89Zr-DFO-REGN3767 in PET Scans in People With Diffuse Large B Cell Lymphoma (DLBCL)

Inclusion Criteria: A patient must be eligible for DLBCL expansion cohort 9 in study 17-421 and, in addition, meet the following criteria to be eligible for inclusion in the study: * Measurable disease by Lugano criteria, with at least one lesion outside of the liver * Patients must have eGFR \>50 mL/min/1.73m2. Exclusion Criteria: A patient must be eligible for DLBCL expansion cohort 9 in study 17-421 and, in addition, a patient who meets any of the following criteria will be excluded from the study: * Patients who have permanently discontinued anti-cancer immune modulating therapies due to drug-related toxicity. * Has not yet recovered (i.e. ≤ Grade 1 or baseline) from any acute toxicities from prior anticancer therapy except for laboratory changes as described in inclusion criteria in study 17-421, except as noted. NOTE: Patients with chronic or stable toxicity following approved therapy, such as mild persistent neuropathy, are allowed. * Has received radiation therapy within 14 days of first administration of study drug or has not recovered (i.e. ≤ Grade 1 or baseline) from adverse events, except for laboratory changes as described in inclusion criteria, except as noted. NOTE: Patients with chronic or stable toxicity, such as mild persistent neuropathy, are allowed. * Women who are pregnant, breastfeeding \*Postmenopausal women must be amenorrhoeic for at least 12 months in order not to be considered of childbearing potential. Pregnancy testing and contraception are not required for women with documented hysterectomy or tubal ligation. Pregnancy testing and screening will be performed per MSK and Department of Radiology standard pregnancy screening guidelines.