Fibroblastic osteosarcoma, light micrograph. Osteosarcoma is a malignant neoplasm in which the tumour cells produce immature bone or osteoid. Based on the morphology, the most common types of osteosarcomas are osteoblastic, chondroblastic, and fibroblastic. These patterns often coexist although their relative proportions vary from case to case. This is a fibroblastic osteosarcoma. Approximately 25% of osteosarcomas consist predominantly of spindle tumour cells and can be classified as fibroblastic osteosarcoma. Osteoid production is minimal and seen focally. Fibroblastic osteosarcomas are highly vascular and may resemble hemangiopericytoma. Haematoxylin and eosin stain.