White spot disease is a common side effect on the skin in the context of cancer treatments with immunotherapies. There is no standard therapy yet for white spot caused by cancer therapies. There is already research in humans on the treatment of white spot disease. Previous human studies with ruxolitinib cream have shown that approximately 30% of patients with white spot disease have achieved a 75% improvement (re-pigmentation) of their affected areas after 24 weeks and approximately 50% of patients have achieved a 75% improvement after 52 weeks. It is also know that a 50% improvement was observed in over 50% of patients after 24 weeks and a 90% improvement in over 15% of patients. However, these studies to date have not included patients with white spot disease caused by cancer therapy. Therefore, we do not know how well the cream works in this situation. In this study, the investigators are therefore examining whether the investigational substance ruxolitinib is effective and well tolerated in white spot disease caused by cancer therapy. The test substance has not yet been approved in Switzerland. Only once the efficacy of the investigational substance ruxolitinib has been scientifically investigated and proven can it be approved in Switzerland and used to treat white spot disease caused by cancer therapy.