The global incidence rate of non-small cell lung cancer (NSCLC) is notable, accounting for ~85% of newly diagnosed cases of lung cancer annually and contributing to a large number of cancer-associated mortalities worldwide (1,2). The 5-year overall survival (OS) rate for NSCLC remains relatively low at ~15%, due to the high proportion of cases diagnosed at advanced stages (3). For patients with unresectable advanced NSCLC, the standard treatment is concurrent chemoradiotherapy (CRT). However, the long-term prognosis for this treatment remains limited. According to previous studies, the 5-year OS rate for concurrent CRT in these patients is 15–30% (4,5). Immune checkpoint inhibitors (ICIs) targeting the programmed cell death protein-1 (PD-1)/programmed cell death ligand-1 (PD-L1) axis have markedly improved the survival rates of patients with advanced NSCLC. Numerous clinical trials have demonstrated the clinical advantages of these therapies (6–9). Various ICIs such as nivolumab, pembrolizumab, durvalumab and atezolizumab have already been approved for NSCLC treatment by the US Food and Drug Administration (10). However, the efficacy of ICIs for NSCLC remains limited, as the majority of patients have no tumor response and the objective response rates is only ~20% in previously treated patients (6,11,12). In order to improve efficacy, a number of studies are exploring the combination of immunotherapy and radiotherapy (RT) for advanced NSCLC (13–15), usually referred to as ‘combined radio-immunotherapy’.

Local RT can induce the regression of metastatic cancer at a distance from the field of radiation, known as the abscopal effect (16,17). Although it has been recognized for several years, the abscopal effect remains a rare event (18). RT modulates the immune system through complex mechanisms including local tumor cell death, antigen release, activation of dendritic cells (DCs), recruitment of cytotoxic T lymphocytes (CTLs) and promotion of the antitumor response (17). On other hand, ICIs help to mitigate the immunosuppressive tumor microenvironment (TME), allowing for a more robust and sustained antitumor immune response. Combination strategies with RT and immunotherapy, especially ICIs such as PD-1/PD-L1 and CTL associated protein 4 (CTLA-4) inhibitors, can lead to a mutual synergistic effect by unleashing the full potential of the immune system (19–22).

Moreover, it seems that the timing and sequence is important for the beneficial synergic activity of ICIs plus RT according to previous research. The randomized PACIFIC trial recommended that immunotherapy treatment is initially performed with subsequent RT (13). Certain studies support the concurrent use of RT and immunotherapy (23–25). When administered together, RT not only directly kills tumor cells but also boosts the efficacy of ICIs (23). However, concurrent therapy requires careful monitoring for adverse effects, such as pneumonitis and other immune-related adverse events (AEs) (24). Furthermore, a phase 1 trial of a randomized comparison of concurrent or sequential immunotherapy and stereotactic body RT (SBRT) in patients with stage IV NSCLC reports that administering immunotherapy concurrently with SBRT is not more toxic compared with sequential therapy, allowing for systemic therapy to be initiated earlier in the patient's treatment plan (25). Therefore, the optimal timing of combining ICIs with RT to maximize synergistic effects needs to be further investigated.

In previous years, the complex integration of immunotherapy and RT has advanced rapidly, with a greater expectation of further therapeutic benefit. With the advent of new efficient immunotherapeutic agents, a large area of active research has emerged, following considerable interest in treating RT as a potential combination partner for immunotherapy. The present review discusses the current status of combining immunotherapy and RT for advanced NSCLC, the optimal combination strategies, the utility of RT for overcoming resistance to immunotherapy and summarizes the main problems as well as future perspectives.

Preclinical evidence has demonstrated that focal RT has diverse immunomodulatory effects (26–28). The mechanism of action of RT is complex and involves both direct cytotoxic effects of RT on cancer cells and immunomodulatory components (29). The therapeutic effect of RT is mediated by the generation of ROS resulting direct breakage of DNA, which serves an important role in changes of molecular signaling pathways (30). RT enhances the release of tumor-associated antigens from tumor cell apoptosis and necrosis, which are subsequently taken up by antigen-presenting cells. This upregulates major histocompatibility complex class I and adhesion molecule expression and increases chemokine secretion, leading to APC recruitment, thereby rendering tumor cells more susceptible to recognition by T cells and augmenting the antitumor effect (22,31,32).

