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

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. 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. 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. 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. (orig.)