We performed a retrospective analysis to assess the effects of radiation during immunotherapy on various organs in patients with multiple metastatic SCLC. Our objective was to investigate the selection process and the differences in immune activation effects across irradiated sites, as well as the survival outcomes associated with different stages of RIT in a real-world cohort.
Among the selected blood indices, namely NLR, PLR, and SII, we observed correlations with different irradiated organs. Notably, the mean ranks of delta-NLR, delta-PLR, and delta-SII were consistently lower in the brain-irradiated group during immunotherapy. Previous research identified these three indicators as prognostic markers of poor patient outcomes [28]. Hence, we can infer that RT to brain has the strongest activation effect and therapeutic response in ES-SCLC during immunotherapy. Furthermore, our analysis of prognostic factors for 9-month OS and survival rates across different treatment stages revealed that first-line immunotherapy combined with RT yielded a more favorable curative effect.
Brain RT improves the immune microenvironment indicated by IBs
Gene expression profiling of SCLC patients has provided valuable insights into its heterogeneity, leading to its classification into four major subtypes: SCLC-A, SCLC-N, SCLC-P, and SCLC-Y. Most of these are immune-desert phenotypes characterized by a deficiency of CD8+T cells and an accumulation of regulatory T (Treg) cells and myeloid-derived suppressor cells (MDSCs) within the tumor microenvironment (TME). Consequently, the curative effect of immunotherapy alone is limited [29, 30]. A previous study showed that poly (ADP-ribose) polymerase inhibitors (PARPi) combined with RT reshaped the immune microenvironment of SCLC [31]. ICIs sensitize tumor cells to RT by normalizing the tumor vasculature and reducing hypoxia, thereby ameliorating the immune-desert TME [32]. Regardless of whether the patient undergoes prophylactic cranial irradiation (PCI), SRS, or WBRT, brain RT consistently plays a crucial role in systemic therapy strategies [33, 34]. Therefore, RT to the brain, lungs, or bones has a significant effect on immune activation during immunotherapy in patients with ES-SCLC.
Elevated NLR, PLR, and SII values from baseline are associated with shorter OS and progression-free survival [15, 35]. Consistent with these findings, a previous study suggested that lymphocytopenia is an unfavorable factor affecting immunotherapy outcomes in NSCLC patients [36]. This highlights the importance of improving the immune status by increasing the lymphocyte count, followed by a reduction in the above-mentioned ratios. In our study, among the brain-, bone-, and lung-irradiated groups, the median delta-ratios in the brain RT group were consistently the lowest during immunotherapy, suggesting that irradiation to brain exerts the strongest systemic activation effect in patients with ES-SCLC. In line with our findings, in advanced NSCLC, RT to the brain has been shown to exhibit the best immune activation effect and patient outcomes compared with other organs [37]. However, we did not find a significant correlation between the different irradiated groups and patient survival in ES-SCLC.
PCI is recommended for ES-SCLC during first-line immunotherapy
Owing to the extensive capillary network in the lungs, a significant number of anastomotic branches exist among the pulmonary vessels, vertebral veins, and cerebral vessels [38], providing a direct route for cancer cells to retrogradely enter the brain through the systemic circulation. However, in the case of other types of tumor cells, their initial destination is typically the lungs where they are transported via the venous system, and most cells are filtered through lung capillaries [39]. Consequently, the incidence of brain metastases (BMs) is substantially higher in SCLC. Compared with NSCLC, small cell carcinoma is more invasive and has a higher probability of intracranial metastasis, and approximately 60%–80% of patients who survive for more than 2 years develop brain metastases.
It is evident that RT to the brain not only activates the systemic immune state but also reduces the risk of brain metastasis. PCI is associated with a decrease in brain metastases in ES-SCLC, without a demonstrable impact on OS [40]. However, it should be noted that this reduction is primarily observed when RT is administered sequentially after chemotherapy.