The immune response must be initiated with a unique type of programmed cell death commonly known as immunogenic cell death (ICD). RT induces ICD which involves the release of various immunogenic signals, known as damage-associated molecular patterns (DAMPs), including calreticulin (CALR) exposure on the cell membrane, extracellular adenosine triphosphate (ATP) and high mobility group box 1 (HMGB1) (33–35). In the TME, ICD could also trigger a pathogen response with the co-release of chemokines such as C-X-C motif chemokine ligand 10, TNFα and type I interferon (IFN) (36). Mechanistically, IFN produced by CD8⁺ T cells serves a key role in driving the upregulation of PD-L1 on tumor cells after delivery of fractionated RT (37). In addition, the TME is remodeled by vascular, stromal and immunological changes that are induced by irradiation and contribute to an antitumor response (27,38). The reprogrammed TME induced by RT contributes to turning ‘cold’ tumors lacking lymphocytic infiltration into ‘hot’ tumors with high levels of immune cell infiltration and raises response rates achievable with ICIs (39–41).

The phase 3 randomized clinical trial PACIFIC study compared durvalumab vs. placebo in 713 patients with locally advanced NSCLC with no progressive disease after ≥2 rounds of platinum-based CRT. Progression free survival (PFS) was significantly improved in the durvalumab arm vs. placebo [16.8 months vs. 5.6 months; hazard ratio (HR), 0.52; 95% CI, 0.42–0.65; P<0.001], as well as the median time to mortality or distant metastasis (28.3 months vs. 16.2 months; HR 0.53; 95% CI, 0.41–0.68; P<0.001) (13,52). Updated OS data from the PACIFIC trial reported a long-term clinical benefit in the durvalumab group vs. placebo with a 36-month OS rate (57.0% vs. 43.5%) (53). AEs of any cause were similar in durvalumab vs. placebo for any grade (96.8% vs. 94.9%), grade 3 or 4 AEs (29.9% vs. 26.1%) and serious AEs (28.6% vs. 22.6%). The most frequent AEs leading to discontinuation of durvalumab and placebo were pneumonitis (in 6.3 and 4.3%, respectively) and pneumonia (in 1.1 and 1.3%, respectively), and mortality due to AEs occurred in 4.4 and 6.4%, respectively (13,52). Hence, the safety profile was similar between the groups.

A non-randomized controlled phase 1 trial of pembrolizumab administered concurrently with CRT for locally advanced NSCLC showed that the 12-month PFS rate was 81.0% (95% CI, 49.3–90.2%). Among the 21 patients included in the analysis, no dose-limiting toxic effects in any cohort were observed, and 18% of patients (4/21) experienced immune-related AEs of grade ≥3 (54). These results indicate that combined treatment with PD-1 inhibitors and RT for locally advanced NSCLC is tolerable. The comparable randomized trial, RTOG 3505 (NCT02768558), on nivolumab in unresectable stage III NSCLC patients was terminated and the results have not been reported (55).

In a secondary analysis of the KEYNOTE-001 trial, patients with advanced NSCLC received pembrolizumab after previous RT treatment. The PFS and OS with pembrolizumab were significantly longer in patients who previously received any RT compared with in patients without previous RT (PFS, 4.4 months vs. 2.1 months; P=0.019; OS, 10.7 months vs. 5.3 months; P=0.026). Pulmonary toxicity was recorded in 15 patients of 24 patients (63%) with previous thoracic RT vs. 29 patients of 73 patients (40%) with no previous thoracic RT. A total of 3 patients (13%) with previous thoracic RT developed immune-related pulmonary toxicity compared with 1 patient (1%) without previous RT. The safety profile was considered to be acceptable.

