Osteosarcoma arising from preexisting Paget's disease of bone

Paget's disease of bone (PDB) was first described by Sir James Paget in 1877 and is a relatively benign metabolic bone disease. It is believed to be a more common among European populations but is rare among Scandinavian and Asian populations. Malignant transformation is a rare, but fatal complications of PDB can occur, and most patients are elderly. Herein, we present a case of a tumor detected incidentally at the left proximal femur that was initially treated as metastatic bone disease, but the final pathology was proven to be secondary malignant transformation from preexisting PDB. Recent studies suggest that PDB may be under-represented in Asian populations. Thus, it is important that physicians pay more attention to PDB and its treatments. Early interventions and novel therapeutic agents can achieve better disease control and also prevent the complication of malignant transformation, which still has a poor prognosis.