Efficacy and Safety of Immune Checkpoint Inhibitors Combined With Chemoradiotherapy in Limited-Stage Small Cell Lung Cancer

Introduction

Small cell lung cancer (SCLC) is a highly aggressive neuroendocrine malignancy accounting for approximately 13% to 15% of all lung cancers, characterized by rapid proliferation, early metastatic dissemination, and initial chemosensitivity followed by near-universal relapse.¹ Limited-stage disease (LS-SCLC), defined as tumor confined to 1 hemithorax encompassable within a single tolerable radiation field, represents approximately 30% of newly diagnosed SCLC cases.²

The standard of care for LS-SCLC has been concurrent chemoradiotherapy (CRT) with platinum-etoposide and thoracic radiotherapy, followed by prophylactic cranial irradiation (PCI) in responders. Despite initial response rates exceeding 80%, the 5-year overall survival (OS) remains a dismal 15% to 30%, with a median OS of approximately 25 to 30 months in landmark trials.^3,4 This treatment paradigm remained essentially unchanged for nearly 4 decades, representing one of the longest therapeutic stagnation periods in thoracic oncology.

The advent of immune checkpoint inhibitors (ICIs) transformed the extensive-stage (ES)-SCLC landscape. The IMpower133 (NCT02763579) and CASPIAN (NCT03043872) trials established anti–PD-L1 agents (atezolizumab and durvalumab, respectively) as first-line standards in ES-SCLC.^5,6 The landmark PACIFIC trial (NCT02125461), demonstrating durable OS improvements with consolidation durvalumab after CRT in unresectable stage III non–small cell lung cancer (NSCLC), provided a compelling biological rationale to investigate analogous strategies in LS-SCLC.⁷

Several randomized controlled trials (RCTs) have since evaluated ICIs in LS-SCLC, employing divergent strategies: concurrent ICI during CRT, consolidation ICI following CRT, and combined approaches. The phase 3 ADRIATIC trial (NCT03703297) established consolidation durvalumab as a new standard of care, earning FDA approval in December 2024.⁸ However, the phase 3 NRG-LU005 trial (NCT03811002) reported no benefit, with a numerically inferior OS in the atezolizumab arm (HR, 1.11; 95% CI, 0.85-1.45), and the phase 2 STIMULI (NCT02046733) and ACHILES (NCT03540420) trials also failed to demonstrate significant improvements with consolidation nivolumab-ipilimumab and atezolizumab, respectively.^9,10

Although a prior single-arm meta-analysis reported promising outcomes with ICI plus radiotherapy in LS-SCLC,¹¹ no comparative meta-analysis synthesizing RCT data exclusively has been published. This gap is clinically critical given the conflicting trial results and the pivotal question of whether the benefit of immunotherapy is universal or confined to specific agents and sequences. We therefore conducted this systematic review and meta-analysis to comprehensively evaluate the efficacy and safety of ICIs combined with CRT in LS-SCLC, with prespecified subgroup analyses examining ICI timing, agent selection, ICI class, and radiation fractionation.

Materials and Methods

Search Strategy and Study Selection

The systematic review and meta-analysis were conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines.¹² A comprehensive literature search was performed across PubMed/MEDLINE, Embase, Web of Science, and Cochrane Central Register of Controlled Trials (CENTRAL) from inception through March 31, 2026. The search strategy combined MeSH terms and free-text keywords: (“small cell lung cancer” or “SCLC”) and (“limited stage” or “LS-SCLC”) and (“immunotherapy” or “immune checkpoint” or “PD-1” or “PD-L1” or “durvalumab” or “atezolizumab” or “nivolumab”) and (“radiotherapy” or “chemoradiotherapy”). Conference abstracts from the American Society of Clinical Oncology (ASCO), European Society for Medical Oncology, and American Society for Radiation Oncology annual meetings (2020-2026) were also searched. Reference lists of included studies were manually screened. The full search strategy is provided in Supplementary Table S1.

This systematic review was not prospectively registered in PROSPERO or a comparable registry before data collection. This constitutes a methodological limitation that the authors acknowledge. The review protocol was registered with the Menoufia University Research Registry. In line with PRISMA-P (PRISMA for Protocols) recommendations, the authors commit to prospectively registering any future updated iteration of this meta-analysis in PROSPERO before data collection.

