Background: Only few data on microsatellite instability in rectal cancer are available in literature, and dMMR role in pre-operative chemoradiotherapy response is under debate. The aim of our study was to evaluate the frequency and therapeutic implications of dMMR status in patients (pts) with locally advanced rectal cancer belonging to our Center. Methods: Data were retrospectively collected from 231 patients belonging to the Medical Oncology Unit of the University Hospital of Cagliari from 2011 to 2021. All patients were affected by locally advanced rectal adenocarcinoma (cT3-4 +/- N1-2). All patients included in the study underwent neoadjuvant chemoradiotherapy treatment with capecitabine and RT long course (total dose of Gy 50.4) and subsequently underwent total mesorectal excision (TME) followed by adjuvant chemotherapy. Mismatch repair (MMR) expression was evaluated through immunohistochemistry on surgical samples. Results: Of the 231 patients, 213 were suitable for final analyzes. Patients median age was 68 years (range 34-89). 145/201 were male and 68 were female. 66 (31%) had stage II disease and 147 (69%) had stage III disease. Considering MMR, 205/213 (96%) patients had proficient mismatch repair (pMMR), while 8/213 (4%) had dMMR. In dMMR patients defective proteins were: MSH2 in 4 patients, MLH1 and PMS2 combined in 2 patients and MSH6 in 2 patient. dMMR patients showed, unlike pMMR patients, poor or no response to chemoradiotherapy. Responses were assessed through TRG evaluation (Ryan and Dworak scoring systems) on the primary tumour. 5 patients presented a TRG-3 and 3 patients showed a TRG-4, according to Ryan score. All of them had a grade 1 regression, according to Dworak. Conclusions: The results of our study, albeit with limitations related to the retrospective nature and the limited number of dMMR cases, might indicate a correlation between microsatellite instability and little or no response to preoperative chemo-radiotherapy. It would be useful to analyze the data prospectively and further evaluate MMR as a predictor of response to combined chemo-radiotherapy.

*¹Ryan tumor regression (TRG) score: TRG-1 no visible cancer cells; TRG2 single cells or small group of cancer cells; TRG3 residual cancer outgrown by fibrosis; TRG4 significant fibrosis outgrown by cancer; TRG5 no fibrosis with extensive residual cancer.*²Dworak regression score: grade 0, no response; grade 1, minimal response; grade 2, moderate response; grade 3, near complete response; grade 4, complete response.