Detection of tumor immune checkpoints: from pathological analysis to functional imaging

Abstract Tumor immunotherapy has emerged as a transformative approach following conventional treatments such as surgery, radiotherapy, and chemotherapy. The discovery and application of immune checkpoints, particularly through inhibitors targeting PD-1 and CTLA-4, have led to remarkable clinical successes in cancers including melanoma and non-small cell lung cancer (NSCLC). However, not all patients respond to immune checkpoint inhibitors (ICIs), and treatment is often accompanied by immune-related adverse events, highlighting the need for predictive biomarkers. Assessing immune checkpoint expression prior to therapy is essential, yet current methods face limitations: immunohistochemistry (IHC) is invasive and hampered by tumor heterogeneity, while next-generation sequencing (NGS), circulating tumor cell (CTC) assays, and microsatellite instability (MSI) testing are constrained by cost and sample accessibility. Positron emission tomography (PET), a non-invasive functional imaging technique, offers a promising alternative by enabling real-time, whole-body quantification of target expression using radiolabeled probes. Recently, PET probes targeting PD-1, PD-L1, and CTLA-4 have been developed, facilitating patient stratification, treatment monitoring, and personalized immunotherapy. This review systematically examines methods for detecting tumor immune targets—from traditional pathology to advanced functional imaging—evaluating their principles, applications, and limitations, with the aim of guiding future research and clinical practice in precision immuno-oncology.