Mapping the evolution of immune checkpoint inhibitor research in small cell lung cancer: A bibliometric analysis (1997-2025).

Introduction
Small cell lung cancer (SCLC), a highly aggressive subtype of lung cancer, constitutes approximately 14% of all lung cancer cases.1 This cancer is distinguished by its rapid proliferation, early metastasis, and initial responsiveness to chemotherapy and radiotherapy, yet it is marked by a high recurrence rate and poor long-term survival. The therapeutic arsenal for SCLC primarily encompasses platinum-based chemotherapy, concurrent radiotherapy for local tumor control, prophylactic cranial irradiation to mitigate the risk of brain metastasis, and, in select cases, surgical intervention.2,3 Despite these interventions, the 5-year survival rate for SCLC is less than 7%, underscoring the imperative for novel therapeutic strategies.4,5 Although SCLC exhibits initial sensitivity to conventional chemotherapy and radiotherapy, its rapid relapse and resistance to treatment render it one of the most formidable malignancies to manage, prompting a shift toward exploring immunotherapeutic strategies as a potential avenue for improved patient outcomes.
Immunotherapy has transformed cancer treatment, with immune checkpoint inhibitors (ICIs) leading the way as innovative therapeutics. These agents target immune checkpoint proteins on tumor cells, such as PD-1, PD-L1, and CTLA-4, to enhance immune recognition and attack on tumors.6,7 In SCLC, ICIs have opened new therapeutic horizons, especially for patients with advanced disease unresponsive to conventional chemotherapy.8 Landmark trials like IMpower133 and CASPIAN have confirmed the efficacy of ICIs combined with chemotherapy as the standard first-line therapy for extensive-stage SCLC (ES-SCLC), showing improved median overall survival compared to chemotherapy alone.9,10 Despite these advances, predicting ICI efficacy with reliable biomarkers and managing immune-related adverse events (irAEs) remain challenges. Research is also exploring combination therapies, such as ICIs with anti-angiogenic agents, and comprehensive treatment strategies for ES-SCLC patients with brain metastases, including the integration of radiotherapy, chemotherapy, and ICIs.11,12 There is a need for a comprehensive analysis of ICIs in SCLC to understand the current state of research in this field.
This study was guided by a pre-specified research question: how have immune checkpoint inhibitors been investigated in small cell lung cancer, and what are the publication trends, key contributors, and emerging hotspots in this field? This pre-defined question shaped the search strategy and analytical scope. Bibliometric analysis employs mathematical and statistical methods to systematically evaluate the literature on a specific research topic over time.13 It provides insights into research categories, co-authorship patterns, keyword frequency, and the most cited articles or journals, offering a comprehensive overview of the research landscape.13–16 Recent bibliometric studies have indicated a growing focus on lung cancer, highlighting its increasing prominence in the oncology research field. A recent bibliometric analysis of SCLC research from 2012 to 2021, shows a significant increase in publications, with China leading in productivity and “Heterogeneity and Subtypes” and “Immunotherapy” emerging as key research frontiers.17 However, no bibliometric analysis on the intersection of ICIs and SCLC has been reported to date. This study aims to systematically analyze the bibliometric landscape of ICIs in SCLC to identify research trends, key actors, and potential areas for future investigation.

