I have ra, I had a mass on my chest, so they did ct-guided needle biopsy where the thymus is! Doc has told me its not sarcoid, but could be lymphoma?

Thymus. May be the source of the mass, and it could be invasive ornon-invasive. Thymomais associated with rheumatologic/auto-immune conditions. Lymphoma is another prospect, more rare are germ cell tumors (teratoma) and substernal thyroid. Sarcoid can involve any node bearing areas.
Wait for result. Yes, lymphoma is a possibility. There is also relationship between autoimmune disorder like rheumatoid arthritis and certain type of lymphoma. However, i recommend you to wait until you have the result of the biopsy then you can discuss further in detail about what to do.