TSPO and myeloid sarcoma: • Diffuse microglial activation observed using TSPO PET in progressive MS (Banati et al., 2000; Politis et al., 2012a; Rissanen et al., 2014; Sucksdorff et al., 2020).• TSPO levels can differentiate chronic active and chronic inactive lesions (Rissanen et al., 2014).• TSPO cannot differentiate different phenotypes of microglia (Nutma et al., 2019).• Increased detection of TSPO predominantly reflects microglia/macrophage density in MS patients, and not activation phenotype (Nutma et al., 2021).