Materials and methods

Search strategies and data collection
A systematic literature search was performed using the Web of Science Core Collection (WoSCC), a comprehensive and authoritative database for high-quality academic publications. WoSCC was selected as the sole data source because it provides standardized citation indexing, wide coverage of biomedical literature, and full compatibility with bibliometric tools such as VOSviewer, CiteSpace, and Bibliometrix. Restricting to one authoritative database also minimized duplication across platforms and ensured consistency of analysis.18 The search was conducted on February 17, 2025, and included studies published between 1997 and February 2025. The search formula used was as follows: ((TS = (immune checkpoint inhibitor* OR ICI* OR “PD-1 inhibitor*” OR “PD-L1 inhibitor*” OR Pembrolizumab OR Keytruda OR Nivolumab OR Opdivo OR Cemiplimab OR Libtayo OR Atezolizumab OR Tecentriq OR Hybreza OR Avelumab OR Bavencio OR Durvalumab OR Imfinzi OR “CTLA-4 inhibitor*” OR Ipilimumab OR Yervoy OR Tremelimumab OR Imjudo OR “LAG-3 inhibitor*” OR Relatlimab OR Favezelimab OR “Tim-3 inhibitor” OR “ML-T7” OR “Sabatolimab”)) AND TS = (small cell lung cancer OR SCLC OR small-cell lung carcinoma) NOT TS = (NSCLC OR “non-small cell lung cancer” OR “non-small cell lung carcinoma”)).11,17 To ensure comprehensiveness, the final search strategy was reviewed by both an oncology expert and a medical information retrieval librarian, who provided feedback on the inclusion of relevant terms and the logic of the search strategy. The inclusion criteria were limited to original research articles published in English. The bibliographic data were exported in “Full record and cited references” and “plain text” formats for further analysis. Extracted information included titles, author details, institutions, countries/regions, keywords, publication counts, citations, and journal names.

Statistical analysis
To analyze and visualize the bibliometric data, three tools were employed: VOSviewer (version 1.6.20), CiteSpace (version 6.3.R1), and the R package “bibliometrix” (version 4.3.3).19,20 VOSviewer was used to map co-authorship, institutional collaborations, citation networks, co-citation networks, and keyword co-occurrence. These visualizations revealed collaborative patterns, research hotspots, and thematic clusters. CiteSpace was applied to detect research trends and emerging hotspots using keyword burst analysis. Keyword bursts highlighted temporal shifts in research focus and emerging trends. The R package “bibliometrix” facilitated comprehensive bibliometric evaluations. Key metrics such as the H-index, G-index, and M-index were employed to evaluate the academic influence of authors and journals.21,22 The H-index quantified productivity and citation impact, while the G-index emphasized highly cited works. To assess journal quality and influence, Journal Citation Reports (JCR) quartiles and Impact Factor (IF) were incorporated.

Results

The publication and citation trends
A total of 809 eligible publications were selected for analysis. The data collection and screening process was summarized in Figure 1. A total of 6,536 authors contributed to these publications, with 20.64% involving international co-authorship. The average number of coauthors per document was 10.8. The documents reference 18,711 sources, and the average number of citations per document was 36.47 (Figure 2(A)).

The publication trend remained relatively stable with minimal growth from 1997 to 2015, with annual publications staying below 20. A notable surge began after 2016, with publication counts rising sharply and peaking at 183 in 2024 (Figure 2(B)).

Analysis of countries
The global distribution of publications revealed significant contributions from leading countries. China led with 304 articles (37.6%), followed by the USA (201 articles, 24.8%) and Japan (101 articles, 12.5%) (Table 1). China led in the number of single-country publications (SCP) with 276, but its multiple-country publication (MCP) ratio was relatively low at 0.092. In contrast, the USA had a higher MCP ratio of 0.363. Japan, despite contributing 93 SCPs, showed limited international involvement with an MCP ratio of 0.079. Notably, countries like Spain and Switzerland exhibited high MCP ratios of 0.786 and 1, respectively (Figure 3(A); Figure S1,S2).

The international collaboration network further emphasized the USA as the central node, forming extensive partnerships with countries. Among the 49 countries engaged in international collaborations with at least one publication, the USA led with the highest number of collaborations (total link strength = 414), followed by Germany (total link strength = 268) and Spain (total link strength = 264) (Figure 3(B)).

Analysis of institutions
The University of Texas System led with 105 articles, followed by UTMD Anderson Cancer Center (84 articles) and Harvard University (65 articles). In China, Shandong First Medical University similarly ranked in the top positions, with 65 articles (Figure 4(A)). Among the 135 institutions engaged in international collaborations with a minimum of 5 articles, Astra Zeneca had the highest number of collaborations with other countries (total link strength = 94), followed by the University of Texas MD Anderson Cancer Center (total link strength = 92) and Semmelweis University (total link strength = 84) (Figure 4(B)).