Immunotherapy is very different from CHT; in particular, its combination with PCI to improve the TME is much stronger than chemotherapy alone. Consequently, in patients with ES-SCLC undergoing first-line systemic immunotherapy, PCI is recommended as the preferred approach over regular magnetic resonance imaging for monitoring intracranial metastases. Nevertheless, the absence of a significant correlation between different irradiation groups and patient survival in our study may be attributed to the limited number of cases examined. Further fundamental and prospective investigations are warranted to provide more robust evidence and elucidate the underlying reasons for these outcomes.
Earlier initiation of RIT in ES-SCLC leads to improved survival outcomes
The unique characteristics of the immunotherapy survival curve, known as the "late separation" and "long tail" effects, suggest that OS milestones such as 9- or 12-month OS rates may be associated with 5-year survival benefits [41]. In our analysis of factors associated with the 9-month OS, treatment stage was an independent prognostic factor, and subgroup analyses revealed a statistically significant correlation only between the first-line therapy and OS.
With the publication of results from the IMpower133, CASPIAN, ASTRUM-005, and CAPSTONE-1 studies, ICIs have been shown to greatly improve the survival of patients with ES-SCLC [42]. Although immunotherapy combined with chemotherapy (ICT) plays a significant role in ES-SCLC, data related to the combined efficacy and safety of RT is insufficient, and the results are controversial. A single-center retrospective study analyzing the safety and efficacy of first-line ICT combined with CTRT for ES-SCLC showed estimated OS rates of 97.1%, 80.2%, and 53.3% at 6 months, 1 year, and 2 years, respectively, with controlled safety [43]. However, another study indicated that the combination of first-line anti-PD-L1 blockade with BRT did not confer a significant survival benefit in patients with ES-SCLC with BMs [44]. These results suggest that the addition of RT to ICT may yield different outcomes. However, in our real-world analysis, RT combined with immunotherapy showed a trend of survival improvement in first-line RIT regardless of the irradiated site, and patients who received RIT earlier achieved higher survival rates.
The underlying cause of the diminished efficacy of immunotherapy combined with RT in ES-SCLC when treatment initiation is delayed remains unclear. Prospective studies on NSCLC, such as the KEYNOTE-189 and KEYNOTE-407 trials, demonstrated that patients transferred to the immunotherapy group after chemotherapy failure experienced significant improvements in survival outcomes [45, 46]. These findings suggest that earlier intervention leads to better treatment efficacy. However, studies focusing on SCLC are lacking. Although CHT eliminates tumor cells using cytotoxic drugs, it can induce an inflammatory response and promote the expression of drug-resistant tumor cells and factors that facilitate tumor metastasis [47].
Research has shown that a decrease in interleukin (IL)-2, IL-12, and interferon-γ coupled with an increase in lL-6, IL-8, IL-10, and tumor necrosis factor, can lead to the expansion of regulatory immune cells, such as Tregs, MDSCs, and M2 macrophages [48]. The accumulation of these regulatory immune cells elevates the expression of PD-1/CTLA-4, inhibits T-cell proliferation, and impairs the cytotoxicity of natural killer cells, resulting in immunosuppression [49]. Patients with SCLC typically exhibit weakened immune responses and stronger immunosuppression. Therefore, it is postulated that following prior chemotherapy, the body's immune system may be partially damaged, and immune cells within the TME may not be abundant. Consequently, in a heavily compromised immune system, immunotherapy combined with local RT may no longer achieve maximum effectiveness. In this real-world analysis, the OS rates of first-line immunotherapy combined with RT demonstrated a significant improvement compared to those of second- or third-line and subsequent therapies. Therefore, for patients with a favorable immune status, early administration of RIT is likely to yield enhanced local control and an extended estimated survival duration.
Therefore, considering the impact of radiation on the TME, we suggest that the integration of RT should be considered earlier in the era of immunotherapy for ES-SCLC.