I just want to add a caution here about tumors on the thymus. My CT scan and MRi didn't show a tumor either, but when I had to have some swollen lymph nodes biopsied and they were going into my chest, I had them remove the thymus also (being as they were there anyway). This was 5 years after diagnosis. Turns out I had a Stage II malignant tumor in my thymus. Fortunately, they got clean margins, but I had 28 radiation treatments, just in case.

So a tumor doesn't always show up on traditional testing.