Pearls & Oy-sters: Acute Cerebral Occlusion due to Parosteal Osteosarcoma Embolus.

Tumor embolism is a rare cause of acute ischemic stroke. We present a case of a 46-year-old man with a history of distal femoral amputation 11 years prior due to parosteal osteosarcoma who developed acute onset left-sided weakness. Computed tomography (CT) angiography revealed occlusion of the right middle cerebral artery (MCA). Mechanical thrombectomy achieved recanalization and retrieved a white, gelatinous embolus. Histopathologic examination revealed atypical pleomorphic tumor cells. Immunohistochemistry demonstrated strong positivity for SATB2 and SOX9, positivity for CDK4, and a Ki-67 proliferation index of approximately 10%-20%. Fluorescence in situ hybridization (FISH) confirmed amplification of the MDM2 gene, supporting the diagnosis of metastatic dedifferentiated parosteal osteosarcoma. Postoperative MRI revealed infarcts in the right basal ganglia and temporal lobe, while chest CT showed bilateral pulmonary metastases. Transthoracic echocardiography identified a mass on the cardiac valve. This case illustrates an exceptionally rare clinical course in which a low-grade bone tumor underwent dedifferentiation after long-term survival and subsequently developed cardiopulmonary metastases. Tumor fragments arising from the valvular lesion likely entered the systemic circulation and embolized to the cerebral arteries, resulting in right MCA occlusion. Mechanical thrombectomy was pivotal not only for restoring cerebral perfusion but also for obtaining tissue for definitive histopathologic diagnosis.