To visualize molecular interactions a novel technique is used in which positron emission tomography (PET) is combined with labeled monoclonal antibodies. Because ipilimumab induces activation of T-lymphocytes it is hypothesized that uptake of 89Zr-ipilimumab in tumor lesions and normal tissue is different (i.e. higher) after the second administration of ipilimumab (3 weeks after first injection). Therefore immuno-PET scans will be performed after the first and after the second injection of ipilimumab.