To compare using immuno-PET/CT the distribution of 89Zr-labelled rituximab without and with a preload of unlabelled rituximab to assess the impact of preloading with unlabelled rituximab on tumour targeting and radiation dose of subsequent radioimmunotherapy with 90Y-labelled rituximab in CD20+ B-cell lymphoma. This evidence concerns the gene MS4A1 and B-cell non-Hodgkin lymphoma.