Reframing the paradigm: stereotactic body radiation therapy as an engineer of the tumor immune microenvironment

Abstract

Background

Stereotactic Body Radiation Therapy (SBRT) is undergoing a paradigm shift from a purely local ablative tool to a systemic immunomodulatory agent. However, a significant knowledge gap remains in understanding how to precisely “engineer” the tumor immune microenvironment (TME) to overcome resistance to immune checkpoint inhibitors (ICIs). Current approaches often rely on empirical combinations rather than biologically guided strategies.

Methods

We conducted a comprehensive narrative review of literature published up to January 2026 using PubMed and Web of Science databases. Keywords included “SBRT,” “immunotherapy,” “cGAS-STING,” “abscopal effect,” and “tumor microenvironment.” We specifically synthesized evidence comparing the immunobiological impacts of ablative versus immunogenic doses (e.g., the TREX1-cGAS-STING axis) and analyzed organ-specific immune tolerance mechanisms.

Results

Evidence suggests a “dose-dependent immunomodulatory window,” where moderate hypofractionation (e.g., 8 Gy x 3) optimally induces type I interferons via the cGAS-STING pathway, whereas single high doses (>12-18 Gy) may dampen immunity through TREX1 induction. Beyond direct cytotoxicity, SBRT remodels the TME by depleting regulatory T cells and recruiting effectors, though this is often counterbalanced by a biphasic influx of MDSCs. Clinical translation faces challenges such as organ-specific immune tolerance, necessitating tailored triplet therapies.

Conclusion

Future success lies in a precision medicine approach: moving beyond generic combinations to “organ-specific triplets” (e.g., adding macrophage-targeting agents in liver disease) and implementing adaptive “closed-loop” protocols where real-time liquid biopsy feedback dictates the timing of the next SBRT pulse.

1. Introduction

Stereotactic Body Radiation Therapy (SBRT) has historically been defined by its physical precision and ablative potency (1). However, a paradigm shift is underway, redefining SBRT not merely as a local cytoreductive tool, but as a systemic immunomodulatory agent capable of “engineering” the tumor immune microenvironment (TME) (2). Despite the promise of combining SBRT with immune checkpoint inhibitors (ICIs), clinical results have been mixed, suggesting that a “one-size-fits-all” approach to dosing and scheduling is insufficient (3). A significant knowledge gap remains in understanding how to precisely tailor radiation schemes to overcome organ-specific immune tolerance and resistance mechanisms. Current approaches often rely on empirical combinations rather than biologically guided strategies (4).

To ensure a robust synthesis of evidence addressing these gaps, we conducted a comprehensive narrative review of literature published up to January 2026 using PubMed and Web of Science databases. Keywords included “SBRT,” “immunotherapy,” “cGAS-STING,” “abscopal effect,” and “tumor microenvironment.” We specifically synthesized evidence comparing the immunobiological impacts of ablative versus immunogenic doses (e.g., the TREX1-cGAS-STING axis) and analyzed organ-specific immune tolerance mechanisms to propose a new framework for precision radio-immunotherapy.

2. The foundational immunobiology of SBRT: the in situ vaccine effect

This section establishes the core biological tenet of SBRT’s immunomodulatory power: its ability to convert an irradiated tumor into an immune-activating hub, an effect termed the in situ vaccine.

2.1. Induction of immunogenic cell death and pro-inflammatory signal release

SBRT, through the delivery of high-dose radiation, induces a specialized form of apoptosis known as immunogenic cell death (ICD) (5). Unlike conventional apoptosis, ICD is accompanied by the release of potent immunostimulatory molecules.

Damage-Associated Molecular Patterns (DAMPs): During SBRT-induced ICD, critical DAMPs are released. These include surface-exposed calreticulin (CALR) and heat-shock proteins (HSPs), which act as “eat-me” signals for antigen-presenting cells (APCs), and extracellularly released High Mobility Group Box 1 (HMGB1) and adenosine triphosphate (ATP) (6–8). These molecules function as danger signals, binding to pattern recognition receptors (PRRs) on APCs to initiate an inflammatory cascade.

Tumor-Associated Antigens (TAAs): SBRT-induced cell death results in the release of a large bolus of TAAs into the TME and circulation (9). This flood of antigens provides the raw material for the immune system to recognize and mount a specific response against the tumor, effectively creating a personalized in situ vaccine.

2.2. Activating antigen-presenting cells and priming the adaptive immune response

The combination of DAMPs and TAAs provides a powerful stimulus for dendritic cells (DCs)—the most critical APCs for priming T-cell responses (10). SBRT promotes the maturation of DCs and enhances their cross-presentation of tumor antigens in draining lymph nodes, leading to the activation of naive T-cells (11). This process is fundamental to activating a robust, tumor-specific adaptive immune response that links innate immune activation at the tumor site to systemic, durable immunity.

2.3. Upregulation of MHC-I and enhanced tumor cell recognition

Surviving tumor cells within the radiation field are also modulated. SBRT has been shown to increase the expression of Major Histocompatibility Complex Class I (MHC-I) molecules on the surface of cancer cells (12). This upregulation makes tumor cells more “visible” to the immune system, enhancing their recognition and subsequent killing by cytotoxic CD8+ T lymphocytes (CTLs) (13). This mechanism is critical for the effector phase of the anti-tumor immune response.

