The main purpose of this study is to find out if a type of CT (computed topography) scan can help to know if pembrolizumab is working. This CT scan (called Texture Analysis) looks at the tumour in fine detail. The use of this type of CT scan has not been studied in patients with metastatic melanoma. Investigators want to see if this CT scan will show if some patients being treated with pembrolizumab respond better than others.