Defining ctDNA Metrics in Posttransplant Lymphoproliferative Disorder (PTLD)

Inclusion Criteria: * Histologically confirmed CD20+ PTLD including the below subtypes: * Polymorphic * Monomorphic * Age ≥ 15. * Participants must have measurable disease, defined as lymph node ≥ 1.5 cm in greatest diameter per Lugano Classification * Patients must have a PET-CT scan (preferred; alternatively CT chest, abdomen and pelvis with IV contrast) performed within 28 days prior to the start of the study. * All participants must be screened for chronic hepatitis B virus (HBV) within 28 days prior to registration. Participants with known HBV infection (positive serology) must also have a HBV viral load performed within 28 days prior to registration, and participants must have an undetectable HBV viral load on suppressive therapy within 28 days prior to start of treatment. Participants found to be HBV carriers during screening are eligible and must receive standard of care prophylaxis. Participants with active Hepatitis B (HBV viral load \> 500 IU/mL) within 28 days prior to registration are not eligible. * Participants with a history of hepatitis C virus (HCV) infection must have been treated and cured. Participants with an active HCV infection who are currently on treatment must have an undetectable HCV viral load within 28 days prior to registration. * Participants with known human immunodeficiency virus (HIV)-infection are eligible providing they are on effective anti-retroviral therapy and have undetectable viral load at their most recent viral load test (mus…