18F-FDG-PET detects complete response to PD1-therapy in melanoma patients two weeks after therapy start.

PURPOSE: The aim of the study was to evaluate if 18F-FDG-PET has the potential to detect complete responders to PD1-therapy in patients with unresectable metastasized melanoma two weeks after therapy initiation.

METHODS: Between September 2014 and May 2016, ten patients (four females; 65 ± 12 y) received a whole-body 18F-FDG-PET/MRI examination at three time points: Before therapy start (t0 , base-line), two weeks (t1 , study examination) and three months after treatment initiation (t2 , reference standard). Therapy response was assessed with PET response criteria in solid tumors (PERCIST). Time to progression and overall survival (OS) were obtained for all patients.

RESULTS: Three patients with partial metabolic response in PET at t1 turned out to have complete response at t2 . No tumor relapse was observed in those patients so far (observation period: 265, 511 and 728 days, respectively). At t2 , progressive metabolic disease (PMD) was seen in six patients from whom four showed PMD and two showed stable metabolic disease (SMD) at t1 . OS in patients with PMD at t2 varied between 148 and 814 days. SMD at both t1 and t2 was seen in one patient, tumor progress was observed after 308 days.

CONCLUSION: Our study indicates that whole-body 18F-FDG-PET might be able to reliably identify complete responders to PD1-therapy as early as two weeks after therapy initiation in stage IV melanoma patients. This might help to shorten therapy regimes and avoid unnecessary side effects in the future.