Rationale: Dabrafenib is an oral protein tyrosine kinase inhibitor which specifically targets mutated BRAF protein. It is used in the treatment of metastatic melanoma with evidence of a BRAF V600 mutation in genomic material. However, in this group of patients often a heterogenic response to treatment is seen. Heterogeneity in drug accumulation in the tumor could be responsible for the observed differences in treatment response between lesions and between patients. Besides poor tumor accumulation of the drug, heterogeneous expression of the drug target, B-Raf protein, between patients and between lesions within a single patient could account for heterogeneity in treatment response.