CS.01.1 TUMOR INFILTRATING MACROPHAGES AS A POTENTIAL PREDICTIVE BIOMARKER OF RESPONSE TO 5-FU IN PATIENTS WITH MICROSATELLITE-STABLE COLORECTAL CANCER

stains of endoscopic biopsies by both qualitative tests (paired Wilcoxon) and categorical tests (true negative if the stain was negative in both stains, false negative if one of the stains was positive and the other negative). Results: 74 patients were included, of whom 56 (75.7%) had positive Bethesda criteria. 11 tumors (14.9%) were located in the rectum and all were treated with neoadjuvant chemoradiation. 15 (20.2%) cases show no expression of one or more MMR proteins. Six patients had loss of MLH1 and PMS2 expression, 6 patients had loss of MSH2 and MSH6 expression, two patients had loss of MSH6 expression and one patient had loss of PMS2 expression. Compared to the biopsies obtained during colonoscopy, the mean score was significantly lower in surgical specimens for MLH1, MSH2, MSH6 and PMS2 in both the colon and the rectum (P<0.05 for all). The mean decrease was significantly greater among the rectal cancer cases for MLH1, MSH2, MSH6 (P<0.05). Surgical resection specimens from 3 patients (4.1%) with colon cancer and from 3 patients with rectal cancer (27%) demonstrated loss of expression of one or more MMR proteins (false negative). In all these cases, immunoexpression of MMR proteins was demonstrated on endoscopic biopsies. Conclusions: IHC stains from biopsies obtained during colonoscopy appear to be superior to stains performed on surgical specimens. Care should be taken when interpreting IHC stains after chemoradiation as there is a significant drop in the quality of the stains.