Leukocoria and retinoblastoma: Viva

This is a branching oral built to probe the reasoning that holds the red reflex test and the urgent referral at the centre, the RB1 two-hit biology and the International Intraocular Retinoblastoma Classification, and to expose the candidate who has memorised the headline without the safety-critical corners. The questions escalate from the framing to the red reflex test, the molecular biology, the treatment pathway and the heritable disease, with deliberate probes into the trilateral retinoblastoma and the global disparity. [1]

Opening question: framing the problem

The examiner opens with the white glow in the photograph and asks: how do you frame this problem in a single sentence, and what is your first step? [1]

A strong answer names the leukocoria as retinoblastoma until proven otherwise, confirms the finding with the red reflex test, and states that the first step is the same-day referral to the ophthalmology service. [1][2]

Model answer. This child has leukocoria, the white pupillary reflex, and it is retinoblastoma until proven otherwise. The first step is to confirm the finding with the red reflex test in the dim room and to refer the child the same day to the ophthalmology service, because the delay of weeks can convert the curable intraocular tumour into the lethal extraocular disease. [1]

Probe one: the red reflex test

The examiner presses: describe the red reflex test, and tell me what is abnormal. [1]

A strong answer describes the technique and the interpretation. The test is performed in the dim room with the direct ophthalmoscope held close to the examiner's eye at thirty centimetres from the child, with the ophthalmoscope at the zero or a low dioptre, and the light is shone at both pupils simultaneously while the examiner compares the colour and the brightness of the reflex in the two eyes. The normal reflex is the warm orange-red, symmetric and filling the pupil. The abnormal reflex is the white, the yellow-white, the asymmetric or the absent, and any abnormality demands the urgent referral. The test is performed at the newborn examination, at the six-week check and at every well-child visit. [1][2]

Pitfall probe. What if the only finding is a new strabismus with a normal red reflex? Any new strabismus in a child, particularly under two years and particularly the unilateral, demands the fundus examination, because the tumour behind the fovea may destroy the central vision and cause the drift before the leukocoria appears, and the red reflex may be normal in the certain gaze. [1]

Probe two: the RB1 gene and the two-hit hypothesis

The examiner asks: where is the RB1 gene, and what is the two-hit hypothesis? [5]

A strong answer names the RB1 gene on the long arm of chromosome thirteen at the band thirteen-q-fourteen, encoding the pRB protein that binds the E2F and arrests the cell cycle. The Knudson two-hit hypothesis explains the heritable and the non-heritable forms: the heritable form carries one germline hit in every cell and needs only one further somatic hit, so it is bilateral, multifocal and early; the non-heritable form needs two somatic hits in a single cell, so it is unilateral, unifocal and later. The hypothesis predicted the tumour suppressor gene two decades before the gene was cloned. [5][7]

Pitfall probe. Can a child with the unilateral retinoblastoma carry the germline mutation? Yes, in roughly fifteen percent, and the genetic testing is therefore performed on every child, including the unilateral disease, because the germline mutation changes the surveillance and the offspring counselling. [7]

Probe three: the classification and the treatment

The examiner asks: describe the International Intraocular Retinoblastoma Classification and the treatment. [1]

A strong answer names the five groups and the risk-adapted treatment. Group A is the small tumour confined to the retina away from the disc and the foveola, treated with the focal therapy. Group B involves the macula or the disc, treated with the chemotherapy and the focal consolidation. Group C carries the focal seeds, group D the diffuse seeds, treated with the ophthalmic artery chemosurgery with the melphalan and the topotecan at approximately three to five milligrams of melphalan per eye per session. Group E is the unsalvageable eye, treated by the enucleation, and the histology guides the adjuvant chemotherapy for the high-risk features. [1][10]

Pitfall probe. What is the ophthalmic artery chemosurgery and why has it replaced the radiotherapy? It delivers the melphalan and the topotecan directly into the ophthalmic artery through the femoral catheter, achieving the high intraocular concentration with the low systemic exposure, and it has reduced the reliance on the external beam radiotherapy and its second-malignancy risk in the heritable disease. [10]

Branch one: the bilateral heritable disease

The examiner pivots: imagine instead a child with the bilateral disease and the affected father. What does this change? [7]

A strong answer names the heritable germline RB1 disease, passed with the near-complete penetrance and the fifty-percent offspring risk. The genetic counselling addresses the diagnosis, the offspring risk, the prenatal and the preimplantation testing, and the lifelong surveillance for the second malignancy, including the osteosarcoma, the soft-tissue sarcoma, the melanoma and the brain tumour. The surveillance includes the avoidance of the radiation, the sun protection and the prompt assessment of any new mass or bone pain. The at-risk infant of the affected parent is enrolled in the surveillance from the birth, with the examinations under anaesthesia that detect the tumour before the leukocoria. [7][2]

Pitfall probe. What is the leading cause of the death in the adult survivor of the heritable retinoblastoma? The second primary malignancy, because the mutated RB1 copy sits in every cell and the external beam radiotherapy increases the risk several-fold. [7]

Branch two: the trilateral retinoblastoma

The examiner pivots again: the bilateral child returns months later with the headache, the vomiting and the sixth-nerve palsy. What is this, and what is the prognosis? [8]

A strong answer names the trilateral retinoblastoma, the primitive neuroectodermal tumour of the pineal or the suprasellar region that complicates the bilateral heritable disease. The presentation is the raised intracranial pressure with the headache, the vomiting and the focal deficit, and it may appear months to years after the ocular diagnosis. The prognosis is poor, with the survival under twelve months despite the treatment, and the routine brain magnetic resonance imaging at the diagnosis and through the surveillance is the standard for the heritable disease. [8][9]

Closing question: the global disparity

The examiner closes: why does the outcome of retinoblastoma differ so much across the world, and what is the survival here? [2]

A strong answer describes the Global Retinoblastoma Study of the JAMA Oncology, which analysed over four thousand children worldwide and showed that the national income is the single greatest determinant of the outcome. The high-income setting presents with the leukocoria and the intraocular disease and carries the survival above ninety-five percent, while the low-income setting presents with the proptosis and the extraocular disease and carries the survival below forty percent, because the income determines the speed of the referral and the access to the specialist care. The fellow who links the red reflex test to the global equity demonstrates the reasoning the boards reward. [2][1]