Analysis of journals
The collected papers were distributed across 282 journals, with Frontiers in Oncology leading the list with 38 total publications, followed by Thoracic Cancer (32 total publications) and Lung Cancer (28 total publications) (Table 2). Among the high-impact journals, the top three journals with the highest H-index values were the Journal of Thoracic Oncology (H-index = 19, IF = 21.1, Q1), Journal for Immunotherapy of Cancer (H-index = 13, IF = 10.3, Q1), and Lung Cancer (H-index = 12, IF = 4.5, Q1). In terms of total citations (TC), the Journal of Clinical Oncology ranked first (TC = 2,102), while Journal of Thoracic Oncology (TC = 1,298) occupies the third position. Notably, journals with high impact factors, such as Annals of Oncology (IF = 56.7) and Journal of Clinical Oncology (IF = 42.1), did not necessarily have the highest TP counts, suggesting that articles on this topic may be more commonly found in specialized journals focusing on thoracic and pulmonary diseases.
The co-occurrence networks of journals included 104 journals with at least 2 occurrence. The three key journals with the highest total link strength in these networks were the Journal of Clinical Oncology (425), Journal of Thoracic Oncology (388), and New England Journal of Medicine (330) (Figure 5(A)). The coupling networks of journals consisted of 113 journals with at least 2 couplings. The three key journals with the highest total link strength in the co-occurrence networks were Frontiers in Oncology (24,802), Lung Cancer (21,425), and Journal of Thoracic Oncology (16,020) (Figure 5(B)).

Analysis of authors
A total of 6,536 authors contributed to this research field. Authors with high impact, as determined by their publication and citation metrics, were identified. Notably, Wang J ranked first in total publications (21 articles) with a significant citation count of 1,706 and an M-index of 1.375, reflecting a consistent and impactful contribution since 2018. Reck M, with 15 publications and 4,341 citations, ranked second in productivity and fifth in TC. Other prominent authors included Losonczy G (10 publications, 5,789 citations) and Havel L (9 publications, 6,148 citations) (Table 3).
Among the 100 authors involved in international collaborations with a minimum of 4 articles, Taniguchi Hirokazu had the highest number of collaborations with other authors (total link strength = 71), followed by Kagamu Hiroshi (69) and Kaira Kyoichi (69) (Figure 6).

Analysis of keywords
The keyword co-occurrence network (Figure 7(A)) further visualized these relationships, where larger nodes represented frequently occurring terms, and clusters reflected distinct research focuses. The red cluster (mechanisms of ICIs), centered on “expression” and “blockade,” highlighted research related to the mechanisms of ICIs, particularly focusing on the expression patterns of immune-related molecules and their blockade strategies in cancer treatment. The green cluster (chemotherapy combinations), with keywords like “etoposide” and “survival,” represented studies on chemotherapy combinations and their clinical outcomes. The yellow cluster (clinical trial designs), led by “open-label” and “multicenter,” focused on clinical trial designs and methodologies, emphasizing the importance of transparent and inclusive study protocols. The blue cluster (efficacy/safety profiles), with keywords like “nivolumab,” “efficacy,” and “safety,” highlighted research on the therapeutic potential and clinical outcomes of ICIs, particularly in terms of their effectiveness and safety profiles.

The analysis of citation bursts highlighted emerging trends, with keywords like “nivolumab” (2016 to 2019, strength = 13.5), “trial” (2017 to 2018, strength = 3.93) and “docetaxel” (2016 to 2019, strength = 10.77) experiencing strong bursts during this period. Recent bursts included “overall survival” (2023 to 2025, strength = 3.46), “radiation therapy” (2023 to 2025, strength = 2.76), and “placebo” (2023 to 2025, strength = 2.72) (Figure 7(B)). Beyond identifying clusters and burst terms, the temporal shift of keywords reflects how the field has evolved from broader concepts to more outcome-oriented endpoints. For instance, early burst terms such as “trial” (2017–2018, strength = 3.93) marked the initiation of clinical investigations, while more recent terms such as “overall survival” (2023–2025, strength = 3.46) indicate the growing emphasis on measurable patient outcomes. This evolution illustrates the maturation of SCLC-ICI research, where the scientific focus has moved from the initiation of clinical studies to the evaluation of their impact on survival.

