26O Maintenance dostarlimab after chemoradiation (CRT) for patients (pts) with high-risk locally advanced cervical cancer (LACC): GEICO 78-C/ATOMICC randomised phase II trial

Background: When ATOMICC was designed (2018), CRT was standard for LACC and the impact of adding immunotherapy was unknown.Methods: This ongoing open-label randomised phase II trial is investigating maintenance dostarlimab in pts with high-risk LACC (FIGO 2009 definition) with complete (CR)/partial response after standard concurrent CRT.Pts were randomised 1:2 to observation or dostarlimab (500 mg q3w for 4 cycles → 1000 mg q6w for up to 2 y in total), stratified by histology, response to CRT and combined FIGO 2009 stage/nodal status.The primary endpoint is progression-free survival (PFS) per RECIST v1.1, with final analysis prespecified after 74 PFS events.The protocol was amended to add an interim analysis after ≥32 PFS events, reported here.Overall survival (OS) is a secondary endpoint.Results: In 134 pts randomised Aug 2019-Jun 2023, FIGO 2009 stage at diagnosis was IB-IIB/III/IVA in 58%/37%/5%, 69% had pelvic and 28% paraaortic positive lymph nodes, and 69% were in CR.Pts received current standard CRT and 79% underwent 3D HR-CTV brachytherapy.Median cisplatin duration was 4.3 (IQR 4.1-5.1)wks.By the data cutoff (17 Sep 2025; median follow-up 3.4 [95% CI 3.0-3.8]y), all pts had discontinued dostarlimab (56% after 2 y treatment, 18% disease progression, 13% toxicity related).Median dostarlimab duration was 24.7 (IQR 8.4-25.1)mo.At the interim analysis with 34/74 (46%) of the PFS events required for final analysis, PFS numerically but non-significantly favoured dostarlimab (stratified HR 0.64, 95% CI 0.31-1.31;3-y PFS rate 67% [control] vs 77% [dostarlimab]) (Table ).The most common grade ≥3 adverse events with dostarlimab were diarrhoea (5%) and asthenia (3%); 1 pt died from duodenal perforation unrelated to treatment. Conclusions:In pts with high-risk LACC, PFS at the interim analysis exceeded expectations in both arms.PFS numerically favoured dostarlimab, although not reaching statistical significance. Clinical trial identification: NCT03833479.