ICAM-1 promotes T cell glycolytic reprogramming and tumor infiltration to drive 18F-FDG PET flares following radiotherapy.

18F-fluorodeoxyglucose (18F-FDG) is the most widely used radiotracer for positron emission tomography (PET) imaging in clinical oncology, owing to the elevated glycolytic activity of tumor cells. However, transient post-radiotherapy (RT) "metabolic flares" of 18F-FDG uptake are frequently observed in patients and are traditionally attributed to localized inflammatory responses. Whether these flares are linked to immune cell dynamics, particularly tumor-infiltrating T cells, and the mechanisms involved remain poorly understood. Here, we demonstrate that RT markedly upregulates intracellular adhesion molecule-1 (ICAM-1) expression and promotes T cell infiltration in tumors, as observed in both patients and mouse models. Genetic ablation of ICAM-1 significantly attenuates RT-induced metabolic flares in irradiated tumors, primarily due to reduced 18F-FDG uptake by tumor-infiltrating T cells rather than myeloid cells. Mechanistically, ICAM-1 engages with lymphocyte function-associated antigen 1 (LFA-1) to facilitate T cell clustering, thereby promoting their intratumoral accumulation and activating glycolysis and the tricarboxylic acid cycle via the PI3K-AKT-mTOR signaling pathway. These findings identify ICAM-1 as a critical regulator of T cell metabolic reprogramming and tumor infiltration following RT, offering a mechanistic explanation for 18F-FDG PET flares. Clinical monitoring of post-RT tumor ICAM-1 expression may enhance PET interpretation and aid in distinguishing pseudoprogression from true tumor progression.