The purpose of this study is to find out if there is a benefit to giving rituximab with etoposide, prednisone, vincristine, cyclophosphamide and doxorubicin (R-EPOCH) in patients who have high-risk Bcell posttransplant lymphoproliferative disorder (PTLD), while those with low-risk disease will be spared of chemotherapy and treated with rituximab alone. Concurrently this study also wants to see how helpful a new blood test called circulating tumor DNA (ctDNA) is. This test has already helped doctors make better predictions and treatment decisions in other types of lymphoma. The goal is to...