The PEMBRO-RT randomized trial of 76 patients with advanced NSCLC reported that pembrolizumab combined with SBRT produced a greater overall response rate (ORR) at 12 weeks compared with pembrolizumab alone (36% vs. 18%; P=0.07), particularly in PD-L1-negative patients. Grade 3 to 5 immune-related AEs were reported in 12 patients (17%), with no notable differences between arms (56). A pooled analysis including PEMBRO-RT and MDACC trials was performed to evaluate the responses of pembrolizumab with or without RT for metastatic NSCLC. Overall, 148 patients were included in the pooled analysis. The abscopal response rate was markedly higher in the pembrolizumab plus RT cohort vs. in the pembrolizumab alone group (41.7% vs. 19.7%; P=0.0039), as was the abscopal disease control rate (65.3% vs. 43.4%, P=0.0071). Compared with pembrolizumab alone, adding RT to pembrolizumab significantly increased survival outcomes, with a longer PFS (9.0 months vs. 4.4 months; HR, 0.67; 95% CI, 0.45–0.99; P=0.045) and OS (19.2 months vs. 8.7 months; HR, 0.67; 95% CI, 0.54–0.84; P=0.0004) (24). These findings indicate that adding PD-1inhibitors to RT for advanced NSCLC can provide survival benefits.

Numerous retrospective studies have evaluated the benefit and safety of combining ICIs with RT. In a multicenter retrospective cohort study of patients with NSCLC treated with nivolumab after the second line systemic chemotherapy, there was no difference in PFS between those without past RT and those who received RT in the past 6 months (adjusted HR 0.65; 95% CI, 0.37–1.14) (57). Efficacy and toxicity of immunotherapy with SRT for brain metastases from NSCLC were evaluated in a previous multicentric retrospective study from the Italian Association of RT and Clinical Oncology. Patients receiving SRT with immunotherapy vs. SRT-alone had a significantly higher 1-year intracranial local progression-free survival rate (83.9% vs. 53.5%, respectively; P=0.007). The combined treatment immunotherapy and SRT was well tolerated, and no notable differences in radio-necrosis rates between patients who received SRT + immunotherapy and patients who received SRT-alone were observed (58).

Taken together, these studies suggest that combining RT and immunotherapy is associated with improved survival benefit and does not increase toxicities beyond each administrated strategy independently. However, the combination therapy needs to be further optimized using the results of numerous ongoing clinical trials.

The sequence of administration of combined radio-immunotherapy is a crucial factor for its efficacy. However, currently, the optimal sequence remains unclear. Since different types of ICIs target different stages of the immune response and activate multiple immune cells, the ideal sequence and timing must be meticulously planned to achieve synergistic effects (59–61).

A few preclinical studies have demonstrated that concurrent therapeutic strategies have been associated with superior therapeutic activity. In preclinical models of melanoma, colorectal and breast cancer, concurrent but not sequential PD-1/PD-L1 blockade and RT was shown to be more effective at improving local tumor control and survival (37). The mechanistic hypothesis was that RT leads to upregulation of PD-L1 expression secondary to CD8⁺ T cell production of IFN. A comprehensive meta-analysis of treatment of brain metastases with stereotactic radiosurgery and ICIs suggested that concurrent administration RT and ICIs may be associated with improved safety and efficacy over sequential therapy (62). However, data from a mouse model of colorectal cancer demonstrated that an anti-CTLA-4 inhibitor was the most efficient when given before RT (63). In addition, a meta-analysis of 20 clinical trials that enrolled 2,027 patients with NSCLC who received combination therapy using PD-1/PD-L1 inhibitors and RT demonstrated that PD-1/PD-L1 inhibitor administration following RT outperformed concurrent treatment (64). This finding was in line with the theory established by preclinical studies which demonstrated that RT could regulate PD-L1 expression in cancer cells through the repair of DNA double-strand breaks and induce enrichment of CD8⁺ T-cells. The upregulation of PD-L1 expression has been shown to improve the efficacy of ICIs and increase CD8⁺ T-cell infiltration (65,66). Thus, the different results between various studies suggest that the optimal schedule depends on a number of factors, such as tumor type and the mechanics of action of immunotherapy agents, which should be considered when designing clinicals trials.

