CD8 PET to stop or switch immunotherapy early

Scan for T cells inside the tumour a few weeks after starting immunotherapy, and change course if they have not arrived. Tracers such as 89Zr-crefmirlimab, a CD8 minibody, and 18F-AraG image T-cell infiltration and activation, and an early increase has tracked response in phase 2 studies. Response to checkpoint blockade requires T-cell infiltration, CT changes lag by months and pseudoprogression confounds RECIST, so an absent CD8 PET signal at week 4 could identify non-responders early. The test is a prospective trial in NSCLC or melanoma with week-4 CD8 PET, randomising PET non-responders to continue or switch. At early-clinical maturity it addresses the bottleneck No one can predict who responds to immunotherapy.