A case report from osteosarcoma to SEF: A diagnostic pitfall with a rare metastatic pattern.

Sclerosing epithelioid fibrosarcoma (SEF) is a rare and aggressive variant of fibrosarcoma with a deceptively bland histologic appearance but high metastatic potential, often leading to misdiagnosis and delayed treatment. We present a case of a 21-year-old male initially diagnosed with osteosarcoma due to progressive right proximal thigh pain and swelling. He was treated with perioperative chemotherapy (cisplatin and doxorubicin) and underwent limb-salvage surgery, but postoperative molecular testing revealed an Ewing Sarcoma Receptor 1 (EWSR1) gene rearrangement, confirming SEF. Despite surgery and systemic chemotherapy, the patient developed metastatic disease involving the lungs, adrenal glands, and spine. Aggressive treatments including multiple surgeries, radiotherapy, and chemotherapy (gemcitabine-docetaxel, ifosfamide) failed to halt disease progression. The patient transitioned to palliative care and eventually passed away. SEF is known for its slow progression, high metastatic potential, and resistance to conventional therapies. This case highlights the diagnostic challenges of SEF, the importance of molecular testing for accurate diagnosis, and the need for novel treatment strategies, as SEF remains highly resistant to current therapeutic options.