Immunohistochemical spectrum of GISTs at different sites and their differential diagnosis with a reference to CD117 (KIT)

Gastrointestinal (GT) stromal tumor (GIST) is the designation for the major subset of GI mesenchymal tumors and encompasses most tumors preciously cla ssified as GI smooth muscle tumors. Although GISTs typically express CD117 (KIT), often express CD34, and sometimes express alpha -smooth muscle actin (SMA), the relative frequency of these markers has not been characterized in large series of GISTs of different sites, and the CD117 expression has n ot been fully characterized in intra-abdominal tumors. in this study, we im munohistochemically analyzed 292 GISTs throughout the GI tract, including o mentum and mesentery, and compared the immunoreactivities with 211 other tu mors that may enter in the differential diagnosis. GISTs were defined in th is study as CD117-positive primary spindled or epithelioid mesenchymal tumo rs of the GI tract, omentum, or mesentery, The CD34 positivity of GISTs var ied from 47% in small bowel to 96 to 100% in rectum and esophagus, whereas SMA expression showed the opposite patterns and was most frequent in the GI STs of small bowel (47%) and rarest in the GISTs of rectum and esophagus (1 0-13%), Desmin was seen only occasionally. S100 positivity was rare but was seen most frequently in small intestinal GISTs (15%). True leiomyomas from esophagus, muscularis mucosae of colorectum, and pericolic leiomyomas simi lar to uterine leiomyomas were negative for CD117 and CD34 and positive for SMA and desmin (46 of 46), Inflammatory fibroid polyps of stomach and smal l intestine were negative for CD117 but were often positive for CD34 (6 of 8) and variable for SMA (3 of 8). Inflammatory myofibroblastic tumors invol ving gastric or colonic wall were negative for CD117 but some showed CD117- positive endothelia GI schwannomas were all negative for CD117 and positive for S100 protein (11 of 11). Extremely focal CD117 positivity was seen in the neoplastic cells of some retroperitoneal leiomyosarcomas and liposarcom as. Among other CD117-positive tumors were intestinal metastatic melanomas (8 of 11) and extraskeletal Ewing's sarcomas (5 of 11), two of which were a bdominal, In conclusion, strong CD117 expression defines most primary GI me senchymal tumors as GISTs, which show different patterns for CD34 and SMA i n various parts of the GI tract, Some unrelated CD117-positive tumors (mela nomas, Ewing's sarcomas) should not be confused with GISTs.