Evaluating Consolidative Thoracic Radiotherapy with Atezolizumab Maintenance in Extensive-Stage Small Cell Lung Cancer: Insights from the Phase 2 TREASURE Trial

Highlight

• Consolidative thoracic radiotherapy (TRT) combined with atezolizumab maintenance did not improve overall survival in extensive-stage small cell lung cancer (ES-SCLC) compared to atezolizumab alone.
• Combination therapy resulted in significantly higher severe adverse events, including fatal outcomes, predominantly related to infections and respiratory complications.
• Radiation-induced lymphocyte depletion was noted as a potential mechanism facilitating infections and toxicity in patients receiving TRT.
• TRT in ES-SCLC with immunotherapy requires cautious patient selection and further research to identify beneficiaries without excessive risks.

Study Background

Small cell lung cancer (SCLC) accounts for approximately 15% of lung cancer diagnoses and is characterized by aggressive progression and early metastasis. Extensive-stage SCLC (ES-SCLC) encompasses cases where the disease has spread beyond a limited stage and is treated primarily with systemic therapy. The current standard first-line treatment includes a combination of platinum-based chemotherapy and immunotherapy maintenance, such as atezolizumab, which has shown survival benefits. Despite systemic control, intrathoracic disease progression remains common, leading to morbidity and death. Consolidative thoracic radiotherapy (TRT) has previously demonstrated benefits in selected patients post-chemotherapy by improving local control and survival. However, in the era of chemoimmunotherapy, the added value and safety of TRT combined with immunotherapy maintenance are unclear, particularly given potential overlapping toxicities and immunomodulatory effects. Thus, the TREASURE trial addresses the critical unmet need to clarify whether adding consolidative TRT to atezolizumab maintenance improves outcomes safely in ES-SCLC.

Study Design

The TREASURE trial (ClinicalTrials.gov: NCT04462276) was a multicenter, open-label, phase 2 randomized clinical study conducted in Germany and Austria. Recruitment occurred from September 2020 to August 2022, with follow-up completed in September 2024 and data locked in April 2025. A subsequent post hoc survival update was performed in April 2026.

Inclusion criteria targeted patients with ES-SCLC who demonstrated at least stable disease following induction chemoimmunotherapy consisting of carboplatin, etoposide, and atezolizumab. Participants (N=68) were randomized 1:1 into two arms: Arm A received consolidative TRT (30 Gy delivered over 10 fractions) alongside atezolizumab maintenance, while Arm B received atezolizumab maintenance alone.

The primary endpoint was overall survival (OS), defined as the time from randomization to death from any cause. Secondary endpoints included progression-free survival (PFS) and safety outcomes. The trial was terminated early due to emerging safety concerns.

Key Findings

The trial’s findings revealed that adding consolidative TRT to atezolizumab maintenance did not confer a survival advantage in an unselected ES-SCLC population. Median OS was numerically shorter in the combination arm (6.7 months; 95% CI, 5.1–9.0) versus the atezolizumab-only arm (13.4 months; 95% CI, 10.7–17.5), although this did not reach statistical significance (hazard ratio [HR], 1.55; 95% CI, 0.90–2.69; P=0.34). Median PFS was comparable between arms (2.4 months in the combination arm vs 2.6 months in atezolizumab alone; HR, 0.92; 95% CI, 0.54–1.55; P=0.85).

Safety analysis highlighted a markedly increased incidence of severe adverse events (SAEs) in patients receiving TRT plus immunotherapy (61.3%) compared to immunotherapy alone (18.2%; P < 0.001). Fatal adverse events occurred more frequently in the combination group (19.4% vs 3.0%; P=0.04), with infections and respiratory disorders accounting for most serious complications. Mechanistic investigations suggested radiation-induced depletion of lymphocytes in the combination arm may have compromised host immunity, predisposing to infections. Additionally, patients experiencing fatal events showed lower single-breath diffusing capacity for carbon monoxide (DLCO), indicating reduced lung reserve which might have heightened vulnerability to pulmonary toxicity.

Baseline characteristics were well balanced across groups and between patients with or without SAEs, limiting confounding effects. The premature trial termination was driven by concern over heightened toxicity paired with lack of observed survival benefit.

Expert Commentary

These findings underscore the complexities of integrating thoracic radiotherapy with immune checkpoint inhibition in ES-SCLC. While TRT historically contributed to local disease control post-chemotherapy, the immunomodulatory effects of radiotherapy may also impair lymphocyte populations crucial for effective immunotherapy. The predominant infectious and respiratory toxicities associated with combination treatment highlight the need for careful patient selection, particularly evaluating baseline pulmonary function and immune competence.

Notably, the trial’s early termination and limited sample size constrain definitive conclusions. However, the safety signals merit caution. Future studies might explore tailored radiotherapy dose regimens, advanced radiation techniques that spare healthy lung tissue, or sequencing strategies optimizing immune recovery. Biomarkers predictive of lymphocyte recovery or pulmonary reserve could help identify patients who might safely benefit from consolidative TRT.

Current clinical guidelines continue to support consolidative TRT in selected patients with ES-SCLC after chemotherapy, but the addition of immunotherapy maintenance warrants further investigation before becoming routine practice.

Conclusion

The Phase 2 TREASURE trial demonstrated that consolidative thoracic radiotherapy combined with atezolizumab maintenance in unselected ES-SCLC patients increased severe toxicities and fatal adverse events without improving overall survival. Radiation-induced lymphocyte depletion appears to compromise host immunity, favoring infections and respiratory complications. These results emphasize the necessity of cautious patient selection and further research to delineate subsets of patients who might derive benefit from combined radiotherapy and immunotherapy approaches with acceptable safety profiles.

Funding and Clinical Trial Registration

This investigator-initiated trial was conducted across multiple centers in Germany and Austria. The study is registered at ClinicalTrials.gov under identifier NCT04462276.

References

1. Bozorgmehr F, Chung I, Behnisch R, et al. Consolidative Thoracic Radiotherapy With Atezolizumab Maintenance in Extensive-Stage Small Cell Lung Cancer: The Phase 2 TREASURE Randomized Clinical Trial (AIO-TRK-0320). JAMA Oncol. 2026 Jul 9. doi:10.1001/jamaoncol.2026.42424047.

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3. Slotman BJ, van Tinteren H, Praag JO, et al. Use of thoracic radiotherapy for extensive stage small-cell lung cancer: A phase 3 randomised controlled trial. Lancet. 2015;385(9962):36-42.

4. Antonia SJ, Villegas A, Daniel D, et al. Durvalumab after Chemoradiotherapy in Stage III Non–Small-Cell Lung Cancer. N Engl J Med. 2017;377(20):1919-1929.

5. Palma DA, Olson R, Harrow S, et al. Stereotactic Ablative Radiotherapy Versus Standard of Care Palliative Treatment in Patients With Oligometastatic Cancers (SABR-COMET): A Randomised, Phase 2, Open-Label Trial. Lancet. 2019;393(10185):2051-2058.

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