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An approach to radiographic interpretation of bone lesions

The systematic examination of radiographs will allow  the reader to create a differential diagnosis and allow for recognition of classic findings.  This systematic examination follows a series of questions, as follows:

Is this the right radiograph of the right patient? This question comprises, at the minimum, the verification that the subject is intended patient; that the date and body part examined are correct; and that orthogonal views (typical Anterior-Posterior and lateral) showing the entire lesion (and preferably the entire bone) were obtained. 

In this figure (modified from PORT notes), an AP radiograph reveals the typical appearance of osteosarcoma--but is cut off, not showing the extent of the lesion.

Your description then begins, "This is an AP and lateral radiograph dated (date) of (body part)"

The next question is Is there any soft tissue swelling?

Your description then continues, "There IS / IS NO soft tissue swelling"

Next, ask yourself of the apparent bone mineral density and gross alignment of the limb is normal.  (Generalized osteoporosis evident in the radiograph may be a feature of diffuse multiple myeloma; fibrous dysplasia often warps the bone.)

Your description should include "The bone mineral density and alignment are NORMAL"  (or explain how not)

Next comes the "Four Questions"; these will combine to offer the meat of your description.

The first question is (1) Where is the lesion?  

This has three parts. First, ask yourself, Which bone harbors the lesions? There are some useful rules that can be applied:

  • Is the lesion distal to the knees or elbows? That makes metastatic disease less likely (except possibly metastatic lung cancer, renal cell cancer and melanoma).
  • The phalanges are a common site for enchondromas.
  • The metaphyses of the distal femur, proximal tibia and proximal humerus, representing areas of rapid longitudinal growth, are common locations for primary bone malignancies.
  • Chondrosarcoma is more frequently found in the proximal skeleton (pelvis, scapula, proximal humerus and femur) than in the distal extremities.
  • Chordoma is most commonly found in the sacrum and at the base of skull.  
  • The anterior spine is frequently involved with metastatic disease. The primary tumors found there include giant cell tumor and aneurysmal bone cyst 
  • Osteoid osteoma and osteoblastoma are found in the posterior elements of the spine.

Next, determine the portion of the bone involvedThe skeletal anatomic regions that may be affected by bone tumors are defined as follows:

  • Epiphyseal. (shown in green, below)  These lesions in adults may extend across the growth plate scar but are clearly centered in the epiphysis. This is typical of a chondroblastoma.  Note that if the epiphysis of both bones near a joint are affected, this is termed not "epiphyseal" but Peri-articular. Changes are present on both sides of the joint or present in the soft tissue bursae surrounding the joints are suggestive of  pigmented villonodular synovitis, or infectious, inflamatory, and metabolic (gout) joint disease.  
  • Epiphyseal-metaphyseal. This is a classic location for locally aggressive tumors such as giant cell tumor of bone.
  • Metaphyseal or metaphyseal-diaphyseal. (show in purple, below) Lesions that do not cross the growth plate and tend to grow away from the physis (enchondroma, unicameral bone cyst) are described by this location. Bone abscesses may be metaphyseal but may extend toward the physis. The metaphysis is the most common site for primary malignancies such as osteosarcoma or chondrosarcoma.
  • Diaphyseal. (shown in red, below) This is a relatively uncommon location for bone tumors. Infections and fractures may cause tumor-like changes in the diaphysis. Ewing's sarcoma, eosinophilic granuloma, osteoid osteoma, and metastases are found (not exclusively) in this region.

Last, note where within the bone the lesion lies:

  • Parosteal or cortical. This location describes lesions that are attached to the surface of the bone. Examples include osteochondroma and parosteal osteosarcoma. Other lesions may grow directly within the cortex (osteoid osteoma, periosteal chondroma). It is often necessary to obtain cross-sectional imaging to determine this precisely. Note whether there is cortical erosion.
  • Soft tissue. Some lesions with calcified or ossified matrix may be present in the soft tissues but are well visualized on conventional radiographs. Examples include myositis ossificans and soft tissue sarcoma with ossification that might include malignant fibrous histiocytoma and synovial sarcoma.
  • Central or eccentric? Central lesions include enchondroma and fibrous dysplasia; typical peripheral eccentric peripheral lesions include non-ossifying fibroma and chondromyxoid fibroma.

            

It is often worthwhile to then inquire about the size of the lesionIn general, the larger the lesion, the more likely it is to be aggressive or malignant (though this is not universally true, of course; some very extensive lesions, such as fibrous dysplasia, may be quite large and yet benign).

Your description next includes "There is a (--SIZED) lesion with in the (REGION) of the (NAME OF BONE), located in (AREA)"

The second question is (2) What is the lesion doing to the bone? In general, bone tumors have to destroy native bone to make space for themselves; this question, then, is really asking for a description of the osteolysis. A  well defined lesion is termed "geographic" as one could easily draw the boundary around the lesion on the radiograph with certainty as to where the lesion ends and normal bone begins. This change is typical of unicameral bone cysts, chondromyxoid fibroma, and non-ossifying fibroma. The permeative pattern is one where the borders of the lesion are non-descript and there is no certainty where the lesion ends and the normal bone begins.  The lysis 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..

Your description next includes "There is [No / Geographic / Permeative] osteolysis."

Following that, ask, (3) What is the bone doing to the lesion?

In essence, the bone has only one response to a lesion: to form new bone (if at all). How fast this forms and how organized it is may be instructive. 

The Marginal sclerosis response is characterized by a dense layer of lamellar bone forming in the medullary canal around the lesion. Because this sclerosis requires time to form, it  is typical of benign (or slowly-growing) aggressive lesions.  

Periosteal new bone formation is a complex response to a lesion, and two basic 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 cortical response to this lesion may be massive, making it difficult to find the nidus of the original lesion itself. This reactive "walling off" process suggests a non-aggressive benign lesion such as an  osteoid osteoma. A localized infection, such as Brodie's abscess, may also provoke this response. The second pattern is orderly periosteal new bone formation coupled with endosteal erosion--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.

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. Onion skin periosteal reaction refers to a laminated pattern of periosteal reaction which is made by the sequential growth of the tumor and subsequent attempt at containment.  

Poorly organized periosteal new bone formation is characterized by the bone's 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. Hair on end periosteal reaction is mineralized line perpendicular to the long axis of the bone which represents calcification of the anchoring fibrils collagen at the periosteum-bone interface as the periosteum is rapidly elevated by a growing extramedullary mass.  These robust periosteal reactions are associated with lesions with rapid growth either neoplastic or infectious.

The description next includes "The bone has responded to this lesion by......"

The last question is (4) What type of matrix is being produced by the lesion? 

  • No matrix seen suggests a fluid matrix that could be either cystic or myxoid. 
  • The typical matrix of fibrous tumors is described as ground glass as small trabeculae of woven bone are scattered through the fibrous stroma
  • Calcium deposition in cartilage is typically less well organized than bone and usually has a punctate appearance, with small dots, circles, or whorls of calcification in an otherwise uncalcified background . The nodules of cartilage tumor matrix often undergo calcification at their periphery, leading to an arc-ring pattern of calcific matrix.
  • Bone formation usually is recognized by the greater degree of organization of the matrix, with resultant higher radiographic density. Note: bone formation in some tumors, eg osteoid osteoma, is mainly due to host bone response to the lesion (nidus) rather than true tumor bone formation. Note further that the absence of bone matrix on radiographs does not preclude the presence of bone formation, as the osteoid formed by the malignant cells may not have been calcified yet. 
The description then concludes "The matrix within the lesion is characterized as......"


 

 

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