Telangiectatic 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. Less common subtypes include epithelioid, giant cell-rich, clear cell, and telangiectatic osteosarcomas. Telangiectatic osteosarcoma resembles aneurysmal bone cyst and frequently results in pathologic fractures. It consists of blood-filled spaces separated by septa containing highly pleomorphic tumour cells and multinucleated giant cells. Haematoxylin and eosin stain.