Describing a bone lesion
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Describing a bone lesion is an essential skill for the radiologist, used to form an accurate differential diagnosis for neoplastic entities, and occasionally non-neoplastic. In addition to patient demographics, the radiographic features of a bone lesion are often the primary determinant of non-histological diagnosis. This applies even among the vast amount of additional information provided by other modalities such as MRI.
The main features that should be assessed when a potentially neoplastic bone lesion is discovered include:
- location in the body (i.e. which bone)
- location within a bone
- zone of transition
- periosteal reaction
- cortical involvement
Location in the body
Most bone lesions have a typical distribution within the body, and being cognizant of this is helpful in narrowing in on a diagnosis or differential.
Location within the bone
Bone lesions tend to have a characteristic location within the affected bone. The description should include a statement as to its location medial to lateral (medullary, endosteal, cortical, or periosteal, or more simply concentric vs eccentric) as well as proximal to distal (diaphyseal, metaphyseal, or epiphyseal).
Zone of transition
Arguably the most important determinant of aggressive vs non-aggressive lesions.
Also referred to as well-defined, well-marginated, or narrow zone-of-transition. These lesions are typically benign and indolent.
Also referred to as poorly-defined, poorly-marginated, or wide zone-of-transition. These lesions are typically aggressive and are more likely malignant. Permeative and moth-eaten lesions morphologies fit this description.
Depending on the progenitor tissue, the lesion may have a variety of mineralization patterns that point to its histological origin, and characterizing this appropriately can be very useful in narrowing the differential diagnosis. Characteristic matrix mineralization patterns include:
- fibrous – ground glass e.g. fibrous dysplasia
- osteoid – cumulus cloud e.g. osteosarcoma
- chondroid – popcorn or arcs and rings e.g. chondrosarcoma
Morphology includes descriptors of the degree of osseous expansion, whether it's circular or ovoid, permeative, motheaten, etc.
Characterizing a periosteal reaction, if present, can also narrow or focus a differential diagnosis. For example, a solid periosteal reaction typically indicates a benign lesion. Codman triangles indicate a more rapid rate of growth than a laminated periosteal reaction, etc.
This is important not only for assessing interval change but also in differentiating between lesions of similar histology that are distinguished based on radiographic size, such as osteoid osteoma vs osteoblastoma, and fibrous cortical defect vs non-ossifying fibroma.
The extent and pattern of cortical involvement are important in determining risk for pathologic fracture and lesion aggressiveness (respectively) and may be useful in simply detecting the presence of a lesion which may demonstrate prominent endosteal scalloping and little else.