Extragastrointestinal stromal tumor of the abdominal subcutaneous tissue: Report of a very rare case at an unusual site.

Extragastrointestinal stromal tumors (EGISTs) are rare tumors that arise outside the digestive tract. We report a case of an EGIST arising in the subcutaneous tissue of the abdominal wall, which at this site can often be misdiagnosed as dermatofibrosarcoma protuberans. The tumor was surgically resected from a 72-year-old male Chinese Han patient, and pathological examination revealed spindle-shaped tumor cells with eosinophilic cytoplasm and an oval nucleus. Immunohistochemically, the tumor cells showed strong cytoplasmic positivity for CD34, c-KIT (CD117), and DOG1. Tests for activating mutations of GISTs showed that the tumor cells carried an in-frame deletion (NP_000213.1:p.Lys550_Gln556del) in exon 11 of c-KIT (CD117). Thus, an EGIST should be considered in patients with abdominal subcutaneous tumors with an epithelioid, spindle-shaped, or mixed morphology. Immunohistochemistry of c-KIT (CD117) and DOG1 and genetic testing for activating mutations are recommended to aid in the differential diagnosis of subcutaneous tumors. In short, although EGISTs are rare in the abdominal subcutaneous tissue, pathologists must be aware of their possibility.