Dostarlimab (PD-1) + Pelvic IMRT in MMR-D Endometrial Cancer: ctDNA Clears in Most, Flags Distant Relapse

D-RT (NCT04774419) is an investigator-initiated, single-arm Phase II study testing adjuvant dostarlimab (anti–PD-1 checkpoint inhibitor) given concurrently with standard pelvic IMRT after surgery in newly diagnosed stage III/IVA MMR-D endometrial cancer. In the ctDNA subset (n=15; 58 samples), 40% (6/15) were ctDNA+ post-surgery at baseline and ctDNA positivity largely tracked macroscopic residual disease (5/6 had visible/residual disease); ctDNA cleared post-treatment in all but 2 patients (both dedifferentiated histology) and no patient without recurrence had detectable ctDNA post-treatment. Among recurrences (n=5; 4 distant), ctDNA preceded or coincided with imaging in 80% (4/5) of distant relapses but missed an isolated local vaginal cuff recurrence—suggesting better performance for systemic/distant MRD than for purely local events; the parent trial previously reported 2-year PFS 80.6% in 31 evaluable patients (data cutoff August 1, 2025), but this abstract does not report Grade ≥3 immune- or RT-related toxicities for the ctDNA subset, limiting a safety vs SOC comparison.