A 14-year-old girl with osteosarcoma of the proximal tibia has completed neoadjuvant MAP chemotherapy. Surgical resection shows only 5% viable tumor (>90% necrosis). What is the preferred adjuvant chemotherapy regimen?

Explanation

## Adjuvant Chemotherapy in Good Responders to Neoadjuvant MAP **Key Point:** Patients with good histologic response (>90% tumor necrosis, i.e., <10% viable tumor) to neoadjuvant MAP should receive **continuation of the same MAP regimen** for the planned total duration (typically 10 cycles: 5 neoadjuvant + 5 adjuvant). ### Histologic Response and Prognosis **High-Yield:** Histologic response to neoadjuvant chemotherapy is the strongest prognostic factor in osteosarcoma: - **Good responders** (>90% necrosis): 5-year event-free survival ~70–80% - **Poor responders** (<90% necrosis): 5-year event-free survival ~40–50% This patient's 95% necrosis (5% viable) indicates excellent chemosensitivity and excellent prognosis with standard MAP continuation. ### Adjuvant Strategy by Response | Response Category | Histologic Necrosis | Adjuvant Regimen | Rationale | |-------------------|---------------------|------------------|----------| | **Good responder** | >90% | Continue MAP (5 more cycles) | Proven efficacy; no need to change | | **Poor responder** | <90% | MAP + ifosfamide/etoposide | Intensification; salvage agents | | **Very poor responder** | <50% | Consider clinical trial; MAP + ifosfamide ± other agents | High risk; experimental approaches | **Clinical Pearl:** The decision to intensify therapy (add ifosfamide/etoposide) is made *at the time of surgical resection* based on the pathology report. Good responders do not benefit from intensification and should complete the planned MAP course. ### Why Not Intensify in Good Responders? **Warning:** Adding ifosfamide to good responders increases toxicity (nephrotoxicity, hemorrhagic cystitis, secondary malignancy) without improving survival. This is a common exam trap — students may think "more chemotherapy = better outcome," but this is false in good responders. **Mnemonic:** **Good response = Good prognosis = Stick with MAP** — do not escalate therapy in patients already showing excellent chemosensitivity.