Periosteal osteosarcoma is an exceedingly rare type of chondroblastic osteosarcoma, showing a rather good prognosis, and secondary bone marrow involvement is unusual. However, there have been some reports describing periosteal sarcoma involving medullary bone. We encountered a patient, a 38-year-old man, who had a bone surface tumor in the left tibia. An X-ray showed an erosive cortical mass extraosseous portion, located in the diaphysis of the tibia. Other images revealed a thin cortex, periosteal reactions, coarse mineralization in the extraosseous portion, and bone marrow involvement. Grossly, surgical materials showed that the tumor mainly existed at the periosteal portion, only a part of the cortex was destroyed, and there was medullary involvement throughout. Histological examinations showed a predominantly chondroid component with malignant osteoid formation. On the basis of the histological macroscopic and microscopic findings, we made the diagnosis of periosteal osteosarcoma with secondary bone marrow involvement.
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