Gastrointestinal stromal tumor (GIST)

Biopsy specimen from a GIST showing two histologic patterns. Part of the tumor (upper panel) is composed of spindle cells with elongated nuclei and eosinophilic cytoplasm. Other portions of the tumor (lower panel) consist of epithelioid cells having round central nuclei within eosinophilic or clear cytoplasm. On immunostaining, the tumor is positive for CD117 (C-kit) and CD34, and negative for the smooth muscle markers actin and desmin and the neuronal marker, S100 protein. This profile is diagnostic of a GIST. CD117 is the marker for the interstitial cells of Cajal, which may be the cell of origin of gut stromal tumors. CD34 is a less specific marker and is not positive in all GISTs.