Osteoblastic osteosarcoma in monostotic Paget's disease.

Paget's disease is a disorder of bone remodelling affecting 1-2% of the general population, most frequently men over 50 years of age. Sarcomatous degeneration occurs in 0.1-0.95% of patients with Paget's disease. It can be observed in monostotic and polyostotic disease. Osteosarcomas account for more than 80% of pagetic sarcomas; osteoblastic pagetic osteosarcomas are rare. In this article, we present the case of a 75-year-old man with secondary osteosarcoma arising in monostotic Paget's disease, initially misdiagnosed as rotator cuff tendinopathy. Imaging studies and biopsy were diagnostic.