A 5-year-old boy with a history of unilateral retinoblastoma (diagnosed at age 2) now presents with a new orbital mass. Which investigation is most appropriate to confirm the diagnosis and assess for bilateral disease?

Question

Accepted Answer

B. RB1 gene mutation analysis and bilateral retinal imaging (MRI/ultrasound). ## Investigation of Choice for Retinoblastoma Recurrence and Bilateral Disease

### Clinical Context
- Unilateral retinoblastoma diagnosed at age 2 (early onset suggests possible germline RB1 mutation)
- New orbital mass at age 5 (raises concern for bilateral disease or metastasis)
- Need to confirm diagnosis AND assess for contralateral involvement

**Key Point:** RB1 gene mutation analysis combined with bilateral retinal imaging is the gold standard for confirming hereditary retinoblastoma and detecting bilateral disease, which occurs in 20–30% of RB1 mutation carriers.

### Why This Combination?

| Investigation | Purpose | Yield in RB1 Carriers |
|---|---|---|
| **RB1 sequencing** | Identifies germline mutation; confirms hereditary form; enables cascade testing in siblings | ~90% mutation detection in familial cases |
| **MRI/Ultrasound (bilateral)** | Detects contralateral retinal tumors; assesses orbital extension; evaluates for metastasis | Detects bilateral disease in 20–30% of RB1 carriers |
| **LDH/Catecholamines** | Nonspecific markers of disease burden; not diagnostic for RB | Cannot distinguish RB from neuroblastoma |
| **Bone marrow/CSF** | Assesses for metastatic disease (late-stage workup) | Not first-line for diagnosis confirmation |
| **Fluorescein angiography** | Functional retinal imaging; does not detect structural lesions | Useful for monitoring but not diagnostic |

**High-Yield:** RB1 is the prototype tumor suppressor gene. Germline mutations confer:
- ~90% risk of retinoblastoma (often bilateral, multifocal)
- Increased risk of second malignancies (osteosarcoma, melanoma, lung cancer) in adulthood
- Autosomal dominant inheritance with high penetrance

**Clinical Pearl:** Early-onset unilateral RB (age <18 months) or any bilateral RB should raise suspicion for germline RB1 mutation. Genetic counseling and testing of parents/siblings is mandatory.

**Mnemonic: RB1-RETINA** — RB1 mutations require Retinal imaging (bilateral) to detect Early, Treatable Intraocular Neoplasia in At-risk carriers.

[cite:Robbins 10e Ch 7]

Answer

A. Bone marrow biopsy and cerebrospinal fluid cytology

Answer

C. Fluorescein angiography and electroretinography

Answer

D. Serum lactate dehydrogenase (LDH) and urinary catecholamine metabolites

A 5-year-old boy with a history of unilateral retinoblastoma (diagnosed at age 2) now presents with a new orbital mass. Which investigation is most appropriate to confirm the diagnosis and assess for bilateral disease?

Explanation

Investigation of Choice for Retinoblastoma Recurrence and Bilateral Disease

Clinical Context

Why This Combination?

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Investigation	Purpose	Yield in RB1 Carriers
RB1 sequencing	Identifies germline mutation; confirms hereditary form; enables cascade testing in siblings	~90% mutation detection in familial cases
MRI/Ultrasound (bilateral)	Detects contralateral retinal tumors; assesses orbital extension; evaluates for metastasis	Detects bilateral disease in 20–30% of RB1 carriers
LDH/Catecholamines	Nonspecific markers of disease burden; not diagnostic for RB	Cannot distinguish RB from neuroblastoma
Bone marrow/CSF	Assesses for metastatic disease (late-stage workup)	Not first-line for diagnosis confirmation
Fluorescein angiography	Functional retinal imaging; does not detect structural lesions	Useful for monitoring but not diagnostic