A 2-year-old boy presents with leukocoria (white pupillary reflex) in the right eye noticed by his mother 3 weeks ago. Dilated fundoscopy shows a white intraretinal mass with calcification. What is the investigation of choice to confirm the diagnosis of retinoblastoma and assess for bilateral involvement?

Explanation

## Investigation of Choice for Retinoblastoma Staging and Bilateral Assessment **Key Point:** MRI is the gold standard imaging modality for retinoblastoma because it provides superior soft tissue contrast, detects calcification (though less sensitive than CT), and is the preferred modality for assessing bilateral disease, optic nerve involvement, and extraocular extension without radiation exposure. **High-Yield:** In a child with suspected retinoblastoma, MRI of both orbits is the investigation of choice for: - Confirming diagnosis (shows tumor morphology and calcification) - Detecting bilateral involvement (essential for prognosis and treatment planning) - Assessing optic nerve head involvement - Evaluating extraocular extension - Avoiding ionizing radiation in a pediatric patient **Clinical Pearl:** While CT is more sensitive for detecting calcification (present in 90% of cases), MRI is preferred as the primary staging investigation because: 1. Bilateral disease is present in 25–30% of cases and changes management 2. MRI does not expose the child to radiation (critical in retinoblastoma, which carries increased risk of secondary malignancies) 3. Superior soft tissue contrast for assessing optic nerve and extraocular spread ### Comparison of Imaging Modalities in Retinoblastoma | Modality | Calcification Detection | Bilateral Assessment | Soft Tissue Contrast | Radiation | Role | | --- | --- | --- | --- | --- | --- | | **MRI** | Moderate (70–80%) | Excellent | Excellent | None | **Gold standard for staging** | | **CT** | Excellent (90%) | Good | Fair | Yes (high risk) | Adjunct; reserved for specific indications | | **Ultrasound B-scan** | Poor | Limited | Fair | None | Screening; not diagnostic | | **OCT** | Not applicable | Not applicable | High resolution (anterior) | None | Anterior segment only; not for diagnosis | **Mnemonic:** **MRI FIRST** for Retinoblastoma - **M** = Modality of choice (gold standard) - **R** = Radiation-free (pediatric safety) - **I** = Identifies bilateral disease - **F** = Full staging (optic nerve, extraocular) - **I** = Intraocular tumor assessment - **R** = Retinal and choroidal involvement - **S** = Soft tissue contrast superior - **T** = Treatment planning guide ## Why MRI Over Other Modalities While clinical diagnosis is often made on dilated fundoscopy (white intraretinal mass with calcification), imaging confirmation and staging require MRI because: 1. **Bilateral detection:** 25–30% have bilateral disease; missing this changes prognosis and treatment (enucleation vs. globe-salvaging therapy) 2. **Optic nerve involvement:** Critical for determining if chemotherapy or radiation is needed 3. **Radiation safety:** Retinoblastoma survivors have increased risk of secondary malignancies; avoiding CT in initial staging is essential 4. **Extraocular extension:** Determines if systemic chemotherapy is needed [cite:Boyd & Melia, Retinoblastoma (Pediatric Oncology); Shields & Shields, Intraocular Tumors]