Did you have a thymectomy already (sorry, I can't remember!)? If so. I would think it would be thymic tissue that was missed when the original surgery was done. Does that sound like what you found in your research?

I know if you have a thymus with hyperplasia (tissue that has spread outside of the thymus gland itself, some may be missed during the thymectomy, depending on the procedure and the thoroughness of the surgeon.

Here's hoping you get your answer soon.