The affected bone has a limited ability to respond to the lesion, just as the tumor has a limited number of potential effects on the skeleton. Often, the bone responds by making new bone, which can be seen in either the medullary bone or in the cortex and periosteum.
The pattern of bone response, especially combined with the pattern of lytic destruction, is very useful in determining whether the lesions is latent, active, aggressive or malignant. This information helps order the differential diagnostic list generated from question one.
Four Patterns of Bone Response
#1. Marginal sclerosis
Marginal sclerosis is characterized by a dense layer of lamellar bone forming in the medullary canal around the lesion, and it is typical of the response to benign latent, active, and even slow-growing aggressive lesions. This pattern may be observed with non-ossifying fibroma or fibrous cortical defect, chondro-myxoid fibroma, or in some cases of fibrous dysplasia. The process of marginal sclerosis may not be complete, but usually suffices to create the pattern of highly geographic lytic change.
Periosteal new bone formation is a complex response to a lesion, and a variety of patterns can be observed. These patterns are very useful in classifying the biology and activity of the lesion.
- In the first type of periosteal response, the bone responds to a very slowly-growing focus with cortical thickening. The best example is the cortical osteoid osteoma. The cortical response to this lesion may be massive, making it difficult to find the nidus of the osteoid osteoma within the very extensive sclerosis. This reactive "walling off" process suggests a non-aggressive benign lesion. A localized infection, such as Brodie's abscess, may also provoke this response.
- The second pattern is orderly periosteal new bone formation that results in remodeling of the bone in response to a slowly growing bone tumor. In some cases of slow-growing aggressive or low-grade malignant tumors, periosteal new bone is formed in response to endosteal erosion caused by the tumor. The combination of endosteal erosion and periosteal new bone apposition results in an appearance best characterized as "endosteal expansion": Both the medullary canal and the outer cortex of the bone increase in diameter. This pattern is typical of the lower-grade chondrosarcoma. Rapidly growing malignant lesions do not allow time for this well-ordered expansion and remodeling to occur.
#3. Periosteal neo-cortical response
A periosteal neo-cortical response is observed with rapidly expanding, benign aggressive lesions, such as giant cell tumor or aneurysmal bone cyst. There is destruction of the overlying cortex and rapid expansion of the tumor. However, as these benign aggressive lesions expand, the periosteum remains intact over the surface of the lesion and a thin neo-cortex is formed.
This pattern of periosteal response is characterized by its inability to encompass the rapidly proliferating tumor, and it is typically seen in osteosarcoma or Ewing's sarcoma. It is variously observed as a Codman's triangle, starburst appearance, or laminated/onion skinning, and it produces radiographic evidence that the tumor growth has outstripped the bone's potential for responding to the lesion. There is often an associated soft tissue mass on top of the intact cortex with surrounding periosteal new bone.
The Other Questions to Ask
