Elbow #2 - Answers

1. Metastatic renal cell carcinoma, proximal radius.

These films show a large bubbly, expansile lucent lesion of the proximal radius. No periosteal reaction is noted, and no matrix formation is seen. The lesion appears to extend to the joint surface of the proximal radius. The differential diagnosis here is the generic FEGNOMASHIC or FOGMACHINES differential.

When going through this differential, my reasoning would go something like this:

• Fibrous dysplasia: possible, but less likely in someone this old.
• Enchondroma/Eosinophilic granuloma: Without any visible chondroid matrix, enchondroma would be a lot less likely. E.G. is unlikely in this age group.
• Giant cell tumor: possible, especially extending to the joint space like this.
• Non-ossifying fibroma: very unlikely in this age group. Also, they tend to be more eccentric than this.
• Osteoblastoma: unlikely. They tend to occur mostly in the posterior elements of the spine.
• Met/myeloma: very likely. Certain mets, like renal cell and thyroid carcinoma can be extremely expansile.
• Aneurysmal bone cyst: possible, for same reason as giant cell tumor.
• Simple Cyst: very unlikely. This patient is a bit old for a simple cyst, and this lucent lesion appears a bit too complex for a simple cyst.
• Hyperparathyroidism/Hemangioma: possible.
• Infection: unlikely.
• Chondroblastoma: possible, for same reason as giant cell tumor.

Of the entities listed above, metastasis and myeloma are the most likely in this age group, on a statistical basis alone. However, several of the other items listed above are possible and could be listed lower in the differential.

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