A 16-year-old girl from rural Maharashtra presents with a 6-week history of progressive pain and swelling of the right tibia. She denies trauma. On examination, there is a firm, fixed swelling over the proximal tibia with overlying skin erythema. Plain radiographs show a mixed lytic and sclerotic lesion in the diaphysis with an 'onion-skin' periosteal reaction. MRI reveals a large soft tissue component with marrow infiltration. A biopsy is performed, and histopathology shows sheets of small, round, uniform cells with high mitotic activity and scanty cytoplasm. Immunohistochemistry is positive for CD99 and FLI1. What is the next most important step in management?

Explanation

## Management of Ewing Sarcoma: Multimodal Approach ### Diagnosis Confirmation **Key Point:** The histopathology (small round cells with high mitotic activity, scanty cytoplasm) combined with CD99 and FLI1 positivity on immunohistochemistry is diagnostic of Ewing sarcoma. CD99 is the most sensitive marker; FLI1 is part of the EWSR1-FLI1 fusion gene product. ### Treatment Algorithm for Localized Ewing Sarcoma ```mermaid flowchart TD A[Confirmed Ewing Sarcoma]:::outcome --> B{Metastatic disease?}:::decision B -->|Yes| C[Palliative chemotherapy + local control]:::action B -->|No| D[Localized disease]:::outcome D --> E[Neoadjuvant chemotherapy]:::action E --> F{Chemotherapy response?}:::decision F -->|Good response| G[Wide local excision or limb-sparing surgery]:::action F -->|Poor response| H[Consider amputation]:::action G --> I[Adjuvant chemotherapy]:::action H --> I I --> J[Surveillance imaging]:::action J --> K[Long-term follow-up]:::outcome ``` ### Why Neoadjuvant Chemotherapy First? **High-Yield:** Neoadjuvant (preoperative) chemotherapy is the standard of care for Ewing sarcoma because: 1. **Tumor downsizing:** Reduces tumor volume, improving the feasibility of limb-sparing surgery and reducing surgical morbidity. 2. **Early systemic treatment:** Addresses micrometastases that are present in ~20% of patients at diagnosis (occult metastases). 3. **Prognostic assessment:** Chemotherapy response (>90% necrosis = good response) is the strongest independent prognostic factor. 4. **Improved survival:** Multimodal therapy (chemotherapy + surgery + chemotherapy) achieves 5-year survival rates of 60–75% in localized disease. ### Chemotherapy Regimen **Mnemonic: VAC/IE** — **V**incristine, **A**driamycin (doxorubicin), **C**yclophosphamide alternating with **I**fosfamide and **E**toposide. - **Standard regimen:** 14 weeks of neoadjuvant chemotherapy (5–6 cycles). - **Timing of surgery:** Typically 2–3 weeks after completion of neoadjuvant chemotherapy. - **Adjuvant chemotherapy:** Continued for 12–14 weeks post-surgery based on chemotherapy response. ### Surgical Management **Clinical Pearl:** The goal is **wide local excision with limb-sparing surgery** whenever feasible. Amputation is reserved for: - Tumors with vascular invasion that cannot be resected with adequate margins. - Tumors involving critical neurovascular structures. - Poor chemotherapy response with extensive local progression. - Pathological fracture with significant soft tissue involvement. In this case, the tumor is in the proximal tibia — a site where limb-sparing surgery (e.g., endoprosthetic reconstruction) is often possible after neoadjuvant chemotherapy downsizing. ### Why Not the Other Options? | Option | Why It's Wrong | |--------|---------------| | **Wide local excision first** | Upfront surgery without neoadjuvant chemotherapy misses the opportunity for tumor downsizing, early systemic treatment, and prognostic assessment. This approach is associated with worse outcomes. | | **Amputation** | Reserved for cases with vascular invasion, critical neurovascular involvement, or poor chemotherapy response. Limb-sparing surgery is preferred when feasible. | | **Radiation alone** | Ewing sarcoma is radiosensitive, but radiation monotherapy has poor outcomes. It is used as adjuvant local control in cases where surgery is not feasible or as palliative therapy. | ### Prognosis & Follow-Up - **Good prognostic factors:** Localized disease, small tumor size (<100 mL), good chemotherapy response (>90% necrosis), distal location. - **Poor prognostic factors:** Metastatic disease, large tumor size, poor chemotherapy response, pelvic location. - **5-year survival:** ~70% localized, ~30% metastatic. - **Surveillance:** Regular clinical examination, imaging (chest X-ray, local MRI) every 3 months for 2 years, then every 6 months for 3 years. **Warning:** Do not perform upfront surgery without neoadjuvant chemotherapy — this significantly worsens outcomes and misses the window for systemic treatment of micrometastases.