Many mucinous adenocarcinomas occur in patients with hereditary nonpolyposis colorectal cancer (HNPCC or Lynch syndrome) and thus represent high-level MSI (MSI-H) tumors (12). These tumors are expected to behave in a low grade fashion. In contrast, mucinous adenocarcinomas that are microsatellite stable (MSS) are expected to behave more aggressively, particularly when detected at an advanced stage. Figure

4 Mucinous adenocarcinoma showing abundant extracellular mucin (original magnification ×200) Signet ring cell adenocarcinoma In contrast Inhibitors,research,lifescience,medical to that in the stomach, signet ring cell adenocarcinoma is rare in the colorectum, representing <1% of all colorectal carcinomas. Similar to mucinous carcinoma, signet ring cell carcinoma is defined by the presence of >50% of tumor

cells showing signet ring cell features characterized by a prominent intracytoplasmic mucin vacuole that pushes the nucleus to the periphery (Figure 5). Signet ring cells may show an infiltrative growth Inhibitors,research,lifescience,medical pattern or are present within the pools of extracellular mucin. By definition, signet ring cell carcinoma is poorly differentiated (high grade) and carries a worse outcome than conventional adenocarcinoma Inhibitors,research,lifescience,medical (11,13,14). However, some signet ring cell carcinomas may be MSI-H tumors and thus may behave as low grade tumors biologically (3). Figure 5 Signet ring cell carcinoma (original magnification ×400) Medullary carcinoma Medullary carcinoma is extremely rare, constituting approximately 5-8 cases for every 10,000 colorectal cancers diagnosed, with a mean annual incidence of 3.47 (±0.75)

per 10 million Inhibitors,research,lifescience,medical population (15). This tumor is characterized by sheets of epithelioid neoplastic cells with large vesicular nuclei, prominent nucleoli, and abundant cytoplasm. It typically has a pushing border on resection specimens (Figure 6), and is characteristically associated with marked tumor-infiltrating lymphocytes (Figure 7). Medullary carcinoma is a CCI-779 cost distinctive histologic subtype that is strongly associated with MSI-H (16,17). It usually has a favorable prognosis despite its poorly differentiated or undifferentiated Inhibitors,research,lifescience,medical histology. Figure 6 Medullary carcinoma showing a pushing border at the tumor edge (original magnification ×40) Figure 7 Medullary carcinoma showing poorly differentiated histology and tumor-infiltrating lymphocytes (original magnification ×400) Immunohistochemical phenotype The most widely used immunohistochemical CYTH4 markers for colorectal adenocarcinoma are cytokeratin (CK) 20, CK7 and CDX2. The most common immunophenotype of colorectal adenocarcinoma is positivity for CK20 and negativity for CK7, which is a relatively specific staining pattern for colorectal origin (18). However, up to 20% of the tumors may exhibit a CK7-positive/CK20 negative or CK7-negative/CK20-negative staining pattern. It has been suggested that reduced or absent CK20 expression in colorectal carcinoma is associated with MSI-H (19).