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: MDM2, CDK4

• FISH: MDM2 amplification

Chondroid differentiation is present in roughly half of cases

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: MDM2, CDK4

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