Dedifferentiated Low-Grade Surface Parosteal Osteosarcoma into a High-Grade Osteosarcoma: Case Report and a Brief Review of the Literature

Dedifferentiated parosteal osteosarcoma (D-POS) is an isolated form of osteosarcoma in which low-grade parosteal osteosarcoma (LG-POS) progresses into a high-grade sarcoma (HGS). In this case report, the clinical, radiological, pathological, and molecular evolution of D-POS in a 24-year-old male is described. The patient initially presented with a classic LG-POS of the proximal tibia, with amplification of MDM2 and CDK4, which was treated with a wide surgical resection and no neoadjuvant or adjuvant treatment. Three years later, he developed a progressively growing large mass with severe pain, and imaging appearance of a lytic tumor, necrosis, and medullary invasion at the site of prior surgery. The LG-POS had dedifferentiated into an HGS.

The dedifferentiation of the LG-POS into high-grade sarcomatous component with severe atypia, high rate of mitosis, new mutations of TP53, and the amplification of MDM2 and CDK4 was found in histopathological examination of the recurrent mass. However, the patient died of progressive metastatic disease 2.5 years after the dedifferentiation despite the aggressive multimodal therapy, including neoadjuvant and adjuvant therapies, amputation, and pulmonary metastases metastasectomy. The case implies the highest importance of close follow-up and early detection of dedifferentiation, as well as the intricate nature of the prognosis of such aggressive variant of the tumor and necessity of developing standardized treatment plans.