[Undifferentiated sarcoma of the ulna in a patient with Paget's disease].

The development of bone sarcoma in a patient with Paget disease is a classic but rare event. The authors describe the XR, CT and MR features of an undifferentiated Paget sarcoma arising in the proximal ulna, a very rare location of this tumoral complication. Clinical symptoms, radiological pattern and epidemiology are discussed. In every Paget patient presenting with a new or different pain, with or without mass effect, the radiologist has to search for signs of sarcomatous transformation, i.e. cortical osteolysis, soft tissue mass and sometimes an associated fracture.