A case report of a rare retroperitoneal sarcoma in an adolescent.

Adolescent retroperitoneal sarcoma (RPS) poses significant decision-making challenges due to complex subtypes and limited molecular diagnosis. A 16-year-old male with a 13.8 cm retroperitoneal mass underwent R0 en bloc resection (with left adrenal gland). Pathology favored high-grade fibrosarcoma. He completed 5 cycles of adjuvant Ifosfamide and Doxorubicin. At 17 months post-surgery, surveillance shows no disease recurrence. When molecular testing is unavailable for large, high-grade adolescent RPS, combining R0 en bloc resection with intensive adjuvant chemotherapy remains an effective strategy for favorable outcomes.