Figure 2 Histopathology of primary pulmonary osteosarcoma with hematoxylin and eosin staining(A) Magnification ×100. Low-power view of a hypercellular spindle-cell neoplasm within lung parenchyma, with irregular dense eosinophilic osteoid deposits (arrow named “osteoid”) rimmed and permeated by tumor cells (labeled “spindle cells”). Residual alveolar septa/fibrous stroma are present at the periphery, indicating adjacency to native lung. (B) Magnification ×200. Higher-power view demonstrates a triphasic pattern with focal chondroid differentiation (labeled “Cartilage, chondroid”) showing basophilic matrix containing lacunae with chondrocyte-like tumor cells, transitioning to abundant lace-like/trabecular eosinophilic osteoid (labeled “osteoid”). The osteoid is directly produced by and rimmed and permeated by atypical spindle tumor cells (labeled “spindle cells”), exhibiting nuclear pleomorphism, hyperchromasia, and frequent mitotic figures. The intimate association of malignant spindle cells with tumor osteoid, together with focal chondroid differentiation, supports an osteoid-forming high-grade sarcoma (osteoblastic osteosarcoma phenotype).