Pathology: parosteal osteosarcoma

Parosteal osteosarcoma

  • Fascicles of spindle cells

  • Pale eosinophilic cytoplasm

  • Monomorphic ovoid nuclei with mild pleomorphism; inconspicuous mitotic activity

  • Broad and anastomosing bone trabeculae with variable osteoblastic rimming

  • Sheets of chondrocytes with mild pleomorphism set in abundant hyaline matrix

  • IHC: CDK4

  • FISH: MDM2 amplification

Chondroid differentiation is present in roughly half of cases

Parosteal osteosarcoma

  • Lobulated; osteo-chondroid neoplasm

  • Chondroid matrix: mildly cellular with polygonal cells arranged in small clusters; eosinophilic cytoplasm; hyperchromatic monomorphic nuclei with inconspicuous mitotic activity; patchy enchondral-type ossification

  • Osteoid matrix: sheets of predominantly woven bone punctuated by small lacunae; occasional small hyperchomatic cells; patchy necrosis

  • IHC: MDM2

  • FISH: MDM2 amplification

Chondroid differentiation is present in roughly half of cases

Parosteal osteosarcoma

  • Broad anastomosing trabeculae with lamellar and woven bone and variable osteoblastic rimming

  • Fascicles of spindle cells with pale eosinophilic cytoplasm; monomorphic ovoid nuclei with mild pleomorphism and inconspicuous mitotic activity

  • FISH: MDM2 amplification

Parosteal osteosarcoma

  • Broad anastomosing trabeculae with predominantly lamellar bone and inconspicuous osteoblastic rimming

  • Fascicles of spindle cells with pale eosinophilic cytoplasm; monomorphic ovoid nuclei with mild pleomorphism and inconspicuous mitotic activity

  • FISH: MDM2 amplification

Parosteal osteosarcoma

  • Fascicles of spindle cells

  • Pale eosinophilic cytoplasm

  • Ovoid nuclei, with mild pleomorphism; inconspicuous mitotic activity

  • Irregular woven bone trabeculae with variable osteoblastic rimming

  • IHC: MDM2

  • FISH: MDM2 amplification