The PACIFIC trial showed marked improvements in both PFS and OS for patients treated with the sequential use of durvalumab after CRT compared with those receiving a placebo (13). As shown in the study, the sequential approach has notable benefits in NSCLC, with a manageable toxicity profile. Similarly, the clinical trials assessing the safety and efficacy of pembrolizumab and nivolumab recommended administration as adjuvant therapy after CRT (55,67). Furthermore, the updated data from PACIFIC study suggest that administering immunotherapy within 14 days of completing concurrent CRT seemed to have markedly greater PFS compared with starting immunotherapy after 14–42 days (14). However, a retrospective study of patients with stage IV NSCLC from the National Cancer Database reported a different optimal time for combined radio-immunotherapy. The study demonstrated patients treated with immunotherapy ≥21 days after start of SBRT had a longer OS compared with those treated thereafter (median OS, 19 months vs. 15 months; P=0.0335) (68). This finding suggests that the synergistic effect is not only associated with the sequence, but also with the time interval between RT and ICI administration.

Sequential administration of immunotherapy after RT has previously been assessed, and several studies have begun investigating whether the use of concurrent administration would achieve improved results. The KEYNOTE-799 study, a phase-II trial evaluating the safety and efficacy of pembrolizumab plus concurrent CRT in patients with unresectable, locally advanced, stage III NSCLC, reported an incidence rate of 4.0% for grade ≥3 pneumonitis, which was slightly higher compared with the 3.4% reported in the PACIFIC trial (15). This indicates that concurrent use of ICIs with CRT or RT may lead to an elevated risk of immune-related pneumonitis. The balance between efficacy and safety is crucial when combining ICIs with RT or CRT. A single center prospective trial of concurrent anti-PD-L1 durvalumab and palliative RT demonstrated an acceptable safety profile of the combined treatment (23). Furthermore, the ETOP NICOLAS phase II clinical trial assessed treatment with nivolumab concurrently with standard first line CRT regimen in unresectable locally advanced NSCLC and provided evidence that the addition of nivolumab to concurrent CRT is safe and tolerable (69). The increasing experimental evidence indicates that concurrent immunotherapy and RT is more effective compared with sequential therapy (37). In addition, there are ongoing clinical trials investigating the concurrent administration of ICIs and RT in NSCLC to determine if this approach can provide superior outcomes compared with sequential treatment. The PACIFIC 2 (NCT03519971) study was designed to assess the benefit and safety of concurrent durvalumab and platinum-based CRT in patients with unresectable stage III NSCLC (70). Similarly, another trial (NCT03589547) is exploring the effects of concurrent pembrolizumab with RT in stage III NSCLC. Until more conclusive data is available, the choice between concurrent and sequential administration remains individualized, based on patient factors, such as tumor characteristics and treatment goals.

Immunotherapy has emerged as a breakthrough treatment for NSCLC by enhancing the immune system's ability to recognize and destroy cancer cells. ICIs have provided durable responses in some patients. However, resistance to immunotherapy, whether primary or acquired, remains a notable challenge, limiting its overall effectiveness in the majority of patients (89,90). Primary resistance, which is associated with various intrinsic and extrinsic tumor factors, occurs when patients fail to respond to immunotherapy from the outset (91). Intrinsic factors that lead to primary resistance include a lack of antigenic mutations, loss of tumor antigen and human leukocyte antigen (HLA) expression, alterations in antigen processing machinery, alterations of several signaling pathways (MAPK, PI3K, WNT, IFN, EGFR or anaplastic lymphoma kinase) and constitutive PD-L1 expression (89,92).

The major extrinsic factors which contribute to immunotherapy resistance include immunoregulatory factors within the TME. The TME serves a critical role in shaping the immune response to cancer. In NSCLC, the TME often contains a high density of immunosuppressive cells, such as Tregs and myeloid-derived suppressor cells (MDSCs). These cells inhibit the activity of cytotoxic T cells and promote an environment that supports resistance to ICIs and tumor growth (93). Besides, tumor-associated macrophages (TAMs) may also contribute to immune suppression by secreting anti-inflammatory cytokines and promoting immune evasion (94,95). In addition, tumors can evade immune detection by upregulating immune checkpoint molecules such as PD-L1, which binds to PD-1 on T cells, leading to T cell exhaustion and an impaired immune response (96). The acquired resistance may be associated with loss of the target antigen and HLA expression and altered IFN signaling, as well as loss of T-cell functionality (89). These mechanisms contribute to the complex and multifactorial nature of immunotherapy resistance in NSCLC, highlighting the need for novel approaches to overcome these challenges. A study included patients with advanced NSCLC who developed acquired resistance to PD-1 inhibitor and demonstrated that acquired resistance to PD-1 inhibitors is often limited to one or two sites and lymph nodes metastases appear to be particularly susceptible to acquired resistance (97). Therefore, when patients develop oligo-progression after responding to PD-1 inhibitor, local therapy to oligoprogressive disease may prolong ICI effectiveness.

