Which of the following is NOT included in the IHC panel for primary gastric adenocarcinoma?

• A. CK7
• B. CDX2
• C. MUC2
• D. MUC1

Answer: (D)

Understanding how immunohistochemistry helps classify gastric adenocarcinoma relies on recognizing markers that reflect gastric versus intestinal differentiation. In practice, the panel used to characterize primary gastric adenocarcinoma focuses on markers that reveal mucin expression patterns and intestinal phenotype: CK7 and CK20 help establish epithelial origin and pattern, while CDX2 and MUC2 indicate intestinal differentiation, which is common in the intestinal-type of gastric cancer. These markers provide actionable information about the tumor’s lineage and differentiation pattern.

MUC1, although a mucin expressed in many epithelial tissues and various cancers, is not specific to gastric origin and is not routinely part of the standard gastric cancer panel. It’s less informative for distinguishing gastric versus intestinal differentiation in this context, so it isn’t typically included alongside CK7, CDX2, and MUC2 in the primary gastric adenocarcinoma workup.