Introduction: Primary intraosseous synovial sarcoma is an exceptionally rare malignant tumor of bone and may closely mimic other aggressive primary bone malignancies on imaging, particularly Ewing sarcoma, thereby posing a significant diagnostic challenge for radiologists. 

Case Report: We report the case of a 15-year-old female adolescent who presented with a rapidly enlarging painful mass of the left proximal femur associated with limping and functional limitation following a minor fall, for which she initially did not seek medical care and instead underwent traditional massage therapy. Plain radiographs demonstrated a mixed lytic-blastic lesion of the proximal femur with permeative bone destruction and an associated soft tissue mass, suggesting an aggressive primary bone tumor. Magnetic resonance imaging revealed extensive bone marrow replacement of the proximal femur with a large extraosseous soft tissue component, showing heterogeneous signal intensity, intralesional hemorrhage, marked diffusion restriction with low apparent diffusion coefficient values, and an early wash-in–wash-out enhancement pattern on dynamic contrast-enhanced imaging, reflecting high tumor cellularity and neovascularity. The lesion infiltrated adjacent muscle compartments and encased major femoral vessels. Chest imaging demonstrated bilateral pulmonary metastases with pleural involvement, consistent with advanced-stage disease. Histopathological examination and immunohistochemical analysis confirmed the diagnosis of primary intraosseous synovial sarcoma. 

Discussion: This case highlights the substantial overlap in imaging features between primary intraosseous synovial sarcoma and Ewing sarcoma and underscores the pivotal role of advanced multimodal MRI in refining the differential diagnosis and guiding definitive histopathological confirmation.  

Conclusion: Recognition of this rare entity is essential for accurate diagnosis, staging, and optimal management planning in adolescent patients presenting with aggressive malignant bone tumors.