RT is traditionally used for local tumor control by directly damaging DNA and causing cell death in targeted cancer cells. However, previous evidence suggests that RT has notable immunomodulatory effects that extend beyond the immediate vicinity of the irradiated tumor, affecting both local and distant immune responses. A primary immunomodulatory effect of RT is the induction of ICD. When cancer cells die in response to RT, they release danger signals known as DAMPs, such as CALR, IFN-I and HMGB1 (50,98,99). These DAMPs serve as signals to the immune system, alerting DCs and other antigen-presenting cells to process and present tumor antigens, thereby activating cytotoxic T cells (100). When combined with ICIs, these T cells are more equipped to recognize and eliminate tumor cells, overcoming one of the key mechanisms of immunotherapy resistance.

The immunosuppressive nature of the TME is a notable contributor to immunotherapy resistance. RT could remodel the TME by reducing the population of immunosuppressive cells such as Tregs, MDSCs and TAMs. Additionally, RT increases the infiltration of effector immune cells, such as cytotoxic T cells and natural killer (NK) cells, into the tumor, further enhancing the antitumor immune response (101,102). By shifting the balance of immune cells within the TME, the combination of RT and immunotherapy can overcome the immunosuppressive barrier that often limits the effectiveness of ICIs.

Traditionally, a phenomenon known as the abscopal effect has garnered notable interest in the context of RT and immunotherapy. The abscopal effect refers to the observation that, following local irradiation, distant (non-irradiated) tumors may shrink or become more responsive to immune attack (16). This systemic effect is considered to occur due to RT's ability to stimulate a systemic antitumor immune response, mediated by activated T cells that circulate and target cancer cells at distant sites (73,103). It is widely accepted that CTLs induced by RT migrate to the irradiated tumor and eliminate residual cancer cells and also to metastatic sites, a key mechanism transforming local effects into abscopal responses (104,105). Several preclinical trials have demonstrated high-dose RT causes ICD and leads to a systemic antitumor response. The endogenous cytosolic DNA can be sensed by the cGAS-STING pathway leading to IFN production along this pathway which has a crucial role in T-cell cross-priming to initiate an antitumor response (20,106). Local RT triggers the systemic antitumor immune response through these pathways.

RT has been shown to modulate the expression of immune checkpoint molecules, such as PD-L1 on tumor cells. While increased PD-L1 expression can contribute to immune escape, when combined with PD-1/PD-L1 inhibitors, the upregulation of these checkpoints by RT can actually enhance the effectiveness of ICIs (107,108). By blocking the PD-1/PD-L1 pathway, ICIs prevent tumor cells from evading immune detection, leading to a more robust and sustained antitumor response. A clinical study provided early evidence that the combination of RT and ICIs can overcome resistance in NSCLC. The NRG-LU002 study is investigating the use of SBRT combined with ICIs in patients with oligometastatic NSCLC. Preliminary data indicates that this combination leads to improved PFS, especially in patients with resistance to initial immunotherapy (109). As this field evolves, combination strategies may also include the use of other immunomodulatory agents, such as vaccines, oncolytic viruses or targeted therapies, to further enhance the immune response and overcome resistance.

While the combination of RT and immunotherapy offers potential, several challenges and considerations must be addressed to ensure safe and effective implementation in clinical practice. For instance, the treatment approach of combing ICIs with RT could potentially be utilized for any stage of NSCLC. However, it is unclear which stage of NSCLC patients benefit most from this strategy. In addition to locally advanced and metastatic NSCLC, there is randomized evidence to support that patients with early-stage disease also benefit from this combination treatment. A recent study reported the addition of immunotherapy to SABR in patients with early (stage I–II) or recurrent NSCLC with negative lymph node involvement led to notable improvements in event-free survival with tolerable toxicity (110). For early-stage NSCLC, patients have high survival benefit from postresection adjuvant therapy and post SBRT maintenance therapy, and there are numerous ongoing randomized trials to evaluate safety and efficacy of the combination of SBRT with adjuvant immunotherapy.

