There is a large soft tissue mass in the right leg below the knee.
The right fibula shows extensive and aggressive long segment periosteal reaction along the proximal diaphysis which is perpendicular to the shaft, forming Codman triangle and extends perpendicularly from the inner cortex to the outer margin of the tumour. There is cortical erosion and cortical thickening suggestive of possible intramedullary extension.
Malignant periosteal reaction, Codman triangle, cortical erosion and thickening. In view of the age of the patient, imaging features are suggestive of periosteal osteosarcoma. Differential diagnosis include a high grade surface osteosarcoma.
Open arrow shows Codman triangle (which is a characteristic, but not specific feature of periosteal osteosarcoma).