This gastric adenocarcinoma is positive for cytokeratin, with brown-red reaction product in the neoplastic cell cytoplasm, with immunohistochemical staining. This is a typical staining reaction for carcinomas and helps to distinguish carcinomas from sarcomas and lymphomas. Immunohistochemical staining is helpful to determine the cell type of a neoplasm when the degree of differentiation, or morphology alone, does not allow an exact classification. Traditionally, the tumor cell morphology on light microscopy has been used to predict tumor behavior and prognosis. Further developments in molecular biology provide additional methods to determine tumor cell characteristics that can indicate how the tumor will act, how it can be treated, and what the prognosis for the patient may be.

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