The in situ vaccine effect is not a side effect of SBRT but one of its core mechanisms of action, providing the biological rationale for its combination with immunotherapy. SBRT generates the antigen (the “what to attack”) and the inflammatory signals (the “danger alert”), while ICIs “release the brakes” on the subsequently activated T-cells. This reveals a powerful, multi-step synergy: SBRT creates the ideal upstream conditions (antigen release and DC activation) for ICIs to be effective, and ICIs enhance the downstream effector function of T-cells (14, 15). Without the SBRT-induced immune priming, the population of tumor-specific T-cells for an ICI to act upon may not exist, explaining SBRT’s ability to convert immunologically “cold” tumors into “hot” ones (Figure 1).

Figure 1.

The ‘in situ vaccine’ effect of SBRT. Stereotactic Body Radiation Therapy (SBRT) induces immunogenic cell death (ICD) in cancer cells, leading to the release of Damage-Associated Molecular Patterns (DAMPs)—such as surface-exposed calreticulin (CALR), High Mobility Group Box 1 (HMGB1), and ATP—and a bolus of Tumor-Associated Antigens (TAAs). These molecules act as danger signals that promote the maturation of antigen-presenting cells (APCs), such as dendritic cells. Mature APCs then present the TAAs to naive T-cells, priming their activation and differentiation into cytotoxic T lymphocytes (CTLs), which can mediate a systemic anti-tumor immune response.

3. The dose-response conundrum: engineering the optimal immunogenic regimen

This section delves into the most critical and nuanced aspect of SBRT as an immune modulator: the profound and often paradoxical effects of dose and fractionation. The central thesis is that maximizing immune stimulation is not synonymous with maximizing radiation dose.

3.1. The immunogenic “sweet spot”: why hypofractionated regimens (e.g., 8 Gy x 3) may outperform single ablative doses

Preclinical studies have consistently demonstrated that moderate hypofractionated regimens, such as 8 Gy x 3 fractions, are potent inducers of a Type I Interferon (IFN-I) response, which is a cornerstone of effective anti-tumor immunity (16). In contrast, single, very high doses of radiation can trigger counter-regulatory immunosuppressive mechanisms (17). This introduces the central concept of an “immunomodulatory dose window” that may differ from the dose required for maximal direct cytotoxicity.

3.2. The TREX1-cGAS-STING axis: a dose-dependent molecular switch for immunogenicity

The cGAS-STING Pathway: This pathway is a critical sensor of cytosolic DNA. SBRT-induced DNA damage leads to the accumulation of micronuclei and cytosolic DNA fragments, which are detected by cGAS. This triggers the production of cGAMP, activation of STING, and ultimately a robust IFN-I response (18). The IFN-I cascade is essential for recruiting and activating DCs and priming CD8+ T-cells.

TREX1 Induction as a Negative Feedback Loop: Three Prime Repair Exonuclease 1 (TREX1) is a cytosolic nuclease that prevents aberrant activation of the cGAS-STING pathway and autoimmune responses by degrading DNA (19).

The Dose Threshold: Critically, research has identified a dose threshold. Single radiation doses above 12-18 Gy strongly induce the expression of TREX1 in cancer cells (20).

The Immunosuppressive Consequence: By degrading the cytosolic DNA generated by high-dose radiation, TREX1 effectively short-circuits the cGAS-STING pathway, blunting the IFN-I response and limiting the immunogenicity of the treatment (21). Conversely, hypofractionated regimens like 8 Gy x 3 have been shown to downregulate TREX1 expression, maximizing cytosolic DNA accumulation and leading to a more potent immune response (Figure 2) (16).

Figure 2.

Dose-dependent regulation of the cGAS-STING pathway by TREX1. (Top) Moderate doses of radiation induce the accumulation of cytosolic DNA fragments in tumor cells. This cytosolic DNA is sensed by cGAS, which activates the STING pathway to drive a robust Type I Interferon (IFN-I) response essential for anti-tumor immunity. (Bottom) In contrast, single high doses of radiation (>12-18 Gy) strongly upregulate the exonuclease TREX1. TREX1 degrades cytosolic DNA, thereby preventing cGAS-STING activation and blunting the subsequent IFN-I production, which limits the immunogenicity of the treatment.

3.3. Other nucleic acid sensing pathways

It is critical to note that cGAS-STING is not the sole mechanism of radiation-induced sensing. SBRT can also trigger the transcription of endogenous retroviruses (ERVs) and induce mitochondrial DNA leakage (22). These events activate RNA-sensing pathways, specifically RIG-I and MDA5, which operate in parallel with cGAS-STING to amplify Type I interferon signaling. Neglecting these alternative sensing mechanisms may lead to an incomplete understanding of the radiation-induced inflammatory cascade (23).

