Is the cortex eroded

Cortical erosion is the hallmark of the active, aggressive or malignant tumor, and the pattern of cortical erosion may be highly correlated with the histology of the lesion:

• In chondrosarcoma, (unicameral bone cyst, non-ossifying fibroma) may cause cortical erosion with minimal periosteal response

• In an aneurysmal bone cyst or giant cell tumor, the cortex may be completely destroyed, but a thin layer of periosteal neo-corticalization may surround the lesion.

• The pattern of endosteal cortical erosion in chondrosarcoma tends to be scalloped, with “thumbprints” of tumor eroding into the bone. The erosion of the endosteum caused by chondrosarcoma is often accompanied by well ordered periosteal bone formation on the bone’s surface, leading to a pattern of endosteal expansion.

• High-grade malignant tumors, be they primary or metastatic, may erode through the cortex with only an ineffective periosteal response to the erosion. This periosteal response may produce a Codman’s triangle, starburst, or onion skinning appearance.

The answers to this question and question 4 are very useful in determining in the biological activity of the lesion. In general, lower-grade tumors will produce endosteal erosion with orderly periosteal response, leading to endosteal expansion. Higher-grade tumors will erode through the endosteal surface without an adequate response from the bone, resulting in a significant risk of fracture.

The Other Questions to Ask

• Where is the lesion?
• How large is the lesion?
• What is the lesion doing to the bone?
• What is the bone doing in response?
• What kind of matrix is being made?
• Is a soft tissue mass evident?