Not all patients with NSCLC will benefit equally from the combination of RT and immunotherapy. Identifying predictive biomarkers that can guide patient selection is a key area of research. Assessment of PD-L1and PD-1 expression as biomarkers is currently required to determine clinical decisions on use of treatment strategies targeting ICIs (111,112). Since patients with high PD-L1 expression often have an improved response to ICIs, adding RT may not provide a notable benefit compared with patients with low PD-L1 levels, where RT might stimulate an immune response, making them more responsive to ICIs (24,107). The inter- and intra-tumoral heterogeneity of PD-L1 expression, variability in assay platforms and cut-off thresholds and the lack of standardized assessment methods reduce reproducibility and predictive accuracy. Therefore, the utility of PD-L1 testing in the context of combination immunotherapy with RT remains limited.

Preclinical studies have demonstrated that tumor mutational burden (TMB), tumor-infiltrating lymphocytes and DNA mismatch repair (MMR) deficiency are additional important biomarkers of immunotherapy response (113,114). However, the biomarkers are currently under investigation to determine their role in predicting response to combination therapy. Patients with a higher TMB, particularly those with MMR repair deficiency, may be particularly sensitive to RT-induced cellular damage (115). Although the TMB is a promising biomarker, it is less validated and less widely used compared with PD-L1, and its utility for guiding patient selection remains uncertain. There are still advantages and disadvantages associated with the clinical utility of TMB. Personalized treatment approaches based on biomarker analysis may improve outcomes by selecting patients who are most likely to benefit from the combination of RT and immunotherapy, while avoiding unnecessary toxicity in non-responders.

A major challenge in combining RT and immunotherapy is distinguishing between beneficial immune activation and harmful inflammation (116,117). Radiation can stimulate immune responses, but it can also lead to inflammation and tissue damage, particularly in sensitive organs such as the lungs. Thus, it is essential to guard the patients from immune-, radiation- or combined therapy-associated toxicities during combined treatment. With the latest imaging technology, such as image-guided RT for intensity modulated RT, volumetric modulated arc therapy and adaptive radiation therapy, it is possible to detect clinically relevant changes in lung tissue at the early stages of the disease (118–172). Careful monitoring of side effects and the development of strategies to enhance antitumor immunity without triggering excessive inflammation are crucial for the success of this approach.

Designing clinical trials that adequately evaluate the combination of RT and immunotherapy presents unique challenges. These trials need to account for variations in RT dose, fractionation, timing and sequence relative to immunotherapy administration. Furthermore, the heterogeneity of NSCLC, with its diverse molecular subtypes and immune landscapes, complicates the interpretation of trial results. Trial endpoints, such as PFS and OS, must also be carefully selected, particularly in light of the delayed responses often seen with immunotherapy.

Addressing these challenges is essential for optimizing the combination of RT and immunotherapy for clinical use and ensuring that patients receive the maximum benefit from this promising treatment strategy.

Currently available preclinical and clinical data support the pairing of ICIs with RT. This strategy offers new hope for patients who have not responded to immunotherapy alone. Based on the existing literature, the present review systematically integrated and analyzed the latest clinical research progress in the combined use of immunotherapy and RT for advanced NSCLC in previous years. Firstly, the present review assessed the rationale, safety challenges and management strategies of combined therapy and suggestions for optimizing treatment options were put forward. Secondly, this review comprehensively evaluated the effects of different RT doses and fractionation methods on the efficacy of immunotherapy and proposed strategies for individualized adjustment of RT regimens. Thirdly, the present review explored the mechanism of RT-induced immunomodulatory effects and its potential role in overcoming immune resistance. Furthermore, exploring the mechanistic interaction between RT and immunotherapy, and analyzing optimal sequencing of therapies and RT dose, fraction and target volume may provide new treatment perspectives for patients with advanced NSCLC in the future.