A 3-year-old girl with unilateral retinoblastoma is being evaluated for metastatic disease before initiating systemic chemotherapy. Which investigation is most appropriate to detect bone marrow involvement and assess for metastatic disease?

Explanation

## Investigation of Choice for Metastatic Retinoblastoma Staging **Key Point:** Bone marrow aspiration and biopsy is the most appropriate investigation to detect bone marrow involvement in retinoblastoma, as hematogenous spread to bone marrow is a critical site of metastatic disease and determines prognosis and treatment intensity. **High-Yield:** Metastatic workup for retinoblastoma includes: 1. **Bone marrow aspiration and biopsy** — detects marrow involvement (most common site of metastatic disease) 2. **Lumbar puncture with CSF cytology** — detects leptomeningeal involvement 3. **Imaging** (chest X-ray, abdominal ultrasound, or CT) — detects visceral and nodal metastases 4. **Serum and urine catecholamine metabolites** — rules out neuroblastoma (differential diagnosis) **Clinical Pearl:** Retinoblastoma can metastasize to: - Bone marrow (most common hematogenous site) - Leptomeninges (CNS involvement) - Liver, lungs, kidneys (visceral metastases) - Regional lymph nodes Bone marrow involvement is detected in 5–10% of newly diagnosed retinoblastoma cases and is associated with poor prognosis. Bone marrow aspiration and biopsy is more sensitive than peripheral blood smear for detecting micrometastatic disease. ### Metastatic Workup Algorithm for Retinoblastoma ```mermaid flowchart TD A[Retinoblastoma diagnosed]:::outcome --> B{Assess for metastatic disease}:::decision B --> C[Bone marrow aspiration & biopsy]:::action B --> D[Lumbar puncture + CSF cytology]:::action B --> E[Imaging: CXR, abdominal US]:::action C --> F{Marrow involvement?}:::decision D --> G{CNS involvement?}:::decision E --> H{Visceral/nodal mets?}:::decision F -->|Positive| I[High-risk disease]:::urgent G -->|Positive| I H -->|Positive| I F -->|Negative| J[Standard-risk disease]:::outcome G -->|Negative| J H -->|Negative| J I --> K[Intensive systemic chemotherapy]:::action J --> L[Standard chemotherapy ± radiation]:::action ``` **Mnemonic:** **BONE MARROW FIRST** for Retinoblastoma Metastatic Workup - **B** = Bone marrow aspiration (hematogenous spread site) - **O** = Obtain CSF (leptomeningeal involvement) - **N** = Nodal and visceral imaging - **E** = Essential for risk stratification ## Why Bone Marrow Aspiration and Biopsy Bone marrow aspiration and biopsy is the most appropriate investigation because: 1. **Detects micrometastatic disease:** More sensitive than peripheral blood smear for identifying marrow involvement 2. **Prognostic significance:** Bone marrow involvement is a poor prognostic factor and changes treatment intensity 3. **High-yield site:** Hematogenous spread to bone marrow is the most common pattern of metastatic disease in retinoblastoma 4. **Guides therapy:** Positive marrow involvement necessitates intensive systemic chemotherapy and possibly stem cell transplantation ### Comparison of Investigations in Metastatic Retinoblastoma Workup | Investigation | Detects | Sensitivity | Specificity | Timing | | --- | --- | --- | --- | --- | | **Bone marrow aspiration & biopsy** | Marrow involvement | High for micrometastases | High | At diagnosis | | **Lumbar puncture + CSF cytology** | Leptomeningeal involvement | Moderate (80–90%) | High | At diagnosis | | **Chest X-ray** | Pulmonary metastases | Moderate (70–80%) | High | At diagnosis | | **Abdominal ultrasound** | Hepatic, renal, nodal metastases | Moderate | Moderate | At diagnosis | | **PET-CT** | Metabolically active lesions | High | Moderate | Not routine; reserved for advanced disease | [cite:Shields & Shields, Intraocular Tumors; COG (Children's Oncology Group) Retinoblastoma Protocol]