Sensitivity analyses
We performed sensitivity analyses to assess the robustness of our findings by adjusting the minimum thresholds for co-authorship, citation, and keyword occurrence. Specifically, we modified the minimum co-authorship threshold from three publications to either four or two publications and observed the consistency of the core author clusters. The results of these analyses, including any changes to the core clusters, are detailed in the supplemental document Figures S3–S8.
Furthermore, we have included a timeline analysis to illustrate when each thematic cluster emerged and grew. This temporal visualization is available in Figure S9, which provides detailed insights into the evolution of these clusters over time.

Discussion
This bibliometric analysis evaluated global research trends on ICI in SCLC from 1997 to February, 2025. A total of 809 publications demonstrated a steady increase, with a sharp surge after 2016, coinciding with the expanding clinical application of ICIs in SCLC. The rapid growth after 2019 was further stimulated by landmark regulatory approvals, such as atezolizumab in 2019 and durvalumab in 2020, which likely catalyzed subsequent research momentum. At the same time, fluctuations between 2020 and 2021 may partially reflect the disruptive impact of the COVID-19 pandemic. Oncology trial initiations dropped by nearly 50% in early 2020, with phase II-III studies particularly affected,23 while surveys and multicenter analyses reported enrollment declines, widespread protocol deviations, and a shift toward decentralized trial models and telemedicine to sustain progress.24,25 These regulatory milestones and pandemic-related disruptions together help explain both the surge and short-term variability observed in publication volume and collaboration patterns. China, the USA, and Japan have been the primary contributors, with China leading in total output but showing limited international collaboration. Although China is the most prolific in SCLC-ICI research, its MCP ratio remains low, suggesting that studies are largely domestic. Similar patterns have been reported in other oncology domains, where China’s MCP was ~8.9% in tumor immunotherapy research and ~9.1% in colorectal cancer ctDNA studies, reflecting structural features such as funding priorities for national projects and institutional norms favoring domestic collaboration.26,27 In contrast, the USA exhibited extensive partnerships, underpinned by NIH and NCI programs that incentivize international projects, the use of English as the dominant scientific language, and the leadership of US-based pharmaceutical companies in global multi-center ICI trials, all of which structurally enhance cross-border collaboration (Funding for Global Cancer Research and Training was originally published by the National Cancer Institute). Institutions such as the University of Texas System and UTMD Anderson Cancer Center emerged as central players, demonstrating their leadership in SCLC-related immunotherapy studies.28,29 Notably, Shandong First Medical University represents China growing research capacity in this domain. Publication disparities across institutions and countries may be partly explained by differences in research funding and trial-supporting infrastructure. For example, analysis of USA neuro-oncology networks found that more than 57% of the population lacks direct access to clinical trial infrastructure, with most facilities concentrated in urban and socioeconomically advantaged regions.30 Similarly, global data show that high-income countries dominate oncology trial density, while lower-middle-income countries contribute few new trials despite high disease burden.31 Funding inequities also play a critical role as underfunding cancers and regions show significantly fewer clinical trials, with strong correlations between funding levels and trial availability.32 Journals like Frontiers in Oncology and Journal of Thoracic Oncology were prominent publication venues, reflecting the research community’s emphasis on oncology and thoracic cancers.33,34 The Journal of Clinical Oncology and Journal of Thoracic Oncology, with their high impact factors and central roles in co-citation networks, remain key platforms for disseminating significant findings. The concentration of SCLC-ICI papers in a few journals aligns with Bradford’s Law, enhancing visibility but potentially limiting dissemination across broader audiences, underscoring the benefit of more diverse publication venues.35 The research field boasts a robust community of 6,536 contributing authors, among whom a select few have distinguished themselves as leaders in their publication and citation metrics. These high-impact authors, such as Wang J and Reck M, have not only demonstrated prolific publication records but also garnered substantial recognition within the academic community, indicating their pivotal roles in shaping the discourse and advancing knowledge in the field.29,36 Comprehensive keyword analysis revealed four major research clusters representing different topics and frontier trends.

