Bone tumors have a limited number of potential effects on the skeleton. The most common is to produce bone lysis. The pattern of lysis and the extent of the host response to lysis (which is defined in the next question, What is the bone doing in response?), provide major clues as to whether the lesion is latent, active, aggressive, or malignant.
To answering the two questions "what is the lesion doing to the bone?" and "what is the bone doing in response?" it is important to develop an appreciation of the margin or interface between the host bone and the lesion. In benign latent or active lesions, this margin is usually very well defined, either by abrupt cessation of the lytic changes caused by the lesion or, alternatively, by reactive sclerosis to the lesion produced by the host bone.
- The most favorable pattern of bone lysis is that of a geographic lytic lesion, in which it is easy to determine exactly where the bone lysis begins and ends (a narrow zone of transition). This change is typical of unicameral bone cysts, chondromyxoid fibroma, and non-ossifying fibroma.
- A far more worrisome change is the appearance of a moth-eaten or permeative margin. In this situation, there is a a wide zone of transition, meaning it is difficult to determine where the lesion begins and ends. The lytic change seems to percolate through the trabeculae with little host response. These changes are typical of malignant lesions, including mesenchymal tumors, lymphoma, metastatic cancer, and Ewing's sarcoma.
- Aggressive tumors, such as giant cell tumor of bone, may have a full spectrum of these changes. Typically, giant cell tumors are relatively geographic lesions, while on occasion the lesion may have a somewhat ill-defined margin.
When answering the question of what the lesion is doing to the bone, determine whether the lesion is causing destruction of bone and whether this pattern of destruction is geographic, diffuse, or permeative at its margins. This question and the next will determine whether the differential diagnostic list emphasizes benign or malignant lesions.
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