Dedifferentiation of Metastatic Recurrent Chondrosarcoma of the Right Distal Femur Into High-Grade Sarcoma Favouring Leiomyosarcoma: A Report of a Rare Case With Review of Literature.

Dedifferentiated chondrosarcoma (DDCS) is a rare and aggressive biphasic bone malignancy that accounts for a small but distinct proportion of all chondrosarcomas and primary malignant bone tumours. The high-grade dedifferentiated component most commonly resembles undifferentiated pleomorphic sarcoma or osteosarcoma. The prognosis is dismal, with low five-year overall survival rates that are further reduced in the presence of metastatic disease. Local recurrence occurs in a substantial proportion of patients even after wide-margin resection, and distant metastasis, most frequently to the lungs, is common. Systemic therapy offers limited benefit, with poor overall response rates and no durable remissions with currently available doxorubicin-based, ifosfamide-based, or targeted regimens. Leiomyosarcomatous dedifferentiation within DDCS is exceptionally rare, with only a handful of cases documented worldwide. We report a case in which a patient who underwent limb-salvage surgery for DDCS of the right distal femur in 2023 re-presented two years later with extensive metastatic local recurrence requiring palliative hip disarticulation. Immunohistochemical profiling of the resected specimen demonstrated a smooth muscle differentiation profile consistent with high-grade sarcoma favouring leiomyosarcoma arising within the dedifferentiated component, a finding with important implications for prognostication and systemic therapy selection. This case highlights the critical role of comprehensive tissue sampling, immunohistochemical subtyping, and multidisciplinary management in diagnostically complex bone sarcomas.