Case 5. Incidental finding on knee MR for trauma

What about lymphoma? It would explain the MR marrow abnormality and the vague sclerosis on the CT. Lesion doesn't fit well for GCT, hemangioma or other benign cartilage tumor.

I'll take a stab, literally: my main response, after showing this case to my colleagues, would be that this lesion deserves biopsy. That is what the clinicians need to know.

Why? It does not look like trauma (good place for contusion after patella disloc in this pt with mild subluxation on MR, but the configuration is all wrong, and there is no edema under the cortex, etc etc). Does not look like infection. CT rules out an unusual appearance of more common lucent lesions such as GCT, ABC, FD. Metabolic?--no. Vascular?--no vessels. Developmental?--no.

Not good for mets based on age and appearance.

My best guess is this is something which is pathophysiologically sneaky, and the sneakiest of them all is lymphoma, with other marrow malignancies lower on the list. It's not great for that, either. Thus, a weirdo infection would have to make the list as well.

I'd have to agree with the other respondents -- lymphoma would be my #1 choice (or some other funky small round blue cell). I have a similar case done in a 30-something woman with an ACL tear, only she had several lesions. I also had another similar case in an older woman (around 60) who hurt her knee doing track and field. Both were totally incidental findings. The bone scan in the first woman was normal. I've actually seen normal bone scans in several lymphoma/leukemia/Ewing's cases, so I would not let a normal bone scan deter biopsy. Ultimately, I believe that is the next step in the work-up should be. So when do we find out?

Excellent responses! We went down much the same lines of reasoning, and did an MR-guided biopsy to find lymphoma, which staged out as the only lesion and was treated with XRT, with good response on 6 month followup. I like this case because it nicely illustrates that marrow infiltrative lesions can be virtually invisible on CT.