via ice crystal formation
• Plaque brachytherapy (ruthenium-106 or iodine-125): For tumours 3–16 mm in thickness; radioactive plaque sutured to sclera overlying tumour for several days; avoids EBRT in germline carriers

5. Enucleation

• Indication: Group E eyes with no salvageable vision; eyes failing globe-sparing treatment; eyes with suspected extraocular extension; unilateral Group D/E in developed countries when IAC not preferred
• Technique: Long optic nerve section (>10 mm) to ensure clear margins; primary implant placement; custom ocular prosthesis fitting at 6 weeks
• Post-enucleation histopathology: Determines high-risk features guiding need for adjuvant systemic chemotherapy
• Psychological support: Essential for the child and family; prosthetic eye fitting by an ocularist; return to normal activities encouraged

6. External Beam Radiotherapy (EBRT) — Largely Obsolete

Management of ROP Sequelae (Optometry-Relevant)

• High myopia correction: Spectacles or contact lenses; early fitting important to prevent amblyopia
• Amblyopia treatment: Occlusion therapy for anisometropic or strabismic amblyopia in the surviving or better eye
• Low vision rehabilitation: For children with permanent visual impairment from treatment sequelae
• Prosthetic eye fit monitoring: Regular assessment of socket health and prosthesis fit in enucleated children; referral to ocularist for resizing as the child grows

Survival

Globe Salvage and Vision Preservation

• Groups A–B: Globe salvage >95%; useful vision preserved in majority
• Group C: ~75–85% globe salvage with IAC ± IVC
• Group D: ~50–70% globe salvage with aggressive IAC + IVC
• Group E: <15% globe salvage; enucleation is usually primary treatment
• Macular location is the primary determinant of visual acuity outcome even after successful globe salvage

Long-term Considerations

• Germline RB1 carriers face ~35% cumulative lifetime risk of secondary malignancy; highest in those who received EBRT
• Annual surveillance for secondary malignancy recommended — clinical examination, imaging guided by symptoms
• Children surviving retinoblastoma have largely normal quality of life, cognitive function, and educational outcomes with appropriate support
• Genetic counselling and family planning discussions should be offered at transition to adult care