Mechanisms of ICIs (red)
The cluster underscores the critical role of understanding the molecular mechanisms underlying ICIs in SCLC. This cluster highlights research dedicated to elucidating the expression patterns of immune-related molecules, such as PD-L1 and CTLA-4, and the development of effective blockade strategies to enhance antitumor immune responses. This focus is particularly relevant given the high mutational burden and immunogenic potential of SCLC, which can lead to robust immune responses when effectively targeted.37 However, the complex tumor microenvironment and immune evasion mechanisms in SCLC pose significant challenges, necessitating a deeper understanding of the interplay between immune checkpoints and tumor biology.38 Future research should aim to identify predictive biomarkers and optimize combination therapies to overcome resistance and improve clinical outcomes in SCLC. By focusing on these mechanisms, studies within this cluster aim to optimize the therapeutic potential of ICIs, identify predictive biomarkers, and address resistance mechanisms, thereby advancing the clinical application of immunotherapy in SCLC.

Chemotherapy combinations (green)
This cluster focus on chemotherapy combinations in the context of ICI research for SCLC reflects a critical strategy to enhance treatment efficacy. Combining traditional chemotherapeutic agents, such as etoposide, with ICIs aims to leverage the initial sensitivity of SCLC to chemotherapy while simultaneously reactivating the immune response against tumor cells. This strategy has been validated in recent clinical studies, which demonstrate that the addition of ICIs to the conventional etoposide-platinum regimen can significantly improve progression-free survival and overall survival in patients with extensive-stage SCLC compared to chemotherapy alone.39,40 For instance, the meta-analysis by Chen et al. showed that the combination of ICIs with etoposide and platinum chemotherapy reduced the risk of disease progression by 29% and the risk of death by 21%, while increasing the objective response rate by 1.27 times.39 These findings highlight the potential of ICIs to enhance the therapeutic landscape of SCLC by augmenting the efficacy of traditional chemotherapy regimens. However, it is important to note that the combination therapy also increases the incidence of treatment-related adverse events, particularly immune-related adverse events, which necessitates careful patient selection and close monitoring during treatment.39 Continued exploration of these combination strategies is essential for optimizing therapeutic outcomes and addressing the unmet needs of patients with SCLC.

Clinical trial designs (yellow)
This cluster emphasizes the design and methodology of clinical trials in ICI research for SCLC. Open-label trials, while potentially introducing bias, provide pragmatic insights into efficacy and safety in real-world settings. For example, the CheckMate 331 trial evaluated second-line nivolumab in relapsed SCLC and, although it did not improve survival versus chemotherapy, it highlighted potential benefits in selected subgroups.41 Although nivolumab did not demonstrate a significant survival benefit compared to chemotherapy in this study, the trial highlighted the potential for certain patient subgroups, such as those with normal lactate dehydrogenase levels or without liver metastases, to derive improved outcomes from immunotherapy.41 This underscores the importance of open-label trials in identifying specific populations that may benefit from ICI treatment. Multicenter designs also enhance generalizability, as shown in the CAPSTONE-1 trial, which enrolled patients from 47 hospitals across China and demonstrated significant OS benefit for adebrelimab plus chemotherapy in ES-SCLC.36 These examples underscore the importance of rigorous yet adaptable trial designs in advancing ICI therapies. In our keyword analysis, the prominence of “open-label” appears to reflect reporting practices rather than a distinct scientific theme. For instance, the CASPIAN trial (durvalumab ± tremelimumab plus chemotherapy) and the CheckMate 331 trial both adopted randomized open-label designs due to practical and ethical considerations.41,42 Thus, “open-label” emerged as a hotspot largely because of trial design conventions rather than evolving biological concepts.
It is also important to distinguish between early- and late-phase trial activity when interpreting these bibliometric patterns. Early-phase (I/II) studies are often exploratory, focusing on mechanism, biomarker identification, and preliminary efficacy, but they frequently overestimate benefits that are not confirmed in phase III settings.43,44 By contrast, phase III/IV trials provide definitive survival and safety evidence but are resource-intensive and have a high failure rate when built on insufficient early-phase data.45,46 This gap is particularly relevant in immuno-oncology, where phase II response rates for PD-1/PD-L1 inhibitors have often exceeded outcomes later observed in phase III trials. Therefore, the prominence of clinical trial-related keywords in our analysis may partly reflect the transition from hypothesis-generating early-phase studies to confirmatory late-phase programs, highlighting both the opportunities and the risks of overinterpretation.