Eligibility Criteria

Studies were included if they met all of these criteria: (1) RCT design (phase 2 or 3); (2) enrolled patients with histologically or cytologically confirmed LS-SCLC; (3) compared an ICI-containing regimen (concurrent, consolidation, or both) with standard CRT, observation, or placebo; and (4) reported at least 1 primary end point of OS or progression-free survival (PFS). Exclusion criteria included single-arm studies, retrospective analyses, case reports, review articles, duplicate publications, and trials enrolling patients with ES-SCLC exclusively.

Data Extraction and Quality Assessment

Data were independently extracted by 2 reviewers (A.S. and A.A.) with discrepancies resolved by consensus. Extracted variables included trial name, phase, sample size, ICI agent and dosing, CRT regimen, radiation schedule and fractionation, PCI administration, median follow-up, OS and PFS HRs with 95% CIs, objective response rates, and safety data (grade 3-4 adverse events [AEs], treatment discontinuation, treatment-related deaths). Risk of bias was assessed using the Cochrane Risk of Bias 2.0 (RoB 2) tool.¹³ Certainty of evidence for the primary outcome (OS) was assessed using the Grading of Recommendations, Assessment, Development, and Evaluations (GRADE) framework (Supplementary Table S2).

Statistical Analysis

Pooled HRs and 95% CIs for OS and PFS were calculated using the generic inverse-variance method with a random-effects model (DerSimonian-Laird).¹⁴ Heterogeneity was quantified using the I² statistic, with values of 25%, 50%, and 75% indicating low, moderate, and high heterogeneity, respectively. Prespecified subgroup analyses were performed by (1) ICI timing (consolidation-only vs concurrent); (2) ICI agent class (anti–PD-L1 monotherapy vs dual checkpoint blockade); and (3) trial phase (phase 3 vs phase 2). Safety outcomes were pooled as risk ratios (RRs). Publication bias was not formally assessed given the small number of trials (≥ 5 trials required for reliable funnel plot interpretation), consistent with Cochrane recommendations. All analyses were performed using R version 4.3.2 (R Project for Statistical Computing) with the meta and metafor packages. For all trials, HRs and their 95% CIs were extracted from the primary published report; for ACHILES, values for both OS and PFS were taken from the ASCO 2025 primary presentation,¹⁵ in which the PFS HR is reported as 0.88 (95% CI, 0.60-1.28). To ensure full reproducibility and to allow independent verification of the pooled estimates, the trial-level log-HRs and standard errors used as inputs to the generic inverse-variance pooling for both OS and PFS are reported separately in Supplementary Table S3.

Results

Study Selection

The systematic search identified 1247 records from database searches and 38 additional records from conference proceedings and manual reference screening, for a total of 1285 records. After removing 350 duplicates, 935 records were screened by title and abstract, of which 891 were excluded for failing to meet the eligibility criteria. Full-text assessment of the remaining 44 articles resulted in exclusion of 40 records: 22 single-arm studies, 8 review articles, 5 conference abstracts superseded by full publications, 3 duplicate reports, and 2 trials enrolling exclusively patients with ES-SCLC. Four RCTs meeting all inclusion criteria were included in the final qualitative and quantitative synthesis (Figure 1).

Characteristics of Included Studies

The 4 included RCTs enrolled a combined total of 1397 randomly assigned patients (Table 1). The ADRIATIC trial was a phase 3, double-blind, placebo-controlled study randomly assigning 530 patients to consolidation durvalumab 1500 mg every 4 weeks for up to 24 months vs placebo, initiated 1 to 42 days after CRT completion.⁸

NRG-LU005 was a phase 3, open-label trial randomly assigning 544 patients to CRT with concurrent and adjuvant atezolizumab 1200 mg every 3 weeks for up to 17 cycles vs CRT alone, beginning atezolizumab concurrently during the second chemotherapy cycle.⁹ STIMULI was a phase 2, open-label study randomly assigning 153 patients (target: 260) to consolidation nivolumab 1 mg/kg plus ipilimumab 3 mg/kg every 3 weeks for 4 cycles, followed by nivolumab 240 mg every 2 weeks for up to 12 months vs observation; the trial was prematurely closed due to poor accrual.¹⁰ ACHILES was a phase 2, open-label study randomly assigning 170 patients to atezolizumab 1200 mg every 3 weeks for 12 months vs observation.¹⁵

