The study commences with a biodistribution study (phase 1) in 5 subjects. Thereafter the main study (phase 2) starts, in which 25 subjects will receive two [18F]FB-IL2 PET scans at baseline and week 6 of treatment with either ipilimumab, nivolumab, pembrolizumab or the combination of ipilimumab and nivolumab. If [18F]FB-IL2 PET is able to detect a response to treatment, it could serve as a non-invasive early indicator of T cell response to the treatment. Besides, accumulation of the PET tracer in non-target tissue could indicate infiltration of activated T cells in normal organs and thus may predict the development of an immune-related adverse event.