3.4. Beyond ablation: the distinct role of low-dose radiotherapy

It is crucial to distinguish the immunomodulatory mechanisms of ablative SBRT from Low-Dose Radiotherapy (LDRT, typically < 1-2 Gy). While SBRT acts as an in situ vaccine causing immunogenic cell death and antigen release, LDRT functions primarily as a TME modulator. LDRT has been shown to repolarize tumor-associated macrophages (TAMs) from an M2-like immunosuppressive phenotype to an M1-like pro-inflammatory state and improve T-cell infiltration by normalizing aberrant tumor vasculature (24). Therefore, a novel “RadScopal” strategy combining high-dose SBRT to the primary tumor (for antigen release) with LDRT to metastatic sites (to remodel the stroma) represents a promising avenue for overcoming systemic resistance. It is important to note that while this combination strategy is currently supported predominantly by robust preclinical mechanistic inference, emerging early-phase clinical evidence has begun to demonstrate LDRT’s ability to reverse tumor immune desertification and overcome resistance to immunotherapy in patients (24). Ongoing and future prospective clinical trials are necessary to fully establish the clinical efficacy and safety of this RadScopal approach.

3.5. The complexity of the “immunogenic sweet spot”

While the “8 Gy x 3” regimen is highlighted as a robust inducer of IFN-I, it is important to acknowledge that this is not a universal rule. The optimal immunogenic dose is highly context-dependent, varying by tumor histology, intrinsic DNA repair capabilities (e.g., BRCA mutation status), and the baseline immune landscape. For instance, melanoma models may require different fractionation schemes compared to carcinomas to achieve maximal cGAS-STING activation (25). Thus, the “sweet spot” should be viewed as a biologically defined window rather than a rigid prescription.

3.6. Ablative doses and vascular destruction: an indirect pro-inflammatory and cytotoxic mechanism

Beyond direct DNA damage to tumor cells, high-dose SBRT (>8-10 Gy per fraction) has profound effects on the tumor vasculature (26).

Ceramide-Mediated Endothelial Apoptosis: High radiation doses rapidly induce ceramide-mediated apoptosis in tumor endothelial cells, a distinct mechanism from the slower mitotic death observed in tumor cells (27).

Vascular Occlusion and Ischemic Cell Death: This endothelial cell death leads to severe vascular damage, occlusion, and a sharp reduction in tumor perfusion (28). This creates a hypoxic and nutrient-deprived microenvironment, triggering widespread indirect tumor cell death. This vascular mechanism is considered a key contributor to the high efficacy of SBRT, which cannot be fully explained by direct cell-kill models alone (29).

A fundamental tension exists between the two primary goals of SBRT in an immuno-oncology context: direct tumor ablation and immune priming. The very high doses (>12-18 Gy) required for maximal ablation may be suboptimal for immune priming due to TREX1 induction (16). Conversely, the moderate hypofractionated doses that appear most immunogenic may be less ablative. This paradox presents a clinical dilemma, where the choice of dose regimen involves a trade-off between local control and systemic immune potential. Traditional radiobiological concepts like the Biologically Effective Dose (BED), which model direct cell kill, are insufficient to optimize SBRT-immunotherapy combinations (30). A new paradigm, perhaps termed an “Immunologically Effective Dose” (IED), is required. This shifts the therapeutic goal from simply delivering the highest tolerated dose to delivering a dose that is biologically engineered to produce a desired immune phenotype (e.g., high IFN-I, low TREX1). This has profound implications for clinical trial design. As summarized in Table 1, future trials should compare not just different total doses, but different fractionation schemes explicitly designed to test specific immunological hypotheses (16, 31).

Table 1.

The paradigm shift in clinical trial design for SBRT combined with immunotherapy.

Comparison dimension	Traditional trial concept (based on BED)	Future trial concept (based on IED)
Core Therapeutic Goal	Maximizing direct tumor cell kill	Engineering a specific immune phenotype (high IFN-I, low TREX1)
Dose Testing Strategy	Comparing efficacy across different total doses	Comparing specific fractionation schemes explicitly designed to test specific immunological hypotheses
Key Evaluation Metrics	Traditional local tumor control rates	Cytosolic DNA accumulation, ratio of IFN-I to TREX1 expression, and spatial sparing of draining lymph nodes (DLNs)

4. Remodeling the cellular landscape of the TME

This section transitions from molecular mechanisms to the cellular level, examining how SBRT alters the balance of power between pro- and anti-tumor immune cell populations.

4.1. Mobilizing the attack: enhancing effector T-cell and NK cell function

A primary goal of SBRT is to convert a T-cell-poor, “cold” TME into a T-cell-infiltrated, “hot” TME (14). Through chemokine release and cGAS-STING activation, SBRT promotes the infiltration of cytotoxic CD8+ T-cells, the primary effectors of anti-tumor immunity (32). Studies show SBRT can increase the diversity of the T-cell receptor (TCR) repertoire, suggesting the priming of new T-cell clones against a broader array of tumor antigens (33). Furthermore, SBRT also impacts innate immunity, with some evidence suggesting it promotes the infiltration and activation of Natural Killer (NK) cells, providing a rapid, non-specific anti-tumor response (34).

4.2. Dismantling the defenses: the complex dynamics of Tregs and MDSCs

Regulatory T-Cells (Tregs): Marked by FOXP3, Tregs are a major immunosuppressive population that dampens anti-tumor responses. Studies show that SBRT can decrease levels of immunosuppressive markers like FOXP3, indicating a depletion or functional inhibition of Tregs within the TME (35). This “disarming” of the TME is critical for allowing effector T-cells to function.

