PRODIGE 90 - (FFCD 2204) Neoadjuvant Dostarlimab with Short Course Radiotherapy in a Watch-and-wait Strategy for Microsatellite Unstable or Mismatch Repair-deficient Locally Advanced Rectal Cancer Patients

short course of radiotherapy (5 grays × 5 days)

dostarlimab 500 mg intravenous infusion every 3 weeks for 6 months (nine cycles)

"* Tissue collection (3 time points: baseline, w12 and 25) for:~* Exome (on tumor and normal tissue),~* 3'RNAseq,~* Hiplex Immunofluorescence~* In addition, 48 patients will be tested for Spatial transcriptomics assuming that 24 patients may have non-complete response and/or local recurrence and/or metastatic recurrence and will be paired with 24 patients with complete response and free of any disease at 2 years.~* Blood collection (5 time points: baseline, w3, 6, 12 and 24) for:~* ctDNA~* Cytokine dosage~* Proteomic analyses~* Stool collection (3 time points: baseline, w3 and 24) for:~Microbiota analyses"