Efficacy/safety profiles (blue)
The cluster underscores the critical focus on evaluating the therapeutic potential and clinical outcomes of ICIs in the treatment of SCLC. By examining the efficacy and safety profiles of agents, this cluster reflects the ongoing efforts to optimize ICI therapies for SCLC. The emphasis on these parameters is essential for identifying patient subgroups that may benefit most from ICIs and for addressing the challenges associated with their use, including immune-related adverse events. This is further supported by recent meta-analyses, which demonstrate that combining ICIs with chemotherapy significantly improves PFS and OS in patients with ES-SCLC compared to chemotherapy alone, although at the cost of increased adverse events.47
While the clustering analysis captured key research themes related to ICIs and SCLC, we acknowledge some underrepresented but critical topics that merit further attention. These include real-world evidence (RWE) studies on underrepresented patient populations, such as older adults and those with comorbidities, as well as the understanding of immune-related adverse events (irAEs), particularly pneumonitis, which is a significant concern in ICI therapy.48,49 Further research into the molecular heterogeneity of SCLC, including tumor microenvironment (TME) characteristics and genetic alterations, is also crucial for improving therapeutic outcomes.
The pursuit of improving “overall survival” (2023–2025) in patients with SCLC through ICIs remains a critical focus in oncology research. Recent advancements have shown that combining ICIs with chemotherapy can significantly enhance OS compared to chemotherapy alone. For instance, the ASTRUM-005 trial demonstrated that adding the PD-1 inhibitor serplulimab to standard chemotherapy significantly prolonged median OS to 15.4 months, compared to 10.9 months with placebo plus chemotherapy.50 Similarly, the CAPSTONE-1 trial reported that combining the anti-PD-L1 antibody adebrelimab with chemotherapy improved median OS to 15.3 months, compared to 12.8 months with chemotherapy alone.36 These studies underscore the potential of ICIs to alter the treatment paradigm for ES-SCLC by extending survival outcomes. However, the identification of predictive biomarkers and optimization of combination therapies are still needed to further enhance the efficacy and safety of ICIs in this aggressive malignancy.
The integration of ICIs with “radiation therapy” (2023–2025) in the treatment of SCLC has recently attracted considerable research attention. While ICIs have shown modest benefits in ES-SCLC, their role in limited-stage disease remains under investigation. Early-phase studies and ongoing randomized trials, such as NRG LU-005, are evaluating the potential synergy of ICIs with chemoradiotherapy, aiming to enhance immune activation and local tumor control.51 At the same time, radiotherapy optimization studies, including the CONVERT trial, indicate that fractionation schedules continue to be debated.52 Although combining ICIs with TRT is biologically plausible and clinically appealing, it is not yet clear whether this reflects a durable paradigm shift or a transient spike in research interest. Further confirmatory phase III evidence and biomarker-driven approaches will be required before integration into standard practice can be considered.
The investigation of ICIs in combination with chemotherapy versus chemotherapy alone in SCLC underscores the potential benefits of integrating immunotherapy into standard treatment regimens. A recent systematic review and meta-analysis of four randomized controlled trials (RCTs) involving 2,213 patients demonstrated that the addition of ICIs to chemotherapy significantly improved OS compared to chemotherapy plus “placebo” (2023–2025).53 This finding suggests that ICIs may enhance the efficacy of chemotherapy in SCLC. The study also showed a significant improvement in PFS with the combination therapy, indicating a delay in disease progression.53 Importantly, the combination did not significantly increase the incidence of grade 3–4 adverse events compared to chemotherapy alone, suggesting that the addition of ICIs is well-tolerated.53,54 These results highlight the potential of ICIs to improve outcomes in SCLC without a substantial increase in toxicity, supporting their consideration as a valuable component of first-line treatment strategies. However, the limited number of included RCTs suggests that further large-scale, high-quality trials are needed to confirm these findings and to better understand the long-term benefits and risks associated with ICIs in SCLC treatment.
These emerging hotspots are strongly supported by clinical trial evidence. The CAPSTONE-1 trial (adebrelimab + EP) showed improved OS versus chemotherapy alone (15.3 vs 12.8 months; HR 0.72).55 Likewise, the ASTRUM-005 trial (serplulimab + EP) reported OS benefit (15.4 vs 10.9 months; HR 0.63).56 In addition, ongoing phase II work, such as NCT05161533, is evaluating durvalumab-chemotherapy followed by hypofractionated radiotherapy, directly reflecting the “radiation therapy” burst keyword.
Taken together, the bibliometric patterns identified in this study not only delineate current hotspots but also provide guidance for future SCLC-ICI research strategies. First, the concentration of research output in a few high-income regions highlights the importance of fostering broader international collaborations and supporting underrepresented countries to reduce disparities. Second, the emergence of trial-related keywords such as “placebo” reflects a methodological shift toward more rigorous randomized designs, underscoring the need for large-scale phase III/IV studies to confirm early-phase findings. Third, the recurrent focus on multimodal strategies, including integration with radiotherapy, suggests that translational research into biomarkers and mechanisms of synergy should be prioritized to optimize patient selection. Finally, disparities in funding and publication accessibility emphasize the value of transparent reporting, open access, and equitable funding mechanisms. Addressing these areas can transform bibliometric trends into actionable strategies that advance both scientific understanding and clinical outcomes in SCLC.