Myeloid-Derived Suppressor Cells (MDSCs): The effect of SBRT on MDSCs is more complex and seemingly contradictory.

Suppressive Effects: Some preclinical evidence suggests SBRT can suppress MDSCs. One study found SBRT upregulates miR-21, which in turn inhibits MDSC differentiation and promotes their apoptosis (36). Other clinical observations show decreased MDSC counts following SBRT using higher single-fraction doses (37).

Recruitment Effects: However, a substantial body of evidence indicates that the inflammatory environment created by radiotherapy can actively recruit MDSCs to the tumor site. Radiation-induced cGAS-STING activation and cytokine/chemokine release (e.g., CCL2, CCL5) are potent signals for MDSC mobilization from the bone marrow (38). This can create a negative feedback loop where the pro-inflammatory effects of SBRT are counteracted by a new influx of immunosuppressive cells.

Crosstalk: MDSCs and Tregs engage in bidirectional crosstalk, where each cell type promotes the function and accumulation of the other, forming a potent immunosuppressive axis that SBRT must overcome (39).

The apparently contradictory data on MDSCs are not a flaw in the research but point to a complex, biphasic biological process. It is likely that SBRT first exerts a direct, local suppressive/cytotoxic effect on MDSCs within the treatment field, followed by a secondary, systemic recruitment of new MDSCs driven by the inflammatory response (36). This reveals a dynamic therapeutic window of opportunity: immediately following SBRT, the TME may be transiently cleared of MDSCs, but this is followed by a wave of new suppressive cells (36). This understanding of a biphasic MDSC response provides a strong biological rationale for optimizing the timing of combination therapies. Rather than co-administering MDSC-targeting agents (e.g., CXCR2 or CCR2 inhibitors) concurrently with SBRT, a sequential administration several days after SBRT completion, designed specifically to intercept the wave of newly recruited MDSCs, may be a more effective strategy (40). This approach turns a “contradiction” in the data into a testable and clinically meaningful therapeutic hypothesis (Figure 3).

Figure 3.

The biphasic response model of Myeloid-Derived Suppressor Cells (MDSCs) following SBRT. The effect of SBRT on MDSCs can be conceptualized in two distinct stages. Stage 1: Immediate Local Suppression (Hours to Days Post-SBRT) involves the direct cytotoxic effect of radiation on MDSCs and tumor cells within the treatment field, leading to their clearance. Stage 2: Delayed Systemic Recruitment (Days Post-SBRT) is characterized by the release of inflammatory chemokines (e.g., CCL2, CCL5) from the irradiated tumor, which mobilizes new MDSCs from the bone marrow and recruits them back to the tumor microenvironment, where they exert potent immunosuppressive functions.

4.3. Beyond T-cells: neutrophils, macrophages, and cancer-associated fibroblasts

SBRT-induced remodeling extends to the broader stromal compartment. Neutrophils: Radiation can trigger the release of Neutrophil Extracellular Traps (NETs), which may paradoxically shield tumor cells from immune attack and promote metastasis (41). This highlights a potential target for combination therapy using NET inhibitors. Cancer-Associated Fibroblasts (CAFs): SBRT can stiffen the extracellular matrix via CAFs, potentially hindering T-cell migration. However, specific doses may also render CAFs pro-inflammatory, suggesting a complex plasticity that requires further investigation (42). Spatial Distribution: Importantly, effective SBRT does not just increase the number of immune cells but alters their spatial distribution, facilitating the migration of CD8+ T-cells from the invasive margin into the tumor core, overcoming the “immune-excluded” phenotype (43).

Macrophages are a dominant component of the stromal landscape. Radiation exerts a dual effect on TAMs depending on the dose (44). While low doses (LDRT) can promote the repolarization of pro-tumor M2 macrophages toward an anti-tumor M1 phenotype, high ablative doses may trigger the recruitment of distinctive monocytes (CCR2+) that rapidly differentiate into immunosuppressive macrophages to repair tissue damage (45). This necessitates strategies that block macrophage recruitment (e.g., CCR2 inhibitors) or reprogramming (e.g., PI3Kγ inhibitors) when high-dose SBRT is employed.

4.4. The role of draining lymph nodes: to irradiate or to spare?

A critical consideration in SBRT planning is the management of Draining Lymph Nodes (DLNs). The DLNs are the primary sites for T-cell priming by dendritic cells carrying SBRT-released antigens. Elective nodal irradiation (ENI) utilizing high doses may inadvertently sterilize this critical immune hub, blunting the systemic abscopal response. Recent preclinical data suggest that sparing the DLNs—or treating them with strictly low-dose modulation—preserves the specific stem-like T-cell populations necessary for a durable response to immunotherapy (46). This advocates for a more organ-sparing approach to volume delineation in the era of immuno-radiotherapy.

At present, there is no universally accepted clinical dosimetric definition of DLN “sparing,” and numeric thresholds (e.g., mean dose < 2 Gy or V5 < 10%) should be considered hypothesis-generating rather than evidence-based constraints. Nevertheless, converging preclinical and early translational evidence indicates that concomitant irradiation of tumor-draining lymph nodes can disrupt immune priming and attenuate systemic responses, whereas strategies that spare DLNs or temporally delay DLN irradiation may preserve radio-immunotherapy efficacy (47). Therefore, a pragmatic near-term approach is to treat DLNs as immune organs-at-risk: (i) prospectively define the most likely draining basins using anatomical drainage patterns and/or sentinel-node mapping when feasible; (ii) minimize low-dose “bath” to these regions during plan optimization (reduce mean dose and V5–V10 whenever oncologically safe); and (iii) record DLN dose–volume metrics in clinical trials as candidate immune constraints.

