Discordant response comparing 18F-FDG PET/CT with response assessment by RECIST in patients with advanced melanoma treated with immune checkpoint blockade

10046 Background: Immune checkpoint blockade (ICB) has changed the natural history advanced melanoma (AM). Based on phase III trial, which used RECIST criteria, the complete response (CR) rate with anti-PD1 therapy is around 20%. In daily practice, PET/CT is a useful tool to evaluate response to treatment in melanoma patients. Little is known about the number of patients who achieve metabolic CR by PET/CT but with anatomic residual disease and their prognosis. Methods: We conducted a retrospective analysis of patients with AM treated with ICB who achieved metabolic CR by PET/CT but with residual disease on tomography and compared to patients with RECIST CR in a high-volume cancer center. Progression-free (PFS) and overall survival (OS) were obtained by Kaplan Meier method and log-rank test. Results: One hundred seventy pts with AM treated with anti-PD1 (79%) or anti-PD1 + anti- CTLA4 (21%) betweenSeptember 2013 and December 2019 were analyzed. At a median follow-up of 23.6 months, seventy-five (44%) pts achieved CR. RECIST criteria: 22 pts (29.3%) and metabolic CR: 53 pts (70.7%). All patients with metabolic CR had RECIST partial response. The median total time on treatment was 14.8m (95%CI:0.9-42.3). The median time to reach CR was 5.4m (95%CI: 3-39). The median time of treatment after CR was 6.8m (95%CI: 0-21.4). The rate of CR patients off treatment at the moment of this analysis was 69%. The median follow-up after discontinuing treatment was 5.2m. There was no difference in PFS (36 month-rate: 84.4% vs 74%, p:0.64) and OS (36 month-rate: 100% vs 86.3%, p:0.14) between pts with CR based on RECIST or PET-CT, respectively. Median time for PFS and OS have not been reached until the date cut-off. Nine pts have relapsed (12%). Seven of them had residual disease on tomography but with no metabolic active lesion(s) at the time of the end of treatment. Conclusions: Twice more patients achieve complete response considering only metabolic parameters on PET/CT compared to RECIST criteria and they seem to have comparable prognosis.