Toki S, et al. - The clinical behaviour, prognosis, and optimum treatment of dedifferentiated low-grade osteosarcoma (DLOS) diagnosed based on molecular pathology were assessed via retrospectively reviewing 13 DLOS patients (six men, seven women; median age 32 years (interquartile range (IQR) 27 to 38)). The patients were diagnosed using the following criteria: the histological coexistence of low-grade and high-grade osteosarcoma components in the lesion, and positive immunohistochemistry of mouse double minute 2 homolog (MDM2) and cyclin-dependent kinase 4 (CDK4) associated with MDM2 amplification. The DLOS and conventional osteosarcoma (COS) patients had five-year overall survival (OAS) rates of 85.7% and 77.1%, respectively, and the five-year progression-free survival rates of 57.7% and 44.9% (p = 0.368), respectively. Compared to COS, DLOS showed a poorer response to a standard chemotherapy regimen, while no marked differences were evident in the clinical outcomes.