Interim FDG PET-CT in Melanoma Metastatic Patient's Treated by Anti-PD1 Therapy

This study is investigating whether a PET/CT scan done partway through immunotherapy treatment can help predict how well the therapy is working in people with advanced melanoma treated with anti-PD1 drugs (nivolumab or pembrolizumab). The goal is to find a way to identify earlier whether the treatment is effective or needs to be changed. **You may be eligible if...** - You are 18 or older - You have advanced (metastatic) melanoma confirmed by biopsy — cutaneous or unknown primary type - You are starting treatment with nivolumab or pembrolizumab - You have social insurance (required in France where this study is conducted) **You may NOT be eligible if...** - You have ocular (eye) or mucosal (inner lining) melanoma - You have uncontrolled diabetes or severe claustrophobia (PET scan contraindications) - You are HIV, hepatitis B, or hepatitis C positive - Your only tumors are very small (under 8 mm), except for lung nodules - You have active autoimmune disease Talk to your doctor to see if this trial is right for you.