Key directions for future research include standardized DLN contouring atlases, prospective correlation of DLN dose–volume with T-cell priming/IFN–ISG readouts, and clinical testing of DLN-sparing or delayed-DLN strategies in settings where nodal control can be maintained without compromising oncologic outcomes (48, 49).

5. Clinical translation: synergy and challenges of the SBRT-immunotherapy combination

This section bridges preclinical science with clinical reality, evaluating the evidence for SBRT-ICI synergy, the major obstacles to its successful application, and data from key clinical trials.

5.1. Overcoming resistance: converting immunologically “cold” tumors to “hot”

Many tumors are inherently resistant to ICIs due to a lack of pre-existing T-cell infiltration (a “cold” TME) or low PD-L1 expression (50). SBRT, as an immune primer, can generate the inflammation and T-cell infiltration necessary for ICIs to work (51). The PEMBRO-RT trial, for instance, suggested that the benefit of adding SBRT to pembrolizumab was most pronounced in patients with low baseline PD-L1 expression.

The phase II PEMBRO-RT trial in NSCLC randomized patients to pembrolizumab alone or combined with SBRT (8 Gy x 3). Although the study narrowly missed its prespecified primary endpoint, the experimental arm showed a consistent trend toward improvement in Objective Response Rate (ORR: 36% vs. 18%, P = 0.07) and Median Progression-Free Survival (mPFS: 6.6 vs. 1.9 months, HR 0.71, P = 0.19) (52). Subgroup analysis notably revealed that the benefit was driven almost entirely by patients with PD-L1 negative tumors. Similarly, the MD Anderson randomized phase II trial combining SBRT (50 Gy/4 fx) with pembrolizumab reported an ORR of 38% vs. 20% and a significantly improved mPFS (6.4 vs. 1.8 months, P = 0.04), providing further evidence of systemic efficacy outside the radiation field (53). However, not all trials have been positive, underscoring the need to refine patient selection based on biomarkers rather than anatomy alone.

5.2. The organ-specific battlefield: deconstructing the unique immunosuppressive challenge of the liver

Clinical data consistently show that patients with liver metastases respond significantly worse to ICIs, with lower objective response rates (ORR) and progression-free survival (PFS) compared to patients with lung or lymph node metastases (54).

The Liver’s Immune Tolerant Microenvironment: The liver is not just immunologically “cold”; it is an actively immune tolerant organ, evolutionarily designed to prevent excessive immune reactions to antigens absorbed from the gut (55). This pre-existing state of immunosuppression poses a formidable challenge, as SBRT-induced inflammation may trigger powerful homeostatic feedback mechanisms that reinforce, rather than break, this tolerance. (Table 2) (56).

Table 2.

Comparison of immune microenvironments and targeted strategies in lung versus liver metastases.

Features and strategies	Lung cancer immune microenvironment	Liver immune microenvironment
Immune Status Characteristics	Adaptive immune resistance (predominantly PD-L1 expression)	Innate immune tolerance (evolutionarily designed to prevent excessive immune reactions to antigens absorbed from the gut)
Core Therapeutic Challenge	Re-starting a stalled T-cell response (mechanism: releasing the brakes)	Overcoming innate tolerance to permit T-cell priming (mechanism: building the car)
Recommended Combination Strategy	Doublet therapy (SBRT and ICI)	Triplet therapy (SBRT, ICI, and agents targeting myeloid or stromal cells)

The profound differences in immune architecture between organs like the lung and liver mean that a “one-size-fits-all” approach to SBRT-immunotherapy is destined to fail (57). Therapeutic strategies must be tailored to the specific organ microenvironment. For example, the TME in non-small cell lung cancer (NSCLC) often exhibits adaptive immune resistance (e.g., PD-L1 expression), where SBRT+ICI works by re-starting a stalled T-cell response (58). The liver TME, however, is characterized by innate immune tolerance driven by myeloid and stromal cells, where a T-cell response to re-start may not exist in the first place (59). Thus, in the lung, the problem is to “release the brakes” on T-cells; in the liver, the problem is to “build the car” (i.e., overcome innate tolerance to permit T-cell priming) (60). To successfully treat liver metastases, a third agent specifically targeting the liver’s unique biology—such as targeting myeloid cells (e.g., CSF1R inhibitors), stromal cells (e.g., FAK inhibitors), or specific cytokine pathways (e.g., TGF-β inhibitors)—may be required. This suggests that future clinical trials should move beyond simple SBRT+ICI combinations toward rationally designed, organ-specific triplet therapies. To achieve this, identifying precise immune and hypoxic biomarkers through baseline bioinformatic profiling—as demonstrated in recent colorectal cancer models (61, 62) —is crucial to stratify patients who might benefit most from combining SBRT with ICI and stromal-targeting agents.