Across all trials, median patient age ranged from 62 to 66 years, and chemotherapy consisted of platinum-etoposide doublets. Radiation schedules ranged from once-daily (60-66 Gy) to twice-daily (45 Gy in 30 fractions). PCI was administered at investigator discretion in ADRIATIC (~54%) and ACHILES (~67%), was required in STIMULI, and was optional in NRG-LU005. Median follow-up ranged from 21.0 months (NRG-LU005) to 45.1 months (ACHILES), introducing another source of heterogeneity given that late mortality events in LS-SCLC can substantially alter HR estimates (Table 1).

Risk of Bias Assessment

The phase 3 trials (ADRIATIC and NRG-LU005) were judged at low overall risk of bias. ADRIATIC employed double-blinding with placebo control, while NRG-LU005 used centralized randomization and stratification by prognostic factors. STIMULI had some concerns due to premature closure (n = 153/260 planned patients, 59% enrollment) and a protocol amendment altering the primary end point. ACHILES had low risk in most domains but was open-
label. Full domain-level assessments are provided in Supplementary Table S1.

GRADE Evidence Summary (OS): The overall certainty of evidence for OS benefit with ICIs in LS-SCLC was rated low using the GRADE framework, downgraded from RCT-level evidence due to (1) moderate unexplained statistical heterogeneity (I² = 53.1%); (2) imprecision (95% CI crossing 1.0); and (3) inclusion of 2 underpowered phase 2 trials at moderate risk of type II error. The certainty of evidence for the consolidation durvalumab subgroup (ADRIATIC alone) was rated moderate, downgraded from high due to the single-trial, single-agent limitation and lack of individual patient data.

Primary Outcome: OS

All 4 trials reported OS data (Figure 2A; Table 2). The pooled HR for OS was 0.95 (95% CI, 0.75-1.21; P = .67) with moderate statistical heterogeneity (I² = 53.1%; Cochran Q = 6.40, df = 3, P = .094), indicating that the overall pooled estimate should be interpreted with caution given the substantial between-trial variance in treatment effects.

Only ADRIATIC demonstrated a statistically significant OS benefit (HR, 0.73; 95% CI, 0.57-0.93), representing a clinically meaningful 22.5-month improvement in median OS (55.9 vs 33.4 months).⁸ ADRIATIC employed a prespecified α of 0.01321 for OS (hence the reported 98.321% CI of 0.54-0.98 in the trial publication); the conventional 95% CI (0.57-0.93) used in our meta-analysis is derived from the HR and standard error as reported.

NRG-LU005 demonstrated a numerically inferior OS in the ICI arm (median, 33.1 vs 39.5 months; HR, 1.11; 95% CI, 0.85-1.45),⁹ representing a 6.4-month absolute difference favoring the control arm, albeit nonsignificant. STIMULI reported no OS difference (HR, 0.95; 95% CI, 0.59-1.54) at median follow-up of 22.4 months.¹⁰ ACHILES similarly showed no OS benefit (median, 43.3 vs 38.8 months; HR, 1.14; 95% CI, 0.76-1.72) at 45.1 months median follow-up.¹⁵

Secondary Outcome: PFS

PFS was reported in all 4 trials (Figure 2B). The pooled HR for PFS was 0.89 (95% CI, 0.76-1.03; P = .12), with low heterogeneity (I² = 11.1%). The pooled OS and PFS estimates are therefore directionally concordant but numerically distinct; both point estimates favor the ICI-containing arms but neither is statistically significant, and both are influenced predominantly by the ADRIATIC result; the trial-level inputs underlying each pooled estimate are tabulated separately in Supplementary Table S3.

