I've had a tumor in my chest for about 5 years now and a ct scan 2 years ago showed a possible thymoma or lymphoma. my most recent scan is only showing residual thymus. can you help me understand?

I do not have enough clinical info to answer your question such as your symptoms when the mediastinal tumor was detected, the tumor size and what the reason watchful waiting approach was used instead of intervention. Unless you had had a benign but unusual thymus enlargement and it shrunk in adulthood. Whatever the tumor type, I congratulate you for a natural recovery without any intervention.

There are rare cases reported of spontaneously regressing thymomas as well as lymphomas, as many as 10-20% in select histologies of non Hodgkin subtypes. However, best to clarify would be your current oncologist or primary care doc.