Tehran University of Medical Sciences Journal of Orthopedic and Spine Trauma 2538-2330 12 3 2026 07 16 148 50 Sadegh Saberi Associate Professor, Department of Orthopedic Surgery, Joint Reconstruction Research Center, Imam Khomeini Hospital Complex, Tehran University of Medical Sciences, Tehran, Iran Seyyed Saeed Khabiri Assistant Professor, Department of Orthopedic Surgery, Joint Reconstruction Research Center, Imam Khomeini Hospital Complex, Tehran University of Medical Sciences, Tehran, Iran Aysan Valinejad General Practitioner, Center for Orthopedic Transdisciplinary Applied Research, Tehran University of Medical Sciences, Tehran, Iran 2025 08 23 2026 05 31 Background: Soft tissue masses around the knee are commonly presumed to be benign cystic lesions. However, certain malignant tumors may mimic these entities, leading to delayed diagnosis and treatment. This case highlights the diagnostic challenges associated with malignant lesions presenting as common cysts. Case Report: A 26-year-old woman presented with a painless mass in the posteromedial aspect of her left knee that had gradually enlarged over four months. Initial ultrasonography identified the lesion as a Baker’s cyst. Owing to atypical clinical and imaging features, further evaluation was pursued. The case was discussed at ORTHOBIT 2025 Congress (Tehran, Iran), where the diagnostic pathway, differential diagnoses, imaging findings, and management options were reviewed. Advanced imaging raised suspicion for a malignant soft-tissue tumor, emphasizing the limitations of relying solely on conventional imaging modalities.  Conclusion: Malignant tumors such as synovial sarcoma can closely resemble benign cystic lesions around the knee. Careful clinical assessment, recognition of atypical features, advanced magnetic resonance imaging (MRI) evaluation, and multidisciplinary collaboration are essential to avoid misdiagnosis and ensure timely management. https://jost.tums.ac.ir/index.php/jost/article/view/705 https://jost.tums.ac.ir/index.php/jost/article/download/705/479