LAUSR

immunochemotherapy, in comparison with PET-CT at standard time points and with blood based detection of a ctDNA. Patients underwent standard F-fluorodeoxyglucose (FDG) PET-CT during screening and at the conclusion of 2 cycles of PBC per standard clinical practice, and research FDG PET-CT on day 4 and day 21 of cycle 1 of PBC. Treating physicians were not blinded to the research PET-CT results, but were encouraged to manage the patients per standard practice. Results: The protocol has accrued 28 patients from February 2016 to March 2019. The median age was 62 years (range: 25-82), and 36% were female. 61% had poor risk IPI, 71% had advanced stage. 24 patients had a PET-CT on day 4 (4 missing due to patient status factors and logistical issues). The response rates on day 4 were: complete response 21% (CR, n=5), partial response 33% (PR, n=8), stable disease 37.5% (SD, n=9), and progressive disease 8% (n=2). Among the 5 patients with CR on day 4, all had CR at the end of therapy and have not relapsed. Among the 8 patients with PR on day 4, 2 achieved a CR at the end of therapy and 6 had progressive disease. Among the 11 patients with SD or PD on day 4, 2 patients achieved a PR but both had eventual relapse and all others had PD at end of therapy. Additional data from day 21 PET-CT and ctDNA testing may be presented at the meeting. Conclusions: Our pilot study suggests that very early FDG PET-CT may be able to predict for end of therapy response in patients with relapsed DLBCL. Patients who achieve less than a complete response on day 4 of salvage chemotherapy have a high probability of therapeutic failure and could be considered for alternative therapeutic options. The limitations of our study include small patient numbers, heterogeneity of therapy, and availability of PET-CT at very early time points off study.