Mismatch Repair system protein deficiency as a factor of resistance to chemo-radiotherapy treatment in patients with locally advanced rectal adenocarcinoma and its influence on disease-free survival.

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3615 Background: Available data on Mismatch Repair system deficit and microsatellite instability are conflicting and are generally derived from a small number of patients due to the rarity of this condition in rectal cancer. Our study aimed to evaluate the frequency and therapeutic implications of Mismatch Repair proteins (MMR) status in patients with locally advanced rectal cancer (LARC). Methods: We retrospectively collected data from 318 patients affected by LARC adenocarcinoma (cT3-4 +/- N1-2) treated at the Medical Oncology Unit of the University Hospital of Cagliari, Italy, the Medical Oncology Department, Fondazione IRCCS Istituto Nazionale dei Tumori, Milan, Italy and at the Medical Oncology Unit, AOU Ospedali Riuniti, Ancona. Italy. All patients included in the study underwent neoadjuvant concurrent capecitabine and long-course radiotherapy (RT) (total dose of Gy 50.4), afterwards a total mesorectal excision (TME) was performed. MMR expression was evaluated through immunohistochemistry. The primary objective was major TRG (0-1 Ryan’s score) while secondary objectives were pathological complete response, disease-free survival (DFS) and overall survival (OS). Results: 160 patients (148 pMMR and 12 dMMR) were included in exploratory cohort and 158 (146 pMMR and 12 dMMR) were included in validation cohort. A major TRG has been shown in 64/148 (42,6%) and 63/146 (43,1%) patients with pMMR in exploratory cohort and validation cohort, respectively; while no major TRG have been shown in dMMR patients in exploratory cohort nor in validation cohort. Both exploratory and validation cohorts showed a statistically significant higher median DFS in pMMR patients compared to dMMR ones: NR vs 14 months in exploratory cohort ( p = 0,003) and NR vs 17 months in validation cohort ( p = 0,02). Conclusions: Our retrospective study indicated an association between dMMR and poor or no response to preoperative chemoradiotherapy. These results are consistent with the role of MMR as a predictor of poor-response to chemoradiotherapy and represent a hypothesis-generating study for the selection of these patients for immunotherapy in the neoadjuvant setting, in which the available data are immature.

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