Limitations
This study has several limitations. First, data were collected only from the WoSCC database and limited to English-language publications, which may have excluded relevant studies indexed elsewhere or in other languages. Second, inconsistent reporting of funding acknowledgments and lack of granularity in bibliographic records prevented systematic analysis of research drivers and the specific roles of top institutions (e.g., trial coordination vs. mechanistic studies). In addition, time lags between study completion and subsequent publication may shift the apparent peaks of research activity, reflecting delays inherent to peer review and editorial processes. Third, bibliometric indicators may be disproportionately influenced by a few highly cited studies, potentially skewing country-, author-, or journal-level rankings. To mitigate this, we separately examined top-cited articles and validated core findings through keyword co-occurrence and thematic cluster analyses. Finally, although keyword standardization was performed, some nuanced terms may not have been fully captured. Future studies could enhance validity by linking bibliometric data with funding records and trial registries such as ClinicalTrials.gov to assess whether influential publications correspond to active or completed trials and clarify translational trajectories.57 In addition, integrating alternative metrics (e.g., social media attention, news coverage) may provide complementary insights into societal and cross-disciplinary impact beyond traditional citation-based measures.
In conclusion, this study provides a comprehensive bibliometric analysis of ICIs in SCLC from 1997 to 2025. The research hotspots in ICIs for SCLC include mechanisms of ICIs, chemotherapy combinations, clinical trial designs, and efficacy/safety profiles. Emerging trends focus on multimodal strategies, ICI-radiation integration, and open-label trials to improve outcomes. The analysis reveals emerging trends, including a focus on improving overall survival through multimodal treatment strategies, the integration of ICIs with radiation therapy, and the continued investigation of open-label clinical trials. These findings underscore the dynamic evolution of ICIs research in SCLC, emphasizing the need for further exploration of combination therapies, predictive biomarkers, and optimized treatment protocols to enhance clinical outcomes and address the challenges posed by this aggressive malignancy.

Supplementary Material

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