Radiotherapy and tertiary lymphoid structures: balancing immune activation and immune damage in cancer immunotherapy

Cancer immunotherapy, exemplified by immune checkpoint blockade (ICB), remains strongly influenced by the pre-existing immune organization of the tumor microenvironment. Tertiary lymphoid structures (TLSs) are ectopic lymphoid aggregates whose density, maturation state, and spatial localization correlate with clinical prognosis and ICB response across several malignancies. Radiotherapy (RT) can reshape this immune context in opposing directions. By inducing immunogenic cell death, antigen release, cGAS-STING/type I interferon signaling, vascular remodeling, and lymphocyte recruitment, RT can create conditions that support TLS-associated antitumor immunity in selected settings. Conversely, high-dose or large-volume irradiation, poorly timed nodal exposure, and collateral injury to lymphocytes, tumor-draining lymph nodes, stromal scaffolds, and high endothelial venules can disrupt established TLSs or prevent their maturation. This review summarizes current evidence on the bidirectional relationship between RT and TLS biology, differentiating validated mechanisms from indirect evidence and hypothesis-generating translational concepts. We expound on how dose, fractionation, timing, irradiated volume, nodal management and radiation modality may influence antigen presentation, lymphocyte availability, and local immune architecture. Potential strategies such as lymph-node-aware planning, proton or heavy-ion therapy, FLASH RT, vascular normalization, STING or LTβR agonism, and ICB combinations are evaluated as investigational approaches rather than established TLS-directed clinical interventions. Future studies should evaluate whether optimized RT regimens can preserve or promote the functional maturation of TLSs, integrating paired tissue biopsies, spatial transcriptomics, advanced imaging, and circulating biomarkers to definitively correlate TLS remodeling with clinical outcomes. Tertiary lymphoid structures (TLSs) function as clinically relevant immune-organizing niches whose prognostic and predictive value depends on maturation state, density, and spatial localization. Radiotherapy can promote TLS-associated immunity via antigen release, cGAS-STING/type I interferon signaling, and vascular remodeling, but may impair TLSs through lymphopenia, nodal injury, and stromal damage. The immunological impact of radiotherapy on the tumor microenvironment is determined by the dose, fractionation, timing, irradiated volume, nodal exposure, radiation modality, and baseline immune context. Lymph-node-sparing, volume-conscious, and vascular-preserving radiotherapy strategies can minimize collateral immune damage where oncologically feasible. Proton, heavy-ion, and FLASH radiotherapy offer plausible immune-preserving advantages, though direct evidence linking these modalities to TLS induction or maintenance remains limited. Combining radiotherapy with ICB, STING or LTβR agonism, vascular normalization, or stromal reprogramming must be investigated as stratified translational hypotheses with prespecified TLS endpoints.