Background: Small cell lung cancer (SCLC) is an aggressive tumour type accounting for 10-15 % of lung cancers. The role of thoracic radiotherapy (TRT) in extensive stage SCLC (ES_SCLC) in the era of checkpoint inhibitors (CPIs) is undefined.

Methods: SAKK 15-19 is a national, multicenter, single-arm prospective phase II study evaluating consolidative TRT after standard first-line chemotherapy plus durvalumab in ES-SCLC. Patients (pts) with confirmed ES-SCLC and ECOG ≤ 1 received carboplatin AUC5 (day 1), etoposide 100 mg/m² (days 1-3) and durvalumab 1500 mg (day 1) for 4 cycles, followed by maintenance durvalumab every 4 weeks for up to 2 years in pts without progression. TRT (39 Gy in 13 fractions over 2.5 weeks) was delivered within 5 weeks after chemoimmunotherapy. Prophylactic cranial irradiation was permitted. The primary endpoint was 12-month progression-free rate (PFR); secondary endpoints included progression-free survival (PFS), response rate and overall survival (OS). The study aimed at a 12-month PFR ≥ 25 % (H1) vs. ≤ 12.5 %.

Results: Forty-six pts were enrolled; 37 % had ECOG 0, 30 % had asymptomatic brain metastases. At 29-month follow-up, the 12-month PFR was 15.6 (95 % CI 7-27.5), median PFS 6.4 months (95 % CI 4.8-7.2) and median OS 15.0 months (95 % CI 10.2-22.0). Grade 3-4 treatment-related adverse events (TRAEs) occurred in 23.9 % of pts, mainly neutropenia (23.9 %) and thrombocytopenia (8.4 %). One pt (2 %) had grade 5 sepsis; no severe TRT-related toxicities were observed.

Conclusions: Adding consolidative TRT after chemotherapy plus durvalumab in ES-SCLC didn't meet the primary endpoint of the expected 12-month PFR. No additional severe toxicities from TRT were observed. Ongoing larger Phase III trials, including RAPTOR (LNRG LU007), will further define TRT's role in this setting.