Thoracic Radiotherapy Added to Atezolizumab Raises Fatal Toxicity Risk in ES-SCLC

A randomized phase 2 trial has raised significant safety concerns about adding consolidative thoracic radiotherapy (TRT) to atezolizumab (Tecentriq; Genentech) maintenance for patients with extensive-stage small cell lung cancer (ES-SCLC). In the TREASURE trial (NCT04462276), the combination produced substantially more serious and fatal adverse events (AEs) without improving survival, suggesting that it should not be used routinely in unselected patients outside clinical trials.¹

Why the Combination Was Studied

Atezolizumab plus carboplatin (Paraplatin; Bristol-Myers Squibb) and etoposide (Vepesid; Cheplapharm Arzneimittel GmbH), followed by atezolizumab maintenance, became a first-line standard after the phase 3 IMpower133 trial (NCT02763579) demonstrated longer overall survival (OS) than chemotherapy alone.² Separately, the phase 3 CREST trial (NCT04165317), conducted before immunotherapy became standard, found that TRT improved 2-year OS and reduced intrathoracic progression among patients who responded to chemotherapy.³ These findings supported investigation of whether the strategies could be combined safely.

TREASURE enrolled adults with ES-SCLC who achieved at least stable disease following induction carboplatin-etoposide-atezolizumab. At 20 sites in Germany and Austria, 68 patients were randomly assigned 1:1 to atezolizumab maintenance with TRT at 30 Gy in 10 fractions or atezolizumab maintenance alone. Although 104 patients were planned, recruitment ended early after the safety monitoring committee identified more fatal AEs in the TRT arm.¹

No Survival Benefit Emerges

Median OS, the primary end point, was numerically shorter with TRT plus atezolizumab than with atezolizumab alone: 6.7 months vs 13.4 months, respectively (HR, 1.55; P = .34). Median progression-free survival (PFS) was similar between groups at 2.4 vs 2.6 months (HR, 0.92; P = .85).¹

Because early termination reduced the trial’s statistical power, these efficacy findings should be interpreted cautiously. The absence of a PFS benefit and numerical OS disadvantage offered no evidence that TRT improved the risk-benefit profile of maintenance atezolizumab.

Serious and Fatal Toxicities Increase

The clearest finding was the difference in toxicity. Serious AEs occurred in 61.3% of evaluable patients receiving the combination compared with 18.2% receiving atezolizumab alone (P < .001). Fatal AEs occurred in 19.4% and 3.0%, respectively (P = .04), although causal attribution for individual deaths was not always clear. Infections and respiratory disorders predominated; patients receiving TRT experienced more pulmonary and urinary infections, pneumonitis, dysphagia, esophagitis, and fever.¹

A time-to-event analysis associated TRT with increased AE risk (HR, 2.47; P = .01), whereas atezolizumab exposure was not significantly associated with risk. Concurrent vs sequential delivery of TRT relative to atezolizumab also did not explain the difference.¹

The investigators wrote that “combining consolidative [thoracic radiotherapy] with atezolizumab maintenance resulted in an unexpected increase of toxic effects.”⁴

Lymphocyte Depletion Signals a Potential Mechanism

Patients receiving TRT developed pronounced, persistent lymphocyte depletion as AEs accumulated. By maintenance cycle 2, depleted lymphocyte counts were reported in 91.7% of assessed patients in the combination arm vs 13.6% in the control arm (P < .001); leukocyte and neutrophil counts were unaffected. Lower baseline diffusing capacity of the lungs for carbon monoxide was also observed among combination-arm patients with fatal AEs, although the small sample limits firm conclusions.¹

Implications for Oncology Pharmacists

The results reinforce the pharmacist’s role in reviewing pulmonary history and function, trending blood counts, and rapidly evaluating fever, cough, dyspnea, dysphagia, or signs of infection. Multidisciplinary teams should carefully weigh overlapping immune- and radiation-related toxicities. The investigators recommended that future studies consider organ-at-risk radiation exposure, treatment timing, and “prospectively defined safety stopping rules.”⁴

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. doi:10.1001/jamaoncol.2026.2330

2. Horn L, Mansfield AS, Szczęsna A, et al. First-Line Atezolizumab plus Chemotherapy in Extensive-Stage Small-Cell Lung Cancer. N Engl J Med. 2018;379(23):2220-2229. doi:10.1056/NEJMoa1809064

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. doi:10.1016/S0140-6736(14)61085-0

4. Norton A. In SCLC, radiotherapy plus immunotherapy ups fatal events. Medscape. Published July 23, 2026. Accessed July 24, 2026. https://www.medscape.com/viewarticle/sclc-radiotherapy-plus-immunotherapy-ups-fatal-events-2026a1000ozq?form=fpf