Fibrosarcoma of bone masquerading as fibrous dysplasia: diagnostic pitfall and molecular correlates of aggressive behavior.

Low-grade fibro-osseous lesions can be challenging to classify. A 33-year-old man presented with synchronous lesions involving the ilium and T2 vertebra. Because of an impending pathologic fracture, he received several doses of denosumab. Initial biopsies showed bland fibro-osseous lesions lacking GNAS or MDM2 alterations and were favored to represent polyostotic fibrous dysplasia. Subsequent iliac curettage revealed a fascicular spindle cell proliferation with subtle atypia and focal ossification, leading to a revised diagnosis of low-grade fibrosarcoma. Re-biopsy of the T2 lesion demonstrated a similar spindle cell proliferation without ossification. Sequencing identified copy number gains involving KIT, PDGFRA, and TERT. At 16-month follow-up, two metastatic pulmonary nodules developed, one responsive to chemotherapy. The patient remains alive with disease at 27 months after presentation. This case highlights a diagnostic pitfall in which low-grade fibrosarcoma with denosumab-associated ossification may mimic fibrous dysplasia and underscores the prognostic value of molecular profiling beyond histologic grading.