Histopathology of locally advanced colorectal carcinoma, with emphasis on tumor invasion of adherent peritoneal membranes.

Locally advanced colorectal carcinomas are characterized by neoplastic cells that invade beyond the colon and directly into surrounding organs and structures that include the retroperitoneum and abdominopelvic sidewall. These aggressive tumors are prognostically adverse and are categorized with highest possible tumor stage in current cancer staging systems. Recognizing colorectal carcinoma with extensive locoregional invasion is typically straightforward, but some cases can be diagnostically challenging. These include tumors with limited invasion of extracolonic tissue such as the subserosa in which there are no cells or structures that are histologically or architecturally distinct from the colonic primary. Tumor-associated injury of the colonic peritoneum often precedes invasion by the neoplastic cells and can cause the peritoneal membrane of the colon to adhere and subsequently fuse to the peritoneal membrane of a neighboring organ or structure, thereby creating a trans-peritoneal bridge through which a tumor can directly invade the adherent extracolonic tissue. Hence, locally advanced colorectal carcinoma can be narrowly defined by neoplastic cells that completely invade through the fused peritoneal membrane and into the subserosa of the adherent extracolonic tissue. The evaluation of tumor invasion of the fused peritoneal membrane, which is enhanced by the combined use of an elastic stain and an immunostain for pan-keratin, is essential for the proper staging of locally advanced colorectal carcinoma and its clinical management.