Inclusion Criteria:

   - Age ≥ 18 years old

   - Histologically confirmed aggressive B cell NHL including the following types defined
   by WHO 2008:

      - DLBCL not otherwise specified; T cell/histiocyte rich large B cell lymphoma;
      DLBCL associated with chronic inflammation; Epstein Barr virus (EBV)+ DLBCL of
      the elderly; OR

      - primary mediastinal (thymic) large B cell lymphoma

      - transformation of follicular lymphoma, marginal zone lymphoma or chronic
      lymphocytic leukemia to DLBCL will also be included

   - Measurable disease by PET imaging (as defined by Cheson (2014)), that meets all the
   following criteria:

      - At least one measureable lesion away from head & neck, liver, kidneys, GI tract
      and bladder

      - At least one biopsy-accessible lesion or lymph node.

   - Express willingness to undergo low risk FNA or core biopsy of subcutaneous accessible
   lesion or lymph node.

   - Scheduled to receive commercial or research CAR T cell therapy with axicabtagene
   ciloleucel (Yescarta ®) as part of anticancer therapy.

   - Adequate renal and hepatic function, defined as:

      1. Creatinine clearance (as estimated by Cockcroft Gault) ≥ 60 mL/min or Cr < 1.6
      mg/dL

      2. Serum alanine aminotransferase (ALT) or aspartate aminotransferase (AST) ≤ 2.5x
      upper limit of normal (ULN)

      3. Total bilirubin ≤ 1.5 mg/dL, except in cases of Gilbert's syndrome

   - Able to give informed consent. Subjects unable to give informed consent will not be
   eligible for this study

Exclusion Criteria:

   - Women who are pregnant or breastfeeding.

   - Subjects with significant GI disease involvement by PET imaging

   - In the investigator's judgment, have any medical condition likely to interfere with
   assessment of safety or efficacy, be unable to tolerate additional radiation, or be
   unlikely to complete all protocol-required visits and procedures.