Radiological comparison of bone lesion i larg breed dogs

Large breed dog, a Golden Retriever or a Rottweiler, who has been becoming progressively more lame for the last month. The radiograph shows an aggressive bone lesion in the proximal humerus. You suspect a primary bone tumor, but it could also be a fungal osteomyelitis, or metastatic neoplasia. How do you tell them apart?

The characteristics of the aggressive lesion can help you to distinguish between these differential diagnoses. There is considerable overlap between instances of disease, but in general, primary bone tumors are lytic and productive (first image, osteosarcoma), fungal osteomyelitis is primarily productive (second image, coccidiomycosis), and metastatic neoplasia is primarily lytic (third image, metastatic hemangiosarcoma).
Always look at the location of the lesions to refine the differentials.
First, I look to see if it is monostotic or polyostotic. If you can make this distinction, there is a fairly clear dividing line between a primary bone tumor and fungal osteomyelitis or metastatic disease, which tend to be polyostotic.

Second, I look at the distribution of the lesions. Primary bone tumors tend to occur in the metaphyseal region (although the primary bone tumor in image 1 extends distal to the metaphysis), while fungal disease and metastatic disease can occur anywhere in the bone (fourth image shows fungal osteomyelitis proximal to metaphysis). Of course, if the aggressive bone lesion does not follow the expected bone distribution, you will need to rely on other characteristics (images 1, 2 and 3).

Finally, the signalment will help you to priortize differentials. A middle-aged Rottweiler with a monostotic, lytic and productive lesion in the humeral metaphysis is by far more likely to have a primary bone tumor than a medium sized dog with polyostotic lytic and productive lesions in the epiphysis and diaphysis of several long bones.
Fine needle aspirates or biopsies will often provide a definitive diagnosis. Follow-up radiographs can also help to document progression of the lesion or lesions. These procedures can be very helpful when the radiographic signs of the aggressive bone lesions have characteristics of two or three of the main diseases. Unfortunately bone is limited in its responses to insult, and the findings in these diseases often overlap.
Most likely diagnosis for an aggressive bone lesion not involving a joint:

• Primary Bone Tumor – Monostotic, predilection site, destructive and productive, middle-aged large breed dog
  
• Fungal osteomyelitis – Polyostotic, any portion of bone involved, mainly productive
  
• Metastatic neoplasia – Polyostotic, any portion of bone involved, mainly destructive