A biopsy from a retroperitoneal mass in a 40-year-old man shows a tumor with MDM2 gene amplification confirmed by FISH and CDK4 co-amplification. What is the most accurate diagnosis?
- A Dedifferentiated liposarcoma
- B Well-differentiated liposarcoma / atypical lipomatous tumor ✓
- C Myxoid liposarcoma
- D Pleomorphic liposarcoma
Correct answer: B. Well-differentiated liposarcoma / atypical lipomatous tumor
Explanation
MDM2 amplification (12q13–15 region, which also includes CDK4) is the defining molecular hallmark of well-differentiated liposarcoma (atypical lipomatous tumor) and dedifferentiated liposarcoma. When the tumor lacks high-grade non-lipogenic areas it is classified as well-differentiated/atypical lipomatous tumor. MDM2 amplification is absent in myxoid liposarcoma (FUS-DDIT3 fusion) and pleomorphic liposarcoma (complex genomics without specific amplicon).
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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