ADRIATIC demonstrated significantly improved PFS with durvalumab (median, 16.6 vs 9.2 months; HR, 0.76; 95% CI, 0.61-0.95).⁸ NRG-LU005 showed no difference in PFS (median, 12.0 vs 11.5 months; HR, 1.00; 95% CI, 0.80-1.25),⁹ STIMULI reported numerically inferior PFS in the ICI arm (median, 10.7 vs 14.5 months; HR, 1.02; 95% CI, 0.66-1.58),¹⁰ and ACHILES showed numerically improved but nonsignificant PFS with atezolizumab (median, 21.1 vs 15.9 months; HR, 0.88; 95% CI, 0.60-1.28).¹⁵

Subgroup Analyses

ICI Timing: Consolidation vs Concurrent

The 3 consolidation-only trials (ADRIATIC, STIMULI, ACHILES) yielded a pooled OS HR of 0.87 (95% CI, 0.63-1.20; I² = 64.1%), while the concurrent strategy (NRG-LU005) reported an OS HR of 1.11 (95% CI, 0.85-1.45). The test for subgroup differences was not statistically significant (P = .14), precluding definitive conclusions about differential strategy effects. Nevertheless, the directional divergence is consistent with—but does not establish—the hypothesis that simultaneous ICI and thoracic radiation may be immunologically counterproductive; this remains hypothesis generating (Figure 3; Supplementary Table S4).

ICI Agent and Regimen

Among anti–PD-L1 consolidation strategies, in an indirect, cross-trial comparison that cannot support conclusions about agent superiority, durvalumab (ADRIATIC; 24 months) showed a significant OS benefit (HR, 0.73; 95% CI, 0.57-0.93), whereas atezolizumab (ACHILES; 12 months) did not (HR, 1.14; 95% CI, 0.76-1.72). Potential explanations for this agent-specific divergence include: (1) structural differences between durvalumab (fully human IgG1) and atezolizumab (humanized IgG1) in Fc-receptor binding and antibody-dependent cellular cytotoxicity; (2) the nearly double treatment duration in ADRIATIC (24 vs 12 months); (3) differences in patient populations and stratification factors; and (4) potential variation in the immunogenic window post CRT (1-42 days for ADRIATIC vs 3-7 weeks for ACHILES). The dual ICI regimen (STIMULI; nivolumab plus ipilimumab) was associated with substantially higher toxicity-driven treatment discontinuation (~46% received fewer than 4 planned induction cycles), with median time on immunotherapy less than 2 months.¹⁰This abbreviated exposure is likely insufficient to generate durable antitumor immunity and constitutes a major confound for interpreting STIMULI’s efficacy results.

Radiation Fractionation

Exploratory subgroup data from ADRIATIC suggested consistent durvalumab benefit across both once-daily and twice-daily schedules.⁸ In NRG-LU005, OS was longer with twice-daily than once-daily radiation regardless of atezolizumab allocation (median OS, 35 vs 28 months; HR for once-daily vs twice-daily, 1.44; 95% CI, 1.10-1.89). Because fractionation was not randomly assigned, this comparison is nonrandomized and subject to confounding.⁹ This reinforces the importance of radiation intensification in LS-SCLC independent of immunotherapy. It should be emphasized that radiation dose and fractionation are variables independent of the ICI strategy in these trials: The twice-daily OS advantage in NRG-LU005 was observed across both the atezolizumab and control arms and therefore represents a radiation-biology effect rather than an immunotherapy-specific signal. This fractionation effect should not be interpreted as evidence that twice-daily radiation is a synergistic partner specifically for ICI.

Secondary Outcome: Safety

The pooled risk of grade 3 to 4 AEs was not significantly increased with ICI vs control (RR, 1.08; 95% CI, 0.92-1.27; I² = 48.2%). In ADRIATIC, grade 3 to 4 AEs were comparable between durvalumab (24.4%) and placebo (24.2%) arms, and grade 3 to 4 pneumonitis rates were similarly low (3.1% vs 2.6%).⁸ Treatment discontinuation due to AEs was numerically higher with durvalumab (16.4% vs 10.6%), a difference that was not formally tested but may reflect a modest tolerability burden. In ACHILES, grade 3 to 4 toxicity was numerically higher in the atezolizumab arm (34.1% vs 20.0%), and 3 treatment-related deaths were reported.¹⁵ Patients in STIMULI had the greatest ICI-attributable toxicity, with the largest absolute excess in grade 3 to 4 AEs (46.2% vs 30.7%), the highest AE-related discontinuation among reporting trials (17.9%), and a median time on nivolumab-ipilimumab of under 2 months (Table 3).

