639 Background: The objective of this study is to investigate the value of mismatch repair deficiency (MMR) in rectal cancers as a predictive fator of response to chemoradiation in rectal cancer. Methods: We evaluated a consecutive cohort of 109 patients with locally advanced rectal carcinomas, treated with a total dose of 5,040 cGy of preoperative radiation therapy, concomitant with infusional 5-FU-based chemotherapy. Radical surgery (TME) was performed in all patients. MMR status was determined by imunohistochemistry in tumor tissue from biopsies prior to chemoradiation. The primary endpoint was complete pathologic response (pCR). Results: 63 patients (57.8%) were male, and the mean age was 60 years (range from 28 to 93). The rate of pCR was 18.3%. Among clinical variables (age, sex, distance to dentate line, per-treatment CEA level, and clinical assessment of response), only clinical complete response (cCR) was significant associated with cPR (p=0.009). Only two patients (1.8%) presented tumors with MMR deficiency. These 2 patients had cPR. Despite the very low rate of MMR deficiency, the association with cPR was significant (p=0.032). Conclusions: Considering the low rate of MMR deficiency found, we do not consider MMR status as a good tool for predicting pathologic response to chemoradiation in rectal cancer. Further investigation can be considered among the subgroup of Lynch syndrome patients.
1758O Neoadjuvant pembrolizumab in localized/locally advanced solid tumors with mismatch repair deficiency
