Pseudosarcoma in Paget's disease of bone

The appearance of a sarcoma of bone is a well-recognized complication of Paget's disease. The most common type of such a sarcoma is osteosarcoma. Much less common are soft tissue lesions adjoining the pagetic bone that clinically and radiologically simulate sarcoma but histologically represent exaggerated periosteal bone formation as a manifestation of the basic pathologic process. We present a case of a bulky juxtacortical soft tissue mass in the thigh arising from a pagetic femur in a 62-year-old patient with polyostotic Paget's disease that was clinically and radiologically suspected to be a juxtacortical osteosarcoma. Microscopically, the lesion showed features of florid Paget's disease without any evidence of sarcomatous growth. It is important to be aware of this rare manifestation of Paget's disease to avoid unnecessary overtreatment.