Immunohistochemistry staining for mismatch repair proteins: The endoscopic biopsy material provides useful and coherent results

Immunohistochemistry (IHC) testing for mismatch repair proteins (MMRP) in patients with colorectal cancer can be performed on endoscopic biopsy material or the surgical resection material. Data are continuing to accumulate regarding the deleterious effect of neoadjuvant chemoradiation on MMRP expression. However, despite continuing rise in the use of endoscopic biopsies for IHC, most pathology departments still use mainly the surgical materials for IHC testing. In this study we compared the quality of stains among 96 colon cancer subjects with paired endoscopic and surgical material available for MLH1, MSH2, MSH6, and PMS2 stains (96 × 4, yielding 384 paired stains). Each slide received both a quantitative score (immunoreactivity [0-3] × percent positivity [0-4]) and a qualitative score (absent; weak and focal; strong). The quantitative scores of all MMRP were significantly higher among the endoscopic material (P <.001 for all). In 358 pairs (93.2%), both the endoscopic and operative material stained either strong (322, 83.9%) or absent (36, 9.4%). In 26 pairs (6.8%), the endoscopic material stained strong, whereas the operative material stained focal and weak. No endoscopic biopsy materials stained focal and weak. Our findings indicate that the biopsy material may provide more coherent results. Although these results may indicate that biopsy material provides coherent and useful results, it is yet to be determined if the demonstrated differences pose a real clinical problem in interpreting final results of IHC staining of such kind. Hence, we suggest that when available, the endoscopic material rather than the operative one should serve as the primary substrate for IHC staining.