Periosteal Osteosarcoma: A case report of a rare tumor and a brief review of the literature

Periosteal osteosarcoma (PerOS) is a rare intermediate-grade surface osteosarcoma (OS), accounting for fewer than 2% of all OSs. Its unique topographic characteristics and prominent chondroblastic element frequently make diagnosis very challenging, and it should be distinguished from other morphologic pretenders, such as periosteal chondrosarcoma, parosteal osteosarcoma, and high-grade surface osteosarcoma. Timely, accurate diagnosis and treatment require a multidisciplinary approach that considers the combination of clinical, radiological, and pathological features, including immunohistochemistry (IHC) and, if indicated, molecular studies. The presence of malignant osteoid and the absence of MDM2 amplification distinguish PerOS from its closest mimicker, parosteal OS. Here, we report the case of a 42-year-old male with an insidious onset of mass and pain in the right knee. After discussion at the multidisciplinary tumor board, the patient was successfully diagnosed and treated with a wide excision and limb‐sparing reconstruction. The presented case highlights the importance of a multidisciplinary diagnostic and therapeutic approach to achieve appropriate patient outcomes in this rare bone neoplasm.