In this study, invesigators propose to analyze 150 DLBCL patients, 50 MCL patients, and 100 FL patients to determine the clinical utility of ctDNA- as well as circulating tumor cell (CTC)-based MRD assessment in CAR therapy patients. The project detailed in this protocol will utilize the clonoSEQ platform as specific quantification of residual DLBCL/FL/MCL and correlate its results with radiologic assessment of disease and clinical outcomes. Invesitgators predict there will be a strong correlation between ctDNA and PET/CT and dynamic changes in ctDNA will precede radiologic evidence of disease recurrence in patients following CAR therapy.