5.3. Review of landmark and ongoing clinical trials

A systematic review of pivotal trials reveals the evolving landscape of SBRT-ICI combinations, as detailed in Table 3. Early phase studies, such as those in metastatic NSCLC and melanoma, primarily focused on safety and established that concurrent SBRT with PD-1/PD-L1 blockade is well-tolerated without synergistic toxicity (63). However, efficacy data have been heterogeneous. While the PEMBRO-RT trial suggested a benefit in PD-L1 negative patients, larger randomized trials and ongoing phase III studies (e.g., NRG-LU002) continue to investigate the optimal sequencing and patient selection to validate these signals (52). Notably, recent trials are increasingly incorporating “triplet” strategies—combining SBRT, ICI, and a targeted agent (e.g., GM-CSF or cytokine modulators)—to overcome the limitations of doublet therapy (64). The discordant results across trials highlight that distinct tumor histologies and anatomical sites (e.g., lung vs. liver) require distinct radio-immunotherapy prescriptions, moving away from a universal protocol (65).

Table 3.

Summary of landmark and representative clinical trials of SBRT combined with immunotherapy.

Trial phase and name	Target cancer type	Treatment regimen	Key findings and research focus
Early-Phase Studies	Metastatic NSCLC and melanoma	Concurrent SBRT with PD-1/PD-L1 blockade	Established that concurrent SBRT with PD-1/PD-L1 blockade is well-tolerated without synergistic toxicity.
PEMBRO-RT (Phase II)	NSCLC	Pembrolizumab alone or combined with SBRT (8 Gy x 3)	Demonstrated a trend toward improvement in Objective Response Rate and Median Progression-Free Survival; the benefit was driven almost entirely by patients with PD-L1 negative tumors.
MD Anderson (Phase II Randomized)	Metastatic NSCLC	Pembrolizumab with or without SBRT (50 Gy/4 fx)	Reported significantly improved ORR and mPFS, providing further evidence of systemic efficacy outside the radiation field.
Phase III Trials (NRG-LU002)	NSCLC	SBRT and Immunotherapy	Investigating the optimal sequencing and patient selection to validate early phase signals.
Recent “Triplet Therapy” Trials	Multiple solid tumors	SBRT, ICI, and a targeted agent (GM-CSF or cytokine modulators)	Incorporating triplet strategies to overcome the limitations of doublet therapy.

6. New frontiers: advanced strategies and future outlook

This section explores cutting-edge research and concepts that are pushing the boundaries of SBRT as an immune-engineering tool (Figure 4).

Figure 4.

A proposed closed-loop paradigm for personalized adaptive radioimmunotherapy. This model illustrates a future strategy for cancer treatment guided by real-time biological feedback. Step 1: An ablative pulse of SBRT is delivered to the tumor. Step 2: Treatment response is monitored non-invasively using liquid biopsies to analyze circulating tumor DNA (ctDNA) kinetics and T-cell receptor (TCR) repertoire dynamics. Step 3: AI-driven computational models analyze these data to predict tumor response and identify emerging resistance mechanisms. Step 4: The analysis informs the selection and timing of the next therapeutic intervention, such as an adaptive combination therapy (e.g., adding a CD73 inhibitor) with the subsequent SBRT pulse, creating a personalized and dynamic treatment cycle.

6.1. Defining the immunologically effective dose

To advance the field, we propose the formal adoption of the “Immunologically Effective Dose” (IED) concept. Unlike the Biologically Effective Dose (BED) which focuses on cell kill, the IED incorporates parameters such as (1): quantification of cytosolic DNA accumulation; (2) ratio of IFN-I to TREX1 expression and (3) spatial sparing of Draining Lymph Nodes (DLNs). Consequently, the concept of IED should be integrated into a broader “Immunologically Effective Radiotherapy” (IER) plan that optimizes both the dose delivered to the tumor and the dose spared from critical immune tissues.

Practical measurement of IED in patients remains non-trivial, but several translational “toolboxes” are now feasible. First, cytosolic DNA accumulation can be approximated in paired tumor biopsies by quantifying radiation-induced micronuclei and cGAS localization using multiplex immunofluorescence or spatial imaging, and by measuring downstream pathway activation through an interferon/ISG transcriptional program (e.g., CXCL10, IFIT1, ISG15) using RNA-based assays. Notably, biopsy-based assessment of radiation-induced cGAS re-localization and IFN/ISG immune signatures has been linked to clinical efficacy in human HNSCC, providing proof-of-principle that these mechanistic readouts can be captured in patients (66). Similarly, in NSCLC treated with pembrolizumab plus radiotherapy, systemic immune activation has been monitored longitudinally through blood immunophenotyping and immune profiling, supporting a clinically implementable framework for immune response tracking (67).

Second, the IFN-I/TREX1 ratio is conceptually attractive but is unlikely to be reliably inferred from plasma alone because TREX1 is primarily intracellular. A more robust approach is to quantify TREX1 at the tumor level (IHC or RNA) and normalize it to a contemporaneous IFN/ISG signature in the same biopsy specimen, while using blood-based IFN/ISG scores as a complementary systemic surrogate. For minimally invasive monitoring, candidate liquid-biopsy surrogates include (i) whole-blood or PBMC IFN/ISG scores, (ii) serum cytokine/chemokine panels enriched for IFN-driven factors (e.g., CXCL10), and (iii) orthogonal response kinetics from ctDNA and TCR repertoire dynamics, which reflect antigen release and clonal expansion even when direct pathway readouts are not obtainable (67).

