Margin of bone lesion

Margins

This is one of the most important things that you can determine about a solitary, lucent, expansile lesion of bone. Why is this? Because, this is the finding that will give you your best shot at determining the biological activity of the lesion (how fast is it growing?). This is important, because in general, the faster a process grows, the more likely it is to be malignant.

So, how can we determine biological activity from the margin of the lesion? The answer is bone response. Bone is sensitive to a variety of stimuli, and generally responds to one of the processes in FOGMACHINES by either removing bone or creating bone. That's right! The bone itself does the removing or creating of bone - not the disorder involving the bone. At the AFIP, they are fond of saying that the only things that can remove bone are osteoclasts and orthopedic surgeons. I agree with this rule, but would also add talented amateurs to the list (lawnmower and saw accidents, auto crashes, blast injuries, and animal bites are favored mechanisms of bone removal by amateurs -- professionals prefer saws, drills, and osteotomes and confine their efforts to operating rooms). For this reason, the term "expansile lesion" is a bit of a misnomer, since the lesion itself is not expanding the bone. The bone is remodeling itself in response to the stimulus of the lesion.

The other thing to know about bone response is that while it is certain, it is rather slow. If a lesion is growing slowly, then the bone will have plenty of time to retreat from the lesion, removing some bone around the lesion, but also laying down new bone around the margins of the lesion. This generally has the effect of producing a sclerotic and usually distinct margin around the lesion. If process grows more rapidly, the bone may only have time to retreat before the lesion, and not have time to lay down this sclerotic rim. Solitary lucent lesions in bone with a distinct margin are generally called "geographic" lesions, whether or not they have a sclerotic rim.

geographic lesion with ill-defined rim	geographic lesion with well-defined rim

If the lesion grows more rapidly still, there may not be time for the bone to retreat in an orderly manner, and the margin may become ill-defined. Rather than a single discrete lesion, we may see several ill-defined foci of lucency. This has been termed a "moth-eaten" pattern.

a "moth-eaten" lesion

If, alas, the process grows more rapidly still, then the bone's retreat may become disorderly indeed. Continuing this battlefield analogy, the boundary between normal and abnormal bone may be lost altogether, with only a very ill-defined pattern of lucency seen, caused by many small, irregular holes in the bone, left behind by osteoclasts. This is an extremely aggressive pattern, sometimes called a "permeative" pattern.

permeative pattern	normal bone

The presence of a permeative pattern usually means that the patient either has an aggressive infection or a malignant tumor. The most common malignancies that give this pattern are metastases, myeloma, primary histiocytic lymphoma, and Ewing's sarcoma. These lesions are sometimes referred to as "round cell lesions" due to the small, dark, round cells that they display to the pathologist.