Weighing the evidence: Is it a benign or malignant bone lesion?

An owner brings an 8-year-old female, neutered Australian Shepherd with acute right forelimb lameness to your office. You palpate a swelling in the metaphysis of the right radius and recommend taking radiographs of the area.

There is a radiolucency in the distal lateral radius, causing expansion of the cortex. There is a mild periosteal reaction. The zone of transition is short. This lesion has mostly benign radiographic characteristics. However, the radiolucent area may indicate destruction.

Characteristics of aggressive and non-aggressive bone lesions

When interpreting musculoskeletal radiographs, we use certain criteria to group/classify the lesion as an aggressive or non-aggressive bone lesion. This strategy helps narrow the differential diagnoses into a more manageable list with similar diagnostic and treatment plans. Aggressive lesions are diseases such as primary bone tumors and fungal osteomyelitis, while more benign lesions include degenerative disease, trauma and developmental orthopedic disease.

The main characteristics of aggressive bone lesions are an irregular periosteal reaction, lysis of the cortex or any trabecular bone and a long zone of transition. All of these findings indicate a rapidly progressive disease that is destroying bone tissue.

Non-aggressive lesions are more clearly defined, and may appear expansile. Developmental orthopedic disease and trauma can include a change in alignment or productive new bone (sclerosis, periarticular osteophytes). Panosteitis is an example of a developmental disease that causes increased medullary new bone.

As with all radiographic interpretation, there are clear cases easily classified into one of these categories, and there are those that lie in the gray zone. They may have both benign and aggressive characteristics, or be in a region that is atypical of the primary differential diagnosis.

Does the history fit with radiographic signs?

We always interpret radiographs in the context of the patient.

After making your radiographic diagnosis, consider the history and think about whether it fits with your findings. In the case above, the lesion had many benign characteristics, and the dog was relatively young. However, the lameness was acute, and there was an area of lysis. In addition, the distal metaphysis of the radius is a site of predisposition for primary bone tumors.

Treat atypical lesions with suspicion

Because the radiographic appearance was at odds with the clinical presentation, this dog underwent a bone biopsy of the lesion. The first biopsy was non-diagnostic, but the second attempt returned a diagnosis of fibrosarcoma. Bone has a limited range of responses to diseases, and many etiologies can cause the same appearance.

Any lesion that is not typical of a certain disease should be treated with suspicion. Tissue samples and follow-up radiographs are options to present to the owner.

Dr. Zwingenberger is a veterinary radiologist at the University of California-Davis.