Dedifferentiated parosteal osteosarcoma with a novel low-grade leiomyosarcomatous and high-grade conventional osteosarcomatous differentiation: Case report and review of literature.

INTRODUCTION & IMPORTANCE: Parosteal osteosarcoma is an uncommon, low-grade, well differentiated malignant bone neoplasm with a more favorable prognosis compared to other types of osteosarcoma. Dedifferentiation is a well-known phenomenon, observed in 16-24.6 % of cases, leads to a combination of low-grade fibroblastic osteosarcoma and high-grade sarcoma. The dedifferentiated component is typically high-grade conventional osteosarcoma, although unusual variants such as spindle cell sarcoma, telangiectatic osteosarcoma, and rhabdomyosarcomatous dedifferentiation may arise. This case report contributes significant information to the sparse literature on dedifferentiated parosteal osteosarcoma due to its distinctive characteristics. CASE PRESENTATION: We describe a rare presentation of dedifferentiated parosteal osteosarcoma in a 44-year-old male with a lesion in the left femur. The dedifferentiated component included low-grade leiomyosarcoma alongside high-grade conventional osteosarcoma, presenting a diagnostic challenge, particularly with limited biopsy material. Diagnosis was confirmed using fluorescence in situ hybridization (FISH) for MDM2 gene amplification, emphasizing the importance of molecular diagnostic testing in such complex cases. CLINICAL DISCUSSION: This case highlights a rare presentation of dedifferentiated parosteal osteosarcoma (DPO) with low-grade leiomyosarcomatous and high-grade osteosarcomatous components, posing significant diagnostic challenges. Accurate diagnosis required a multidisciplinary approach, including imaging, histopathology, and MDM2 gene amplification testing, especially given the limitations of initial biopsy samples. Early recognition of dedifferentiation is critical due to its impact on metastasis risk and treatment planning. The case underscores the need for personalized treatment strategies and vigilant long-term follow-up in managing rare bone tumors like DPO.

conclusionThis case presenting a distinctive pattern of dedifferentiation is, to our knowledge, the first documented case of low-grade leiomyosarcoma occurring in parosteal osteosarcoma. As it enhances the clinical understanding by broadening the range of dedifferentiated components identified in parosteal osteosarcoma and highlights the diagnostic difficulties associated with these uncommon bone tumors.