FLT-PET at 6 Weeks Predicts Response Assessed by CT at 12 Weeks in Melanoma Patients Treated with Pembrolizumab

Purpose Investigate the ability of ¹⁸F-fluorothymidine (FLT) PET combined with CT at 6 weeks to predict treatment response at 12 weeks after treatment with pembrolizumab. Methods Five patients with unresectable stage IV melanoma were included in this single-institution pilot study. Patients underwent FLT-PET/CT (baseline and 6 weeks) and CT (baseline and 12 weeks). FLT-PET/CT response and CT response were assessed using PET Response Criteria in Solid Tumors and immune Response Evaluation Criteria in Solid Tumors, respectively. Patients were categorized as responders (complete response, partial response) and nonresponders (stable disease, progressive disease). Agreement between 6-week FLT-PET/CT and 12-week CT was calculated using Cohen kappa's agreement. Eight baseline FLT-PET/CT parameters were extracted: SUV_max, SUV_peak, SUV_SD, SUV_mean, proliferative tumor volume, total lesion proliferation, bone marrow-to-liver SUV_max ratio, and spleen-to-liver SUV_max ratio. Eight delta-parameters were extracted at 6 weeks by calculating variation in FLT uptake as percentage change from baseline. Results Agreement between 6-week FLT-PET/CT and 12-week CT was kappa = 0.615, P = 0.025. Three of 5 patients were categorized as responders on CT by immune Response Evaluation Criteria in Solid Tumors. At baseline, responders had a lower mean proliferative tumor volume and a higher bone marrow-to-liver SUV_max ratio. At 6 weeks, responders demonstrated a decrease in tumor volume and tumor proliferation. Conclusions Our study illustrates the potential for FLT-PET/CT as an early predictor of response for patients with metastatic melanoma on anti-PD1 immunotherapy. Larger studies are indicated to confirm these findings.