Third, operationalizing “DLN sparing” for IED requires prospectively defining a pragmatic DLN region (based on anatomical drainage patterns and/or sentinel-node mapping when available) and routinely reporting low-dose metrics (e.g., mean dose, V5–V10) as candidate immune constraints. This concept aligns with the broader movement toward quantifying dose to the immune compartment (e.g., EDIC-style frameworks) and enables prospective correlation between immune-dose metrics and immunologic endpoints (68).

6.2. Optimizing the fourth dimension: the critical question of SBRT-immunotherapy timing

The optimal timing of SBRT administration relative to ICI remains a critical and unresolved question (69). Rationale for SBRT First: Administering SBRT first allows the in situ vaccine effect to prime a T-cell response, which can then be unleashed by subsequent checkpoint inhibition.

Rationale for Concurrent Therapy: Concurrent administration may capture the peak of inflammation and T-cell infiltration, and a pre-existing T-cell population may be critical for radiation to exert its maximal effect (70).

Clinical Evidence: One retrospective study found significantly worse overall survival in patients who completed immunotherapy prior to SBRT/SRS compared to those receiving it concurrently or after, suggesting that irradiating a tumor in an immune-exhausted state is suboptimal (71).

6.3. Personalized adaptive radiotherapy: The PULSAR concept and future scheduling

PULSAR (Personalized Ultrafractionated Stereotactic Adaptive Radiotherapy): This innovative concept abandons standard daily fractionation in favor of delivering ablative doses in “pulses” separated by long time intervals (e.g., 10 days) (72).

Biological Rationale: The long interval is designed to match the kinetics of the adaptive immune response, allowing the T-cell response generated by the first pulse to mature and reach full effect before the next pulse is delivered (72).

Preclinical Evidence: Preclinical models show that PULSAR-style scheduling, when combined with α-PD-L1, achieves superior tumor control and induces immunologic memory compared to traditional daily fractionation (73). This represents a shift in radiotherapy scheduling from being based on radiobiological convenience to being based on immunological timing.

6.4. New combinations for intractable cancers: targeting the adenosine and LIF pathways in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is a classically “cold” tumor, highly resistant to immunotherapy and characterized by a dense, immunosuppressive stroma (74). SBRT plus ICI alone has had limited success (75).

Targeting the Adenosine Pathway: The PDAC TME is rich in extracellular adenosine, a potent immunosuppressive molecule generated by the ectoenzymes CD39 and CD73. Research has shown that SBRT upregulates CD73 on pancreatic cancer cells, creating a resistance mechanism (76). Preclinical studies show that a triple therapy of SBRT + anti-PD-L1 + a CD73 inhibitor significantly improves tumor control and survival in PDAC models, including control of liver metastases (77).

Targeting the LIF Pathway: Leukemia Inhibitory Factor (LIF) is another cytokine implicated in the immunosuppressive TME of PDAC, promoting Treg differentiation and M2 macrophage polarization (78). Preclinical models show that combining chemotherapy with dual LIF and PD-L1 blockade enhances anti-tumor immunity (79). This provides a strong rationale for investigating a triple combination of SBRT + anti-PD-L1 + a LIF inhibitor to comprehensively remodel the PDAC TME.

6.5. Expanding the arsenal: SBRT with cellular therapies and oncolytic viruses

The “engineering” capability of SBRT extends beyond ICIs. CAR-T Therapy: Solid tumors often resist CAR-T therapy due to antigen heterogeneity and physical barriers. SBRT can address this by debulking the tumor mass and inducing “antigen spreading,” potentially preventing antigen-negative relapse. Furthermore, SBRT-induced vascular normalization may improve CAR-T cell trafficking into the tumor core (80). However, CAR-T therapy carries risks of cytokine release syndrome (CRS) and neurotoxicity; thus, SBRT sequencing should be designed to minimize overlapping inflammatory peaks around CAR-T infusion, with SBRT most commonly positioned as cytoreductive/bridging therapy prior to infusion and with careful post-infusion toxicity monitoring (81). Oncolytic Viruses (OVs): The combination of SBRT and OVs represents a dual-danger signal strategy. SBRT can enhance viral replication and oncolysis, while the virus amplifies the inflammatory cascade initiated by radiation, creating a potent synergy particularly in immunologically cold environments (82).

6.6. Dynamic monitoring with liquid biopsies: guiding therapy with ctDNA and TCR repertoire analysis

Traditional imaging provides a slow assessment of response and can be confounded by phenomena like pseudoprogression (83). Liquid biopsies offer a dynamic, real-time window into treatment response.

T-Cell Receptor (TCR) Repertoire Sequencing: Serial TCR sequencing can track the diversity and clonal expansion of T-cells in the peripheral blood. A positive response to SBRT-immunotherapy is associated with decreased or stable TCR diversity (indicating expansion of dominant anti-tumor clones) and the emergence of new, high-abundance TCR clones (reflecting the in situ vaccine effect) (84). Conversely, disease progression is associated with an increase in diversity as these clones contract.

