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How should a bone lesion be characterized radiographically?
Bone lesions should not be defined radiographically as malignant or benign, but rather aggressive or nonaggressive. Some malignant lesions can have a nonaggressive appearance and many benign lesions (osteomyelitis, Langerhans cell histiocytosis of bone) can have a very aggressive appearance. There are a number of criteria that should be assessed when evaluating a bony lesion.
- a. Demographics: Both aggressive and nonaggressive bone lesions occur with a specific propensity for certain age demographics and to a lesser extent sex demographics. When evaluating bone lesions, it is imperative to take in to consideration the age of the patient before forming a differential diagnosis.
- b. Location: The location of the lesion within the bone and skeleton is helpful as specific lesions have a propensity to affect certain locations in the skeleton.
- i. Osseous location: The lesion’s location in the bone as it relates to the diaphysis, metaphysis, or epiphysis of long bones is important for narrowing a differential diagnosis. Special consideration should be given to epiphyseal equivalents (patella) and flat bones (pelvis, ribs) and epiphyseal or metaphyseal equivalents (around apophyses or in flat bones that form by membranous ossification). Most common tumor sites:
- • Epiphysis: chondroblastoma, clear-cell chondrosarcoma, giant cell tumor (GCT), aneurysmal bone cyst (ABC). Of note, GCT and ABC usually arise from the metaphysis and extend into the epiphysis.
- • Metaphysis: osteosarcoma, chondrosarcoma, GCT chondromyxoid fibroma, enchondroma, nonossifying fibroma (NOF), ABC (long bones).
- • Diaphysis: Ewing’s sarcoma, osteoid osteoma, osteoblastoma, enchondroma (phalanges), fibrous dysplasia (FD).
- i. Osseous location: The lesion’s location in the bone as it relates to the diaphysis, metaphysis, or epiphysis of long bones is important for narrowing a differential diagnosis. Special consideration should be given to epiphyseal equivalents (patella) and flat bones (pelvis, ribs) and epiphyseal or metaphyseal equivalents (around apophyses or in flat bones that form by membranous ossification). Most common tumor sites:
Types of Tumor by Osseous Location
| Epiphyseal | Metaphyseal | Diaphyseal |
|---|---|---|
| Chondroblastoma | Osteosarcoma | Ewing’s sarcoma |
| Clear-cell chondrosarcoma | Chondrosarcoma | Osteoid osteoma |
| Giant cell tumor a | Giant cell tumor | Osteoblastoma |
| Aneurysmal bone cyst a | Chondromyxoid fibroma | Enchondroma (phalanges) |
| Enchondroma | Fibrous dysplasia |
a Begin in the metaphysis and extend to the epiphysis.
- ii. Axial location: The lesion’s location should be defined as central intramedullary, eccentric intramedullary, cortically-based, or surface (periosteal)-centered.
- c. Zone of transition: The transition zone between the lesion and the adjacent uninvolved bone should be classified as:
- i. Narrow: well-defined margins that can be easily traced. This is a nonaggressive feature.
- ii. Wide or ill-defined: poorly-defined margins, not easily traceable. This is an aggressive feature.
- d. Pattern of bone destruction: the pattern of bone replacement should be assessed and characterized as:
- i. Geographic: well-defined lesion of the bone. Geographic lesions have a narrow zone of transition by definition. This is a nonaggressive feature.
- ii. Blastic or sclerotic: densely sclerotic lesion of bone. Can be aggressive or nonaggressive.
- iii. Permeative: ill-defined bony destruction. This is an aggressive feature.
- iv. Bubbly: multilocular lucencies, with or without expansile appearance. This is usually a nonaggressive feature.
- e. Presence of matrix: This is one of the most critical and difficult decisions in the radiographic assessment of a bone lesion. The confident presence of matrix can narrow the differential markedly. If present, matrix falls into one of following categories
i. Clear or cystic
ii. Fibrous: uniform intermediate density described as “ground-glass” in appearance. Lesions producing fibrous matrix are usually (but not always) nonaggressive
iii. Chondroid: punctate matrix described as “rings and arcs” or “Cs and Js.” Usually equal or greater density to cortical bone. Can be associated with either aggressive or nonaggressive lesions
iv. Osteoid: amorphous, fluffy, cloudy appearing matrix, usually less dense than cortical bone. Osteoid matrix can be immature and fluffy favoring an aggressive lesion or more mature, resembling bone, and favor a nonaggressive lesion
Types of Nonaggressive Primary Bone Tumors by Matrix Appearance
| Clear | Fibrous | Chondroid | Osteoid |
|---|---|---|---|
| Unicameral bone cyst | Fibroxanthoma | Enchondroma | Osteoma |
| Aneurysmal bone cyst | Fibrous dysplasia | Osteochondroma | Osteoid osteoma |
| Giant cell tumor | Chondroblastoma | Osteoblastoma | |
| Eosinophilic granuloma | Chondromyxoid fibroma |
f.
Periosteal reaction: The bones’ response to insult is to lay down new bone and form protective callous. The periosteum is a remarkable combination of tissues that has progenitor cells ready to differentiate into osteoblasts and chondroblasts. This allows the rapid formation of new bone producing the radiographically visible periosteal reaction. Pathologic processes that are aggressive and rapidly growing produce periosteal reaction indicative of ongoing attempts to wall off the expanding lesion. Periosteal reaction can be described in one of four typical patterns:
- i. Smooth: Favors a nonaggressive, slow growing, or reactive process allowing the formation of solid callous/periosteal new bone
ii. Lamellated (onion skin): Layers of periosteal new bone laid down on one another. Indicative of an aggressive or fast growing lesion as the new periosteal layers are not allowed sufficient time to mature before another is laid down on top
iii. Codman’s triangle: A very aggressive appearance, indicative of osseous soft tissue mass expanding at such a rate that the adjacent margin of periosteal new bone is lifted up, forming a triangle with the underlying cortex
iv. Sun burst: The most aggressive form of periosteal reaction is seen exclusively in very aggressive lesions. Has classically been described in osteosarcoma
g. Presence or absence of a soft tissue mass/cortical breakthrough: Visible cortical breakthrough and/or soft tissue mass are the hallmarks of an aggressive lesion of bone. Despite reasonable assumption to the contrary, a large soft tissue may be present in the absence of profound cortical disruption. This feature is common in small round blue cell tumors (lymphoma and Ewing’s sarcoma) in which tumor cells characteristically spread through cortical tunnels, resulting in an appearance of intact cortex with large surrounding soft-tissue mass.
h. Polyostotic versus monostotic: This alters the differential significantly. Primary bone lesions are rarely polyostotic; therefore, the presence of multiple bone lesions favors secondary processes such as metastatic disease or myeloma. Whole body bone scan, positron emission tomography–computed tomography (CT), or whole body magnetic resonance imaging (MRI) may help clarify this issue.
