A 16-year-old girl presents with a 2-month history of pain in the right tibia and a visible swelling. She denies trauma. Clinical examination reveals a firm, tender mass over the anterior tibia with warmth and mild erythema. Laboratory work shows WBC 11,200/μL, ESR 65 mm/hr, and CRP 8.5 mg/dL. Plain radiographs show a mixed lytic-sclerotic lesion in the diaphysis with 'onion-skin' periosteal reaction. CT chest is normal. Biopsy confirms Ewing sarcoma with EWSR1-FLI1 fusion gene. The patient is started on neoadjuvant chemotherapy. After 10 weeks, repeat imaging shows 90% reduction in tumour size and no new lesions. What is the significance of this radiological response?

Explanation

## Prognostic Significance of Neoadjuvant Chemotherapy Response in Ewing Sarcoma ### Assessment of Chemotherapy Response **Key Point:** Radiological response to neoadjuvant chemotherapy is one of the most important independent prognostic factors in Ewing sarcoma. Good responders (≥90% reduction in tumour volume) have significantly better overall survival. ### Prognostic Stratification | Response Category | Tumour Volume Reduction | 5-Year Survival | Mechanism | |---|---|---|---| | **Good responders** | ≥90% | 65–75% | Chemotherapy-sensitive disease; lower metastatic burden | | **Poor responders** | <90% | 20–30% | Chemotherapy-resistant disease; higher metastatic potential | | **Non-responders** | <50% | <10% | Aggressive biology; early treatment failure | ### Pathological Basis of Response **High-Yield:** Good responders show: - Extensive tumour necrosis (>90%) - Minimal residual viable tumour - Absence of chemotherapy-resistant clones - Lower likelihood of occult metastases **Clinical Pearl:** Histological grading of necrosis in the surgical specimen (Huvos grading) correlates with radiological response and is an independent prognostic factor. Grade III–IV necrosis (>90% necrosis) is associated with 5-year survival >70%. ### Mnemonic for Ewing Sarcoma Prognostic Factors **BEST PROGNOSIS:** - **B**iochemistry: Low LDH, low alkaline phosphatase - **E**arly response: ≥90% necrosis on neoadjuvant therapy - **S**mall tumour: <100 mL volume - **T**umour site: Distal limb > proximal; fibula/tibia > femur - **P**rimary: Non-metastatic at diagnosis - **R**adiosensitivity: Chemotherapy-responsive - **O**utcome: Complete surgical resection with negative margins - **G**enetics: Standard EWSR1-FLI1 (vs. EWSR1-ERG) - **N**o metastases: M0 stage - **O**ncology: Completion of full multimodal therapy - **S**urvival: 5-year disease-free survival correlates with early response - **I**maging: Good radiological response (≥90% reduction) - **S**urgery: Wide margins achieved ### Treatment Implications ```mermaid flowchart TD A[Neoadjuvant Chemotherapy Complete]:::outcome --> B{Radiological Response?}:::decision B -->|≥90% reduction| C[Good Prognostic Sign]:::action B -->|<90% reduction| D[Poor Prognostic Sign]:::urgent C --> E[Proceed to surgery<br/>as planned]:::action D --> F[Consider intensified<br/>adjuvant therapy]:::action E --> G[Adjuvant chemotherapy<br/>completion]:::action F --> G G --> H[Long-term surveillance<br/>Good responders: 65-75% 5-yr OS<br/>Poor responders: 20-30% 5-yr OS]:::outcome ``` **Warning:** Good radiological response does NOT mean cure — adjuvant chemotherapy must still be completed to 12 months total. Stopping early increases relapse risk. **Key Point:** This patient's 90% reduction is a FAVOURABLE prognostic indicator. She should proceed to surgery followed by adjuvant chemotherapy completion, with an expected 5-year survival of ~65–75%. [cite:Robbins 10e Ch 26]