Discussion

The systematic review and meta-analysis are, to our knowledge, the first comprehensive synthesis of RCT evidence evaluating ICIs combined with CRT in LS-SCLC. Our analysis of 4 RCTs involving 1397 patients reveals a nuanced landscape in which immunotherapy benefit is not uniform but critically dependent on treatment strategy, agent selection, sequencing, and treatment duration.

The overall pooled analysis did not demonstrate statistically significant OS or PFS benefit with the addition of ICIs (pooled OS HR, 0.95; PFS HR, 0.89; both P >.10). However, this aggregate result masks moderate between-trial heterogeneity (I² = 53.1% for OS) driven by fundamentally different treatment strategies and outcomes. The central and most practice-relevant finding of this analysis is the divergence between consolidation and concurrent ICI strategies, with the directionality favoring consolidation approaches (pooled HR, 0.87 vs 1.11), though the subgroup difference did not reach statistical significance (P = .14).

The ADRIATIC trial stands as the definitive positive study, delivering a 22.5-month absolute gain in median OS with consolidation durvalumab (55.9 months vs 33.4 months; HR, 0.73).⁸ This represents the first major therapeutic advance in LS-SCLC in 4 decades, exceeding the OS benefits seen with immunotherapy in the ES-SCLC setting and likely reflecting the unique immunological milieu following CRT completion: substantially reduced tumor burden, radiation-induced immunogenic cell death, and recovery of radiation-depleted lymphocyte populations. The magnitude and durability of benefit—with a median OS exceeding 55 months—qualifies consolidation durvalumab as a validated new standard of care.

In contrast, NRG-LU005 did not show an improvement in outcomes with concurrent atezolizumab (HR, 1.11; 95% CI, 0.85-1.45), and OS was numerically inferior in the atezolizumab arm.⁹ This pattern directly parallels the PACIFIC-2 experience in stage III NSCLC, where simultaneous durvalumab and CRT also failed to replicate the survival benefit of the sequential PACIFIC strategy.¹⁶ Multiple biological mechanisms likely explain this finding: (1) thoracic radiation induces profound lymphocyte depletion, potentially eliminating the effector T-cell populations that ICI depends on; (2) the immunosuppressive tumor microenvironment during active cytotoxic therapy may suppress ICI-induced immune activation; and (3) concurrent administration may create temporal competition between cytotoxic cell kill and immune priming.

The differential performance of 2 anti–PD-L1 consolidation strategies—durvalumab (ADRIATIC HR, 0.73, significant) vs atezolizumab (ACHILES HR, 1.14, nonsignificant)—within the same mechanistic class is particularly instructive and requires careful interpretation.^8,15 Beyond agent-specific pharmacological differences, the most biologically plausible explanatory factor is treatment duration: 24 months in ADRIATIC vs 12 months in ACHILES. Extended immunotherapy duration may be necessary to sustain immune surveillance and prevent late relapse in a disease with high clonal evolution capacity. Additional differentiating factors include sample size (ADRIATIC better powered), stratification rigor, and patient selection criteria. Importantly, a direct head-to-head comparison between durvalumab and atezolizumab in LS-SCLC is lacking, and firm conclusions about agent superiority cannot be drawn from indirect comparisons. An alternative interpretation must also be acknowledged: the ACHILES result may reflect, at least in part, a statistical artifact of limited power rather than a true absence of effect. With only 170 patients (n = 85 per arm) vs 530 in ADRIATIC, the point estimate for OS in ACHILES (HR, 1.14) trends unfavorably, but the wide 95% CI (0.76-1.72) is unable to exclude a clinically meaningful benefit of the magnitude observed in ADRIATIC (eg, HR, 0.80). The ACHILES sample size was therefore insufficient to reliably detect an effect of that size, and its null finding should not be overinterpreted as definitive evidence against a class effect for consolidation anti–PD-L1 therapy.

The STIMULI trial’s negative result deserves careful contextualization.¹⁰ Underpowering (59% of planned enrollment reached), high early discontinuation (~46%), and a median active treatment exposure of under 2 months collectively render it impossible to determine whether the null result reflects true absence of efficacy or failed drug delivery. A post hoc landmark analysis in STIMULI showed a numerically higher 36-month OS with nivolumab-ipilimumab (53.9% vs 40.7%); this exploratory observation is hypothesis generating and may warrant further investigation in adequately powered, better-tolerated trial designs, potentially with lower ipilimumab doses or modified sequencing.

