Although PET-CT with 18F-FDG is widely used to assess the response of tumours to traditional therapy, its use is limited in assessing response to immunotherapy agents.129,130 This is due to the complex tumour microenvironment, in which metabolically active immune cell infiltrates can cause an expansion in tumour volume with increased glucose uptake.130 As the generation of an immune response leads to T-cell activation, detecting T-cell subsets (CD8+ and CD4+) within tumours might enable enhanced imaging of this response. The gene discussed is CD4; the disease is neoplasm.