Circulating Tumor DNA (ctDNA): Monitoring ctDNA levels provides a highly sensitive measure of tumor burden. A rapid drop in ctDNA often precedes radiologic response and can help distinguish true progression from pseudoprogression (85). Studies have shown that a transient spike in ctDNA post-SBRT (from cell death) followed by a rapid clearance is a marker of good response (86). Molecular progression detected by ctDNA can precede radiologic progression by weeks or months, providing an early warning of treatment failure (87).

The future strategies discussed in this section—PULSAR, novel molecular combinations, and liquid biopsy monitoring—are not isolated concepts; they converge toward a fully personalized and adaptive treatment paradigm. Liquid biopsies (ctDNA/TCR) provide real-time feedback on a patient’s biological response to therapy (88). This feedback can be used to adjust the therapeutic strategy on the fly. For example, if TCR analysis after a first SBRT pulse shows a failure to expand new clones, a different molecular agent (e.g., a CD73 inhibitor) could be added. The PULSAR framework, with its long inter-fraction intervals, provides the ideal temporal structure to perform this monitoring and adaptation (89). One could measure the immune response after the first pulse and use that data to inform the timing and combination strategy for the second pulse. The ultimate evolution of this trend is a “closed-loop” treatment system. In this paradigm, a patient receives a pulse of SBRT. Days later, a liquid biopsy is performed. The molecular data (ctDNA kinetics, TCR repertoire changes) are fed into a predictive model (perhaps AI-driven, as hinted at in (90)) that determines the optimal timing for the next SBRT pulse and the specific immunotherapy agent that should be paired with it to overcome observed resistance mechanisms. This represents a true TME engineering approach, where the system is actively measured, modeled, and modulated in a continuous, adaptive cycle.

6.7. Challenges in clinical implementation

Despite the promise, the clinical utility of ctDNA/TCR monitoring faces substantial hurdles. First, the “shedding dynamics” vary significantly across tumor types; for example, intracranial metastases often shed minimal DNA into plasma due to the blood-brain barrier (91). Second, differentiating a post-SBRT “flare” (a transient spike in ctDNA caused by cell lysis) from true biological progression remains difficult without standardized time-point definitions (92). Finally, the lack of uniform assay sensitivity and high costs currently limit widespread routine adoption. Future protocols must validate specific “molecular response criteria” before these biomarkers can guide treatment changes outside of clinical trials (93).

7. Conclusion and strategic recommendations

This review synthesizes the evidence for a new paradigm of SBRT as an immune-engineering modulator. It reiterates that the optimal use of SBRT in this context requires a shift from a “maximum dose” philosophy to one of a “biologically tailored dose.”

7.1. Recommendations for clinical practice

Optimize Fractionation: When immune synergy is a primary goal, moderate hypofractionation regimens (e.g., 8 Gy x 3) should be considered to maximize immune stimulation and avoid high-dose-induced TREX1-mediated suppression.

Multidisciplinary Management: For patients with highly immunosuppressive sites like liver metastases, a multidisciplinary approach should be emphasized, considering the addition of a third agent targeting myeloid cells or the stroma to the SBRT and ICI backbone.

Judicious Use of Biomarkers: Dynamic biomarkers like ctDNA can be judiciously used in clinical decision-making to help differentiate pseudoprogression from true progression and provide an early assessment of efficacy.

7.2. Directions for future research

Head-to-Head Fractionation Trials: Prospective, randomized trials are urgently needed to directly compare different SBRT fractionation schemes on immunological endpoints (e.g., IFN-I levels, T-cell infiltration), not just traditional tumor control.

Rational Triplet Therapy Trials: In immunologically challenging settings like pancreatic cancer and liver metastases, trials of “triplet therapies” based on clear biological rationales, such as SBRT + ICI + CD73 or LIF inhibitors, should be designed.

Integrating Adaptive Radiotherapy and Dynamic Monitoring: Clinical trial designs that combine adaptive radiotherapy concepts like PULSAR with intensive liquid biopsy monitoring should be pursued to pioneer the next generation of personalized radioimmunotherapy.

The ultimate goal is to move from empirically combining SBRT and immunotherapy to rationally engineering a desired, systemic anti-tumor immune response. Currently, based on the “Immunologically Effective Dose” (IED) and organ-specific paradigms discussed in this review, our center is initiating a prospective clinical study to validate these combinatorial strategies in metastatic colorectal cancer patients.

Funding Statement

The author(s) declared that financial support was received for this work and/or its publication. This study was supported by the National Natural Science Foundation of China (82560504), Yunnan Fundamental Research Projects (202301AT070611; 202401AY070001-017; 202501AY070001-253; 202401AT070256), Youth Project of Beijing Science and Technology Innovation Medical Development Foundation (KC2023-JX-0288-FQ05). The Medical Discipline Reserve Talent Project of The High-level Health Technology Talents of Yunnan Province(H-2024053).

Author contributions

XW: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft. JD: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft. SF: Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing – review & editing. YZ: Data curation, Formal analysis, Writing – review & editing. RC: Data curation, Formal analysis, Writing – review & editing. XC: Funding acquisition, Project administration, Supervision, Writing – review & editing. ZY: Funding acquisition, Project administration, Supervision, Writing – review & editing.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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The author(s) declared that generative AI was not used in the creation of this manuscript.

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