Evaluation of radiographs of the musculoskeletal system requires systematic evaluation of soft tissues, bones and joints. An assessment of size, shape, location, and where possible, function is made. Serial radiographs provide additional information on the dynamic nature of a lesion, and contribute to a correct diagnosis.

Aggressive bone lesions are evaluated according to the following criteria:

 Periosteal reaction

 Osteolysis / cortical integrity

 Zone of transition

 Location (in the bone / monostotic /polyostotic)

 Serial evaluation

Bone lesions are radiographically characterized by the presence of bone lysis, bone production or a combination of both of these processes. The pattern by which these processes occur forms the basis of classification. Radiographic patterns, although not diagnostic, help us to answer the following questions: Is the lesion:

 Aggressive or non-aggressive

 Active or inactive

 Malignant or benign

 Neoplastic or infectious

 Acute or chronic

Inactive and Non-Aggressive

 Minimal clinical signs

 Well demarcated / defined margins

 Smooth periosteal surfaces

 Dense new bone formation / sclerosis

 Abrupt zone of transition

 Focal osteolysis / medullary expansion

 Intact or thickened cortex

 Slow rate of change or no change

Active and Aggressive

 Prominent soft tissue swelling

 Moth eaten or permeative osteolysis

 Poor defined margins

 Ill-defined bone production

 Pathologic fractures

 Poorly demarcated

 Gradual/long zone of transition

 Rapid changes on follow-up

Radiographic features can be common to many diseases processes, especially in the early stages. This means that a diagnosis based solely on the radiographic appearance will often be erroneous. The most frequent dilemma is the differentiation between infection (fungal or bacterial osteomyelitis) and neoplasia (primary or metastatic bone cancer). The definitive diagnosis depends on bone biopsy but it is not available all the time and may take time to get the results. Radiographs may help the decision making process while biopsy findings are not available.

Consideration of all supporting clinical data (history, signalment, clinical signs and physical examination), additional lab data, response to previous treatment and other imaging findings (thoracic radiographs and/or ultrasound) is necessary.

Solitary metaphyseal aggressive lesions: Primary bone tumor is the most common cause and should be considered until proved otherwise. Primary bone tumors of the appendicular skeleton other that osteosarcoma are uncommon. Other than neoplasia, the major diagnosis to be ruled out for a monostotic aggressive lesion is mycotic osteomyelitis.

Multiple aggressive bone lesions: The major diagnosis to be ruled out for polyostotic aggressive bone lesions are metastatic solid tumors and mycotic osteomyelitis. Patients with mycotic osteomyelitis tend to be younger than patients with metastatic solid tumors. Metastatic bone cancer is more common than once thought. In dogs, mammary, liver, thyroid and prostatic cancer may cause bone metastasis. They produce polyostotic lesions on ribs, vertebra, femur and humerus, as the most common sites. Bacterial hematogenous osteomyelitis may also cause polyostotic aggressive lesions but is rare in dogs and cats. Aggressive monostotic lesion is the most common finding associated to bacterial osteomyelitis, usually a consequence of open fractures, surgical contamination or perforating injury.

Aggressive digital bone lesions: The digit is another location for which it may be a challenge to differentiate between infectious and neoplastic lesions prior to the biopsy results. Subungual tumors are relatively common in the dog and the most common type being squamous cell carcinoma. Most of them involve large breed dogs with black hair coats. Melanomas are another common type. Digital tumors tend to be osteolytic affecting a single digit.

Inflammatory conditions of the digit, as pododermatitis also occur. Regarding radiographic changes, it was concluded that pododermatitis could not be radiographically differentiated from malignant tumors as both conditions resulted in similar aggressive bone lesions.

References

1.  Thrall DT. In: Thrall, DE Textbook of Veterinary Diagnostic Radiology 4th edition. 2002.

2.  Dennis R, et al. Handbook of small animal radiological differential diagnosis. 2001.