Diagnosis of a gastrointestinal stromal tumor presenting as a prostatic mass: a case report.

Gastrointestinal stromal tumors (GISTs) are an unusual and heterogeneous group of spindle cell tumors that can also appear on the exterior of the gastrointestinal tract (extra-GISTs). Despite the fact that extra-GISTs or large rectal GISTs can lead to the clinical impression of a prostatic mass, these tumors are, in general, excluded in the differential diagnosis of spindle cell tumors observed on prostate needle biopsy. Here, we present, in detail, a case of an extra-GIST identified on prostatic biopsy; the tumor was previously believed to be a primary prostatic stromal sarcoma in the differential diagnosis. Every investigator should check for KIT (CD117) in immunohistochemical staining to rule out an extra-GIST prior to diagnosing a solitary prostatic tumor, specialized prostatic stromal tumor, or leiomyosarcoma on prostate needle biopsy.