TABLE II.

Summary of the Most Important Tumor Entities in the Differential Diagnosis of GIST

Tumor entity	Preferential site in GI tract	Patient demographics	Key pathologic features contrasting with GIST
Leiomyoma (intramural)	Esophagus, rare elsewhere	Young adults, also older	Eosinophilic spindle cells with immunohistochemical positivity for smooth muscle markers and negativity for KIT and anoctamin 1
Leiomyoma (of muscularis mucosae	Colon and rectum	Old adults	Small mucosal polyp. Tumor cells histologically similar to intramural leiomyoma
Leiomyoma, Mullerian type	Colon, also in the abdominal cavity	Middle-aged women	Estrogen and progesterone receptor-positive leiomyomas (low mitotic activity and atypia), comparable to uterine smooth muscle tumors and representing their extrauterine counterparts
Leiomyosarcoma	Relatively most common in colon, occurs at all sites	Old adults	Similar to leiomyoma, but contains nuclear atypia and mitotic activity. Negative for KIT and anoctamin 1. Positive for smooth muscle markers, desmin-positivity may vary
Glomus tumor	Stomach, nearly exclusively	Adults, marked female predominance	Round tumor cells with variably eosinophilic tumor cells. Positive for smooth muscle actin and negative for KIT
Schwannoma	Stomach, colon, rare in small intestine and elsewhere	Older adults	Spindle cells forming microtrabeculae or microfascicles. Tumor cells immunohistochemically positive for S100 protein and GFAP and negative for KIT and anoctamin 1.
Inflammatory myofibroblastic tumor	All segments of GI-tract, also other abdominal sites	Infants, children and young adults	Elongated, cytoplasm-rich spindle cells interspersed with lymphocytes and plasma cells. Typically positive for ALK and ALK-gene rearrangement by fluoresce in situ hybridization (FISH). Negative for KIT and anoctamin 1.
Inflammatory fibroid polyp	Small intestine, stomach, rare elsewhere	All ages, rare in children	Polypoid intraluminal mass, often in terminal ileum causing intussusception, or a small gastric polyp. Epithelioid to spindled lesional cells in a loose myxofibrous matrix with abundant capillaries. Contains eosinophils and mixed inflammatory cells. KIT negative.
Desmoid fibromatosis	Stomach and intestines	From young adulthood on	Moderately cellular, collagenous to myxoid spindle cell tumor. KIT negative. Nuclear positivity for beta-catenin is common
Synovial sarcoma	Stomach	Varies	Cellular spindle cell tumor, may involve mucosa or be transmural. Keratin-positive component, negative for KIT
Dedifferentiated liposarcoma	Intra-abdominal space, but may also involve intestines	Middle-aged to old adults	Spindle cell tumor with various appearances. Can simulate other tumors such as GIST, smooth muscle tumors, and fibromatosis. Negative for KIT and anoctamin 1. Often shows nuclear positivity for MDM2