Molecular PET and immuno-PET in gastric cancer immunotherapy: current evidence, clinical utilities, and translational barriers.

This mini-review aims to examine how molecular PET and immuno-PET may complement tissue-based biomarkers in gastric cancer immunotherapy by enabling whole-body assessment of target expression, stromal biology, immune effector activity, and treatment-related inflammatory changes. Gastric cancer remains a major global health burden, while immunotherapy is expanding from advanced disease into perioperative and conversion settings. However, clinical benefit remains heterogeneous, and conventional biomarkers such as PD-L1, HER2, and MSI/dMMR do not fully capture spatial and temporal variation across lesions. Molecular PET broadly encompasses tracers targeting metabolism, receptors, stromal components, and immune biology, whereas immuno-PET uses radiolabeled antibodies or related scaffold proteins to visualize target distribution and, in some settings, pharmacodynamic changes in vivo. Current evidence in gastric cancer suggests potential roles for HER2-targeted PET, FAPI PET, and granzyme B PET in lesion characterization, stromal phenotyping, early response assessment, and prognostic stratification. However, PD-L1 imaging in gastric cancer remains supported mainly by preclinical data, and much of the checkpoint-imaging literature is extrapolated from non-gastric malignancies. Major barriers include limited gastric-cancer-specific prospective cohorts, heterogeneous tracer formats and imaging time points, lack of standardized quantitative thresholds, and a shortage of outcome-driven studies. Overall, molecular PET and immuno-PET should currently be viewed as promising translational tools rather than routine clinical biomarkers, with their greatest near-term value likely lying in multimodal patient stratification and early pharmacodynamic assessment.