Several additional methodological considerations merit emphasis. The timing of ICI initiation relative to CRT completion varied substantially: ADRIATIC allowed initiation 1 to 42 days post CRT, ACHILES waited 3 to 7 weeks, and NRG-LU005 initiated atezolizumab concurrently during chemotherapy. This window of immune reconstitution post CRT may be critical for optimal ICI activity, with earlier initiation after CRT potentially allowing ICI to act on primed, recovering immune effectors.¹⁷ Additionally, the role of PCI in the immunotherapy era remains unresolved. PCI rates varied from mandatory (STIMULI) to approximately 54% (ADRIATIC), and the interaction between whole-brain radiation, cranial immunosurveillance, and systemic ICI activity has not been formally evaluated.

From a radiation oncology perspective, cumulative data from both ADRIATIC and NRG-LU005 continue to support twice-daily fractionation as potentially advantageous in the immunotherapy era.¹⁸ Accelerated fractionation may enhance immunogenic cell death by increasing mitotic catastrophe and neoantigen release, potentially synergizing with ICI-mediated immune priming. This proposed synergy, however, remains a mechanistic hypothesis: The fractionation effects observed to date (notably in NRG-LU005) were independent of ICI allocation and reflect radiation biology rather than a demonstrated radiation-immunotherapy interaction. Prospective evaluation of fractionation schedule as an effect modifier for ICI benefit is warranted.

This meta-analysis has several limitations that must be explicitly acknowledged. First, only 4 trials were available, limiting meta-analytic power and precluding reliable publication bias assessment. Second, substantial clinical heterogeneity (ICI agents, sequencing, radiation schedules, PCI utilization, follow-up duration) introduces sources of variability that statistical models cannot fully resolve. Third, individual patient-level data were unavailable, precluding adjustment for patient-level confounders (ECOG performance status, radiation technique, PCI receipt, and biomarker subgroups). Fourth, 2 of the 4 included trials were phase 2 with small sample sizes, contributing disproportionate uncertainty. Fifth, the absence of prospective PROSPERO registration represents a methodological gap. Sixth, the GRADE assessment rates the overall evidence as low certainty, appropriately tempering the strength of recommendations derived from the aggregate analysis.

Despite these limitations, this meta-analysis provides the most comprehensive RCT-level synthesis currently available and delivers a clear, clinically actionable message: Consolidation durvalumab following CRT is the only validated ICI strategy in LS-SCLC. Future research priorities should include (1) biomarker-driven patient selection (PD-L1 expression, tumor mutational burden, circulating tumor DNA dynamics), (2) prospective evaluation of treatment duration, (3) investigation of novel agents including TIGIT inhibitors and bispecific antibodies, (4) prospective evaluation of radiation fractionation as a potential effect modifier of consolidation ICI benefit, and (5) standardization of PCI practices in the immunotherapy era.

Conclusions

The systematic review and meta-analysis demonstrate that the integration of ICIs into LS-SCLC management is feasible but not uniformly beneficial. Consolidation durvalumab following CRT represents the only validated ICI strategy, supported by phase 3 evidence graded as moderate certainty, with a median OS of 55.9 vs 33.4 months (HR, 0.73; 95% CI, 0.57-0.93), establishing it as a new standard of care. Concurrent ICI administration and consolidation with agents other than durvalumab have not shown significant benefits. Pooled grade 3 to 4 toxicity did not differ significantly between ICI-containing and control arms (RR, 1.08; 95% CI, 0.92-1.27), although dual checkpoint blockade in STIMULI was associated with substantially higher discontinuation. The field must now focus on biomarker identification, optimal treatment duration, and refinement of radiation-immunotherapy synergy to extend the benefits of immunotherapy to a broader LS-SCLC patient population.

Funding

This study received no external funding.

Conflicts of Interest

All authors declare no conflicts of interest.

Author Contributions

All authors contributed equally to this work.

Ethics Statement

Ethical approval was not required, as this is a systematic review and meta-analysis of previously published data.

AI Disclosure

Artificial intelligence (AI) tools were used for grammar correction and text formatting only. No generative AI was used for data extraction, interpretation, or critical appraisal.

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Supplementary Materials