Researchers are moving toward imaging approaches that can verify whether immunotherapy drugs reach tumors and engage targets. A study in the British Journal of Cancer evaluated PET imaging strategies intended to show drug delivery to the tumor microenvironment, addressing a key clinical gap: many checkpoint inhibitor patients do not respond, and clinicians currently lack a reliable way to determine target exposure upfront. The work centers on whether the PET signal can function as a practical proxy for tumor delivery, offering a potential route to earlier treatment selection or discontinuation. For trial designers, the ability to confirm target engagement in vivo also creates a measurable endpoint beyond response rates alone. If validated broadly, such tools may help reduce the time and cost spent on ineffective therapies, while sharpening patient stratification for next-generation immunotherapy combinations.